CA1175106A - Method and apparatus for extending lateral range electrical conductivity logging - Google Patents
Method and apparatus for extending lateral range electrical conductivity loggingInfo
- Publication number
- CA1175106A CA1175106A CA000394520A CA394520A CA1175106A CA 1175106 A CA1175106 A CA 1175106A CA 000394520 A CA000394520 A CA 000394520A CA 394520 A CA394520 A CA 394520A CA 1175106 A CA1175106 A CA 1175106A
- Authority
- CA
- Canada
- Prior art keywords
- magnetometer
- sensor
- sensors
- leg portions
- magnetic field
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Abstract
METHOD AND APPARATUS FOR EXTENDED
LATERAL RANGE ELECTRICAL CONDUCTIVITY LOGGING
Abstract of the Disclosure A method of well logging in petroleum exploration and apparatus for use in the method is described. An improved magnetometer is disposed in a well bore to provide indications of the distribution, filimentation, and distortion of a con-trolled current flow within adjoining strata of interest. The current flow is excited by a distant alternating source located at the surface or at other boreholes, and perturbations in the current flow and accompanying magnetic field caused by the electrical characteristics of the adjoining strata are regis-tered by the magnetometer.
The magnetometer comprises two pairs of elongated U-shaped cores of laminated, high-permeability metal, adjacent portions of each pair being surrounded by a sensing coil. The coil outputs are fed to surface equipment for detection and analysis.
LATERAL RANGE ELECTRICAL CONDUCTIVITY LOGGING
Abstract of the Disclosure A method of well logging in petroleum exploration and apparatus for use in the method is described. An improved magnetometer is disposed in a well bore to provide indications of the distribution, filimentation, and distortion of a con-trolled current flow within adjoining strata of interest. The current flow is excited by a distant alternating source located at the surface or at other boreholes, and perturbations in the current flow and accompanying magnetic field caused by the electrical characteristics of the adjoining strata are regis-tered by the magnetometer.
The magnetometer comprises two pairs of elongated U-shaped cores of laminated, high-permeability metal, adjacent portions of each pair being surrounded by a sensing coil. The coil outputs are fed to surface equipment for detection and analysis.
Description
~S~
Back~round of the Invention __ __ __ The present invention relates, in general, to well loggin~ methods useful in petroleum exploration, and more par--ticularly to an improved, highly sensitive, down hole magne-5 tometer for detecting field perturbations caused by yeologics-tructures in the vicinity oE a well bore.
Geophysical well logging methods are extremely impor-tant in the exploration Eor petroleum deposits and for the systematic development of an oil field for production after it 10 has been located. Historically, electrical conductivity and self potential measurements for these purposes have been impor-tant, and over the years numerous patents have issued on such techniques and the apparatus for carrying them out. Typical examples of the approaches taken in -the prior art are found in lS U.S. patent No. 2,359,894 to Brown et al., U.S. patent No.
;2,723,374 to Williams, and U.S. patent No. 3,697,864 to Runge.
In Brown et al., electrodes are embedded in the earth or are placed in boreholes spaced from a main borehole in which is placed a measuring unit responsive to the current flow between 20 the electrodes. In the Williams patent, current injection is accomplished by electrodes which may be suspended in the main borehole a predetermined distance from the measuring unit, which is also suspended in the borehole. As an alternative, the patent discloses the use of surface electrodes for current 25 injection. The Runge patent discloses a plurality of current electrodes and a plurality of potential measuring electrodes secured at spaced locations along a logging cable or to a drill string within a well bore. In each of the foregoing patents, the current injected into the geological formations surround-30 ing the borehole is either D.C. or very low frequency A.C., and variations in the electrical current flow due to variations in the conductivity or resistivity of the geological formations are measured. Such measurements may be made by way of current or voltage sensitive electrodes, by induction coils or the like.
The patterns of current flow and the variations in potential which exist within the geological formations surround-ing the borehole have a direction and magnitude at any given location which refl~ct the electrical conductivity or resistiv-ity of the stratum in that area. By measuring and recording 40 these variations, it is possible to determine the general . ~ .
,
Back~round of the Invention __ __ __ The present invention relates, in general, to well loggin~ methods useful in petroleum exploration, and more par--ticularly to an improved, highly sensitive, down hole magne-5 tometer for detecting field perturbations caused by yeologics-tructures in the vicinity oE a well bore.
Geophysical well logging methods are extremely impor-tant in the exploration Eor petroleum deposits and for the systematic development of an oil field for production after it 10 has been located. Historically, electrical conductivity and self potential measurements for these purposes have been impor-tant, and over the years numerous patents have issued on such techniques and the apparatus for carrying them out. Typical examples of the approaches taken in -the prior art are found in lS U.S. patent No. 2,359,894 to Brown et al., U.S. patent No.
;2,723,374 to Williams, and U.S. patent No. 3,697,864 to Runge.
In Brown et al., electrodes are embedded in the earth or are placed in boreholes spaced from a main borehole in which is placed a measuring unit responsive to the current flow between 20 the electrodes. In the Williams patent, current injection is accomplished by electrodes which may be suspended in the main borehole a predetermined distance from the measuring unit, which is also suspended in the borehole. As an alternative, the patent discloses the use of surface electrodes for current 25 injection. The Runge patent discloses a plurality of current electrodes and a plurality of potential measuring electrodes secured at spaced locations along a logging cable or to a drill string within a well bore. In each of the foregoing patents, the current injected into the geological formations surround-30 ing the borehole is either D.C. or very low frequency A.C., and variations in the electrical current flow due to variations in the conductivity or resistivity of the geological formations are measured. Such measurements may be made by way of current or voltage sensitive electrodes, by induction coils or the like.
The patterns of current flow and the variations in potential which exist within the geological formations surround-ing the borehole have a direction and magnitude at any given location which refl~ct the electrical conductivity or resistiv-ity of the stratum in that area. By measuring and recording 40 these variations, it is possible to determine the general . ~ .
,
2 '~
characteristics of the formations, ~nd to identify anornalies which would indicate the probable locations and dirnensions of mineral beds, petroleum bearing deposits, and the like.
However, prior systems for the logging of geoloyical 5 formations have been found to be of limi-ted usefulness, since they lack the sensitivity required to make meaningful measure-ments at any appreciable distance :from the current or voltage source. Thus, the measurements have been limited to relatively short lateral distances from the borehole, or where the source 10 electrodes are located at the surface of the earth, at rela-tively shallow depths. As a result, it has been difficult to obtain accurate measurements of deposits which are not actually penetrated by the borehole, and even where the borehole passes through or very close to the edge of a deposit, it has been 15 difficult to determine with any degree of certainty the dimen-sions of the deposit. Thus, although a deposit may disturb a current flow pattern, the lack of sensitivity in prior measuring devices has made it difficult, if not impossible, to determine the nature of the geological anomaly any appreciable distance 20 from the point of measurement.
Attempts have been made to solve this problem through the provision of more sensitive detectors, and to this end magnetometers sensitive to faint magnetic fields have been proposed. However, in order to obtain accurate measurements 25 of subsurface magnetic fields, the horizontal components of the fields must be detected, and in prior devices this required a horizontally oriented magnetometer having significant hori-zontal dimensions. Unfortunately, exploratory well bores have a diameter of between about ~ and 6 inches, and simply do not 30 provide the space needed for conventional magnetometers of the required sensitivity. In order to accommodate prior magne-tometers, then, it was often necessary to redrill existing ex-ploratory wells, or to drill new exploratory wells with much greater diameters. This not only was prohibitively expensive, 35 but did not result in significantly better results.
Summary of the Invention It is, therefore, an object of the present invention to provide a method of geophysical exploration that will provide accurate and reliable information concerning deposits of minerals .
r~l~
~nd of hydrocarhons located a substant:i~l d.istance from a borehole and at substantial depths. It is also an object of the invention to provide an improved magnetometer for use in such exploration which will have a hiyh degree of sensitivity sto variations in maynetic fields due to yeological anomalies, and which will have dimensions tha-t will perm.it its use in small-diameter well bores such as those drilled for exploration purposes.
The present invention is an electromagnetic well log~-lOing method and apparatus fo.r obtaining electrical conductivitydata a considerable distance from a borehole, and finds partic-ular application in the logging of petrolellm bearing strata, which are electrically conductive. The invention provides integration of the measured electrical conductivity over a large 151aterally extending volume, and permits detection of lateral variations in conductivity, measurement of the lateral direction in which the conductivity is changing, and detection of the lateral extent of conduct.ive beds of material. In general, this is accomplished by exciting the strata in the vicinity of 20a borehole by means of an A.C. surface source. A sensitive magnetometer is lowered down the borehole and the depth depen-dence of the A.C. magnetic fields produced by the source is determined by means of a large number of measurements of the magnitude, phase, and direction of the fields in the strata near 25the hole. Any non-uniform electrical conductivity of the strata leads to a concentration of currents and of magnetic fields produced by the source into the more highly conductive strata, and their location and nature can be identified by analysis of the depth variations obtained from measurements made over a 30period of time.
To obtain the required data, the magnetometer of the present invention is used in conjunction with a conventional electronic compass and with a vertical field sensor which may be integrated with the magnetometer or be separate. The magneto-35meter is used to measure the horizontal A.C. magnetic fieldcomponents produced by a surface loop or by electrode injected currents and the compass measures the earth's magnetic field to determine the azimuthal orientation of the instrument. The horizontal field magnetometer consists of a pair of magnetic 40field sensors, each sensor comprising two generally U-shaped .
~, ;
.
~cL~L~7~6 core segments, each segment having two space~ parallel legs extendlng from an lnterme~iate hody portion. The core is con-structed of a lamina-ted, high permeability magnetic material, and the core segments are assembled hack-to-back with a common 5coil oE about 25,000 turns surrounding the adjacent core body portions. The core segmen-ts have highly elongated legs, on the order of 18 inches, as cornpared to a short body portion length of about two to three inches, so that the ef~ective diameter of the sensors allows the magnetome-ter to ~it within an 10 explorator~ well bore.
The core segments each are formed of multiple contin-uous laminations of mu-metal, each lamination ex-tending the full length of the core segment. l~fter formation o~ the two segments into their respective U-shapes, the cores are assernbled 15 back-to-back, with the legs extending in opposite directions, but in the same plane, and are secured by means of a nonmagnetic channel member which is crimped tightly around the laminations to hold them securely together and to hold the core segments in assembled relationship. The assembled segments are then 20 carefully annealed to provide high permeability, and therea~ter the coil is wound around the adjacent intermediate body poxtions.
Suitable spacers are then secured to the free ends of the core legs, as by welding to the channel member, to complete the field sensor.
