GB2328746A - Determining the shape of an earth borehole and measuring the acoustic velocity in the earth formation - Google Patents

Abstract

An acoustic logging tool, useful for measuring the acoustic velocity of ultrasonic waves in formations surrounding an earth borehole, and also as an acoustic caliper tool, determines the shape and dimensions of an earth borehole, and includes one or many acoustic transceivers 56-60 mounted in substantially the same plane, in a drill string sub. The mounting receptacles are located in blocks, flexibly mounted in fixed inserts within the interior of the drill string sub. Each of the transceivers is structured to provide temperature and pressure compensation. The echo signals received by the transceivers are digitized and processed to eliminate extraneous noise created by the ringing of the transducers, by the signals reflected from the material backing the ultrasound transducer element, and by any other sources inherent to the drilling environment. Digitization and further processing also allows for signal enhancement, thus allowing detection of signals in the presence of substantial noise. Temperature and pressure determinations are used to correct the acoustic velocity of the waves passing through the drilling fluid, "on-the-fly", as the drilling process continues. At least one accelerometer 160 and at least one magnetometer 162, preferably axially aligned with one of the transceivers, enables the determination of the orientation of the major axis and the minor axis with respect to Magnetic North or earth's gravity, of any elliptical borehole being drilled.

Description

METHOD AND APPARATUS FOR DETERMINING TEIE SHAPE AND THE DIAMETER OF AN EARTH BOREHOLE, AND FOR MEASURING TBE ACOUSTIC VELOCITY IN EARTH FORMATIONS BACKGROUND OF THE INVENTION Field of the Invention The invention relates to an acoustic tool for measuring dimensions and shape within a borehole, and for measuring acoustic velocity in earth formations surrounding a borehole, and more particularly, to a caliper instrument for use primarily in a measuring while drilling (MWD) environment utilizing acoustic pulses transmitted within a borehole.

History ofthe Prior Art It has long been recognized in the oil and gas industry that the collection of downhole data during the drilling operation is of extreme value. Such information improves the efficiency of the drilling operation by providing critical data concerning downhole conditions. For example, it is desirable that a continuous record of borehole size be provided so that variations in borehole diameter as a function of depth may be recorded for analysis in connection with other measurements of formation parameters.

Acoustic well logging is also used in the geophysical and seismic arts to provide surveys of the various formations traversed by the borehole. In particular, acoustic velocity measurements provide valuable information concerning the type of rocks and the porosity thereof in the formation surrounding the borehole. The most commonly measured acoustic pararneter in the field of well logging has been the velocity of compression waves.

The velocity of shear waves and acoustic impedance has also been of value in determination of both the formation characteristics and the fluid environment.

A myriad of acoustic logging systems for downhole measurements is available in the prior art. One ofthe most critical measurement parameters of such acoustic logging systems is the acoustic velocity in the fluid through which the acoustic pulse is transmitted. A high degree of accuracy in the interpretation of pulse data is only possible with a precise knowledge of the acoustic velocity in the medium of measurement.

Moreover, a lugh degree of resolution and/or accuracy in acoustic velocity measurements is necessary for the accurate identification of various formation strata as well as other critical borehole parameters.

Many prior art attempts to provide accurate acoustic logging instrumentation have encountered serious problems due to the downhole environment. For example, the drilling operation necessitates the flow of high pressure drilling mud which is pumped down through a central bore in the drill pipe, out through apertures in the drill bit and back to the surface through the annular space between the drill pipe and the side walls of the borehole. The mud removes drill bit cuttings and the like and can reveal much information about the formation itself. Such a fluid system, by definition, includes wide variations in drilling mud density and character both along the borehole as well as in a direction across the borehole annulus. For example, gas present in the drilling fluid has a direct bearing on acoustic velocity within the fluid and the presence of gas varies with position and pressure within the borehole.

One prior art technique of determining acoustic velocity includes sampling the drilling mud at the weü-head for purposes of measurement. However, such a measurement cannot accurately reflect the varying conditions of the mud downhole where the acoustic measurements are actually made. Downhole acoustic pulses are generated, generally, by acoustic transducers disposed within the side walls of a sub secured above an operating drill bit within the borehole. The acoustic pulses are transmitted from the sub to the sidewalls of the borehole through the drilling fluid and the reflection time thereof is monitored. The presence of gas or cuttings within the fluid as well as downhole pressures, temperature and turbulence thus has a direct bearing on the acoustic velocity and the pulse-echo amplitude or reflectivity measurements. However, the most convenient location for measuring acoustic velocity is at the wellhead in the passive fluid collection area where the dynamic turbulent downhole conditions are not present. in addition, once received from the borehole, the drilling mud is generally allowed to settle andlor is passed through an out-gassing unit prior to its collection and recirculation. This step drastically alters the acoustic velocity parameters of the drilling fluid from its downhole gaseous and turbulent condition and leads to inaccuracies in the interpretation of the downhole acoustic reflectivity measurements.

A prior art method of overcoming the problems of accurate data collection in a measuring-while-drilling (MWD) environment is the recording of acoustic borehole measurements with a wireline logging tool. Such tools are utilized with the drill string removed from the borehole and the drilling mud being in a settled state. Such a condition lends itself to a more homogeneous configuration and the presence of mud cakes and turbulence related to nonhomogeneous regions are generally eliminated. One such acoustic caliper logging device is set forth and shown in U.S. Patent No. 3,835,953 to Summers wherein a wire line tool is provided for positioning within a borehole. A transducer unit repeatedly generates an acoustic pulse as the transducer system is rotated to scan the walls of the borehole in a full circle. A scan of between 1 and 10 revolutions per second may be provided with the tool itself being generally centered within the boriole. The reflections ofacoustic energy from the borehole wall are then from a small, centralized area whereby the system can be highly definitive of the character of the wall.

Such information is obviously useful in an analysis of the borehole configuration. One distinct disadvantage is, however, the necessity of pulling the drill string from the borehole for utilization of the wireline tool. This operation is both time consuming and expensive from the standpoint of the drilling operation.

In addition, prior art downhole acoustic parameter measurement techniques have obtained acoustic velocity at a downhole location but the acoustic path over which the velocity measurements are made is different from the path over which the parameter of interest is measured. For example, an acoustic caliper measurement made across a borehole annulus which relies on acoustic velocity data obtained in a direction parallel to the borehole axis will not be precise because of the nonlinearity of the flow pattern and flow densities across the borehole.

