JPS61253486A - Induction logging sonde with metallic supporter - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to well logging equipment for investigating the properties of formations, and in particular to guided well logging sondes.

An induction logging device basically has a transmitting coil and a receiving coil mounted on a support and axially spaced from each other in the direction of the borehole. The transmitting fill is excited by an alternating current, typically at a frequency of 20 to 20 Hz, to generate a magnetic field that induces eddy currents in the surrounding formation coaxial with the borehole and The strength is proportional to the conductivity of the formation.

The magnetic field generated by these eddy currents induces an electric force within the receiving coil. By suitably processing the signal from the receiving coil, a measurement of the conductivity of the formation can be obtained.

In a conventional guided logging sonde, the fill support is
in the form of a tubular 8 rod of non-conductive material such as epoxy resin reinforced with glass fibers (see, for example, U.S. Patent No. 3.
/ 7 9 ff 7 9 (Tangny), No. 3
, / lIZ 4t, 2 No. 9 (Moran) and No. 3, 7θl,, 02! Since the signal from the receive coil is at a very low level, to avoid spurious currents that flow near the coil and create spurious components (“sonde errors”), it is necessary to It has always been believed that it is important to minimize the amount of conductive material present in
1 Publication entitled ``Electromagnetic Resistance MWD Tools'' presented by P. F. Rodney et al. at the 3rd Annual Technical Conference and Exhibition at San Francisco.
E, /2/A7C Society of Petroleum Engineers) Please refer to the second paragraph on the left side of page 7. The obvious disadvantage of using synthetic resin supports is that they are very fragile during use;
Induction probes are considered the most fragile of the various types of logging equipment.In addition, the logging section, which measures while drilling, must be constructed around a collar of steel (or other high-strength material). As the drilling mud must be assembled in the collar and the drilling mud is circulated inside the collar, guided tools are not recommended in drilling-while-measuring situations, as pointed out in the S, P, E publications mentioned above. It has been considered unusable.

It must be pointed out that it is impossible to completely exclude metal parts in the vicinity of the coil, since electrical conductors are required to energize the transmitter coil and carry the signal from the receiver coil. In a conventional inductive sonde, the conducting wire is
It is a rigid, pressure-resistant, multilayer coaxial cable.

These cables are made of mutually insulated coaxial gold pA@
layer”, the inner layer acts as a conductor carrying the signal,
On the other hand, the outer layer provides shielding and mechanical strength for the electrical conductors. Discontinuities in the connections with these cables and coils create eddies in the presence of the transmit field that cause errors in the output signal. In the case of low conductivity formations, this error can be of the same order of magnitude as the useful signal. Furthermore, this error is significantly subject to temperature drift and
Its value at room temperature is substantially different from that in the environment of the borehole where temperatures may be above 0°C.
This error may vary with the age of the device due to bending, aging of the synthetic resin, etc. that may affect the support.

U.S. Patent No. 3,2 daqg! uses a metal support for the purpose of increasing mechanical strength. rg issue (Goull
oud). This patent teaches that in order to minimize the generation of spurious currents within the metal support, the metal support should include a diametrically through slot extending substantially along its entire length. This patent also directs that the coil be made of circular turns interconnected by straight conductor segments, and that the midpoints of the segments be located in a plane along the diameter of the slot. ing. However, the improvement in mechanical strength is limited by the slots across the support;
Nor is it directed to problems arising from the presence of electrical conductors.

It is an object of the present invention to provide a guided well logging sonde with good mechanical strength and robustness.

It is a further object of the present invention to provide a guided transverse 1 sonde that exhibits low, stable and predictable sonde errors.

Another object of the invention is to provide an inductive logging sonde in which the use of multilayer coaxial cables for connections to and from the coil is avoided.

Another object of the invention is to provide an inductive logging sonde that can be inserted at any position within a logging tool set. The objective is to provide a suitable guided logging sonde.