Two sensors arranged along a common axis form the mag-netometer of the present invention, with one sensor lying in a first plane and the second sensor lying in a second plane which is perpendicular to the first. The two sensors are secured in end-to-end relationship, but spaced a short distance 30 from each other, and the entire unit is mounted within a non-magnetic housing of low electrical conductivity having an outer diameter of less than about four inches, which is small enough to permit its use in a boreholeO Included in the housing with the magnetometer is an electronic compass, a suitable power 35 supply, and electronic circuiting for detecting the signals produced by the magnetometer and transmitting them to the sur-face for processing.
The magnetometer of the present invention is lowered into a vertical borehole, and by virtue of its exceptional 40 length, totalling about two meters for the two sensors, is capable of accurate and hiyhly sensitive rneclsurements of alte~-nating magnetic fields caused by injected t~urrents. The measurements so obtained are used in conjunction with ver-tical field measurements to provide the required data.
5 Brief Description of the Drawinys The foregoing and additional objects, features and advantages of the present invention will become apparent to -those of skill in the art from a considera~ion of the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings in which:
Fig. 1 is a diagrammatic illustration of a well loygin~
system utilizing a loop source of current;
Fig. 2 is a diagrammatic illustration of a well logging system utilizing a current dipole;
Fig. 3 is a diagrammatic illustration of circuitry for use with the present invention;
Fig. 4 is a partial sectional view of a magnetometer constructed in accordance with one form of the pxesent inven-tion;
Fig. 5 is a sectional view of one sensor unit from the magnetometer of Fig. 4 taken along lines 5-5 of Fig. 7;
Fig. 6 is a cross-sectional view of a sensor unit taken along lines 6-6 of Fig. 5;
Fig. 7 is a sectional view of a sensor taken along 25lines 7-7 of Fig. S;
Fig. 8 is a perspective view of a single laminate folded to form part of a core segment for the sensor of Fig. 5;
Fig. 9 is a more detailed illustration o-f down-hole circuitry for use in the present invention; and Fig. 10 is a more detailed illustration of surface electronics for the present invention.
Description of ~ Preferred Embodiment Turning now to a more detailed consideration of the drawings, there is illustrated in Fig. 1 a well logging system 35Of the type that may be used with the improved magnetometer of the present invention to obtain highly accurate indications of geological formations, and particularly of formations which contain electrically conductive material, to facilitate the location and mapping of petroleum and brine pools, mineral 40 deposits, and the like. Since the present magnetometer is '' ' '~ ' 6 ~ 7~
particulaxly useflll in locating petroleum deposits, the follow-ing clescription of a preferred embodiment will have specific reEerence to such an application. Mowever, it should be un~er-stood that the invention is not limited to the location of 5 petroleum deposits, and that other deposits, such as minerals, which produce anomalies in the underground magne-tic fields and electrical currents may also be det:ected and identified.
The association of gas, oil and brine in porous geo-logical s-tructures is well known, and electrical methods may 10 be used in well logging because of the high electrical conduc-tivity of such brines, because of the much lower conductivity of petroleum, and because of the resistivlty of host rock forma-tions. The stratographic setting of petroleum and the associaked brine, which appear in relatively thin horizontal len-ticular lS bodies w.ith a few kilometers lateral extent, leads to char-;acteristic perturbations of electric current flows or ofmagnetic fields in their proximity. These perturbations depend not only upon the characteristics of the stratographic structure actually pierced by the borehole, but upon the characteristics 20 of neighboring structures as well, and the accurate measurement of these perturbations can give valuable geophysical information.
By means of the sensitive magnetometer of this invention, the distribution, filimentation, and distortion of a controlled subsurface current flow, produced either by distant electrodes 25 to which an A.C. source is applied or by a controlled magnetic field which is in turn caused by a current flow at the surface, can be accurately measured. Since the electric currents so produced will try to follow paths of least resistance, these currents will be concentrated or spread out by the character-30 istic conductivity or resistivity of the strata, and the areasof concentration or separation will be manifested by magnetic field gradients which are detected by the magnetometer of the invention to produce "signatures" in the magnetometer output signals. It has been found that the magnetometer of the present 35 invention will produce such characteristic signatures even in situations where presently available resistivity and conductiv-ity well logs indicate uniformity in the strata. It has further been found that such information can be obtained at a much greater lateral distance from the borehole than was heretofore 40 possible.
~, 7 ~l~Lt75~
Eigs. 1 and 2 lllustrate ~wo methods ~or creatin~ the desired magnetic flelds ln the strata of lnterest In Flg. 1, a magne-tic Eield ls produced by rneans of a slngle -turn current loop formed by a cable 10 located on the surface of the earth 5 and surrounding a borehole 12. The cable forms a loop which may, for example, be aoo meters or more in diameter and excited by a suitable source 14 of alternating current. The A.C. source is supplied by way of a sili.con controlled rectifier inverter 15 which produces an alternating square wave current of about 10 20 amperes. In a test of the magnetometer oE the present in-vention, it was Eound that a suitab].e magnetic field could be produced at frequencies below 60 Hz, with good results being obtained in tests using sources of 7 Hz or 36 Hz.
Borehole 12 is a typical exploration hole which is 15 cased in the normal manner with 5-1/2 inch inner diameter non-magnetic, nonconductive (e.g., fiberglass) pipe over at least a part of its depth. A magnetometer 16 constructed in accord-ance with the invention is suspended in the well hole by means of a standard well logging cable 18 which may for example, be 20 15,000 feet long. The cable includes suitable electric wiring to permit connection of the electronics in the magnetometer 16 with surface processing circuitry 20 which may be mounted in a truck or trailer for portability from one borehole to another.
The flow of current in the single turn loop cable 10 produces a magnetic field generally indicated by lines 22, the lines of magnetic field force being centered on the cable 10 and extending to a considerable depth within the earth. As indicated above, the exact pattern followed by the magnetic 30 field will depend upon the electrical characteristics of the earth's structure. Layers of conductive material or layers of non-conductive material will disturb the regular lines of flux for the magnetic field, producing anomalies, or aberrations, which can be detected by the magnetometer.
Fig. 2 illustrates an alternative mode for obtaining the desired magnetic field within the strata of interest. In this arrangement, the loop 10 is replaced by a pair of elec-trodes 24 and 25 which may be embedded in the ground at the surface or which may be located within additional boreholes 40 in the vicinity of the hole 120 As illustrated, electrodes 24 .. . . .
:; .
8 ~.~'7~
and 25 are located at the surface 2~ and the source of alter-nating current 1~ is connected thereacross ~y wa~ of cab]es 30 and 32. In one test of -the magnetometer of the invention, the electrodes were spaced apart by about 1,200 meters and a current 5Of approximately 20 amperes was fed to them. Again the alter-nating source may be a square wave operated at 60 ~z or less, with 7 Hz and 36 Hz providing suitable results.
The source connected across electrodes 24 and 25 pro-duces a potential difference therebetween which results in a lOcurrent flow extending through the strata beneath the surface of the ground 28. The current flow will follow a mu~titude of paths, as diagrammaticall~v illuslrated by the current flow lines 34, with the exact path followed by the current being dependent upon the conductivity characteristics o~ -the strata 15and the loaation of the electrodes. For example, an oil bear-ing stratum is generall~ indicated at 36, with the stratum including a pool of conductive brine 38 covered by a layer ~0 of lower conductivity petroleum. If this petroleum and brine de-posit occurs in a surrounding stratum of relatively uniform 20conductivity, the disturbance created by the deposit will be substantial, as broadly illustrated in the drawing, and the resulting disturbance in the current flow can be detected by the magnetometer 16. It will be understood that electrical currents illustrated by lines 34 generate aorresponding magnetic fields, 25with the intensity of the current i at any point in space being related to the magnetic field B in accordance with the Maxwell equation: V x B=~Ol,where ~ = 4~ x 10 7 (MKS units). With a Cartesian coordinate system for the systems of Figs. 1 or 2, equation 1 can be written as:
~ jx = dBz dz ~iY = dBX - dBz ~y - dBx ~o~Z dx dy Since in the applications of the invention envisaged/
35the dominant structural variation of the strata is in the hori-zonta~ (x,y) plane, it has been found that many of the strato-graphic configurations of interest produce their most important and most characteristic magnetic field variations in the vertical (z) direction so that field variations with respect to depth are :.
:, , . : , .. . .
9 ~ 7~
sign.ificant. Accordingly, in accorclance with the invent.i.on, the x and y components of the maynetic fi.eld are measured at various depths, and the variations are plotted to provide a field profile in the z, or depth direction. The resulting field 5 chaxacteristics then provide informat.ion concerning the nature of the strata surrounding the borehole.
The downhole magne-tometer equipment 16 is illustrated in diagrammatic form in Fig. 3, to which reference is now made.
The magnetometer includes a conventional electronic fluxgate 10 compass 42 which may include a paix of coils oriented at right angles to each other and mounted in the horizontal ¦x,y) plane to measure the earth's magnetic field. The outputs 44 and 46 represent the horizontal components BN and BE, respec-tivel~, of the earth's magnetic field, and these may be fed through pre-15 amplifiers 48 and 50, respectively, which may be a part of the compass, to corresponding inputs 52 and 54 of a multiplexer 56.
It should be understood that the compass 42 is conventional and that its components are shown diagrammatically for purposes of illustration only.
The magnetometer unit 16 also incorporates a pair of ~.C. field sensors 58 and 60 which include sensor coils wound on specially prepared magnetic cores to be described. The coils and their cores are mounted at right angles to each other so that sensor 58 is responsive to changes in magnetic flux in 25 the x direction and sensor 60 is responsive to changes in mag-netic flux in the y direction. The outputs of the two sensor coils are fed through preamplifiers 62 and 64, respectively, and through filters 66 and.68, respectively, to inputs 70 and 72 of the multiplexer 56. The filters 66 and 68 are notch 30 filters which are selected to pass OUtpllt signals from the coils 58 and 60 which correspond in frequency to the frequency .of source 14.
Multiplexer 56 is designed to sweep through the inputs 70, 72, 52, 54 in sequence with the respective outputs being 35 supplied sequentially by way of line 74 to the input of a vol-tage controlled oscillator 76. Oscillator 76 may operate, for example, at a carrier requency of approximately 100 kilo-Hertz and is modulated by the voltages supplied in sequence from the sensor coils and the compass coils through the multi-40 plexer. The modulated carrier signal is fed to an amplifier .
' ' ' ' .
77 ~y way of line 78, r~nd the outp~t oE the ampli~ier is fedto the borehole cabl~ 18. A power supply 79, which may be a battery pack, provides suitable downhole bias voltages for operating the downhole elec-tronics, with power being supplied 5 by way of line 80 to the multiplexer, the voltage controlled oscillator 76 and preamplifier 77 and by way of line 81 to preamplifiers 62 and 64, as well as to other parts of the cir-cuit. At the surface, the signal is again ampli~ied by amplifier 82 and is fed through detector circuitry 84 to suitable 10 signal processing circuitry 86 for signal averaging, display, recording, and the like. The various signal processing functions may be carried out by means of a microprocessor such as a commercially available 5YM-l, manufactured by Synertek Systems Corp. For signal averaging, the processor may accumulate the 15 input signals from the borehole sensors over an appropriate period of time ~for example, 5 to 30 minutes, averaging the sig-nal and printiny out the results on a computer print-out terminal.