It would be an advantage, therefore, to overcome the problems of the prior art by providing detailed acoustic caliper information of a borehole in a measuring while drilling configuration. This gives the driller immediate feedback as to the quality of the borehole being drilled and can be used to infer insitu stresses, for example, as disclosed in U.S Patent No. 4,599,904 to John E. Fontenot, assigned to Baroid Technology, Inc., the assignee of this present application. This patent discloses that the calipering operation itself can be accomplished through the use of any conventional calipering device, e.g., mechanical, acoustic or neutron calipering device.

Another approach to providing MWD caliper measurements ofthe drilled borehole is disclosed in U.S. Patent No. 4,665,511 to Paul F. Rodney et al., also assigned to Baroid Technology, Inc., the assignee of the present application. This patent discloses a downhole, MWD logging tool in which a plurality of acoustic transceivers are placed on the tool in both azimuthal and longitudinal spacings to provide not only a measurement of the acoustic energy reflected from the borehole wall, but also a measurement of the drilling fluid acoustic velocity, coupled with an inclinometer to determine one period of rotational data for purposes of analysis. The patent suggests that the tool can be used to measure the dimensions and shape ofthe drilled borehole.

The prior art also indudes U.S. Patent No. 5,130,950 which discloses a pulse echo apparatus for measuring borehole standoff and borehole diameter in an MWD ern'irnnment, and includes deformable material such as rubber inside the acoustic sensor stack to allow the sensor stack to move or deform under pressure or due to thermal expansion or contraction.

As additional prior art, U.K Patent Appln. No. GB 2 254 921 A discloses means for pressure equalization in an MWD acoustic borehole caliper device.

The prior art also discloses in U.S. Patent No. 5,341,345 an MWD, ultrasonic stand-off gauge for use in measuring the instantaneous stand-off distance between the drill stem and the borehole wall during drilling.

As additional prior art, U.S. Patent No. 5,469, 736 discloses an MWD, acoustic caliper tool which derives a caliper measurement based on an analog threshold technique.

Finally, European Patent Appln. No. 0 747 732 A2, and also U.S. Patent No.

5,644,186 discloses an MWD, acoustic caliper tool having a movable seal assembly intended to provide temperature and pressure compensation.

Objects ofthe Invention The primary object of the present invention is to provide new and improved methods and apparatus for determining, during a drilling operation an approximation to the borehole shape and the borehole dimensions as the borehole is drilled through various sub-surface earth formations.

Another object ofthe present invention is to provide new and improved methods and apparatus for determining during a drilling operation, the ellipticity of the borehole and the directions of the minor/major axes of the elliptical borehole relative to that of Magnetic North and to the earth's gravity.

Another object of the present invention is to provide new and improved acoustic transducers having temperature and pressure compensation and methods of mounting such transducers in MWD logging tools.

Still another object of the present invention is to provide new and improved methods and apparatus for digitizing the various acoustic signals and for processing such signals to improve the accuracy of the borehole size and borehole shape detemlinations.

Another object ofthe present invention is to provide new and improved methods and apparatus for digitizing the various acoustic signals and for processing such signals to improve the echo detection reliability and extend the operating range of acoustic caliper logging systems.

Yet another object of the present invention is to provide new and improved methods and apparatus enabling the acoustic caliper logging system to provide drilling fluid acoustic velocity corrections on the fly; i.e., as the well is being drilled.

Summary ofthe Invention The objects ofthe invention are accomplished, generally, by new and improved methods and apparatus which generate ultrasonic pulses in an earth borehole, which then measures the return of such ultrasonic pulses as they are reflected off the wall of the earth formations surrounding the earth borehole, which digitizes such reflected ultrasonic pulses, and which then processes such digitized pulses to minimize the effects of ultrasonic transducer ringing.

As another feature of the invention, the ultrasonic pulses are transmitted from a plurality of transducers, with the transmitted signal from said transducers being digitized, computed and utilized to improve the representation of the received echo signals by Digital Signal Processing methods.

As yet another feature ofthe invention, in which an appropriate element such as a piezoelectric crystal is used to generate ultrasonic pulses, a profile is generated of the signal reflection from the backing material behind the ultrasonic element, and the generated profile is utilized to improve the representation of the received echo signals by Digital Signal Processing methods.

Another feature of the present invention is the provision of new and improved methods and apparatus for measuring the elliptical nature of an earth borehole, and the direction of the major axis of such ellipse with respect to Magnetic North and to the earth's gravity.

Still another feature of the invention involves the measurement of downhole pressures and temperatures, and the correction of the caliper determinations, while drilling, based upon such temperature and pressure measurements.

Another feature of the present invention relates to a new and improved ultrasonic transducer having temperature and pressure compensation, and to methods and apparatus for mounting such transducers in an MWD sub intended to be incorporated in a drill string.

As additional features of the invention, each if not all of the foregoing features of the invention can be used not only in acoustic caliper logging, but also in acoustic velocity logging of the formations surrounding the earth borehole. BriefaescriDtion of the Drawinas FIG. 1 is a diagrammic, side elevational view of a borehole drilling operation illustrating the use of an acoustic caliper apparatus in accord with the present invention; FIG. 2 is an elevated, pictorial view of a sub, typically a drill collar, incorporating the ultrasonic caliper apparatus according to the present invention, and configured to be incorporated in a drill string; FIG. 3 is a cross-sectional view taken through the sub illustrated in FIG. 2 and taken along the sectional line 3-2 thereof; FIG. 4(a) is an elevated view, partly in cross-section, of an ultrasonic transducer according to the present invention; FIG. 4(b) is an elevated view, partly in cross-section, of the transducer illustrated in FIG. 4(a), but rotated 90o; FIG. 5 is an elevated view, partly in cross-section, of an ultrasonic transducer mounted within a sub in accord with the present invention; FIG. 6 is a cross-sectional view taken through the sub illustrated in FIG. 2 and taken along the sectional line 6-2; FIG. 7 is a block diagram illustrating the three transducers, an accelerometer and a magnetometer used in practicing the present invention; FIG. 8 is a diagrammatic view of an elliptical-shaped borehole having a major axis and a minor axis; and FIG. 9 is a block diagram of circuits used in practicing the present invention.

Detailed Description of the Preferred Embodiment As discussed below with respect to the prior art, there is a constant need for new and improved MWD caliper tools, as discussed, for example, in Paper A, Transactions of the 30th SPWLA Annual Logging Symposium, Denver, 1989, "Field Experience Using the Full Suite MWD-Combination for Reservoir Logging and Evaluation", presented by K.H. Norre and H. Saether. Also, such a need is discussed in SPE Paper No. 20563, New Orleans, 1990, "The Effect of Wellbore Conditions on Wireline and MWD Neutron Density Logs", presented by D.F. Allen, D.D. Best, M. Evans and J. Holenka.