Inductive logging sondes provided in accordance with the present invention include a generally cylindrical electrically conductive metal elongate support, at least seven transmit solenoid coils, and at least seven transmitter solenoid coils in coaxial spaced relation to the support. and two receiving solenoid coils. The transmitting fill is operated at an external frequency such that the transmitting coil generates an electromagnetic field that is substantially free of dielectric effects;
A suitable frequency range is about 70H! In between, the preferred lower limit of the frequency range is about 8 KHz, and the preferred upper limit is about 200 to 8 KHz. It is. The support, at least in a portion thereof adjacent the coil, has a substantially continuous and axisymmetric outer surface that facilitates vortex flow in a circular manner around said surface.

Preferably, the support has an outer sleeve made of a metal with high electrical conductivity, such as copper, and an inner core made of a less electrically conductive but high strength material, such as stainless steel.

The present invention can be easily understood by reading the following description with reference to the accompanying drawings.

Figure 1 shows the geological formation 1 traversed by the borehole 12.
0 showing a guided logging sonde 10 for investigating 1
The borehole is filled with drilling mud water 18. The equipment is suspended from a multi-conductor cable 14, which is passed over a rope car 15 and wrapped onto a winch 16, which is part of the ground equipment associated with the downhole logging sonde. The ground equipment is downhole equipment 1
Cable 14 provides signals and power for controlling the operation of 0.
The measurement signal is supplied to the downhole device 10 via the downhole device 10, and a measurement signal is received from the downhole device 10.

The ground equipment is provided with a sensor 17a for detecting the movement of the zero cable, which includes a device 7 for processing and recording these signals. The signal coming from sensor 17a is an indication of the instantaneous depth of the downhole device;
The downhole device 10 comprises an electronic cartridge connected to the cable 14 via a cable head 19, which is sent to a processing device to determine the depth consistent with the measurement signal. The cartridge 18 has a telemetry cartridge zO that converts the signals from the ground equipment generated by the downhole equipment into a form suitable for transmission by cable. The downhole equipment 10 also includes an elongate support BO;
The upper end of the support body 80 is fixed to the cartridge 18. A coil system is mounted on the support 80 and includes a transmit coil 81 and a receive filter 82 spaced apart from each other in the longitudinal direction of the support 80 and coaxial.

The transmitting coil 81 is energized to produce a magnetic field that induces a vortex flow within the formation coaxial with the axis of the support. A receiver coil responds to the magnetic field created by these vortices to generate an output signal representative of the conductivity of the formation. The frequency of operation of the transmitter coil is such that the magnetic field created within the formation can be classified as a quasi-static electromagnetic field.

In other words, the operating frequency is such that displacement currents are negligible and small, and conduction currents are dominant. The frequency is suitably between about /θKHz and about 'IO0KH2. Above Hz, the displacement current becomes significantly smaller and the output signal responds not only to the conductivity of the formation, but also to the dielectric constant of the formation, which for the purposes of the present invention. Undesirable. The preferred upper limit of the frequency range is about 0 to HI. The preferred lower limit of the frequency range is approximately 20KH2.

Although in the schematic diagram of FIG. 7 the sonde is shown as having only transmit and receive coils, it is clear that each coil system may include more than two coils. For example, seven or more transmit coils, several receive coils, and bucking coils each associated with a receive coil to counteract the effects of the direct coupling between the transmit coils and the receive coils. be able to.

The sonde may further include several coil systems distributed over its length.

Support 80 generally has the shape of a tube and is preferably made of a metal, non-magnetic metal having excellent electrical conductivity.

Suitable materials include copper, copper alloys, and stainless steel.

The support has a cylindrical longitudinal section 88 with an outer wall 84 in contact with the outside or drilling mud, and a cylindrical longitudinal section 85 having a smaller outer diameter than this section 88 . Thus, portions 85 define recesses 8fia for receiving coils B1 and 82 which are electrically insulated from and coaxial with the respective portions 85. The embodiment shown in FIG. 1 has such a recess for each coil, but
However, it will be appreciated that one section 85 could similarly be fitted with an entire coil system, ie one recess 85a could receive a plurality of axially spaced coils. Intermediate portion 8B preferably has a larger inner diameter than portion 85 to define an interior space 86, and in the embodiment shown in FIG. 1, portions 88 and 85 are joined by a transverse portion 87. The walls of portion 88 have sufficient thickness to withstand the hydrostatic pressure of the fluid within the borehole.