The information thus accumulated for each depth location of the magnetometer unit 16 may then be analyzed to determine the 20 parameters of the magnetic fields at various depths.
A preferred form of the structure of the borehole mag-netometer unit 16 is illustrated in Figs. 4 through 8, to which reference is now made. The unit comprises a housing 90 which is adapted to receive the sensors 58 and 60, the magnetic compass 25 42, the power supply 79, and an electronics package 92 which includes the various amplifiers, the filters, the multiplexer, the voltage controlled oscillator, and like down hole circuitry illustrated in Fig. 3. The housing 90 preferahly is a double-walled tubular container of the vacuum bot-tle type which serves 30 to insulate the contents of the bottle from the heat encountered in the typical down hole environment. rrhus, housing 90 is con-structed with an outer wall 94 spaced from a concentric inner wall 96 to define an interior space 98. The interior space is closed and sealed at both ends of the container by radially 35 extending walls 100 and 102 so that a vacuum may be maintained therein. The outer diameter of the housing is substantially less than the 5-1/2 inch inner diameter of the typical explora-tory well casing for borehole 12, and preferably has a diameter of approximately 4 inches so that the unit 16 may move easily 40 within the borehole.
~ ;
~' :
l:L ~ Sl~
The inner wall o~ the housiny ~e:~ines an i.nterior ca-vity 104 which receives the sensors, compass and electronic e~uip-ment, the cavity being closed by an upper cap 106 and a lower cap 108 which are threaded in-to the housi.ny to hermetically 5 seal cavity 104. Preferably, -the housing 90 is constructed of a non-magnetic metal to provide the strength required for wi.th-standing the hazardous high -temperature environment of a bore-hole, while allowing detection of the surrounding magnetic fields.
The sensor units 58 and 60 are adapted to be removably mounted within housing 90, with the first sensor unit located near the upper end of housing 90 and the second sensor 60 lo~
cated in the lower end of the housing. The two sensors are secured together in spaced, end-to-end coaxial relationship by 15 means of a suitable fastener 114, the fastener serviny to rigidly secure the two sensors for placement in and mounting within the housing. The two sensors are mounted so that their respective planes are at right angles to each other, whereby one sensor is adapted to measure magnetic fields in the "x" directi.on and 20 the other is adapted to measure magnetic ~ields in the "y"
direction. For purposes of reference, sensor 58 may be said to lie in the x plane, or x-measuring plane, and sensor 60 may be said to lie in the y, or y-measuring, plane. Fastener 114, which is shown for purposes of illustration as being a bolt 25 secured to the respective sensors by suitable nuts and lock washers, holds the sensors in an end-to-end relationship so that they can be mounted coaxially within housing 90. Suitable radially-extending spacers (not shown) may be mounted on the sensors to center them with respect to the interior wall 96 30 and to securely hold the sensors within cavity 104 during use o~
the unit.
Secured to opposite ends of the sensors are mounting boards 116 and 118, board 116 being mounted on the upper end of sensor 58 by means of suitable fasteners 120 and board 118 being 35 mounted on the lower end of sensor 60 by means of suitable fas-teners 122. Mounting board 116 carries the power supply 79 and electronics package 92, while mounting board 118 carries the magnetic compass 42, the boards serving to fasten these compon-ents rigidly to the sensor units for secure mounting within 40 housing 90. Again, the boards may be provided with suitable , - , ~
~ D~ 6 radial spacers to properly locate these cornponents within cavity 104.
Elec-tric cables 124, 126 and 128 interconnect the compass 42 and -the sensors 58 and 60, respectively, to the elec-5tronics package 92, while electrical cable 130 connects thepower supply 79 thereto. ~orehole cable 18 includes electrical wiring 132 which passes -through the threaded cap 106 and is sealed therein in known manner to provide electrical communi-cation between the electronics package of magnetometer unit 16 lOand the instrumentation at the surface of the earth. The mag-netometer is supported mechanically by the conventional borehole cable, which may be secured to the unit by means o~ fastener 134.
The structural details of sensor 58 are illustrated in Figs. 5 through 8, but it should be understood that sensor 60 15is identical thereto. The sensor comprises two generally U-shaped magnetic core segments each having a base portion with two elongated, parallel legs extending from opposite ends thereof.
The bases of the two core segments are arranged in back-to-back relationship wtih the legs extending in opposite directions so 20that the two core segments form a generallv H-shaped sensor lying in a single plane, as illustrated in Fig. 5.
The first, or upper, core segment 140 of sensor 58 consists of a base portion 142 and parallel elongated leg portions 144 and 146 extending from opposite ends of base 142 25so that the legs are spaced apart by a distance equal to the length of the base. Although the length of the base portion 142 is exaggerated with respect to the length of leg portions 144 and 146 for purposes of illustrating the structure of sensor 58, it should be understood that the length L of the leg portions 30is large with respect to the length d of the base portion, the legs being on the order of six to ten times the length of the base. The extreme relative length of the legs 144 and 146 provides a large area of interaction between the core segment and the magnetic field to be detected, and thus provides a high 3sdegree of sensitivity to the fields of interest. The short length of the base portion 142 provides a small effective dia-meter for the sensor so that it will fit within the relatively small inner diameter of housing 90, thereby accommodating the magnetometer to use in small-diameter exploratory boreholes.
.
.
.- . , : . .
.i , .
In the preferred form o~ the inventi~o~,7~le core segment 140 ls constructe~ of a plurality of very thin, elongated, con-tinuous laminations of a high permeabili-ty material such as mu-metal, each laminate extending from one end of the core 5segment to the other. A pre~erred sensor utiliæes a core of eleven laminations, each being .004 thick and with each lami-nation being continuous for the full leng-th of the core; that is, each lamination ex-tends the full length of leg 144, is folded to extend across the width of the core to form base 142, lOand is folded again to extend the full length of the opposite leg 146. For clarity of illustration, core segment 140 is shown as being constructed of only two laminations 148 and 150, with laminate 148 being illustrated in perspective in Fig~ 8, but it should be understood that numerous larninations are used. As 15illustrated, laminate 148 forms a part of leg portiorl 144, is folded upwardly at the bottom 152 oE the leg portion to form a part of the base portion 142, and is folded upwardly again at the bottom 153 o~ the leg portion 146 to form a part of that leg. ~lthough a number of fold configurations are possible, 20in one form the laminate 148 is doubled over in the ~ase portion 142 to form a double thickness (see Fig. 6), this double thickness being formed by means of triangular folds 154 at each side of the base portion 142. Similar folds are made at the bottom 153 of leg portion 146 (shown in phantom in Fig. 8).
25By reason of this particular folding arrangement, the width w of the laminate 148, which may be about 1/2 inch, is reduced in the base portion 142 to one-half its width in legs 144 and 146. However, this fold allows the base portion to be perpen-dicular to and centered with respect to the width of laminate 148 30for easier winding of the coil to be described. As seen in Fig.
6, upon assembly of the core segment 140, the folded-over portions which form the base 142 are stacked and folded one over the other, while the laminations forming the leg portions 144 and 146 remain flat.
The sensor 58 includes a second core segment 140' which is a duplicate of the segment 140, and accordingly it is similarly numbered, with the numbers being primed.
The two sensor elements are positioned with the respec-tive base portions 142 and 142' in back-to-back relationship as 40illustrated in Figs. 5 and 6, and with the corresponding leg ' ~ -`
1~
portions ex~encling ln opposi.-te directions. The hase portions define the plane of the s~nsor, and the legs lie in that plane, although the individual lamination in the leg portions are perpendicular to the plarle of the sensor.
After assembly of the laminations to form the two core segments 140 and 140l, and after positioning them in back-to-back relationship, the corresponding legs 144, 144' and 146, 146' on each side of the sensor 58 are secured in corresponding channel members 156 and 158. These channel members are sub~
lOstantially C-shaped and are crimped tightly around the lamina-tions to secure them in assembled relationship, each channel extending the full length of the sensor so that channel 156 sur-rounds and secures legs 144 and 144' while channel 158 surrounds and secures legs 146 and 146'. Preferab:Ly, the channels 156 15and 158 are of a non-magnetic material such as stainless steel.
Assembly of the laminations and of the core segments in the manner described forms the H-shaped core of sensor 58.
This core is then suitably heat-treated, as by firing it to a temperature of approximately 1200C for three hours and then 20allowing it to cool, to produce the desired high permeabilit~.
Thereafter, a sensor coil 160 is wound around the abutting base portions 142 and 142', the coil being formed from approxi-mately 8,000 turns of very fine wire. A pair of spacer blocks 162 and 164 are secured to the outer ends of the core legs, 25spacer 162 spanning legs 144 and 146, and spacer 164 spanning legs 144' and 146'. The spacers 162 and 164 provide mechanical support for the elongated leg portions and serve as well to receive the fasteners 114, 120 and 122 to enable the magnet-ometer unit to be assembled in the manner previously described.
30Therefore, the spacers 162 and 164 must be securely attached to the sensor unit and it is preferred that they be welded to the clamping members 156 and 158. The blocks may be of any non-magnetic material, but again stainless steel is pre-ferred for stren~th and durability.
The circuitry of Fig. 3 is illustrated in greater detail in Figs. 9 and 10, to which reference is now made. As shown, the horizontal components Bx and By of the A.C. field are measured by means of sensors 58 and 60, while fluxgate compass 42 measures the north and east orientation of the probe, 40and provides outputs CN and CE which represent the earth's :. .
~.
.
5~
magnetic field. Only one oE the fiel~ sensors is in operation at any one time, and the output data of each is transmitted to the surface by a frequency modula-ted carrier at about 100 kHz.
The outputs from the sensor coils 58 and 60 are each 5 connected -to their corresponding preampllfiers 62 and 64 by way of a resonant circuit, which includes capacitor 170r and oppositely connected parallel diodes 172, 174. The preamps each include a pair of transistors Ql and Q2 connected in a standard low noise, high performance configuration, the par-10 ticular preamp design not being a part of this invention. Theoutputs from compass 42 may be fed through similar preamps 48 and 50 if necessary.
The outputs from the sensor and co~npass coils are sequentially switched by a suitable multiplexer, which is func-15 tionally illustrated in Fig. 9 as including four analog switches176, 177, 178 and 179 connected to the outputs BX and BY of the A.C. field sensors 58 and 60 and to the outputs CN and CE of the compass 42, respectively. Switching is accomplished by means of a ring counter 180 which sequentially operates the 20 switches 176-179 by way of control outputs, CBX, CBY, CCN and CCE on lines 182, 183, 184 and 185, respectively. The outputs on lines 184 and 185 are also connected through OR gate 188 to activate the fluxgate compass only when one of its outputs is to be selected for transmission. Similarly, the outputs on 25 lines 182 and 183 are connected by way of OR gate 190 to a main sensor switch 192, which is another analog switch connecting the sensor outputs to the voltage controlled oscillator 76.