One ofthe key uses of a caliper tool is for correction of the logging measurements of other FEWD (Formation Evaluation While Drilling) type sensors. These might include the gamma ray, resistivity, neutron, and other acoustic type sensors. Understanding and verifying the effects of washouts on the sensors offers a key quality control mechanism.

Another use of an MWD caliper tool is to offer a method of calculating cement volumes. On a final bit run, an MWD tool can be triggered to collect data while tripping out of the hole to offer accurate cement volume calculations.

Other applications of an MWD caliper tool include real-time assessment of well bore stability, evaluation of hole cleaning, and determination of tight spots or formation ledges. With the addition of "dynamic" directional sensors, bore hole geometry (ellipticity) can also be computed. Borehole ellipticity is some times used to estimate the maximum horizontal stress field for reservoir calculations, for example, as discussed above with respect to U.S. Patent No. 4,599,904.

A need has also been established for an accurate1 highly reliable standalone MWD caliper tool which can be run independent of other expensive MWD sensors. The ability to make measurements over a relatively large operating range is considered a key requirement for fielding a commercially successful tool.

Some ofthe requirements identified as critical to the success of an MWD caliper tool include: operation at elevated temperature, high shock and vibration environment; industry standard connection configuration capable of accommodating industry standard radii of curvature under rotation and/or bending; a power source; read out capabilities; internal processor, data storage memory, communications hardware; and finally, a transceiver design that allows for reliable operation at elevated pressures and temperatures.

Theorv of Operation The tool physics behind the measurement is quite simple. Based on the pulse-echo technique the standoff from the borehole wall can be calculated by the equation: a vj (I) with S the standoff, van the mud acoustic velocity, and t the round-trip time (time difference between pulse emission and echo arrival, or equivalently time difference between transmitter firing and echo detection of the borehole wall signal). Note that for equation (1) the standoff is assumed to include contributions from delay lines, transducer packaging material, etc. All of these need to be taken into account when the actual standoff is calculated, i.e. the distance from the "diameter" of the tool at the transducer, to the borehole wall.

In theoretical terms, whenever a tool of diameter Dt is centered in a borehole of diameter Dh, the hole diameter can be estimated (with the use of6 from equation 1) by the equation: Db = D, + 26 (2) Unfortunately in a drilling environment, a bottom hole assembly (ALIA) reacts much differently. The tool is seldom centered in the borehole, thus the value of & will vary with time. In addition, the borehole is seldom a dean circle, thus the value of Dh (and 6) will vary with the orientation of the transducer and the actual size of the borehole. It is for these reasons that multiple transducers, geometric calculations and averaging techniques must be utilized. The result is an average hole diameter, that represents the diameter of an equivalent circle with approximately the same area as the area of the true borehole that is calculated downhole.

The error in the standoff as well as the borehole diameter can be calculated by standard mathematical and statistical means applied to equations (1) and (2). When arrival time can be estimated accurately, the dominant error factor for the borehole calculation becomes the fractional error in mud acoustic velocity. It is well known to those skilled in the art, that the mud acoustic velocity changes with mud-type and density, mud salinity, pressure, temperature, and finally with the amountlpresence of gas dissolved in the mud.

Recognizing the potentially dominant effect of changes in the mud acoustic velocity, one needs to construct a set of experiments to characterize/measure the mud acoustic velocity versus most of the significant factors that influence it. The result then is a set of data, as well as a set of semiempincal equations that allow for "on-the-fly" corrections to the caliper value. The corrections can either be applied downhole or the downhole data can be reprocessed at the surface once the data is retrieved from the downhole tool.

The error in round-trip time can become significant if the true zero-crossing equivalent ofthe echo signal is missed or estimated at the wrong time. Published sampling techniques (for downhole acoustic calipers) rely on analog signal manipulation (filtering and rectification or comparator-based time sampling). When purely analog techniques are utilized for timing measurements, the probability of detection error for the first arrival increases dramatically, and the error in the round-trip time can become another significant error factor in the caliper calculation. For example, the variation in the detection of the first arrival (for a tool based on analog detection techniques) is around 3 microseconds (a standoff error of about 0.09 inches in water), as discussed in SPE Paper No. 26494, Annual Technical Conference and Exhibition, "Standoff and Caliper Measurements While Drilling Using a New Formation Evaluation Tool with Three Ultrasonic Transducers", presented by G.L. Moake, J.R. Birchak, R.G. Matthews and W.E. Schultz. For a tool constructed in accordance with the present invention, the variation in the detection of the first arrival (using digitization of the signal, Digital Signal Processing and "true-zerocrossing" techniques) is as little as fractions of a microsecond (a standoff error of a few thousandths of an inch in water or a factor of many times smaller).

In one of the preferred embodiments according to the present invention, the tool uses a fast Analog-to-Digital converter to digitize the acoustic signal. Determination of the first arrival is accomplished by Digital Signal Processing, thus minimizing the error associated with analog signal manipulation. Multiple samples are taken per rotation to provide an average borehole diameter and to allow for proper determination of the approximate shape of the borehole.

The existence of digitized signals offers a distinct advantage to the tool according to the present invention Digital Signal Processing techniques are utilized to enhance the capability of echo detection, even at the presence of significant signal attenuation (larger standofXi, heavy muds etc. ), thus resulting to an increased over prior art standoff detection capability even in heavy mud (16 to 18 ppg). In addition, Digital Signal Processing can eliminate proximity effects (no "delay-line" required), as well as calculate an accurate borehole diameter in the presence of drilling cuttings and/or small concentrations of gas.

There are generally two "errors" associated with a caliper tool: first1 the repeatabllity error, i.e. the capability ofthe tool to consistently measure a "fixed" borehole.

For a preferred embodiment constructed in accordance with the principles of the present invention the average repeatability error is in the order of 0.02 inches or lower, over most of the operating range. In fact in many cases, when the tool was relatively centralized the repeatability error was less than 0.005 inches. For comparison, prior art quotes repeatability errors from 0.05 inches to 0.2 inches.

The second error is the accuracy ofthe measurement, i.e. the capability ofthe tool to precisely measure a borehole. For a preferred embodiment constructed in accordance with the prin ples of the present invention arid with the use of circular boreholes (where the diameter is known) the average accuracy error is less than 0.03 inches extending over most ofthe operating range ofthe tool.

