Abstract: A method and an apparatus for recording the depth of a well logging apparatus is provided. The method and apparatus include the direct acquisition of broad depth information from the logging system conversion of the broad depth information to an acceptable digital signal and recording that signal on the audio track of a video cassette recorder. A microcontroller is used to convert broad depth information from either the logging system or the video cassette recorder (in the playback mode) to actual depth and depth rate for use by other displays and plotters.

Abstract: A method for determining the height and length of a fracture (2) in a borehole (1) involves generating pressure waves at closely spaced frequencies by means of a transmitter (3) at the wellhead. The response at the wellhead of the wellbore-fracture system to the resultant standing waves is measured by means of a transducer (6). A theoretical model of the wellbore-fracture system is generated and compared with the measured response to determine the height and length of the fracture.

Abstract: A new "radial" cement bond instrument is disclosed for measuring the quality of cement to casing bond. The instrument is capable of measuring the quality of the cement bond both vertically and circumferentially. Four arrays of sectored transmitters and receivers are arranged longitudinally along the instrument body and provide a compensated radial bond measurement with high azimuthal resolution. The sectored transmitter excites both a compressional wave and a shear wave in the casing, with the beam pattern of the transmitter focused in a radial direction. The sectored receiver provides additional directivity for enhanced detection of channels in cement. The disclosed apparatus provides measurement of the quality of cement bond in eight radial segments. The resolution of the instrument provides 5 to 10 percent overlap to insure "full" coverage of the borehole.

Abstract: A method of testing a water or petroleum well to obtain information about well features, especially that part of the well beneath the ground surface, is described. The method is especially useful in detecting problems or irregularities in the well such as stuck tools, casing damage, damaged zones in an uncased well, or debris in a well. The method involves creating oscillations in the fluid pressure in the well. Transducers measure the pressure oscillations. The measured pressure oscillations are used to determine resonant frequencies. The measured resonant frequencies are used to determine the characteristic impedance and the depth of each reflector in the well, after removing resonances caused by known reflectors. It is then possible to determine the nature of each unknown reflector based on suspected problems or the well's operational history.

Abstract: A borehole televiewer system is disclosed which includes a sonde adapted to be suspended in a borehole containing borehole fluids. A variable speed brushless DC motor within the sonde is utilized to drive a rotatable mount at selected speeds in order to maintain a rotational speed in proportion to the linear speed of the sonde through the borehole. A "quick-disconnect" transducer head, including diametrically opposed acoustic transducers, is mounted to the rotatable mount in direct contact with the borehole fluids and is utilized to repetitively transmit energy pulses at the wall of the borehole throughout each rotation. Receiver circuitry is then utilized to determine the amplitude and transit time for each reflected pulse in order to determine parameters of interest for the borehole wall. In a preferred mode of the present invention, the reflected pulses are then utilized to generate a video representation of the borehole wall in addition to providing a graphic plot.

Abstract: An improved method to determine the quality of a material disposed in an annular space behind a casing (or a liner) in a wellbore. A transit time of a sound wave to travel through a specimen of the material is determined, and a pulse velocity is calculated. The slurry density and mixing rate of the material are determined at a wellsite, and the slurry volume can then be calculated by multiplying the amount of time that the slurry remains at the slurry density by the mixing rate. The slurry has a theoretical acoustical impedance that is equal to the slurry density multiplied by the pulse velocity. The true annular displacement volume and a theoretical acoustical impedance map, an attenuation rate map, and a voltage map are than drawn. After running an acoustical log, a calculated acoustical impedance is derived, and is compared to the theoretical acoustical impedance to detertmine the quality of the material.

Abstract: The growth of a hydraulic fracture increases the period of free oscillations in the well connected to the fracture. Simultaneously, the decay rate of free oscillations decreases. The properties of forced oscillations in a well also change during fracture growth. All of these effects result from the changing impedance of the hydraulic fracture that intersects the well. Hydraulic fracture impedance can be defined in terms of the hydraulic resistance and the hydraulic capacitance of a fracture. Fracture impedance can be determined directly by measuring the ratio of down hole pressure and flow oscillations or indirectly from well head impedance measurements using impedance transfer functions. Well head pressure measurements can also be used to evaluate fracture impedance by comparing them to pressure oscillations computed with hydraulic models that include fractures with different impedances.