Preferably the multiplexing function is performed by a CMOS
CD4017 ring coun-ter and C~OS CD4017 analog switches, although 30 other devices can be used.
The outputs from switches 176 and 177 are supplied by way of lines 194 and 196 to an amplifier 198, the output of which is fed ~y way of line 200 to a suitable notch filter 202.
The version illustrated in Fig. 9 differs from that of Fig. 3 35in that only a single notch filter is required, with the multi-plexer selecting which sensor signal is to be filtered; other-- wise, the function is the same as in Fig. 3. Thus, the notch filter is designed to pass the frequency of the A.C. field applied by the surface excitation, and to effectively block 40any signals that might be produced by other A.C. fields, thereby ~:, .
~ x~
:L~
improving the sensitivity o~ the present system.
The output from f:ilter 202 is Eed to -the voltage con-trolled oscillator 76 by way of lines 204 and 76 whenever switch 192 is activated by the selection of sensors 58 or 60. W~en the S compass g2 is selected, the outputc; CN or CE are supplied by way of switches 178 or 179, respect:ively, to line 206 and -thence to line 74 and VCO 76. The fre~uency of oscillator 76 is varied in accordance with the amplitude of -the signals applied thereto by the sensors or the compass, and the frequency-modulated 10 output on line 78 is amplified in amplifier 77 and supplied by way of isolating transformer 208 to the cable 18. The VCO 76 may be a conventional C~OS CD~046 voltage controlled oscillator.
The counter 180 is advanced by -the surEace controller by means of a D.C. pulse sent down the logging cahle 18. This 15;count advance pulse is detected by an optical isolator Q3 which is connected across a capacitor 210 which is connected in line 18, the output of the isolator driving the counter by way of line 212. The counter is stepped one count each time a D.C. pulse is detected to enable a single probe sensor or compass output at a 20 time.
In addition to the data furnished by sensors 58 and 60 and by compass 42, one other measurernent is needed to obtain the desired information concerning underground strata. That measure-ment is of the vertical component of the A.C. magnetic field, 25 and is obtained by means of a standard low noise mu metal induc-tion coil sensor 220, illustrated in Fig. 9. This sensor, which may be an elongated strip o~ mu metal about one meter in length wrapped at its center by a coil of about 30,000 turns, produces an output corresponding to the vertical field component BZ.
30 This signal is supplied to a preamp 222, an amplifier 224, and a voltage controlled oscillator 226, all of which are similar to the components previously described. The vertical field com-ponent probe may be a separate sensor lowered down the borehole after completion of the horizontal field component measurements 35 described above, in which case it will be provided with its own borehole cable, mav be connected to the horizontal field probe 16 as indicated in Fig. 9, in which case it may share the bore-hole cable 18, or may be integrated with the probe 16. In the latter event, the vertical field probe may be connected through ., .
17 ~7~
the mul~iplexer switching to shar~ the VCO 76. It has ~een found, however, that combinlng the vertical and field sensors makes the unit overly bulky and complicated, and it is pre-ferred that separate probes be used. However, the surface 5 processing of the data remains the same in any of these arrange-ments.
The surface processing of information is accomplished by means of microprocessor 86, illustrated in Fig. 10. The frequency modulated information carried on the borehole line 18 10 is supplied through suitable swi~ching 230, surface line 232, and through isolating transformer 234 to the detector 84, which may comprise an amplifier 236 and a Schmidt trigger 238. The Schmidt trigger is driven by the signals received from the down-hole instruments to produce a train of pulses having a frequency 15 equal to the frequency of ~he output from the voltage controlled oscillator. The pulses are applied by way of line 240 to a frequency meter 242, which is activa-ted when the compass 42 is selected by the downhole counter, and to a counter 244, which is activated when the probe sensors 58 or 60 are selected~ When 20 the compass is selected, the frequency of the signal sent to the surface is linearly related to the amplitude of the appropriate component of the earth's magnetic field, and ~his VCO output is read on the frequency meter 242. This meter is a commerci-ally available ICM 7226A meter supplied by Intersil Corp.
A crystal oscillator 246 synchronizes the data process-ing circuitry b~ way of a data latch 248 and a divider 250 which synchronizes the processor 86 with the signals received from the downhole sensors. The oscillator also drives the SCR in-verter 15 through the divider network 250 to produce the square 30 wave current signals used to generate the A.C. field to be measured, the use of a single oscillator thereby assuring synch-ronization of -the excitation currents and the signal processing.
The excitation current is mounted by means of a differ-ence amplifier 252 connected across a resistor 254 (Fig. 10) 35 connected in series with the excitation current lines leading to coil 10 (Fig. 1) or electrodes 24, 25 (Fig. 2). The output of the difference amplifier, which is proportioned to the vol-tage crop across the resistor, is applied to a voltage controlled oscillator 256, the output of which is supplied through amplifier 40 258 and isolating transformer 260 to a coun~er 262. This counter, .~
( "
'75~
which is similar to counter 244 in the ~ata channel, supplies a count proportional to the excitation current.
The microprocessor functions to averaye the da-ta signals recelved over a preselected period of time. This is accom-5 plished by a simple clata stacking technique wherein each cycleof the primary excitation is divided into 32 "equivalent"
channels. The number of zero crossings generated by the downhole VCO is linearly related to the ins-tarltaneous voltaye output of ~ the selected sensor. The processor samples the output of counter i 10 244, computes the number of zero crossings in each time channel, I and then adds the result to the appropriate channel locations ¦ in its memory.
After adding (i.e., stacking) the data in this manner for a selected period of time, for example, between 5 and 30 15minutes, the contents of the 32 channels are Fourier analyzed and divided by the number of cycles that have been stacked to give the amplitude and phase of the ~.C. maynetic field compon-ent. The processor may then advance the downhole counter by activating switch 230. This may be accomplished, for example, 20by solenoid 264 or by suitable analog switching to disconnect the borehole ]ine 18 from transformer 232 and to connect line 13 to a source 266 of direct current. This produces a D.C. signal on line 18 which activates counter 180 (Fig. 9) as described above. The process then repeats itself for the next sensor or 25Compass outputf accomplishing additional data stacking as de-scribed.
The information collected and processed by the processor 86 may be stored on a suitable recording means such as a cassette recorder 268, or may be displayed by a conventional printer 270, 30for later evaluation. After both the hori~ontal and vertical components of the A.C. magnetic field and the earth's magnetic field components have been measured and stored, the data so obtained provides valuable information concerning the nature of the strata in which the measurements were made. Such data is 35far more accurate than that previously available with prior systems, and permits well logging at depths and at lateral dis-tances from the borehole that were unattainable with prior devices and s~stems.
In testing the magnetometer constructed in accordance 40with the foregoing description, it was found that magnetic field :
. .
.
19 ~L~7511~t~
components excited by a current loop source of the type :i].lus-trated :in F.ig. 1 or by a current dipole source of the type illustrated in Fig. 2 could be measured with a high deyree of sensitivity. It was found that perturbations in the A.C. mag-5 ne-tic field generated by a surface source can be identified even at very great depths, on the order of 4,800 feet, and can produce significant data concerning the structure of the strata in the area of the measurements. Good results were obtained at both 7 Hz and 36 Hz, giving data which permitted accurate interpre-10 tation of the strata adjacent the borehole as well as thatspaced a considerable distance laterally away from the borehole location.
Although the present invention has been described in terms of a preferred embodiment thereof, it will be understood that 15 numeroug variations and modifications can be made without depart-ing from the true spirit and scope thereof as described in the following claims.
characteristics of the formations, ~nd to identify anornalies which would indicate the probable locations and dirnensions of mineral beds, petroleum bearing deposits, and the like.
However, prior systems for the logging of geoloyical 5 formations have been found to be of limi-ted usefulness, since they lack the sensitivity required to make meaningful measure-ments at any appreciable distance :from the current or voltage source. Thus, the measurements have been limited to relatively short lateral distances from the borehole, or where the source 10 electrodes are located at the surface of the earth, at rela-tively shallow depths. As a result, it has been difficult to obtain accurate measurements of deposits which are not actually penetrated by the borehole, and even where the borehole passes through or very close to the edge of a deposit, it has been 15 difficult to determine with any degree of certainty the dimen-sions of the deposit. Thus, although a deposit may disturb a current flow pattern, the lack of sensitivity in prior measuring devices has made it difficult, if not impossible, to determine the nature of the geological anomaly any appreciable distance 20 from the point of measurement.
Attempts have been made to solve this problem through the provision of more sensitive detectors, and to this end magnetometers sensitive to faint magnetic fields have been proposed. However, in order to obtain accurate measurements 25 of subsurface magnetic fields, the horizontal components of the fields must be detected, and in prior devices this required a horizontally oriented magnetometer having significant hori-zontal dimensions. Unfortunately, exploratory well bores have a diameter of between about ~ and 6 inches, and simply do not 30 provide the space needed for conventional magnetometers of the required sensitivity. In order to accommodate prior magne-tometers, then, it was often necessary to redrill existing ex-ploratory wells, or to drill new exploratory wells with much greater diameters. This not only was prohibitively expensive, 35 but did not result in significantly better results.
Summary of the Invention It is, therefore, an object of the present invention to provide a method of geophysical exploration that will provide accurate and reliable information concerning deposits of minerals .
r~l~
~nd of hydrocarhons located a substant:i~l d.istance from a borehole and at substantial depths. It is also an object of the invention to provide an improved magnetometer for use in such exploration which will have a hiyh degree of sensitivity sto variations in maynetic fields due to yeological anomalies, and which will have dimensions tha-t will perm.it its use in small-diameter well bores such as those drilled for exploration purposes.
The present invention is an electromagnetic well log~-lOing method and apparatus fo.r obtaining electrical conductivitydata a considerable distance from a borehole, and finds partic-ular application in the logging of petrolellm bearing strata, which are electrically conductive. The invention provides integration of the measured electrical conductivity over a large 151aterally extending volume, and permits detection of lateral variations in conductivity, measurement of the lateral direction in which the conductivity is changing, and detection of the lateral extent of conduct.ive beds of material. In general, this is accomplished by exciting the strata in the vicinity of 20a borehole by means of an A.C. surface source. A sensitive magnetometer is lowered down the borehole and the depth depen-dence of the A.C. magnetic fields produced by the source is determined by means of a large number of measurements of the magnitude, phase, and direction of the fields in the strata near 25the hole. Any non-uniform electrical conductivity of the strata leads to a concentration of currents and of magnetic fields produced by the source into the more highly conductive strata, and their location and nature can be identified by analysis of the depth variations obtained from measurements made over a 30period of time.