Referring now specifically to the drawings, and first to FIG. 1, there is shown a drilling rig 1 1 disposed atop a borehole 12. A first embodiment of an acoustic caliper tool 10 constructed in accordance with the principles of the present invention is carried by a sub 14, typically a drill collar, incorporated into a drill string 18 and disposed within the borehole 12. The system 10 is provided for the continuous measurement of acoustic velocity and distance within the annular region 16 defined between the sub 14 and the borehole sidewalls 20. A drill bit 22 is located at the lower end of the drill string 18 and carves a borehole 12 through the earth formations 24. Drilling mud 26 is pumped from a storage reservoir pit 27 near the wellhead 28, down an axial passageway 54 (see FIG.

3) through the drill string 18, out of apertures in the bit 22 and back to the surface through the annular region 16. Metal casing 29 is positioned in the borehole 12 above the drill bit 22 for maintaining the integrity of the upper portion of the borehole 12.

Still referring to FIG. 1, the annular 16 between the drill stem 18, sub 14 and the sidewalls 20 of the borehole 12 forms the return flowpath for the drilling mud. Mud is pumped from the storage pit 26 near the well head 28 by a pumping system 30. The mud travels through a mud supply line 3 I which is coupled to a central passageway extending throughout the length ofthe drill string 18 (and 54 when referring to FIG. 3). Drilling mud is, in this manner, forced down the drill string 18 and exits into the borehole through apertures in the drill bit 22 for cooling and lubricating the drill bit and carrying the formation cuttings produced during the drilling operation back to the surface. A fluid exhaust conduit 32 is connected from the annular passageway 16 at the well head for conducting the return mud flow from the borehole 12 to the mud pit 26 as shown in FIG.

1. The drilling mud is typically handled and treated by various apparatus (not shown) such as outgassing units and circulation tanks for maintaining a preselected mud viscosity and consistency. It may be seen that measurements of acoustic velocity of the drilling mud at or within the drilling pit 26 would thus be affected by the treated and stagnant condition of the mud.

The position ofthe acoustic caliper tool 10 upon the drill sub 14 relative to the borehole walls 20 will vary during rotation. The drill string 18 may be rotated for imparting the requisite cutting action to the drill bit 22 and, during rotation, the drill string 18 often rubs against the walls ofthe borehole 12. Such rubbing results in mis-alignments and the non-centralized positioning of the acoustic caliper tool 10 relative to the borehole walls 20. The measurement of distances with the tool 10 by means of acoustic pulses which are reflected from the borehole walls 20 must therefore be extremely precise in order to produce data which accurately depicts the dimensions and shape of the borehole.

This precision of measurement must also be maintained in view of the presence of gas, formation cuttings and non-homogeneous fluid flow conditions as is typical in most drilling operations. Moreover, dimensions of non-uniforrn borehole cross-sections must be measured as well as the variations in acoustic reflectivity which are indicative of different formation materials.

The method and apparatus of the present invention provide a system capable of producing data of an accurate and reliable nature indicative of borehole shape and size by utilizing a common acoustic pulse for both the determination of acoustic velocity within the turbulent flow of non-homogeneous drilling fluid in the borehole annular as well as the distance between the sub 10 and the borehole wall 20. In this manner, all distance measurements will utilize the actual acoustic velocity ofthe fluid medium through which the distance measurements are made.

FIG. 2 illustrates in pictorial form the sub 14 incorporated into the drill string 18 illustrated in FIG. 1. The sub 14 preferably has a box end 40 and a pin end 42 for threadedly engaging the other sections ofthe drill string 18. A wearband 44, having a circumference slightly larger than the circumference of the sub 14 itself, is surfaced with hard-facing, for example, tungsten carbide, and is positioned in near proximity to the transducers to protect the acoustic transducers positioned in the three circular holes 46, 48 and 50, with only the holes 46 and 48 illustrated in FIG. 2. Although wear pad 44 is illustrated as being cylindrically shaped, it can also take other forms, for example, such as three longitudinal pads positioned on the periphery of the sub 14 1200 apart to coincide with the positioning of the transducers in the holes 46, 48 and 50. A port 52 is also provided in the exterior wall of the sub 14 two'access any data stored in a recorder within the sub 14, in a manner well-known in the art, when the drill string 18 is retrieved back to the earth's surface. FIG. 3, described hereinafter, correlates with the section line 3-2 illustrated in FIG. 2.

Referring now to FIG. 3, the view taken along the sectional line 3-2 of FIG. 2, is illustrated as having three circular holes 46, 48 and 50 in the sub 14 and having threadedly engaged therein, three transducers 56, 58 and 60, respectively, 1200 apart. There is also illustrated the axial passageway 54 through which the drilling fluid passes. The transducers 46, 48 and 50 are identical, with transducer 46 illustrated and described in more detail in FIGS. 4(a) and 4(b).

Referring now to FIG. 4(a), the transducer 46 comprises a main body 72, having male threaded connections for threading the sensor 46 into a female threaded receptacle in the sub 14, as illustrated in FIG. 5. The main body 72 has an internal cavity for housing an ultrasonic element 74 and a backing assembly 76, together being an ultrasonic element/backing assembly 78. The upper end of the main body 72 is hexagonally shaped to allow a wrench to tighten the transducer into a threaded receptacle. The material used in the backing assembly 76 can be any material which dampens or absorbs the unwanted ultrasonic signals emitting out of the back of the ultrasonic element 74, but preferably includes a mixture of tungsten powder and elastomer or epoxy material. Electrical connectors 80 and 82 are connected across the ultrasonic element 74. The ultrasonic element and ultrasonic element backing and the electrical connections 80 and 82 are hermetically sealed within the internal cavity of the main body 72 against the hydrostatic pressure within the earth borehole resulting from the weight of the column of drilling fluid.

The electrical connector 80 is tied to a feedthru element 84, whereas the electrical connector 82 is connected to the main body ofthe electrical connector 100 and also to the main housing 72 through set-screws 124, and locking screws 126 to establish a ground connection.