Abstract: The depth of a hydraulic fracture zone is determined by sensing and recording seismic wave motion at a series of spaced depths in a portion of a bore hole subjected to fracturing pressure during a shut-in period after the application of pressure while the fracture zone is unstable, and analyzing such motion recordings by eliminating components not attributable to the fracturing and discrete components from seismic sources attributable to the fracturing to determine an average background level of vertical and horizontal components, and comparing the average background level of the horizontal components of motion to the vertical component of motion to provide an indication of the presence or absence of fracturing at each depth.

Abstract: An apparatus is disclosed for use within a borehole for seismic surveying that has a vertical array of seismic receivers connected to a cable, a top inflatable bladder connected to the cable above the top seismic receiver, and a bottom inflatable bladder connected to the cable below the bottom seismic receiver. The top bladder acoustically isolates all seismic receivers from the borehole environment above the top bladder. The bottom bladder acoustically isolates all seismic receivers from the borehole environment below the bottom bladder. Additional inflatable bladders are placed between each separate seismic receiver in the array.

Abstract: An acoustic logging system for detecting anomalies in formations surrounding a borehole. The system can be used for evaluating the formation for fractures or the bond between a well casing and the cement used to seal the space between the casing and the formation. The system utilizes at least one rotating acoustical transmitter/receiver and at least one receiver vertically spaced from the rotating transducer. The signals from both receivers are analyzed to detect the presence of anomalies, such as fractures or voids in a cement bond.

Abstract: For seismic monitoring of a petroleum deposit at least one signal receiving unit is fixed to an outside of a casing pipe before the casing plate is cemented in a borehole passing through the monitored zone.At least one housing, defined by projecting elements, (longitudinal members, stops), is provided along an outer surface of the casing pipe, with the where the reception unit being embedded and fixed therein so as to protect the reception unit from shocks or compressive forces which may occur during lowering of the pipe into the well. Each reception unit comprises a support assembly with cavities for sensors, electronic cards for amplifying the signals delivered by the sensors and a sealed external case which is easy to position and which is held firmly in position by embedment in the housing. A multiconductor cable is used, for example, for connecting the reception unit to the surface.

Abstract: An acoustic method and apparatus for investigating an earth formation penetrated by a borehole are described. Acoustic transducers are mounted on a tool to accurately determine the distance between a segment of the tool and the wall of the borehole. The acoustic transducers are positioned in such manner that the stand-off distance between individual resistivity measuring electrodes in an array, which is also mounted on the tool segment, and the borehole wall can be measured. The stand-off measurement is recorded and may be used to correct the electrode resistivity measurements. In one embodiment a calibration of acoustic transducers as a function of depth is obtained by employing acoustic calibrating transducers to compensate for borehole environment effects on the performance of the acoustic transducers as well as determine the acoustic velocity of the borehole fluid, such as mud. Several embodiments are described.

Abstract: Logging equipment for use in a borehole has a sensor such as an ultrasonic sensor, oriented radially towards the wall of the borehole and rotatable in one direction during measurement about the longitudinal axis of the equipment. In order to allow the sensor to be recalibrated downhole, the equipment also includes a target which is automatically brought face to face with the sensor upon reversal of the direction of rotation of the sensor. The sensor can thus be calibrated under conditions which are as close as possible to the conditions of measurement.

Abstract: A method and apparatus for measuring the caliber of a borehole while drilling utilizes a borehole compensated downhole measuring system in both a compensated and a non-compensated manner. A transmitter of the measuring system generates a signal which is received by at least one spaced receiver with the time/phase relationship of the transmission and reception of the signal at each receiver being indicative of the borehole caliber.

Abstract: A deviated borehole penetrating a subsurface formation and intersected by a formation fracture is surveyed with a borehole televiewer acoustic logging system. Relative fracture dip angle and dip direction calculated from such survey with respect to the borehole axis is transformed into a true fracture dip angle and dip direction with respect to the vertical of the earth's coordinate system.

Abstract: An acoustic borehole logging system employs an acoustic tranmitter excited by a sweep frequency tone burst to produce an acoustic output having a free-field frequency spectrum of at least one resonant frequency of vibration. A closely-spaced acoustic receiver detects a borehole frequency spectrum that is representative of the acoustic transmitter free-field frequency spectrum altered by the effects of the properties of the subsurface formulation materials comprising the borehole wall. Such detected borehole frequency spectrum is recorded in correlation with depth as a log of the material properties of the subsurface formations surrounding the borehole.