To obtain the required data, the magnetometer of the present invention is used in conjunction with a conventional electronic compass and with a vertical field sensor which may be integrated with the magnetometer or be separate. The magneto-35meter is used to measure the horizontal A.C. magnetic fieldcomponents produced by a surface loop or by electrode injected currents and the compass measures the earth's magnetic field to determine the azimuthal orientation of the instrument. The horizontal field magnetometer consists of a pair of magnetic 40field sensors, each sensor comprising two generally U-shaped .
~, ;
.
~cL~L~7~6 core segments, each segment having two space~ parallel legs extendlng from an lnterme~iate hody portion. The core is con-structed of a lamina-ted, high permeability magnetic material, and the core segments are assembled hack-to-back with a common 5coil oE about 25,000 turns surrounding the adjacent core body portions. The core segmen-ts have highly elongated legs, on the order of 18 inches, as cornpared to a short body portion length of about two to three inches, so that the ef~ective diameter of the sensors allows the magnetome-ter to ~it within an 10 explorator~ well bore.
The core segments each are formed of multiple contin-uous laminations of mu-metal, each lamination ex-tending the full length of the core segment. l~fter formation o~ the two segments into their respective U-shapes, the cores are assernbled 15 back-to-back, with the legs extending in opposite directions, but in the same plane, and are secured by means of a nonmagnetic channel member which is crimped tightly around the laminations to hold them securely together and to hold the core segments in assembled relationship. The assembled segments are then 20 carefully annealed to provide high permeability, and therea~ter the coil is wound around the adjacent intermediate body poxtions.
Suitable spacers are then secured to the free ends of the core legs, as by welding to the channel member, to complete the field sensor.
Two sensors arranged along a common axis form the mag-netometer of the present invention, with one sensor lying in a first plane and the second sensor lying in a second plane which is perpendicular to the first. The two sensors are secured in end-to-end relationship, but spaced a short distance 30 from each other, and the entire unit is mounted within a non-magnetic housing of low electrical conductivity having an outer diameter of less than about four inches, which is small enough to permit its use in a boreholeO Included in the housing with the magnetometer is an electronic compass, a suitable power 35 supply, and electronic circuiting for detecting the signals produced by the magnetometer and transmitting them to the sur-face for processing.
The magnetometer of the present invention is lowered into a vertical borehole, and by virtue of its exceptional 40 length, totalling about two meters for the two sensors, is capable of accurate and hiyhly sensitive rneclsurements of alte~-nating magnetic fields caused by injected t~urrents. The measurements so obtained are used in conjunction with ver-tical field measurements to provide the required data.
5 Brief Description of the Drawinys The foregoing and additional objects, features and advantages of the present invention will become apparent to -those of skill in the art from a considera~ion of the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings in which:
Fig. 1 is a diagrammatic illustration of a well loygin~
system utilizing a loop source of current;
Fig. 2 is a diagrammatic illustration of a well logging system utilizing a current dipole;
Fig. 3 is a diagrammatic illustration of circuitry for use with the present invention;
Fig. 4 is a partial sectional view of a magnetometer constructed in accordance with one form of the pxesent inven-tion;
Fig. 5 is a sectional view of one sensor unit from the magnetometer of Fig. 4 taken along lines 5-5 of Fig. 7;
Fig. 6 is a cross-sectional view of a sensor unit taken along lines 6-6 of Fig. 5;
Fig. 7 is a sectional view of a sensor taken along 25lines 7-7 of Fig. S;
Fig. 8 is a perspective view of a single laminate folded to form part of a core segment for the sensor of Fig. 5;
Fig. 9 is a more detailed illustration o-f down-hole circuitry for use in the present invention; and Fig. 10 is a more detailed illustration of surface electronics for the present invention.
Description of ~ Preferred Embodiment Turning now to a more detailed consideration of the drawings, there is illustrated in Fig. 1 a well logging system 35Of the type that may be used with the improved magnetometer of the present invention to obtain highly accurate indications of geological formations, and particularly of formations which contain electrically conductive material, to facilitate the location and mapping of petroleum and brine pools, mineral 40 deposits, and the like. Since the present magnetometer is '' ' '~ ' 6 ~ 7~
particulaxly useflll in locating petroleum deposits, the follow-ing clescription of a preferred embodiment will have specific reEerence to such an application. Mowever, it should be un~er-stood that the invention is not limited to the location of 5 petroleum deposits, and that other deposits, such as minerals, which produce anomalies in the underground magne-tic fields and electrical currents may also be det:ected and identified.
The association of gas, oil and brine in porous geo-logical s-tructures is well known, and electrical methods may 10 be used in well logging because of the high electrical conduc-tivity of such brines, because of the much lower conductivity of petroleum, and because of the resistivlty of host rock forma-tions. The stratographic setting of petroleum and the associaked brine, which appear in relatively thin horizontal len-ticular lS bodies w.ith a few kilometers lateral extent, leads to char-;acteristic perturbations of electric current flows or ofmagnetic fields in their proximity. These perturbations depend not only upon the characteristics of the stratographic structure actually pierced by the borehole, but upon the characteristics 20 of neighboring structures as well, and the accurate measurement of these perturbations can give valuable geophysical information.
By means of the sensitive magnetometer of this invention, the distribution, filimentation, and distortion of a controlled subsurface current flow, produced either by distant electrodes 25 to which an A.C. source is applied or by a controlled magnetic field which is in turn caused by a current flow at the surface, can be accurately measured. Since the electric currents so produced will try to follow paths of least resistance, these currents will be concentrated or spread out by the character-30 istic conductivity or resistivity of the strata, and the areasof concentration or separation will be manifested by magnetic field gradients which are detected by the magnetometer of the invention to produce "signatures" in the magnetometer output signals. It has been found that the magnetometer of the present 35 invention will produce such characteristic signatures even in situations where presently available resistivity and conductiv-ity well logs indicate uniformity in the strata. It has further been found that such information can be obtained at a much greater lateral distance from the borehole than was heretofore 40 possible.
~, 7 ~l~Lt75~
Eigs. 1 and 2 lllustrate ~wo methods ~or creatin~ the desired magnetic flelds ln the strata of lnterest In Flg. 1, a magne-tic Eield ls produced by rneans of a slngle -turn current loop formed by a cable 10 located on the surface of the earth 5 and surrounding a borehole 12. The cable forms a loop which may, for example, be aoo meters or more in diameter and excited by a suitable source 14 of alternating current. The A.C. source is supplied by way of a sili.con controlled rectifier inverter 15 which produces an alternating square wave current of about 10 20 amperes. In a test of the magnetometer oE the present in-vention, it was Eound that a suitab].e magnetic field could be produced at frequencies below 60 Hz, with good results being obtained in tests using sources of 7 Hz or 36 Hz.
Borehole 12 is a typical exploration hole which is 15 cased in the normal manner with 5-1/2 inch inner diameter non-magnetic, nonconductive (e.g., fiberglass) pipe over at least a part of its depth. A magnetometer 16 constructed in accord-ance with the invention is suspended in the well hole by means of a standard well logging cable 18 which may for example, be 20 15,000 feet long. The cable includes suitable electric wiring to permit connection of the electronics in the magnetometer 16 with surface processing circuitry 20 which may be mounted in a truck or trailer for portability from one borehole to another.
The flow of current in the single turn loop cable 10 produces a magnetic field generally indicated by lines 22, the lines of magnetic field force being centered on the cable 10 and extending to a considerable depth within the earth. As indicated above, the exact pattern followed by the magnetic 30 field will depend upon the electrical characteristics of the earth's structure. Layers of conductive material or layers of non-conductive material will disturb the regular lines of flux for the magnetic field, producing anomalies, or aberrations, which can be detected by the magnetometer.
Fig. 2 illustrates an alternative mode for obtaining the desired magnetic field within the strata of interest. In this arrangement, the loop 10 is replaced by a pair of elec-trodes 24 and 25 which may be embedded in the ground at the surface or which may be located within additional boreholes 40 in the vicinity of the hole 120 As illustrated, electrodes 24 .. . . .
:; .
8 ~.~'7~
and 25 are located at the surface 2~ and the source of alter-nating current 1~ is connected thereacross ~y wa~ of cab]es 30 and 32. In one test of -the magnetometer of the invention, the electrodes were spaced apart by about 1,200 meters and a current 5Of approximately 20 amperes was fed to them. Again the alter-nating source may be a square wave operated at 60 ~z or less, with 7 Hz and 36 Hz providing suitable results.
The source connected across electrodes 24 and 25 pro-duces a potential difference therebetween which results in a lOcurrent flow extending through the strata beneath the surface of the ground 28. The current flow will follow a mu~titude of paths, as diagrammaticall~v illuslrated by the current flow lines 34, with the exact path followed by the current being dependent upon the conductivity characteristics o~ -the strata 15and the loaation of the electrodes. For example, an oil bear-ing stratum is generall~ indicated at 36, with the stratum including a pool of conductive brine 38 covered by a layer ~0 of lower conductivity petroleum. If this petroleum and brine de-posit occurs in a surrounding stratum of relatively uniform 20conductivity, the disturbance created by the deposit will be substantial, as broadly illustrated in the drawing, and the resulting disturbance in the current flow can be detected by the magnetometer 16. It will be understood that electrical currents illustrated by lines 34 generate aorresponding magnetic fields, 25with the intensity of the current i at any point in space being related to the magnetic field B in accordance with the Maxwell equation: V x B=~Ol,where ~ = 4~ x 10 7 (MKS units). With a Cartesian coordinate system for the systems of Figs. 1 or 2, equation 1 can be written as:
~ jx = dBz dz ~iY = dBX - dBz ~y - dBx ~o~Z dx dy Since in the applications of the invention envisaged/
35the dominant structural variation of the strata is in the hori-zonta~ (x,y) plane, it has been found that many of the strato-graphic configurations of interest produce their most important and most characteristic magnetic field variations in the vertical (z) direction so that field variations with respect to depth are :.
:, , . : , .. . .
9 ~ 7~
sign.ificant. Accordingly, in accorclance with the invent.i.on, the x and y components of the maynetic fi.eld are measured at various depths, and the variations are plotted to provide a field profile in the z, or depth direction. The resulting field 5 chaxacteristics then provide informat.ion concerning the nature of the strata surrounding the borehole.
The downhole magne-tometer equipment 16 is illustrated in diagrammatic form in Fig. 3, to which reference is now made.
The magnetometer includes a conventional electronic fluxgate 10 compass 42 which may include a paix of coils oriented at right angles to each other and mounted in the horizontal ¦x,y) plane to measure the earth's magnetic field. The outputs 44 and 46 represent the horizontal components BN and BE, respec-tivel~, of the earth's magnetic field, and these may be fed through pre-15 amplifiers 48 and 50, respectively, which may be a part of the compass, to corresponding inputs 52 and 54 of a multiplexer 56.
It should be understood that the compass 42 is conventional and that its components are shown diagrammatically for purposes of illustration only.