The ultrasonic element and backing sub-assembly 78 containing the ultrasonic element 74 and the backing material 76 are enclosed within a pair of thermoplastic housings 86 and 88, preferably formed using "PEEK" (poly-ether-ether-keton) or a similar thermoplastic or thermoset material. The ultrasonic element 74 is adhered to the backing material 76 using a highly flexible adhesive material. The ultrasonic element 74 and the backing material are adhered to the top housing 86 using the same or similar adhesive material. The upper housing 86 is attached to the lower housing 88 using a plurality of bolts or screws through their respective abutting flanges 92 and 94. A pair of wave springs 96 and 98 encirCle the upper and lower housings 86 and 88, respectively, and ride against the housing flanges 92 and 94, respectively. The spring 90 maintains a compressive preload between the ultrasonic element 74 and the backing material 76 against axial shock and vibration of the downhole environment when the housings 86 and 88 are bolted together. The wave springs 96 and 98, riding against the flanges 92 and 94, respectively, cause the ultrasonic element/backing sub-assembly to be in a null position, and also allow the sub-assembly to be preloaded to enable the sub-assembly to withstand handling, tripping in and out of the borehole, and the drilling conditions themselves. An increase of temperatures and pressures, usually a result of drilling deeper into the earth formations, causes an increase in the preload and thus an increased resistance to drilling shock and vibration.

The electrical connector assembly 100, housing the electrical feedthru element 84, uses an O-ring 102 to seal against the internal diameter of the main body 72, thus sealing the high compensation fluid pressure. An wring 104 between the main body 72 and the lower housing 88 helps to centralize the ultrasonic elementlbacking assembly in the lateral direction. A pair of O-rings 106 and 108 in the external sidewall portion 110 of the main body 72 provide a static seal to withstand the extremely high differential pressure between the weilbore fluid and the atmospheric conditions inside the sub 104.

A piston 110, exposed to the wellbore fluid, is positioned in the annular spacing between the upper housing 86 and the main body 72, and is held against the housing 86 using a retaining ring 116 illustrated in FIG. 4(b). An O-ring 112 is positioned between the main body72 and the piston 110. An O-ring 114 is positioned between the piston 110 and the upper housing 86. The O-rings 112 and 114 separate the internal oil, i.e., the compensation fluid, from the external borehole fluid.

The electrical connector assembly 100 is held in place within the main body 72 through the use of a pair ofset screws 124 and a pair of locking screws 126 used also for a signal return connection.

Referring now to FIG. 4(b), merely being rotated 90C from FIG. 4(a), a pair of ports 120 and 122 are used to add compensation fluid, which can be any suitable oil which typically increases in volume with increases in temperature. In the assembly of the device illustrated in FIGS. 4(a) and 4(b), the inside of the main body 72 is first evacuated and then fined with a compensation fluid through the ports 120 and 122. The ports 120 and 122 are both fitted with high pressure Springs 130 and backup rings 132.

In the operation of the device illustrated in FIGS. 4(a) and 4(b), the oil volume within the interior of the main body 72 expands and contracts with changes in the ambient pressure and temperature conditions. Upon increase in the interior oil volume due to temperature, the ultrasonic element/backing assembly acts as a piston unit and moves outward towards the borehole annulus, thus expanding the interior oil volume. If however, the interior oil volume is contracted due to an increase in hydrostatic pressure, the piston 110 and the ultrasonic element/backing assembly will move inwardly, away from the borehole annulus, as a compensating piston to reduce the interior oil volume.

Adequate amounts of electrical lead wire lengths and strain relief is provided to allow for movement of the ultrasonic element/backing sub-assembly for temperature/pressure compensation motion between the ultrasonic element /backing assembly and the electrical connector assembly 100.

Referring now to FIG. 5, there is illustrated a preferred apparatus for mounting the transducer of FIGS. 4(a) and 4(b) within the sub 14. A circular hole, for example, hole 48, is cut through the side wall 140 of sub 14. An insert 142 which mates to the hole 48 includes a base/block unit 144, with the base block unit 144 providing for the support and electrical connection of the transducer to the driver electronics. The block 144 is designed to "float" with respect to the insert 142 to allow compensation for length changes, whether in the sub 14 or in the insert 142 due to tolerance stack up. As a result, the transducer can always be threadedly mounted to the floating base/block 144.

Referring now to FIG. 6, which is a view taken along the sectional lines 6-2 of FIG. 2, there is illustrated the placement of up to four (4) electronic circuitry boards 150, 152, 154 and 156 that may be implemented in one of the preferred embodiments used to practice the present invention. At least one of the circuit boards, not illustrated, fitting in one of the cavities 150, 152, 154 and 156, includes at least one accelerometer 160 and at least one of the boards includes at least one magnetometer 162. It is preferable that the accelerometer and the magnetometer be included on the same board, and that one board be aligned axially with a particular transducer. For example, the transducer 60 can be axially aligned with an electronic board 154 containing both at least one accelerometer 160 and at least one magnetometer 162.

Referring now to FIGS. 7 and 8, there is described first in FIG. 7 a block diagram ofthe effect of using the three transducers 56, 58 and 60 in conjunction with at least one accelerometer 160 and at least one magnetometer 162 with the digital processing circuits 164, 166, 168, 170 and 172. When the borehole is being drilled in a circular pattern and for a centralized tool, the time between the source pulse and the return or echo from the borehole wall back to the transducers remains essentially the same for each of the three transducers. However, when the borehole becomes elliptical, as illustrated in FIG. 8, the processing circuits will plot the shape of the ellipse, but will not provide the directions of the major axis and of the minor axis with respect to Magnetic North. By also using an accelerometer and a magnetometer having known orientations with respect to one of the transducers, one can readily plot the direction of the major axis and of the minor axis with respect to Magnetic North or earth's gravity. As is illustrated in FIG. 7, the output of the magnetometer 162 as the sub 14 rotates with the drill string is a sine wave, with the peak signal being at Magnetic North, and the minimum signal being at Magnetic South. The output of the accelerometer 160 is also a sine wave having peaks coinciding with the earth's gravitational pull as the sub 14 rotates. However, because many horizontal wells are being drilled, it is important to have a tool which can measure the caliper of boreholes in horizontal wells, vertical wells, and all angles therebetween.

Also illustrated in FIG. 7 is a block diagram of a downhole temperature measurement sensor 200, a downhole pressure measurement sensor 202, and processing circuits 204 and 206, respectively, for the provision of"on-the-fly" corrections to the detenninations of acoustic velocity in the Acoustic Velocity processing circuitry 208.

It is important to recognize that an accelerometer will not function very well when the tool is nearly vertical. A magnetometer will not function very well if the angle of the tool is direcdy towards Magnetic North. By generating measurements taken by both the accelerometer 160 and the magnetometer 162, and as the drilled borehole takes on an elliptical shape through different angles or directions, one or the other of the accelerometer and the magnetometer will generally function well, and provide an indication of the direction of the major axis and of the minor axis of the ellipse, and thereby provide a major advantage for the calculation of stresses around the borehole.