Abstract: An acoustic transducer having a curved surface to match the inner or outer surfaces of a target surface such as a cylindrical borehole tubular is disclosed. A sequence of excitation pulses causes the transducer to launch a series of high-frequency, short duration acoustic energy pulses toward the tubular. The transducer face receives echos reflected from the surface of the tubular, including multiple echo structures indicative of the repeated echoing of the acoustic energy between the inner and outer faces of the tubular. The characteristics of the tubular surfaces, including the location, area and depth of any corrosion pits on the tubular surfaces, are determined from the full echo waveform. A rule-based Expert System analyzes the full waveform of the echo pulses returned from the tubular to determine the most likely characteristics of the surfaces which produced the full echo waveform.

Abstract: A system and methods for the inspection of a well borehole and the formation around said borehole. The system includes a coiled tubing unit for injecting flexible coiled tubing into a wellbore through a wellhead, a pump and valves for control of injection of fluids such as water, nitrogen, light-hydrocarbons, natural gas, and carbon dioxide through the coiled tubing into the wellbore, and a sensor for visually inspecting and/or acoustically examining the wellbore and a region around the sensor within a slug of fluid injected into the wellbore from the coiled tubing. The method includes the steps of injecting coiled tubing having an inspection sensor into a wellbore to a selected location, injecting an optically transparent or acoustically homogenous fluid into the wellbore through the coiled tubing to form a slug of such fluid around the sensor, and transmitting signals from the sensor representative of well conditions to the surface.

Abstract: The present invention is directed to a method of calculating the impedance of a material behind the section of a casing. An acoustic excitation pulse is directed towards a section of the casing, resulting in a return waveform having a reverberation segment and an initial reflection segment. The return waveform is analyzed to choose a resonance frequency indicative of the casing's nominal thickness. The return waveform is bandpass filtered about the chosen frequency. A time window of the reverberation segment is selected, and the energy content in the time window of the filtered reverberation segment is calculated. Both the bandpass filter and time windows are selected based on the chosen resonance frequency, thereby removing variations in the thickness of the casing. The energy content calculation produces a cementation signal indicative of the impedance of the cement behind the section of the casing. The impedance value is indicative of the cement quality.

Abstract: An ultrasonic logging apparatus is provided with a receiver for receiving acoustic returns of varying amplitudes and a digitizer for digitizing the acoustic returns. The receiver utilizes the same transducer as the transmitter, and has input switching protection to protect the amplifiers from the transmitter signal. The receiver amplifiers have gain switching, utilizing junction field-effect transistors to produce rapid gain switching during an acoustic return. The gain switching allows the digitizer to digitize with satisfactory resolution.

Abstract: A novel method for detecting and measuring elastic anisotropy in a sample of the earth's formations is provided. A dyad of time series signal representative of the sample's response to imparted shear waves is recorded. The dyad of time series signals can then be processed to detect and measure elastic anisotropy in the sample. More particularly, the dyad of time series signals are collected by imparting shear waves having first and second polarizations into the samples with a shear wave transducer and recording the sample's response to each of the imparted shear waves by shear wave transducers having first and second polarizations. In one embodiment of the invention, the dyad of time series signals can be diagonalized to detect and measure elastic anisotropy in the sample. In another embodiment, the dyad of time series signals can be processed for a plurality of rotation angles and the resulting rotated dyad of time series signals can be displayed to detect and measure elastic anisotropy in the sample.

Abstract: An acoustic borehole logging tool traverses a fluid-filled cased well with an acoustic energy transmitter. Pressure waves created in the well fluid by the transmitter generate tube waves in the well casing. A closely spaced receiver detects tube waves that have traveled along the cased wellbore and been reflected by fracture interfaces. These reflected tube waves are recorded and used to identify the top and bottom of any fracture interval in the formation behind the well casing.

Abstract: A downhole tool carries an inflatable packer by means of which radial pressure may be applied to the surrounding formation. Transducers within the packer measure the radial direction and extent of formation displacements responsive to such pressure. In this way one can determine the directions of maximum and minimum horizontal in-situ formation stress.

Abstract: An acoustic logging tool traverses a fluid-filled cased well with an asymmetric acoustic energy transducer. Pressure waves created in the well fluid by the transducer generate asymmetric tube waves in the well casing. At least one spaced-apart transducer receives these asymmetric tube waves after they have traveled directly to the receiver through the well casing. Changes in tube wave ampltiude as detected by the receiver are used to identify the azimuth and height of any fracture interval in the formation behind the well casing.