Respective electrical conductors 88 with passages inside the support connect coils 81 and 82 to electronic cartridge 18 .

The support in the part adjacent to the coil is made of a highly conductive material and has a continuous, axially symmetrical outer surface, so that it is almost a perfect electrical conductor. This aids in the generation of vortices in the presence of the electromagnetic field generated (by the transmitter coil), which vortices flow around the surface of the support. As a result, the tangential electric field is forced to zero on the surface of the support and no electromagnetic field is generated in the closed space defined by the interior of the support. The support therefore constitutes a very efficient electromagnetic shield. For example, 20K
6. operating frequency of H2; g X /θ7s/m (copper)
For a conductivity of , the thickness of the third layer represents the IO penetration thickness. Thus, the spurious effects of a direct connection between the conductor and the coil are eliminated and it is possible to use a simple conductor wire rather than the multi-layer coaxial cables used heretofore.

Furthermore, since the contact electric field is substantially extinguished on the surface of the support, the sonde errors caused by the eddies flowing around the support are low. For a perfectly axially symmetrical metallic support, the sonde error is due to the decreasing function of the electrical conductivity of the support material and the frequency.
on). The mathematical representation of this variation is E = -%, t-V2. However, E: Sonde error: Electric conductivity f: Frequency: Coefficient.

Therefore, if high conductivity metals are used, sonde errors will be minimized. A typical value of the sonde error of a support measured in air is λ millisiemens, which is of the same order of magnitude as the output signal obtained for the highest resistivity formations. Furthermore, a very significant advantage of the present invention is that
This sonde error exhibits a very low temperature drift and is well predicted. Therefore, it is easy to correct the output signal for the effects of the metal support by subtracting a well-defined sonde error from the output signal.0 The fact that the coil is mounted around a metal part
This has the effect of reducing the cross-sectional area available for the magnetic flux transmitted by coil 81 and received by coil 82, the surface area being independent of the spacing between the outer surface of portion 85 and the coil.

This results in a reduction in the sensitivity of the measurement, but this reduction is easily compensated by the correct design of the coil, i.e. by increasing the number of turns of the fill with respect to a conventional arrangement with a non-conductive support. Ru.

It should be noted that the axial spacing between the ends of the coil and the adjacent walls of the transverse portion 87 is maintained appropriately above a predetermined value. The response of each of the sondes, when measured along the outer surface of the sonde, exhibits a sharp peak opposite the coil, the average width of the peak being approximately equal to one diameter of the respective coil. In order to avoid substantial changes in the response of the sonde, the spacing between the ends of the coils and adjacent transverse sections is chosen equal to at least one diameter of the respective coil.

If multiple coils are received in one recess 85a, the spacing between each transverse section and the end of the coil located closest to the transverse section is at least about one diameter of the coil. should be.

In other words, the cylindrical part of the support on which the individual coils or the entire coil system are mounted has a smaller axial dimension than the axial dimensions of the coils (K for each of the coil systems) on each side of the coil (for each of the coil systems). (by the diameter of each end coil of the coil system).

In view of the foregoing, a support with a completely axially symmetrical and continuous outer surface in the area adjacent to the coil is optimal, however, designs that deviate slightly from this optimum are within the scope of the present invention. . Provided, however, that the flow of the vortex around the support is not substantially affected. For example, a generally similar but different cross-section to a circular cross-section, for example a polygonal cross-section, can be used. Also, small holes provided through the support, for example for passing electrical conductors into the coil, do not substantially alter the flow of the vortex flow. On the other hand, longitudinal slots through the support resist the flow of vortices and are detrimental to the shielding effect of the support.