The magnetometer unit 16 also incorporates a pair of ~.C. field sensors 58 and 60 which include sensor coils wound on specially prepared magnetic cores to be described. The coils and their cores are mounted at right angles to each other so that sensor 58 is responsive to changes in magnetic flux in 25 the x direction and sensor 60 is responsive to changes in mag-netic flux in the y direction. The outputs of the two sensor coils are fed through preamplifiers 62 and 64, respectively, and through filters 66 and.68, respectively, to inputs 70 and 72 of the multiplexer 56. The filters 66 and 68 are notch 30 filters which are selected to pass OUtpllt signals from the coils 58 and 60 which correspond in frequency to the frequency .of source 14.
Multiplexer 56 is designed to sweep through the inputs 70, 72, 52, 54 in sequence with the respective outputs being 35 supplied sequentially by way of line 74 to the input of a vol-tage controlled oscillator 76. Oscillator 76 may operate, for example, at a carrier requency of approximately 100 kilo-Hertz and is modulated by the voltages supplied in sequence from the sensor coils and the compass coils through the multi-40 plexer. The modulated carrier signal is fed to an amplifier .
' ' ' ' .
77 ~y way of line 78, r~nd the outp~t oE the ampli~ier is fedto the borehole cabl~ 18. A power supply 79, which may be a battery pack, provides suitable downhole bias voltages for operating the downhole elec-tronics, with power being supplied 5 by way of line 80 to the multiplexer, the voltage controlled oscillator 76 and preamplifier 77 and by way of line 81 to preamplifiers 62 and 64, as well as to other parts of the cir-cuit. At the surface, the signal is again ampli~ied by amplifier 82 and is fed through detector circuitry 84 to suitable 10 signal processing circuitry 86 for signal averaging, display, recording, and the like. The various signal processing functions may be carried out by means of a microprocessor such as a commercially available 5YM-l, manufactured by Synertek Systems Corp. For signal averaging, the processor may accumulate the 15 input signals from the borehole sensors over an appropriate period of time ~for example, 5 to 30 minutes, averaging the sig-nal and printiny out the results on a computer print-out terminal.
The information thus accumulated for each depth location of the magnetometer unit 16 may then be analyzed to determine the 20 parameters of the magnetic fields at various depths.
A preferred form of the structure of the borehole mag-netometer unit 16 is illustrated in Figs. 4 through 8, to which reference is now made. The unit comprises a housing 90 which is adapted to receive the sensors 58 and 60, the magnetic compass 25 42, the power supply 79, and an electronics package 92 which includes the various amplifiers, the filters, the multiplexer, the voltage controlled oscillator, and like down hole circuitry illustrated in Fig. 3. The housing 90 preferahly is a double-walled tubular container of the vacuum bot-tle type which serves 30 to insulate the contents of the bottle from the heat encountered in the typical down hole environment. rrhus, housing 90 is con-structed with an outer wall 94 spaced from a concentric inner wall 96 to define an interior space 98. The interior space is closed and sealed at both ends of the container by radially 35 extending walls 100 and 102 so that a vacuum may be maintained therein. The outer diameter of the housing is substantially less than the 5-1/2 inch inner diameter of the typical explora-tory well casing for borehole 12, and preferably has a diameter of approximately 4 inches so that the unit 16 may move easily 40 within the borehole.
~ ;
~' :
l:L ~ Sl~
The inner wall o~ the housiny ~e:~ines an i.nterior ca-vity 104 which receives the sensors, compass and electronic e~uip-ment, the cavity being closed by an upper cap 106 and a lower cap 108 which are threaded in-to the housi.ny to hermetically 5 seal cavity 104. Preferably, -the housing 90 is constructed of a non-magnetic metal to provide the strength required for wi.th-standing the hazardous high -temperature environment of a bore-hole, while allowing detection of the surrounding magnetic fields.
The sensor units 58 and 60 are adapted to be removably mounted within housing 90, with the first sensor unit located near the upper end of housing 90 and the second sensor 60 lo~
cated in the lower end of the housing. The two sensors are secured together in spaced, end-to-end coaxial relationship by 15 means of a suitable fastener 114, the fastener serviny to rigidly secure the two sensors for placement in and mounting within the housing. The two sensors are mounted so that their respective planes are at right angles to each other, whereby one sensor is adapted to measure magnetic fields in the "x" directi.on and 20 the other is adapted to measure magnetic ~ields in the "y"
direction. For purposes of reference, sensor 58 may be said to lie in the x plane, or x-measuring plane, and sensor 60 may be said to lie in the y, or y-measuring, plane. Fastener 114, which is shown for purposes of illustration as being a bolt 25 secured to the respective sensors by suitable nuts and lock washers, holds the sensors in an end-to-end relationship so that they can be mounted coaxially within housing 90. Suitable radially-extending spacers (not shown) may be mounted on the sensors to center them with respect to the interior wall 96 30 and to securely hold the sensors within cavity 104 during use o~
the unit.
Secured to opposite ends of the sensors are mounting boards 116 and 118, board 116 being mounted on the upper end of sensor 58 by means of suitable fasteners 120 and board 118 being 35 mounted on the lower end of sensor 60 by means of suitable fas-teners 122. Mounting board 116 carries the power supply 79 and electronics package 92, while mounting board 118 carries the magnetic compass 42, the boards serving to fasten these compon-ents rigidly to the sensor units for secure mounting within 40 housing 90. Again, the boards may be provided with suitable , - , ~
~ D~ 6 radial spacers to properly locate these cornponents within cavity 104.
Elec-tric cables 124, 126 and 128 interconnect the compass 42 and -the sensors 58 and 60, respectively, to the elec-5tronics package 92, while electrical cable 130 connects thepower supply 79 thereto. ~orehole cable 18 includes electrical wiring 132 which passes -through the threaded cap 106 and is sealed therein in known manner to provide electrical communi-cation between the electronics package of magnetometer unit 16 lOand the instrumentation at the surface of the earth. The mag-netometer is supported mechanically by the conventional borehole cable, which may be secured to the unit by means o~ fastener 134.
The structural details of sensor 58 are illustrated in Figs. 5 through 8, but it should be understood that sensor 60 15is identical thereto. The sensor comprises two generally U-shaped magnetic core segments each having a base portion with two elongated, parallel legs extending from opposite ends thereof.
The bases of the two core segments are arranged in back-to-back relationship wtih the legs extending in opposite directions so 20that the two core segments form a generallv H-shaped sensor lying in a single plane, as illustrated in Fig. 5.
The first, or upper, core segment 140 of sensor 58 consists of a base portion 142 and parallel elongated leg portions 144 and 146 extending from opposite ends of base 142 25so that the legs are spaced apart by a distance equal to the length of the base. Although the length of the base portion 142 is exaggerated with respect to the length of leg portions 144 and 146 for purposes of illustrating the structure of sensor 58, it should be understood that the length L of the leg portions 30is large with respect to the length d of the base portion, the legs being on the order of six to ten times the length of the base. The extreme relative length of the legs 144 and 146 provides a large area of interaction between the core segment and the magnetic field to be detected, and thus provides a high 3sdegree of sensitivity to the fields of interest. The short length of the base portion 142 provides a small effective dia-meter for the sensor so that it will fit within the relatively small inner diameter of housing 90, thereby accommodating the magnetometer to use in small-diameter exploratory boreholes.
.
.
.- . , : . .
.i , .
In the preferred form o~ the inventi~o~,7~le core segment 140 ls constructe~ of a plurality of very thin, elongated, con-tinuous laminations of a high permeabili-ty material such as mu-metal, each laminate extending from one end of the core 5segment to the other. A pre~erred sensor utiliæes a core of eleven laminations, each being .004 thick and with each lami-nation being continuous for the full leng-th of the core; that is, each lamination ex-tends the full length of leg 144, is folded to extend across the width of the core to form base 142, lOand is folded again to extend the full length of the opposite leg 146. For clarity of illustration, core segment 140 is shown as being constructed of only two laminations 148 and 150, with laminate 148 being illustrated in perspective in Fig~ 8, but it should be understood that numerous larninations are used. As 15illustrated, laminate 148 forms a part of leg portiorl 144, is folded upwardly at the bottom 152 oE the leg portion to form a part of the base portion 142, and is folded upwardly again at the bottom 153 o~ the leg portion 146 to form a part of that leg. ~lthough a number of fold configurations are possible, 20in one form the laminate 148 is doubled over in the ~ase portion 142 to form a double thickness (see Fig. 6), this double thickness being formed by means of triangular folds 154 at each side of the base portion 142. Similar folds are made at the bottom 153 of leg portion 146 (shown in phantom in Fig. 8).
25By reason of this particular folding arrangement, the width w of the laminate 148, which may be about 1/2 inch, is reduced in the base portion 142 to one-half its width in legs 144 and 146. However, this fold allows the base portion to be perpen-dicular to and centered with respect to the width of laminate 148 30for easier winding of the coil to be described. As seen in Fig.
6, upon assembly of the core segment 140, the folded-over portions which form the base 142 are stacked and folded one over the other, while the laminations forming the leg portions 144 and 146 remain flat.
The sensor 58 includes a second core segment 140' which is a duplicate of the segment 140, and accordingly it is similarly numbered, with the numbers being primed.
The two sensor elements are positioned with the respec-tive base portions 142 and 142' in back-to-back relationship as 40illustrated in Figs. 5 and 6, and with the corresponding leg ' ~ -`
1~
portions ex~encling ln opposi.-te directions. The hase portions define the plane of the s~nsor, and the legs lie in that plane, although the individual lamination in the leg portions are perpendicular to the plarle of the sensor.
After assembly of the laminations to form the two core segments 140 and 140l, and after positioning them in back-to-back relationship, the corresponding legs 144, 144' and 146, 146' on each side of the sensor 58 are secured in corresponding channel members 156 and 158. These channel members are sub~
lOstantially C-shaped and are crimped tightly around the lamina-tions to secure them in assembled relationship, each channel extending the full length of the sensor so that channel 156 sur-rounds and secures legs 144 and 144' while channel 158 surrounds and secures legs 146 and 146'. Preferab:Ly, the channels 156 15and 158 are of a non-magnetic material such as stainless steel.
Assembly of the laminations and of the core segments in the manner described forms the H-shaped core of sensor 58.
This core is then suitably heat-treated, as by firing it to a temperature of approximately 1200C for three hours and then 20allowing it to cool, to produce the desired high permeabilit~.
Thereafter, a sensor coil 160 is wound around the abutting base portions 142 and 142', the coil being formed from approxi-mately 8,000 turns of very fine wire. A pair of spacer blocks 162 and 164 are secured to the outer ends of the core legs, 25spacer 162 spanning legs 144 and 146, and spacer 164 spanning legs 144' and 146'. The spacers 162 and 164 provide mechanical support for the elongated leg portions and serve as well to receive the fasteners 114, 120 and 122 to enable the magnet-ometer unit to be assembled in the manner previously described.