Abstract: Acoustical coupling is proposed as a means of determining the presence and acoustical impedance of materials behind casing in cementation evaluation. Expressions for computing coupling and attenuation in multi-receiver/transmitter logging systems are analyzed in an exponential model for the spatial decay of elastic wave energy propagating axially along the casing. Amplitude change between transmitter(s) and receiver(s) is separated into a change dependent on coupling into and out of the casing wave, as well as a change related to attenuation along the casing. Coupling which is dependent on geometrical factors such as casing size and thickness, and on distribution and mechanical properties of the material outside the casing, shows principally the mechanical impedance of the material outside. Significantly, the bonding of this material to the casing is not a dominant factor for the coupling. Thus an estimation of cement strength may be made even in the presence of microannulus.

Abstract: In accordance with principles of the present invention methods and apparatus are provided for evaluating the quality of the cement bond in cased boreholes. Acoustic energy is used to excite the borehole-casing-annulus-formation system and the quality of the cement bond is determined by examining the ratios of the signals received by two longitudinally spaced apart receivers supported on a sonde. The acoustic energy is generated by two transmitters symmetrically disposed above and below the receivers along the sonde. The spacings between the receivers themselves and between the receivers and the transmitters are selected so as to enhance the correlation between the ratios of the received signals and the quality of the cement bond log. An additional receiver, supported on the sonde at a small distance from one of the transmitters, is employed to determine the quality of the cement bond in hard formations.

Abstract: In the field of the echography, the present invention is a system and method of preprocessing and transmitting information (such as echo waveform information) from a source (such as a transducer) to a destination (such as a rule-based computer system). The present invention reduces the absolute amount of information that is transmitted without reducing the amount of meaningful information that is conveyed. In this way, the present invention is effectively able to transmit more meaningful information than would be expected for conventional transmission systems. It also acts to reduce the amount of information processing that must be done at the destination.

Abstract: Apparatus and methods are described for detecting fractures in a wall of a borehole penetrating an earth formation. An acoustic transducer produces pulses of acoustic energy at beam forming frequencies with the direction of the beam being so oriented as to preferentially excite transverse waves in the wall of the borehole. Discontinuities such as fractures cause a reflection of the transverse waves and these in turn are detected so that a positive identification of fractures is obtained. Fractures having various inclination angles are detected by employing apparatus and methods for scanning the acoustic beam while maintaining its orientation which preferentially enhances transverse waves. In this manner the transverse waves may be directed perpendicular to the fractures to enhance detectable reflections. A transducer employing an array of individually excitable acoustic elements is described with associated controls.

Abstract: An acoustic well logging method and apparatus utilizes a rare earth acoustic transducer to provide low frequency acoustic energy within the borehole, and characteristics of a subsurface geological formation, such as the formation dip, may be obtained by measuring the amplitude of the shear wave and combining that amplitude with known data comprising a graph of shear wave amplitude plotted against incidence angle at known frequencies.

Abstract: An ultrasonic logging tool (borehole televiewer) which utilizes a rotating transducer which is exposed directly to the fluids in the borehole during logging operations.

Abstract: A flexcompressional acoustic transducer is set forth. In the preferred and illustrated embodiment, multiple parallel magnetostrictive rods provided with coils are coupled through a plate affixed to a radiating element. The radiating element in one embodiment is an elongate hollow cylinder which is closed at both ends. Vibrations from the magnetostrictive members cause the cylinder to radially flex, creating an omnidirectional acoustic wave transmitted radially outwardly, the apparatus being positioned in a sonde for connecting an acoustic wave front through the wall of the sonde for transmission into the surrounding wellbore fluids and the adjacent formations.

Abstract: An apparatus useful in investigating the wall surfaces of a structure defining a cavity comprising: a tool having a periphery and being capable of being at least partially located in the cavity; and a plurality of transducers located at different points on the periphery of the tool, each of the transducers being capable of sending first signals toward the wall surfaces of the cavity and receiving reflected second signals. An improved method for investigating the wall surfaces of a structure defining a cavity is also disclosed.

Abstract: The microstructure of thinly laminated earth formations is acoustically determined with a logging tool having an acoustical transducer pad mounted for measuring while in contact with the borehole wall.