Another advantage of the metal support is that it gives the sonde improved mechanical strength and solidity, allowing the portion 88 of the support to withstand pressure. The space 86 formed inside the portion 88 allows the sonde to can be used to accommodate some of the electrical circuits. In this case, instead of arranging all the circuits in the cartridge 18 as shown in FIG. There is a receiving block connected to the side and similarly mounted in the space adjacent to the receiving coil 0. Due to the mechanical strength of the support 80 and the possibility of wiring conductor wires inside the support, The above-mentioned inductive sondes are of different types (sound wave,
Nuclear) can be combined with one (or more) logging devices. This device is indicated in dotted lines in FIG. 1 by the reference numeral 40. The electrical conductor connecting the device to the telemetry cartridge 20 through the interior space of the support 80 is also shown in dotted lines with reference numeral 41. Therefore, the inductive sonde of the invention can be inserted anywhere within the logging sonde combination, while a conventional inductive sonde with a non-metallic support can be placed at the bottom of the combination.

FIG. 2 shows a suitable embodiment of coils 81 and 82 in more detail. Each coil is in the form of a fill unit 50. The coil unit is insulated from part 85 of the support by an insulating sleeve 15, for example a ceramic sleeve. Sleeve 51 is secured to portion 85 by means such as pins 52. The sleeve 51 is
There are a number of rings 58 and a number of rings 54 screwed to each end of the tubular portion 51 on each side of the coil unit to hold the coil unit in place.

The coil unit comprises a fill shape made of one approximately annular support part 561, 57 of an insulating material, for example ceramic, the support parts 56 and 57 - together having a rectangular cross-section and an annular internal space 58. is defined. space 58
receives a winding 6o of the coil, which winding is attached to the support 8
It is coaxial with the zero axis. A plurality of conductive wires 61 are wrapped around the support portions 66, 57 in a toroidal arrangement. Each wire is cut so that it does not form a closed loop, and all wires are connected to the ground ring (
(not shown) and are therefore at the same ground potential. The wires 61 form an electrostatic shield that prevents electrostatic coupling of the respective coil with other coils or drilling fluid. This arrangement is incorporated by reference in U.S. Patent Application No. 1, entitled "Shielded Solenoid Coil for Well Logging," assigned to the present applicant! r
! No. 1-1239 (pending with the original U.S. patent application of this application), which is hereby incorporated by reference.

A fluid-tight sleeve 62 made of a non-conductive material such as glass fiber reinforced epoxy resin is mounted around the coil unit with pressure seals 62a provided at both ends to protect the coil unit from contact with drilling mud. It is being Rings 58.54 support the sleeve 62 on its inner surface, allowing the sleeve 6z to withstand the pressure of muddy water. A hole 6B is provided in the part 87 of the support for the passage of the electrical conductor connected to the coil.

In the case described above where several coils are mounted on the same section 85, an additional ring similar to ring 58 is provided in the space between the coils to hold the coils in place and It supports a sleeve 6z that closes the recess 85a.

Additionally, FIG. 2 shows sections 85 and 85 as separate sections, screws 64 are provided at each end of section 85 for attaching section 85 to transverse section 87, and the coil unit is attached to section 85. After mounting, the support is assembled.

Regarding the material of the support, it will be appreciated that parts 88 and 85 can be made from different materials, for example stainless steel for part 88 and steel or a copper alloy for part 85.

A modified embodiment is shown in FIG.

The coil 70 is connected to the portion 56,5? of FIG. It is surrounded in a coil shape 71 similar to the coil shape constructed by. The coil is electrically shielded by a slotted cylindrical member 72 made of a conductive material, with the slot 7z filled with an insulating material, as in the embodiment described above.
The coil shape is held in place by a ring screwed onto the tubular portion, which engages the inner wall of the slotted member and pushes the slotted member against the pressure of the fluid in the borehole. There is.

A modification to the embodiment described above is to pressurize the interior of the recess housing the coil unit to the pressure of the borehole fluid. Fluid lines are routed inside the support 80 to connect these recesses to conventional pressure compensators. In this case, a fluid-tight electrical feed-through connection is provided to make an electrical connection through the wall of the recess. Figure 3 shows an inductive sonde according to the invention adapted to measuring operations during excavation. The support for the coil located above the drill pit unit 82 is provided by the drill collar 88.
and a steel tubular member conventionally connected to the lower end of the drill string for mounting above the pit unit. Drilling mud is circulated through the center hole 84 of the drill collar during drilling operations. In order to provide adequate weight on the bit for drilling purposes, the drill collar has a large thickness.