Referring now to FIG. 9, there is illustrated in block diagram the basic electronics of the system according to the invention. Although only transducer 60 is illustrated, identical circuits are used for each of the transducers 56 and 58. Although the expression "transducer" is used throughout this specification, each such transducer is actually a transceiver, because each is used to transmit the ultrasonic pulses and also to receive the pulses reflected from the borehole wall. The pulsing or firing of the transducer is controlled by the Pulsing (Firing) Control Circuit 170, which in the preferred embodiment includes an oscillator operating in the range of 125 Khz to 400 Khz, which triggers the ultrasonic element 74 illustrated in FIG. 4(a). The output of the ultrasonic element 74 passes through the drilling fluid to the borehole wall, where it is bounced back to the crystal 74 for processing The return analog signal, sometimes referred to as an "echo," is then processed as required in the Signal Conditional Circuit 172, such as being amplified or filtered , and coupled into an Analog to Digital Conversion Circuit 174, the output of which is coupled into a Digital Signal Processor Circuit 176. The Digital Signal Processor Circuit 176 also sends a trigger signal to the Pulsing Control Circuit 170. The Digital Signal Processing Circuit 176 processes the digitized waveforms with a multitude of techniques including but not limited to: filtering (simple or multirate); waveform mathematical manipulation (adding, subtracting, averaging etc.); waveform auto and cross-correlation; convolutions and deconvolutions; rnultiple waveform processing from one or may acquisitions from one or all of the transducers; signal decimation or upsampling; and many others known to those skilled in the art The input waveform to the Processor Circuit 176 is stored in a downhole recorder 178 for further processing at the earth's surface, if desired; the output results of the Processor Circuit 176 are stored in a downhole recorder 178 for further (re) processing at the earth's surface, if desired, and also may be used as inputs to a conventional mud pulse telemetry system (not illustrated), commonly referred to as MWD, for transmitting real time data to the earth's surface for processing in a conventional manner, such as in the conventional surface Data Processor System 190-illustrated in FIG. 1.

The Digital Signal Processor Circuit 176 provides various functions, including: Processing the signal (from one or many acquisitions from one or many transducers) and then manipulating each individual pulse echo waveform, thus yielding an accurate representation of the actual echo signal minimizing the effects of the transmitter signal to the pulse-echo time calculation. In contrast to the prior art, such methods allow for accurate echo arrival time determination even in cases where the tool lies substantially against the borehole wall and thus the separation between the transmitted signal and the received signal is too small to be detected by analog threshold detection. In addition, since the signals can change with pressure and/or temperature the described technique is effectively an adaptive technique.

Processing the ultrasonic transducer ringing and then manipulating each individual received signal minimizing the effects of transducer ringing to the pulseecho time calculation. In contrast to the prior art, such a method allows for accurate echo arrival time determination even in cases where due to signal attenuation the signal to noise ratio is very small.

Processing the reflection noise from the imperfect backing material and then manipulating each individual received signal minimizing the effect of the backing material reflection noise to the pulse-echo time calculation.

t Utilizing standard techniques (digital filtering, correlation, convolutionldeconvolution, up or down sampling of signals etc.) to substantially improve the accuracy of the determination of the arrival time.

In contrast to the prior art which is based on analog threshold detection, the current method yields a significantly more accurate determination of the arrival time, thus significantly improving the accuracy of the standoff measurements used for borehole diameter or shape determination.

In addition to the foregoing description of the preferred embodiment of the present invention, the invention also contemplates the downhole measurement of the pressure and temperature of the drilling fluid (mud) column, and thus more accurate mud acoustic velocity can be determined while drilling, i.e., "on-the-fly," to correct the caliper measurements based upon such temperature and pressure measurements.

It should be appreciated that while the foregoing description of the preferred embodiment is primarily focused on acoustic caliper logging, most if not all of the features of the invention are equally applicable to the measurement of acoustic velocity in the formations themselves.

Claims (34)