Abstract: An ultrasonic mine survey probe is disclosed for providing quantitative profile data in cylindrical coordinates of mine stopes, caves and other underground openings. The probe comprises a probe body adapted to be lowered at several vertical positions in an underground opening and containing a non-rotating transducer assembly adapted to generate a beam of ultrasonic energy longitudinally in the probe body, a beam focussing assembly for focussing the beam of ultrasonic energy generated by the transducer assembly, a beam reflecting device for directing the focussed beam of ultrasonic energy radially outward from the probe body toward the wall of the underground opening during transmission and to redirect the echo signals from the wall of the underground opening toward the transducer assembly. The probe body further contains means for moving the beam deflecting device at predetermined angular positions over a 360.degree.

Abstract: In a method for locating fractures in subsurface earth formations traversed by a borehole a first signal is generated representative of low frequency Stoneley waves (that is tube waves) propagating along the borehole from a source mounted in a logging tool. A second signal is generated by an array of detectors on the tool, representative of low frequency Stoneley waves propagating along the borehole and arising from reflection from fractures of the waves propagating from the source. An indication of the presence of fractures is derived by deconvolving the second signal with the first signal and identifying the time and magnitude of the peak value of the envelope of the deconvolved signal. The variation that would be expected in the presence of fracture and the actual variation are compared and a fracture detected when the actual and expected variations match. Alternatively the first and second signals can be cross-correlated and the time of the peak negative value of the cross-covariance determined.

Abstract: In an acoustic logging system for providing information regarding selected parameters of the wall of a borehole in the earth, and of the rock formation which is adjacent the borehole, in which a single transmit/receive transducer system (T/RTS) mounted on a rotating assembly probes the wall of the borehole in a circular scanning pattern as a function of depth, the improvement which includes at least a second T/RTS mounted on the rotating assembly in known geometrical relation to the first T/RTS, and means to process two or more analog electrical scan signals for transmission over one or more electrical transmission channels in a logging cable. In particular, methods and means are described for the transmission of multiple scan signals, whereby more bottom hole parameters are transmitted, including composite electrical scan signals, in which the early part provides a gated high frequency scan signal and the latter part, the gated flow frequency scan signal, for greater penetration into the rock.

Abstract: A method for monitoring the relative goodness of a cement bond to a borehole casing by observing the relative change in amplitude of a converted-compressional wave as a function of depth in the borehole.

Abstract: The present invention is directed to an apparatus and method for obtaining azimuthally dependent measurements for evaluating the casing cement bond quality and, more particularly, for detecting and specifically locating channels or fissures between the casing and cement. The quality of the bond behind one or more arcs of known location on the casing is determinable by the present invention. The apparatus and method of the present invention employ a plurality of sonic transducers disposed in near contact with the casing in defined segmented locations about or around the exterior surface of a downhole tool. The transducers are arranged in a plurality of arrays, each array comprising four transducers, two transmitting and two receiving transducers, disposed along a single arc about or around the exterior of the tool.

Abstract: Data received from a sonde in a borehole is enhanced by first measuring the relative travel time of a signal transmitted from the sonde and reflected from an area in the wall of a borehole. This measurement is repeated for a plurality of angles in a horizontal plane. From this plurality of travel times a best fit ellipse or circle is calculated and the travel times are correlated with the best fit ellipse or circle to provide corrected travel times and to determine the eccentricity of the sonde in the borehole. The corrected travel times are displayed as images to indicate the detailed shape properties of the borehole.

Abstract: The growth of a hydraulic fracture increases the period of free oscillations in the well connected to the fracture. Simultaneously, the decay rate of free oscillations decreases. The properties of forced oscillations in a well also change during fracture growth. All of these effects result from the changing impedance of the hydraulic fracture that intersects the well. Hydraulic fracture impedance can be defined in terms of the hydraulic resistance and the hydraulic capacitance of a fracture. Fracture impedance can be determined directly by measuring the ratio of down hole pressure and flow oscillation or indirectly from well head impedance measurements using impedance transfere functions. Well head pressure measurements can also be used to evaluate fracture impedance by comparing them to pressure oscillations computed with hydraulic models that include fractures with different impedances.

Abstract: An apparatus and method of use thereof are disclosed herein for determining cement conditions adjacent a casing set within a wellbore. At least one acoustic transmitting and receiving transducer and at least one acoustic receiving transducer are positioned into contact with the interior surface of the casing. At least one pulse of acoustic energy is transmitted into the media surrounding the wellbore from the at least one acoustic transmitting and receiving transducer. Electrical signals are generated in response to acoustic energy received by both transducers from the surrounding media. Representations of the generated electrical signals are recorded as a function of the depth within the wellbore and are indicative of the cement conditions, including the casing-cement bond, the cement-formation bond, cement thickness, and the acoustic velocity associated with the cement.