FIG. 7 shows only λ coils, namely the transmit coil /l/85 and the receive coil 86, however, a bucking coil, not shown, is associated with the receive coil for the purpose described above, and the sonde It will be appreciated that this may involve an arrangement of several coils with different spacing from the transmit coil. All these coils are arranged in the same way as =Ile 85.86.

Drill collar portion 88 has a portion of reduced outer diameter;
Thus defining a circular recess 87 for receiving the coils, each coil having a recess-filling sleeve 88 of fum or similar material.
be fitted into. The filling of the drill collar f88 involves the drilling collar part 8 where the outer surface of the three f88 is exposed to the borehole fluid.
Section 890 of 8 is filled to be substantially flush with the cylindrical outer surface. Each recess 8 has an outer diameter of 0
It has a central cylindrical portion 90 with an outer diameter 01 of less than 2, around which each cylinder is mounted, and a frustoconical portion 91 joining the central portion 90 with a portion 811 of the drill collar. The axial dimension of the truncated conical part is
Suitably it is equal to at least about one diameter of each coil.

Signals applied to the transmitter coil and received from the receiver coil are carried by electrical conductors (not shown), which include vertical conductors formed on the outer surface of the drill collar and filled with a suitable conformal protective material. Preferably threaded through the grooves, these conductors are connected to an electronic cartridge (not shown) located inside the drill collar at the top of the inductive sonde.

In a preferred embodiment, steel or other highly conductive
A layer of material is formed in at least the central portion of each recess to provide a highly conductive surface near the coil, and the drill collar portion 8B is otherwise made of steel.

FIG. S shows a preferred embodiment of the inductive sonde according to the invention in a partial sectional view.

In the embodiment of FIG. 3, there are ten transmitter coil units.
0, a plurality of arrays of solenoid coils are provided, each row of fills including a receive coil and a bucking coil, the packing coils having the effect of coupling the transmit fill directly to the respective receive coil. 5 designed and positioned to offset each other. The receive coils with different spacing from the transmit coil are shown at 101% lOa, and the backing coils associated with the receive fills are shown at 101% lOa.
', 10z'. All the coils are mounted around a central support 105 which has an outer cylindrical surface of one circular cross section as seen in the cross-sectional view of FIG. The enlarged diameter ends 106 and 107 of the support 10
50 is fixed at both ends.

A glass fiber reinforced epoxy backed tubular sleeve 108 is mounted around the coil to prevent contact with the borehole fluid. The sleeve has ends 106 and 10? and held in place between the ends 106.10? has the same outer pole as . The free space within the ring 110 formed between the central support 105 and the three f108 is filled with pressurized oil;
As a result, the space is the lower end 10? Ill adjacent to
It is in communication with the pressure compensator indicated by .

The compensation device, which is a conventional element of a drilling well logging sonde, is [1
10 to pressurize the oil present in the borehole fluid to a pressure slightly higher than the pressure of the borehole fluid and hence
The differential pressure acting on the top is small.

The cross-sectional view of FIG. 6 shows the preferred embodiment of the central support 105. The support 1os+i comprises one part, an outer sleeve 115 of preferably a highly conductive material such as steel or a copper alloy, and an inner core 116 of a high strength metal, preferably stainless steel. ,
It is mounted on top of the inner core with a loose fit that takes into account the differences in thermal expansion between the steels. The inner core lie has a plurality of longitudinal grooves 11 formed on its outer periphery. with a conductor passed through it, 5K as shown in FIG.

The groove is a conductor 118 passed inside the tubular shield 119.
accept. Although each groove receives a pair of conductors inside the shield, FIG.
Only one shield is shown.

The purpose of the shield 1190 is to minimize interference between conductors placed in adjacent grooves. The shield can be suitably made from a ferromagnetic material such as mu-metal. In addition to tR117, the inner core has a central wellbore 130 that is connected to a power line and possibly other logging equipment suspended from the logging sonde of the present invention. Used to pass through conductors. Groove 1
1? and the central bore 120 are in fluid communication with the fil O through radial bores (not shown), so that the ring 11
Filled with oil at the same pressure as inside.

A suitable method for manufacturing the inner core is extrusion through a die of suitable design.