30Therefore, the spacers 162 and 164 must be securely attached to the sensor unit and it is preferred that they be welded to the clamping members 156 and 158. The blocks may be of any non-magnetic material, but again stainless steel is pre-ferred for stren~th and durability.
The circuitry of Fig. 3 is illustrated in greater detail in Figs. 9 and 10, to which reference is now made. As shown, the horizontal components Bx and By of the A.C. field are measured by means of sensors 58 and 60, while fluxgate compass 42 measures the north and east orientation of the probe, 40and provides outputs CN and CE which represent the earth's :. .
~.
.
5~
magnetic field. Only one oE the fiel~ sensors is in operation at any one time, and the output data of each is transmitted to the surface by a frequency modula-ted carrier at about 100 kHz.
The outputs from the sensor coils 58 and 60 are each 5 connected -to their corresponding preampllfiers 62 and 64 by way of a resonant circuit, which includes capacitor 170r and oppositely connected parallel diodes 172, 174. The preamps each include a pair of transistors Ql and Q2 connected in a standard low noise, high performance configuration, the par-10 ticular preamp design not being a part of this invention. Theoutputs from compass 42 may be fed through similar preamps 48 and 50 if necessary.
The outputs from the sensor and co~npass coils are sequentially switched by a suitable multiplexer, which is func-15 tionally illustrated in Fig. 9 as including four analog switches176, 177, 178 and 179 connected to the outputs BX and BY of the A.C. field sensors 58 and 60 and to the outputs CN and CE of the compass 42, respectively. Switching is accomplished by means of a ring counter 180 which sequentially operates the 20 switches 176-179 by way of control outputs, CBX, CBY, CCN and CCE on lines 182, 183, 184 and 185, respectively. The outputs on lines 184 and 185 are also connected through OR gate 188 to activate the fluxgate compass only when one of its outputs is to be selected for transmission. Similarly, the outputs on 25 lines 182 and 183 are connected by way of OR gate 190 to a main sensor switch 192, which is another analog switch connecting the sensor outputs to the voltage controlled oscillator 76.
Preferably the multiplexing function is performed by a CMOS
CD4017 ring coun-ter and C~OS CD4017 analog switches, although 30 other devices can be used.
The outputs from switches 176 and 177 are supplied by way of lines 194 and 196 to an amplifier 198, the output of which is fed ~y way of line 200 to a suitable notch filter 202.
The version illustrated in Fig. 9 differs from that of Fig. 3 35in that only a single notch filter is required, with the multi-plexer selecting which sensor signal is to be filtered; other-- wise, the function is the same as in Fig. 3. Thus, the notch filter is designed to pass the frequency of the A.C. field applied by the surface excitation, and to effectively block 40any signals that might be produced by other A.C. fields, thereby ~:, .
~ x~
:L~
improving the sensitivity o~ the present system.
The output from f:ilter 202 is Eed to -the voltage con-trolled oscillator 76 by way of lines 204 and 76 whenever switch 192 is activated by the selection of sensors 58 or 60. W~en the S compass g2 is selected, the outputc; CN or CE are supplied by way of switches 178 or 179, respect:ively, to line 206 and -thence to line 74 and VCO 76. The fre~uency of oscillator 76 is varied in accordance with the amplitude of -the signals applied thereto by the sensors or the compass, and the frequency-modulated 10 output on line 78 is amplified in amplifier 77 and supplied by way of isolating transformer 208 to the cable 18. The VCO 76 may be a conventional C~OS CD~046 voltage controlled oscillator.
The counter 180 is advanced by -the surEace controller by means of a D.C. pulse sent down the logging cahle 18. This 15;count advance pulse is detected by an optical isolator Q3 which is connected across a capacitor 210 which is connected in line 18, the output of the isolator driving the counter by way of line 212. The counter is stepped one count each time a D.C. pulse is detected to enable a single probe sensor or compass output at a 20 time.
In addition to the data furnished by sensors 58 and 60 and by compass 42, one other measurernent is needed to obtain the desired information concerning underground strata. That measure-ment is of the vertical component of the A.C. magnetic field, 25 and is obtained by means of a standard low noise mu metal induc-tion coil sensor 220, illustrated in Fig. 9. This sensor, which may be an elongated strip o~ mu metal about one meter in length wrapped at its center by a coil of about 30,000 turns, produces an output corresponding to the vertical field component BZ.
30 This signal is supplied to a preamp 222, an amplifier 224, and a voltage controlled oscillator 226, all of which are similar to the components previously described. The vertical field com-ponent probe may be a separate sensor lowered down the borehole after completion of the horizontal field component measurements 35 described above, in which case it will be provided with its own borehole cable, mav be connected to the horizontal field probe 16 as indicated in Fig. 9, in which case it may share the bore-hole cable 18, or may be integrated with the probe 16. In the latter event, the vertical field probe may be connected through ., .
17 ~7~
the mul~iplexer switching to shar~ the VCO 76. It has ~een found, however, that combinlng the vertical and field sensors makes the unit overly bulky and complicated, and it is pre-ferred that separate probes be used. However, the surface 5 processing of the data remains the same in any of these arrange-ments.
The surface processing of information is accomplished by means of microprocessor 86, illustrated in Fig. 10. The frequency modulated information carried on the borehole line 18 10 is supplied through suitable swi~ching 230, surface line 232, and through isolating transformer 234 to the detector 84, which may comprise an amplifier 236 and a Schmidt trigger 238. The Schmidt trigger is driven by the signals received from the down-hole instruments to produce a train of pulses having a frequency 15 equal to the frequency of ~he output from the voltage controlled oscillator. The pulses are applied by way of line 240 to a frequency meter 242, which is activa-ted when the compass 42 is selected by the downhole counter, and to a counter 244, which is activated when the probe sensors 58 or 60 are selected~ When 20 the compass is selected, the frequency of the signal sent to the surface is linearly related to the amplitude of the appropriate component of the earth's magnetic field, and ~his VCO output is read on the frequency meter 242. This meter is a commerci-ally available ICM 7226A meter supplied by Intersil Corp.
A crystal oscillator 246 synchronizes the data process-ing circuitry b~ way of a data latch 248 and a divider 250 which synchronizes the processor 86 with the signals received from the downhole sensors. The oscillator also drives the SCR in-verter 15 through the divider network 250 to produce the square 30 wave current signals used to generate the A.C. field to be measured, the use of a single oscillator thereby assuring synch-ronization of -the excitation currents and the signal processing.
The excitation current is mounted by means of a differ-ence amplifier 252 connected across a resistor 254 (Fig. 10) 35 connected in series with the excitation current lines leading to coil 10 (Fig. 1) or electrodes 24, 25 (Fig. 2). The output of the difference amplifier, which is proportioned to the vol-tage crop across the resistor, is applied to a voltage controlled oscillator 256, the output of which is supplied through amplifier 40 258 and isolating transformer 260 to a coun~er 262. This counter, .~
( "
'75~
which is similar to counter 244 in the ~ata channel, supplies a count proportional to the excitation current.
The microprocessor functions to averaye the da-ta signals recelved over a preselected period of time. This is accom-5 plished by a simple clata stacking technique wherein each cycleof the primary excitation is divided into 32 "equivalent"
channels. The number of zero crossings generated by the downhole VCO is linearly related to the ins-tarltaneous voltaye output of ~ the selected sensor. The processor samples the output of counter i 10 244, computes the number of zero crossings in each time channel, I and then adds the result to the appropriate channel locations ¦ in its memory.
After adding (i.e., stacking) the data in this manner for a selected period of time, for example, between 5 and 30 15minutes, the contents of the 32 channels are Fourier analyzed and divided by the number of cycles that have been stacked to give the amplitude and phase of the ~.C. maynetic field compon-ent. The processor may then advance the downhole counter by activating switch 230. This may be accomplished, for example, 20by solenoid 264 or by suitable analog switching to disconnect the borehole ]ine 18 from transformer 232 and to connect line 13 to a source 266 of direct current. This produces a D.C. signal on line 18 which activates counter 180 (Fig. 9) as described above. The process then repeats itself for the next sensor or 25Compass outputf accomplishing additional data stacking as de-scribed.
The information collected and processed by the processor 86 may be stored on a suitable recording means such as a cassette recorder 268, or may be displayed by a conventional printer 270, 30for later evaluation. After both the hori~ontal and vertical components of the A.C. magnetic field and the earth's magnetic field components have been measured and stored, the data so obtained provides valuable information concerning the nature of the strata in which the measurements were made. Such data is 35far more accurate than that previously available with prior systems, and permits well logging at depths and at lateral dis-tances from the borehole that were unattainable with prior devices and s~stems.
In testing the magnetometer constructed in accordance 40with the foregoing description, it was found that magnetic field :
. .
.
19 ~L~7511~t~
components excited by a current loop source of the type :i].lus-trated :in F.ig. 1 or by a current dipole source of the type illustrated in Fig. 2 could be measured with a high deyree of sensitivity. It was found that perturbations in the A.C. mag-5 ne-tic field generated by a surface source can be identified even at very great depths, on the order of 4,800 feet, and can produce significant data concerning the structure of the strata in the area of the measurements. Good results were obtained at both 7 Hz and 36 Hz, giving data which permitted accurate interpre-10 tation of the strata adjacent the borehole as well as thatspaced a considerable distance laterally away from the borehole location.
Although the present invention has been described in terms of a preferred embodiment thereof, it will be understood that 15 numeroug variations and modifications can be made without depart-ing from the true spirit and scope thereof as described in the following claims.
Claims (20)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A magnetometer for use in measuring subsurface A.C.
injected magnetic fields for borehole logging, comprising:
first and second magnetic field sensors, said first sensor lying in a first plane and said second sensor lying in a second plane;
fastening means for securing said sensors in axial align-ment with the plane of said first sensor perpendicular to the plane of said second sensor;
an elongated heat-insulating housing adapted to receive said sensors, said housing having a relatively small diameter and being adapted for insertion vertically into a borehole;
said first and second sensors each including:
first and second generally U-shaped core segments each having first and second spaced, parallel, elongated leg portions extending from opposite ends of a relatively short base portion, said first and second core segments being arranged in back-to-back relationship with their base portions abutting each other;
channel means securing corresponding leg portions of said core segments to hold said segments in assembled relationship to form a generally H-shaped core having elongated leg portions extending parallel to, and defining, the sensor axis; and coil means surrounding said abutting base portions;
circuit means connected to the coil means of each said sensor for producing signals representing A.C. magnetic field components detected by said first and second sensors; and means for securing said circuit means on at least one of said sensors.
injected magnetic fields for borehole logging, comprising:
first and second magnetic field sensors, said first sensor lying in a first plane and said second sensor lying in a second plane;
fastening means for securing said sensors in axial align-ment with the plane of said first sensor perpendicular to the plane of said second sensor;
an elongated heat-insulating housing adapted to receive said sensors, said housing having a relatively small diameter and being adapted for insertion vertically into a borehole;
said first and second sensors each including:
first and second generally U-shaped core segments each having first and second spaced, parallel, elongated leg portions extending from opposite ends of a relatively short base portion, said first and second core segments being arranged in back-to-back relationship with their base portions abutting each other;
channel means securing corresponding leg portions of said core segments to hold said segments in assembled relationship to form a generally H-shaped core having elongated leg portions extending parallel to, and defining, the sensor axis; and coil means surrounding said abutting base portions;
circuit means connected to the coil means of each said sensor for producing signals representing A.C. magnetic field components detected by said first and second sensors; and means for securing said circuit means on at least one of said sensors.