Abstract: An apparatus and method of use thereof are disclosed herein for determining cement conditions adjacent a casing set within a wellbore. At least one acoustic transmitting and receiving transducer and at least one acoustic receiving transducer are positioned into contact with the interior surface of the casing. At least one pulse of acoustic energy is transmitted into the media surrounding the wellbore from the at least one acoustic transmitting and receiving transducer. Electrical signals are generated in response to acoustic energy received by both transducers from the surrounding media. Representations of the generated electrical signals are recorded as a function of the depth within the wellbore and are indicative of the cement conditions, including the casing-cement bond, the cement-formation bond, cement thickness, and the acoustic velocity associated with the cement.

Abstract: According to the method an acoustic transmitter/receiver tool is lowered in measured manner down a borehole. The tool possesses separate transducers for transmitting and receiving sound waves and includes at least one pair of spaced apart transmitter and receiver spacing. As it is lowered, the tool is caused to repetitively transmit sound waves and on each occasion the waveforms of the echoes received by the tool are recorded in relation to the spacing. Fractures are detected by processing portions of the waveforms which are representative of at least one of two different local effects corresponding to fractures, which effects are amplitude peaks due to a local increase in acoustic coupling, and criss-cross patterns due to mode conversion.

Abstract: Disclosed herein are methods for canceling acoustic reverberations caused when acoustic energy impinges a medium. In one method, a series of positive pulses of acoustic energy are transmitted and the amplitudes of the resulting acoustic reverberations are measured. Negative pulses of acoustic energy are then transmitted having amplitudes equal to the immediately preceding received acoustic reverberation and at a time later than the positive pulses so that the acoustic reverberations are driven to a null.

Abstract: Two downhole measurement-while-drilling tools, at least one of which is borehole sensitive, are used to make standard measurements of borehole conditions while drilling. From these measurements, and with some prior knowledge of the lithology, the caliber of the borehole is determined by iteration. The preferred tools are a neutron porosity and a density tool with lithology having been determined by prior seismic survey techniques.

Abstract: Using waveforms or travel time acquired by multitransmitter and/or borehole sonic tools, acoustic velocity (or slowness) is computed for compressional, shear, and Stoneley waves using direct phase determination (DPD), the slowness time coherence (STC) method, and multiple shot processing (MSP). There is a nonzero difference between transmitter and receiver delta-t's (slownesses) when the sonic tool is in the vicinity of a borehole feature, such as a fracture or a borehole enlargement. The amount of the difference, T/R delta-t, for compressional, shear and Stoneley waves depends on the size and nature of the fracture(s). It is possible to determine the height, width, and dip angle of the fracture when the T/R delta-t's are used in conjunction with other model data. It is also possible to distinguish between borehole enlargements and fractures with the method of this invention.

Abstract: The growth of a hydraulic fracture increases the period of free oscillations in the well connected to the fracture. Simultaneously, the decay rate of free oscillations decreases. The properties of forced oscillations in a well also change during fracture growth. All of these effects result from the changing impedance of the hydraulic fracture that intersects the well. Hydraulic fracture impedance can be defined in terms of the hydraulic resistance and the hydraulic capacitance of a fracture. Fracture impedance can be determined directly by measuring the ratio of down hole pressure and flow oscillations or indirectly from well head impedance measurements using impedance transfer functions. Well head pressure measurements can also be used to evaluate fracture impedance by comparing them to pressure oscillations computed with hydraulic models that include fractures with different impedances.

Abstract: A borehole televiewer uses an acoustic fluid which is lubricating, non-toxic, thermally conductive, electrically insulating, non-reactive, tolerant of high temperatures and pressures, impedance-matched to that of the borehole fluids and the tool window, and which has a low kinematic viscosity at low temperatures.

Abstract: A borehole logging tool is rotated within a borehole and also advanced along the borehole. A log of the formations surrounding the borehole is recorded. An interval of the borehole intersected by a formation fracture is identified on the recorded log. The fracture dip angle and dip direction are determined from the location of the center of the fracture with respect to the borehole and the peak amplitude and phase angle of the recorded borehole signal at the fracture location.

Abstract: In connection with a borehole televiewer system (10), the delta time (51) between low threshold (54) crossing (T.sub.1) and signal peak arrival (T.sub.2) provides a measure of mudcake (43) thickness.