1. What is Claimed is:

   Apparatus for determining the dimensions of an elliptical earth borehole and the orientation of the major axis of said borehole with respect to Magnetic North, comprising: a main tool body configured to be incorporated in a drill string used in drilling an earth borehole; a plurality of ultrasonic transceivers positioned about the periphery of said main tool body for measuring the dimensions of an elliptical earth borehole; and one or more magnetometers positioned in said main tool body and one being substantially axially aligned with said one transceiver, the outputs of one or all of said magnetometers providing indications of the orientation of the major axis of said elliptical borehole with respect to Magnetic North.
2. 2. The apparatus according to Claim 1, wherein the outputs of one or all said magnetometers also provide indications of the orientation of the minor axis of said elliptical borehole with respect to Magnetic North.
3. 3. Apparatus for determining the dimensions of an elliptical earth borehole and the orientation of the major axis of said borehole with respect to earth's axis, comprising: a main tool body configured to be incorporated in a drill string used in drilling an earth borehole; a plurality of ultrasonic transceivers positioned about the periphery of said main tool body for measuring the dimensions of an elliptical earth borehole; and one or more accelerometers positioned in said main tool body and one being substantially axially aligned with one of said transceivers; the outputs of one or all of said accelerometers providing indications of the orientation of the major axis of said elliptical borehole with respect to earth's gravity.
4. 4. The apparatus according to Claim 3, wherein the outputs of one or all said accelerometers also provide indications of the orientation of the minor axis of said elliptical borehole with respect to earth's gravity.
5. 5. A transducer for generating ultrasonic waves in earth boreholes, comprising: a main body, portions of which are cylindrically-shaped, and having a side wall and an end surface and wherein the external surface of said side wall is at least partially threaded, said end surface having a circular opening for receiving an electrical connection insert; a cup-shaped upper housing having a side wall and an end surface positioned within said main body and having a first flange, the positioning of said upper housing creating an annulus between the interior surface ofthe side wall of said main body and the side wall of said upper housing; a cup-shaped lower housing positioned within said main body and having a second flange, said first and second flanges being fixedly attached together; a spring positioned within said lower housing prior to fixedly attaching together said first and said second flange; an annular piston being positioned in the said annulus between said main body and said upper housing; first and second springs positioned on the exterior surfaces of said upper and lower housings, respectively, and on said first and second flanges, respectively; an ultrasonic-generating element fixedly attached to the interior end surface of said upper housing; a backing material fixedly attached to said element, said backing material being at least partially absorptive of ultrasonic waves; an electrical connection insert positioned within said circular opening in said end surface of said main body; and at least one port in the end surface of said main body for adding compensation fluid into the interior of said main body between said annular piston and the end surface of said main body.
6. 6. The apparatus according to Claim 5, including in addition thereto, compensation fluid, whereas said fluid, said upper housing, said lower housing and said annular piston, comprise an assembly that compensates for pressure and temperature variations.
7. 7. Apparatus for determining the dimensions of an earth borehole, comprising: a cylindrically-shaped main tool body having a side wall and having at least one circular hole in said side wall through which at least one ultrasonic transducer can be positioned, and configured to be incorporated in a drill string used in drilling an earth borehole; an insert fixedly positioned within the interior of said main tool body, said insert having a block non-fixedly attached to said insert, thereby allowing movement between said insert and said block, said block being positioned in proximity to the circular hole in the side wall of said main tool body and having an internally threaded receptacle for receiving an externally threaded ultrasonic transducer.
8. 8. The apparatus according to Claim 7, including in addition thereto, an ultrasonic transducer having external threads and being threadedly engaged with the threaded receptacle in said block.
9. 9. A method for determining the dimensions of an earth borehole, comprising: generating an ultrasonic wave from a drill string in an earth borehole; constructing a digitized waveform substantially corresponding to the ringing of said generated ultrasonic wave; detecting in said drill string an ultrasonic wave reflected from the borehole wall; digitizing said detected ultrasonic wave; processing the digitized constructed waveform and then manipulating said digitized detected wave, to thereby result in a processed digitized echo wave substantially free of the ringing effect of said generated ultrasonic wave; and determining the actual pulse-echo time of the reflected wave from said processed digitized echo wave.
10. 10. A method for determining the dimensions of an earth borehole, comprising: generating ultrasonic waves from a drill string in an earth borehole; detecting in said drill string ultrasonic waves reflected from the borehole wall; determining the distance from the drill string to the borehole wall as a function of the velocity of said waves passing through the borehole fluid; determining the temperature of the borehole fluid in proximate location to the depth in the borehole at which the ultrasonic waves are being generated; determining the hydrostatic pressure of the borehole fluid in proximate location to the depth at which the ultrasonic waves are being generated; and correcting the velocity of the ultrasonic waves passing through the drilling fluid as a function of the determined temperature and pressure of the borehole fluid as the drill string continues to drill the borehole deeper into the earth formations.
11. 11. A method for determining the dimensions of an earth borehole, comprising: generating ultrasonic waves from a plurality of ultrasonic transceivers positioned in a drill string in an earth borehole; digitizing the transmitted ultrasonic wave from each of said transceivers; detecting in each of said plurality of transceivers the ultrasonic waves reflected from the borehole wall; digitizing in each of said plurality of transceivers the ultrasonic waved reflected form the borehole waH; manipulating said digitized ultrasonic waves, thereby yielding a processed digitized echo wave substantially being an accurate representation of the actual echo signals reflected from the borehole wall; and determining the actual pulse-echo time of the reflected wave from said processed digitized echo wave
12. 12. A method for measuring the dimensions of an earth borehole, comprising: generating an ultrasonic wave from a drill string in an earth borehole; constructing a digitized waveform substantially corresponding to reflection of the transmitted ultrasonic wave from the backing material behind the source generating the ultrasonic wave; detecting in said drill string an ultrasonic wave reflected from the borehole wall; digitizing said detected ultrasonic wave; processing the digitized constructed waveform and then manipulating said digitized detected wave, to thereby result in a processed digitized echo wave substantially free of the effect of the ultrasonic waves reflected back from the backing material; and detennining the actual pulse-echo time of the reflected wave from said processed digitized echo wave.
13. 13. Apparatus for determining the acoustic velocity of the earth formations surrounding an earth borehole and the orientation of the major axis of said borehole with respect to Magnetic North, comprising: a main tool body configured to be incorporated in a drill string used in drilling an earth borehole; a plurality of ultrasonic transducers positioned in said main tool body for measuring the velocity of acoustic waves in an earth formation; and one or many magnetometers positioned in said main tool body and being positioned in a known spatial relationship with one of said transducers, the outputs of one or all of said magnetometers providing indications of the orientation of the major axis of said elliptical borehole with respect to Magnetic North.
14. 14. The apparatus according to Claim 13, wherein the outputs of one or all said magnetometers also provide indications of the orientation of the minor axis of said elliptical borehole with respect to Magnetic North.
15. 15. Apparatus for determining the acoustic velocity of the earth formations surrounding an earth borehole and the orientation of the major axis of said borehole with respect to earth's gravity, comprising: a main tool body configured to be incorporated in a drill string used in drilling an earth borehole; a plurality of ultrasonic transducers positioned in said main tool body for measuring the velocity of acoustic waves in an earth formation; and one or many accelerometers positioned in said main tool body and being positioned in a known spatial relationship with one of said transducers; the outputs of one or all of said accelerometers providing indications of the orientation of the major axis of said elliptical borehole with respect to earth's gravity.
16. 16. The apparatus according to Claim 15, wherein the outputs of one or all said accelerometers also provide indications of the orientation of the minor axis of said elliptical borehole with respect to earth's gravity.