The coil may be in the form of a coil unit similar to that described with reference to FIG. The receiving coil and its associated backing coil are coils 101, 101.
5 can be mounted on the same insulating sleeve of ceramic (with each sleeve 122
) or coil 108.108/ Separate insulating three as shown! Can be installed on top. A radial hole for passing the 4-N body connected to the receiving coil and the bucking coil is provided in the outer sleeve 1l of the support.
ls, while another itN body 24 connects each pair of coils to each other.

In order to reduce the amount of oil to be pressurized by the compensating device and thereby shorten the length of this device, filler elements 125, for example of epoxy resin, are provided in the annulus 110 between the coils. .

The electronic cartridge necessary for the operation of the transducer is shown schematically at 180 adjacent to the lower end of the sonde, with a pressure bulkhead 181 arranged between the compensating device 111 and the cartridge 130. An electronic cartridge 182 connected to the receiver coil is mounted adjacent to the upper end of the sonde, and similarly a pressure bulkhead 18B is connected to the cartridge 1.
Power is distributed between B2 and the coil portion of the sonde. FIG. 3 shows an embodiment of the upper and lower ends of the coil portion of the sonde. At the lower end, two half rings 186, 186 are inside 4I! Flanged over the end of the core, the inner core has a circumferential groove 187 for engaging the inner collar 188 of the half ring. The half-rings abut the ends of the copper sleeve and are secured to each other by bolts (not shown) K, so that the half-rings form a sleeve that rotates around the end of the support core. The sleeve 140 is screwed onto the half ring and snaps together with the support. j-fl 40 and the key that engages with the key groove formed in the support body = 7
It is restrained from rotating about 116. Sleeve 14G has a reduced outer diameter portion 141 over which outer sleeve 17-7" 108 fits. A compensator housing 146 connects sleeve 140 to
is coupled to the sleeve 140 by a nut 148IC positioned between the sleeve f140 and the end of the housing 145 and in threaded connection with the housing;
8 is fixed by a retainer 149 so as not to be displaced along the axis with respect to the sleeve 14G. When the nut is turned with a special wrench, the housing will move into the sleeve 14.
0 and is therefore axially displaced with respect to the support of the coil. Moreover, the grooves 117 are connected at their ends to inclined passages 111 that open into the central hole 120.

The arrangement at the upper end of the core portion includes a screw 16 which engages a screw formed on the core 116 of the support abutting that end.
It has a sleeve 16G with 1.

The sleeve has a portion 168 of reduced outer diameter over which portion 163 the outer sleeve f108 fits. Sleeve 160 was threadedly engaged with sleeve f160 and restrained from axial displacement with respect to housing 165. With nut 166 similar to nut 148, 11: force bulkhead 1
83 housing 165.

The pressure bulkhead itself is shown schematically at 170, but is
A conventional piece of equipment in a well logging sonde, having an axial passage (not shown) for receiving a pressure-tight feedthrough, with electrical conductors coupled to the feedthrough on both sides. An intermediate tubular member 1 is provided between the wall 170 and the sleeve 160 on the one hand and between the end 1 of the fill support and the partition 170 on the other hand. End 1 of support core 116
7■ has a reduced outer diameter, the intermediate tubular member 171f)
Engaged within the annular recess. Helical compression spring 17B
The support member 173 is attached between the end of the support body 173 and the intermediate member 171, and applies an elastic force to the Li wall. An axially oriented passageway 175.1 flow is formed in intermediate member 191 and sleeve 160, respectively, for passing electrical conductors connected to the coil. A passageway 176 passes through a respective groove 11?of the support through a portion 178 that is fixed to the support and has a respective plurality of radial slots for passing electrical conductors.
communicate with.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of a logging sonde according to the present invention, and
FIG. 2 is a schematic diagram showing ground equipment connected thereto. FIG. 2 shows the first coil unit of the device shown in FIG.
2 is an enlarged detail view of an embodiment; FIG. FIG. 3 shows a modification of the coil unit. FIG. 7 shows an inductive logging sonde according to the invention suitable for measuring while drilling. FIG. 3 is a partial longitudinal section view of a preferred embodiment of the guided logging sonde. FIG. 6 is a sectional view taken along A-AIs in FIG. 3 in the direction of the arrow. 10・0. Induction logging sonde 80...Support 81...Transmission coil 85a...Engraving of receiving coil drawing (no change in content) Procedural amendment (method) % formula %, Title of invention Induction with metal support Relationship with well logging sonde and amendment cases Applicant, agent