2. The magnetometer of Claim 1, wherein each said core segment comprises a plurality of thin, narrow, flat, elongated, continuous sheets secured in stacked relationship by said channel means to form a laminated segment, each sheet extending the full length of said core segment, each said sheet being folded into a U-shape to form said elongated leg portions and said base portion.
3. The magnetometer of Claim 2, wherein each of said elongated leg portions is about 10 times the length of said base portion.
4. The magnetometer of Claim 3, wherein said laminated sheets are formed from high permeability metal.
5. The magnetometer of Claim 3, wherein said channel means is generally C-shaped in cross-section and is crimped around said laminated sheets.
6. The magnetometer of Claim 3, further including spacer means fastened to and extending between the free ends of the leg portions of each said core segment.
7. The magnetometer of Claim 6, wherein said fastener means is connected between a spacer means on said first sensor and a spacer means on said second sensor, said fastener means securing said sensors in coaxial spaced relationship whereby the sensors have an effective diameter smaller than the inner diameter of said housing and wherein said magnetometer has a substantial axial length.
8. The magnetometer of Claim 3, wherein said coil means for each sensor includes on the order of 8,000 turns, said coil means surrounding each of said continuous lamination sheets for said first and second core segments.
9. The magnetometer of Claim 8, wherein the elongated leg portions of each of said folded continuous lamination sheets lie in parallel spaced planes and wherein said base portion lies in a plane which includes the sensor axis and which is per-pendicular to said parallel, spaced planes.
10. The magnetometer of Claim 9, wherein said base portion of each lamination sheet is formed by folding the lami-nation sheet longitudinally, whereby said base portion is double the thickness of said leg portions.
11. The magnetometer of Claim 1, wherein said circuit means comprises a voltage controlled oscillator, multiplexer means for connecting said coils sequentially to modulate the output signal from said oscillator; and means for connecting said output signal to data proces-sing equipment.
12. The magnetometer of Claim 11, wherein said surface data processing equipment comprises detector means for demodu-lating said oscillator output signals and signal averaging means for accumulating said demodulated output signals over a prede-termined period of time.
13. The magnetometer of Claim 1, further including means for producing an A.C. magnetic field in subsurface strata; and means for positioning said magnetometer within a borehole passing through said strata.
14. A magnetometer system for measuring A.C. injected magnetic fields for borehole logging, comprising:
surface means for producing in subsurface strata an A.C. magnetic field;
first and second magnetic field sensors, each sensor including first and second generally U-shaped core segments each having first and second spaced, parallel, elongated leg portions extending from opposite ends of a relatively short base portion, said first and second core segments being arranged in back-to-back relationship with their base portions abutting each other, channel means securing corresponding leg portions of said core segments to hold said segments in assembled relationship to form a generally H-shaped core having elongated leg portions extending parallel to, and defining, the sensor axis, and coil means sur-rounding said abutting base portions;
fastening means for securing said sensors in axial align-ment with the plane of said first sensor being perpendicular to the plane of said second sensor;
an elongated housing for said sensors, said housing having a relatively small diameter and being adapted for inser-tion vertically into a borehole;
notch filter means for passing signals having the fre-quency of said A.C. magnetic fields;
multiplexer switching means connecting the output of a selected one of said sensor coils to the input of said notch filter;
surface data processing equipment;
cable means for connecting said magnetometer to said surface data processing equipment; and transmitter means responsive to the output from said notch filter to transmit to said surface equipment signals corresponding to said A.C. magnetic fields.
surface means for producing in subsurface strata an A.C. magnetic field;
first and second magnetic field sensors, each sensor including first and second generally U-shaped core segments each having first and second spaced, parallel, elongated leg portions extending from opposite ends of a relatively short base portion, said first and second core segments being arranged in back-to-back relationship with their base portions abutting each other, channel means securing corresponding leg portions of said core segments to hold said segments in assembled relationship to form a generally H-shaped core having elongated leg portions extending parallel to, and defining, the sensor axis, and coil means sur-rounding said abutting base portions;
fastening means for securing said sensors in axial align-ment with the plane of said first sensor being perpendicular to the plane of said second sensor;
an elongated housing for said sensors, said housing having a relatively small diameter and being adapted for inser-tion vertically into a borehole;
notch filter means for passing signals having the fre-quency of said A.C. magnetic fields;
multiplexer switching means connecting the output of a selected one of said sensor coils to the input of said notch filter;
surface data processing equipment;
cable means for connecting said magnetometer to said surface data processing equipment; and transmitter means responsive to the output from said notch filter to transmit to said surface equipment signals corresponding to said A.C. magnetic fields.
15. The magnetometer of Claim 14, wherein said trans-mitter means is a voltage controlled oscillator.
16. The magnetometer of Claim 15, wherein said multi-plexer switching means includes counter means for selectively connecting one of said sensors to said notch filter.
17. The magnetometer of Claim 16, further including compass means mounted in said housing, said compass means having at least one output representing the direction and strength of the earth's magnetic field, said multiplexer switching means further including switch means connecting the output of said compass to said transmitter means.
18. The magnetometer of Claim 14, wherein said surface data processing equipment comprises:
means responsive to said signals from said transmitter for producing a train of pulses having a frequency corresponding to said transmitted signals;
counter means for receiving said train of pulses; and processor means for receiving and accumulating received pulses over a preselected time period and for averaging the accumulated values to obtain the amplitude and phase of said A.C. magnetic field.
means responsive to said signals from said transmitter for producing a train of pulses having a frequency corresponding to said transmitted signals;
counter means for receiving said train of pulses; and processor means for receiving and accumulating received pulses over a preselected time period and for averaging the accumulated values to obtain the amplitude and phase of said A.C. magnetic field.
19. The magnetometer of Claim 18, wherein said surface data processing equipment further includes crystal oscillator means for synchronizing said processor means and said A.C.
magnetic field.
magnetic field.
20. The magnetometer of Claim 19, wherein said surface data processing equipment further includes means for advancing said magnetometer multiplexer means.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000394520A CA1175106A (en) | 1982-01-20 | 1982-01-20 | Method and apparatus for extending lateral range electrical conductivity logging |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000394520A CA1175106A (en) | 1982-01-20 | 1982-01-20 | Method and apparatus for extending lateral range electrical conductivity logging |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1175106A true CA1175106A (en) | 1984-09-25 |
Family
ID=4121862
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000394520A Expired CA1175106A (en) | 1982-01-20 | 1982-01-20 | Method and apparatus for extending lateral range electrical conductivity logging |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1175106A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6988566B2 (en) | 2002-02-19 | 2006-01-24 | Cdx Gas, Llc | Acoustic position measurement system for well bore formation |
| US7321293B2 (en) | 2004-08-06 | 2008-01-22 | Halliburton Energy Services, Inc. | Integrated magnetic ranging tool |
| US7878270B2 (en) | 2004-11-19 | 2011-02-01 | Halliburton Energy Services, Inc. | Methods and apparatus for drilling, completing and configuring U-tube boreholes |
-
1982
- 1982-01-20 CA CA000394520A patent/CA1175106A/en not_active Expired
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6988566B2 (en) | 2002-02-19 | 2006-01-24 | Cdx Gas, Llc | Acoustic position measurement system for well bore formation |
| US7321293B2 (en) | 2004-08-06 | 2008-01-22 | Halliburton Energy Services, Inc. | Integrated magnetic ranging tool |
| US7878270B2 (en) | 2004-11-19 | 2011-02-01 | Halliburton Energy Services, Inc. | Methods and apparatus for drilling, completing and configuring U-tube boreholes |
| US8146685B2 (en) | 2004-11-19 | 2012-04-03 | Halliburton Energy Services, Inc. | Methods and apparatus for drilling, completing and configuring U-tube boreholes |
| US8272447B2 (en) | 2004-11-19 | 2012-09-25 | Halliburton Energy Services, Inc. | Methods and apparatus for drilling, completing and configuring U-tube boreholes |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4323848A (en) | Plural sensor magnetometer arrangement for extended lateral range electrical conductivity logging | |
| US4502010A (en) | Apparatus including a magnetometer having a pair of U-shaped cores for extended lateral range electrical conductivity logging | |
| US4845434A (en) | Magnetometer circuitry for use in bore hole detection of AC magnetic fields | |
| CA2102768C (en) | Apparatus and method for determining thickness of magnetic pipe by measuring the time it takes the pipe to reach magnetic saturation | |
| CA1146220A (en) | Electromagnetic sensing device | |
| CA1263440A (en) | Oil well logging tools measuring paramagnetic logging effect for use in open boreholes and cased well bores | |
| US3617867A (en) | Method and apparatus for well logging utilizing resonance | |
| US6342784B1 (en) | Method for resistivity well logging utilizing nuclear magnetic resonance | |
| US6534986B2 (en) | Permanently emplaced electromagnetic system and method for measuring formation resistivity adjacent to and between wells | |
| US6934635B2 (en) | Apparatus and method for measurement of the magnetic induction tensor using triaxial induction arrays | |
| Trushkin et al. | Surface NMR applied to an electroconductive medium1 | |
| US6400148B1 (en) | Use of redundant data for log quality measurements | |
| CA1053755A (en) | Dielectric induction logging system for obtaining water and residual oil saturation of earth formations | |
| EP0019091A1 (en) | Method and apparatus for pipe inspection | |
| CA1040708A (en) | Combination radio frequency dielectric and conventional induction logging system | |
| CA1062333A (en) | Movable oil measurement combining dual radio frequency induction and dual induction laterolog measurements | |
| CN101319611A (en) | Devices, systems and methods for assessing porous media properties | |
| US6081116A (en) | Nuclear magnetic resonance apparatus and method for geological applications | |
| US20050049791A1 (en) | Deep resistivity transient method for MWD applications using asymptotic filtering | |
| CA2500337A1 (en) | A method for resistivity anisotropy determination in near vertical wells | |
| EP2539745A2 (en) | Method and apparatus for determining geological structural dip using mulitaxial induction measurements | |
| Jackson | Nuclear magnetic resonance well logging | |
| JP2939334B2 (en) | Sub-audio low-frequency magnetometer | |
| US7737697B2 (en) | Method and apparatus for use of the real component of a magnetic field of multicomponent resistivity measurements | |
| US6445187B1 (en) | System for the measurement of electrical characteristics of geological formations from within steel cased wells using magnetic circuits |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEC | Expiry (correction) | ||
| MKEX | Expiry |