17. 17. Apparatus for measuring the velocity of acoustic waves in an earth formation, comprising: a cylindrically-shaped main tool body having a side wall and having a circular hole in said side wall through which at least one ultrasonic transducer can be positioned, and configured to be incorporated in a drill string used in drilling an earth borehole; and an insert fixedly positioned within the interior of said main tool body, said insert having a block non-fixedly attached to said insert, thereby allowing movement between said insert and said block, said block being positioned in proximity to the circular hole in the side wall of said main tool body and having an internally threaded receptacle for receiving an externally threaded ultrasonic transducer.
18. 18. The apparatus according to Claim 17, including in addition thereto, an ultrasonic transducer having external threads and being threadedly engaged with the threaded receptacle in said block.
19. 19. A method for measuring the velocity of acoustic waves in an earth formation, comprising: generating an ultrasonic wave from a drill string in an earth borehole; constructing a digitized waveform substantially corresponding to the ringing of said generated ultrasonic wave; detecting in said drill string an ultrasonic wave reflected from the earth formation; digitizing said detected ultrasonic wave; processing the digitized constructed waveform and then manipulating said digitized detected wave, to thereby result in a processed digitized echo wave substantially free of the ringing effect of said generated ultrasonic wave; and determining the actual amplitude and formation reflectivity of said echo wave from said processed digitized echo wave.
20. 20. A method for measuring the velocity of acoustic waves passing through an earth formation, comprising: generating an ultrasonic wave from a drill string in an earth borehole; constructing a digitized waveform substantially corresponding to the reflection of the transmitted ultrasonic wave from the backing material behind the source generating the ultrasonic wave; detecting in said drill string an ultrasonic wave received from the earth formation; digitizing said detected ultrasonic wave; processing the digitized constructed waveform and then manipulating said digitized detected wave, to thereby result in a processed digitized ultrasonic wave substantially flee of the effect of the ultrasonic waves reflected back from the backing material; and determining the actual amplitude and formation reflectivity of said received wave from said processed digitized echo wave.
21. 21. A method for measuring the velocity of acoustic waves passing through an earth formation, comprising: generating ultrasonic waves from a plurality of ultrasonic transceivers positioned in a drill string in an earth borehole; digitizing the transmitted ultrasonic wave from each of said transceivers; detecting in each of said plurality of transceivers the ultrasonic waves reflected from the borehole wall; digitizing in each of said plurality of transceivers the ultrasonic waved reflected form the borehole wall; manipulating said digitized ultrasonic waves, thereby yielding processed digitized echo waves substantially accurate representations of the actual echo signals reflected from the borehole wall; and determining the actual amplitude and formation reflectivity of said received waves from said processed digitized echo waves.
22. 22. A method for measuring the dimensions of an earth borehole, comprising: generating an ultrasonic wave from a drill string in an earth borehole; constructing a digitized waveform substantially corresponding to said generated ultrasonic wave; detecting in said drill string an ultrasonic wave reflected from the borehole wall; digitizing said detected ultrasonic wave; and determining the actual pulse-echo time of the reflected wave from said digitized echo wave.
23. 23. A method for measuring the shape of an earth borehole, comprising: generating an ultrasonic wave from a drill string in an earth borehole; constructing a digitized waveform substantially corresponding to said generated ultrasonic wave; detecting in said drill string an ultrasonic wave reflected from the borehole wall; digitizing said detected ultrasonic wave; and determining the actual pulse-ccho time of the reflected wave from said digitized echo wave.
24. 24. A method for measuring the velocity of acoustic waves passing through an earth formation, comprising: generating ultrasonic waves from a plurality of ultrasonic transceivers positioned in a drill string in an earth borehole; digitizing the transmitted ultrasonic wave from each of said transceivers; detecting in each of said plurality of transceivers the ultrasonic waves reflected from the borehole wall; digitizing in each of said plurality of transceivers the ultrasonic waved reflected form the borehole wall; manipulating said digitized ultrasonic waves, thereby yielding processed digitized echo waves substantially accurate representations of the actual echo signals reflected from the borehole wall; storing in appropriate electronic storage device said digitized ultrasonic waves reflected from the borehole wall; storing in appropriate electronic storage device said processed digitized echo waves; recovering said stored waves; and determining the actual amplitude and formation reflectivity of said received waves through post-processing of said stored waves.
25. 25. A method for measuring the dimensions of an earth borehole, comprising generating an ultrasonic wave from a drill string in an earth borehole; constructing a digitized wavefonn substantially corresponding to said generated ultrasonic wave; detecting in said drill string an ultrasonic wave reflected from the borehole wall; digitizing said detected ultrasonic wave; storing in appropriate electronic storage device said digitized ultrasonic waves reflected from the borehole wall; storing in appropriate electronic storage device said processed digitized echo waves; recovering said stored waves; and determining the actual pulse-echo time of the reflected wave through postprocessing of said stored waves.
26. 26. A method for measuring the shape of an earth borehole, comprising: generating an ultrasonic wave from a drill string in an earth borehole; constructing a digitized waveform substantially corresponding to said generated ultrasonic wave; detecting in said drill string an ultrasonic wave reflected from the borehole wall; digitizing said detected ultrasonic wave; and storing in appropriate electronic storage device said digitized ultrasonic waves reflected from the borehole wall; storing in appropriate electronic storage device said processed digitized echo waves; recovering said stored waves; and determining the actual pulse-echo time of the reflected wave through postprocessing of said stored waves.
27. 27. Apparatus for calculating cement volumes for an earth formation comprising: a cylindrically-shaped main tool body having a side wall and having a circular hole in said side wall through which at least one ultrasonic transducer can be positioned, and configured to be incorporated in a drill string used in drilling an earth borehole; an insert fixedly positioned within the interior of said main tool body, said insert having a block non-fixedly attached to said insert, thereby allowing movement between said insert and said block, said block bcing positioned in proximity to the circular hole in the side wall of said main tool body and having an internally threaded receptacle for receiving an externally threaded ultrasonic transducer.
28. 28. The apparatus according to Claim 27, including in addition thereto, an ultrasonic transducer having external threads and being threadedly engaged with the threaded receptacle in said block.
29. 29. A method for calculating cement volumes for an earth formation, comprising: generating an ultrasonic wave from a drill string in an earth borehole; constructing a digitized waveform substantially corresponding to said generated ultrasonic wave; detecting in said drill string an ultrasonic wave reflected from the earth formation; digitizing said detected ultrasonic wave; and determining the actual pulse-echo time of the reflected wave from said digitized echo wave.
30. 30. The method of Claim 29, where said signals are generated during drillstring's trip into the earth's borehole.
31. 31. The method of Claim 29, where said signals are generated during drilling the earth's borehole.
32. 32. The method of Claim 29, where said signals are generated during drill-string's trip out of the earth's borehole.
33. 33. Method for correcting measurements for the dimension of an earth borehole comprising: generating a value for the dimensions of an earth borehole from a measuring instrument in an earth borehole; transmitting said value to a computerized unit at the earth's surface, utilizing pressure and temperature measurements generated from said measuring instrument and transmitted to said computerized unit; and applying at said computerized unit correction methods to correct said transmitted value for the earth borehole dimensions for the change in the mud acoustic velocity due to said pressure and temperature in the borehole.
34. 34. Method for correcting measurements for the shape of an earth borehole comprising: generating a value for the shape of an earth borehole from a measuring instrument in an earth borehole; transmitting said value to a computerized unit at the earth's surface; utilizing pressure and temperature measurements generated from said measuring instrument and transmitted to said computerized unit; and applying at said computerized unit correction methods to correct said transmitted value for the earth borehole shape for the change in the mud acoustic velocity due to said pressure and temperature in the borehole.