Claims (1)

[Claims] 1. In an inductive logging sonde adapted to be displaced within a borehole for investigating a formation traversed by the borehole: generating an electromagnetic field substantially free of dielectric effects; at least one transmitting solenoid coil for, in which an electromagnetic field induces currents in the formation; in response to the electromagnetic fields induced by these currents, an indication of the conductivity of the formation; at least one receive solenoid cord spaced axially from the transmit coil for generating a signal; an elongated support for mounting the coil in coaxial and spaced relationship; an inductive testing sonde comprising: a support made of an electrically conductive material and having a continuous, axisymmetric outer surface at least near the coil to facilitate the flow of the vortex around said surface; . 2. The substantially continuous, axisymmetric outer surface extends longitudinally beyond each end of the coil by at least about one diameter of the coil on each side of the coil.
The sonde described in section. 3. The support includes an outer sleeve of a first material of high electrical conductivity and an inner core of a second material of higher mechanical strength than the first material, the inner core substantially extending within the inner diameter of the outer sleeve. Sonde according to claim 1, having equal outer diameters. 4. The sonde according to claim 3, wherein the outer sleeve is made of copper or a copper alloy and the inner core is made of stainless steel. 5. The inner core has longitudinal grooves formed around it, said grooves forming respective passageways with the inner surface of the outer sleeve, thereby permitting the passage of electrical conductors through the support.
A sonde according to claim 3. 6. The sonde of claim 5, wherein the inner core further defines a central longitudinal hole. 7. The operating frequency of the transmitter coil is approximately 10KHz and approximately 40KHz.
0 KHz, the sonde according to claim 1. 8. a support having at least one first longitudinal section for mounting said coil in coaxial and spaced relationship and second longitudinal sections on opposite sides of said first section; Claim 1, wherein the first portion has a substantially continuous outer surface to assist the vortex flow in a circular manner around the surface, and the second portion is exposed to the pressure of the borehole fluid. The sonde according to item 1 or 7. 9. The sonde according to claim 8, wherein the second portion of the support has a larger outer diameter than the first portion. 10. Claim 9, wherein the first and second parts of the support are connected by a transverse portion spaced from the ends of adjacent coils by a distance at least equal to the diameter of the coil. The sonde mentioned. 11. The sonde according to claim 10, wherein the second portion of the support has a larger inner diameter than the first portion. 12. The sonde according to claim 8, wherein the support is made of copper or a copper alloy. 13. In a sonde measuring during drilling, adapted to be coupled to a drill string for providing a signal representative of the conductivity of the formation traversed by the borehole being drilled: substantially at least one transmitting solenoid coil for generating an electromagnetic field without dielectric effects, the electromagnetic field inducing currents in the formation; in response to the electromagnetic fields induced by these currents; at least one receiver solenoid coil axially spaced from the transmitter coil for generating a signal representative of the conductivity of the formation; a generally tubular elongated support of conductive metal adapted to couple to the drill string; a body, the support body comprising:
a portion for mounting said coil in coaxially spaced relationship, said portion having a substantially continuous outer surface to facilitate circular flow of the vortex flow around said surface; A sonde for measuring during excavation, characterized by comprising; 14. The sonde of claim 13, wherein said portion has a reduced outer diameter to form a recess for receiving a coil. 15. The sonde of claim 14, wherein the coil is embedded in a rubber-like material. 16. The sonde according to claim 13, wherein said portion has a surface layer of metal having a higher electrical conductivity than the support. 17. The sonde according to claim 16, wherein the support is made of stainless steel and the surface layer is copper. 18. The operating frequency of the transmitter coil is about 10KHz and about 4
14. The sonde according to claim 13, wherein the frequency is between 00 KHz and 00 KHz.