Patent Description:
Well logging systems and methods may be used to inspect and evaluate many characteristics of the wellbore, wellbore casing, and the formations through which the wellbore traverses. For both open hole logging and logging-while-drilling ("LWD"), it is important to obtain high resolution images of the borehole to identify dips angles, fractures, washouts and breakouts. The pulse-echo technique employs a transducer which fires an ultrasonic pulse and receives the echo from the borehole. This technique offers high resolution borehole images in both oil-based mud ("OBM") and water-based mud ("WBM") and has been used in wireline tools for more than <NUM> years.

However, there are multiple challenges applying the pulse-echo technique to LWD, and one major challenge is eccentricity and motion compensation due to vibration. The challenges are presented because pulse-echo signals are sensitive to multiple parameters, such as beam divergence, reflection angle, attenuation along propagation path, etc. All the factors need to be taken into account to acquire a complete deterministic compensation, which requires accurate motion sensing and motion algorithms. Conventional approaches have been unable to solve these challenges, hence there is no acoustic tool commercially available for LWD borehole imaging. Moreover, conventional high resolution imaging tools for LWD are mainly based on micro-resistivity, which can only be used in water based mud.

A prior art acoustic borehole imaging tool and method is disclosed in <NPL>.

A prior art shear wave acoustic logging method and system are disclosed in <CIT> wherein repetitive time-spaced pulses of acoustic energy from a source are each detected at a detector moved along a borehole in fixed space relationship to the source, and wherein the acoustic energy arriving at the detector comprises a series of wave trains, each including a first-arriving compressional wave event and a later-arriving shear wave event.

According to the present invention there is provided an acoustic borehole imaging method as defined in the appended independent method claim. Further preferable features of the method of the present invention are defined in the appended dependent method claims.

According to a further aspect of the present invention there is provided an acoustic borehole imaging tool as defined in the appended independent apparatus claim. Further preferable features of the apparatus of the present invention are defined in the appended dependent apparatus claims.

Illustrative embodiments and related methods of the present disclosure are described below as they might be employed in a downhole method and tool generating acoustic borehole images using the amplitudes of refracted waves. In the interest of clarity, not all features of an actual implementation or method are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. Further aspects and advantages of the various embodiments and related methods of the disclosure will become apparent from consideration of the following description and drawings.

As described herein, the present disclosure provide downhole tools and methods to generate acoustic borehole images using the amplitude of ultrasonic waves. An acoustic imaging tool is positioned along a borehole. An ultrasonic pulse is transmitted toward the borehole wall, wherein the ultrasonic pulse interacts with the borehole wall to produce a refracted ultrasonic wave which is received by one or more receivers along the tool. The amplitude of the refracted waves are then calculated and used to generate the image of the borehole wall. The borehole images are then used to identify fractures in the borehole wall or other discontinuities beneficial in the planning or analysis of downhole operations. Accordingly, the present disclosure provides logging tools and methods which use refracted wave amplitudes to compute the borehole image, thereby providing a method that is highly sensitive to borehole discontinuities (e.g., fractures) or enlargements of the borehole wall due to washout, and can be used independently or in conjunction with velocity imaging methods.

As described in the background section above, conventional imaging tools are challenged because of tool eccentricity and vibration, which results in suboptimal imaging. In the present disclosure, however, a pitch-catch technique is applied that is much less prone to eccentricity and vibration, thus providing a more robust option for LWD and wireline applications. The pitch-catch technique overcomes these challenges by firing refracted waves which are received using multiple receivers. Since the standoff variation affects all received signals almost equally, the standoff effect can be removed by processing signals from multiple receivers. "Standoff," as defined herein, means the distance between the external surface of a logging tool and the borehole wall, while "offset" means the distance between the transmitter and the receiver along the external surface of a logging tool. In addition, the pitch-catch method uses refracted waves, which are less sensitive to eccentricity compared to the pulse-echo approach used in many conventional tools.

The fundamental principle of the present disclosure is that the amplitude of refracted acoustic waves decays as the waves propagate along the borehole. However, the rate of amplitude decay should not vary significantly for a borehole section without fractures. As will be described in more detail below, when the refracted acoustic wave encounters a fracture along its propagation path, acoustic energy will be scattered out near the fracture location leading to an increase in the refracted wave amplitude, and after which, a decrease of amplitude with a higher rate of decay will be observed after the fracture location. In certain embodiments, this sudden change of refracted wave amplitude near a fracture location is then detected by an array receiver or multiple receivers when the transmitter and receivers are shifted incrementally along azimuthal or axial directions. The amplitude readings are then used to generate a reliable borehole image as described herein.

In certain illustrative methods, the amplitude of compressional or shear waves can be measured by average acoustic energy density within the time window [t<NUM>, t<NUM>]. The amplitude equation may be expressed as: <MAT> in the unit of J/m<NUM>, where p is the mud pressure, c is the sound speed in mud, and ρ is the density of mud. For a discrete signal of pressure pi, Equation <NUM> becomes <MAT> where Δt is the sampling interval and n is the number of sampling interval in the time window.

<FIG> is a graph showing a sample ultrasonic waveform based on Finite-Difference Time-Domain ("FDTD") simulation. Here, the graphed model simulates a transmitter firing an ultrasonic pulse at <NUM>. The refracted wave propagates along a segment of uniform borehole and is received by <NUM> array receivers. The standoff is linch (<NUM>), offset is <NUM>. 8inch (<NUM>), and the receivers are spaced equally at. 06inch (<NUM>).

As shown, the compressional wave arrives earlier than shear wave, but with much less amplitude. The time windows used to compute wave amplitude are identified in brackets for compressional and shear waves. In certain illustrative methods, the time window is chosen to cover an adequate time behind the first wave arrival while avoiding interference with other wave arrivals. For example, as shown in <FIG>, the time window for the compressional waves is selected from the first compressional wave arrival to the first shear wave arrival. The time window for the shear waves start from the first shear wave arrival to a time duration which is calculated, for example, based on wave speed, pulse duration, number of receiver, or receiver spacing. In certain embodiments, it is also important to isolate arrivals of other waves such as, for example, tool arrival, direct fluid arrival, and multiple reflections between transmitter and receiver - which can be achieved by tool design or post processing.

In the case of borehole imaging after a washout, the washout area we will have little or no return signal at the expected time window. Therefore, there will be a blackout zone in terms of the image, and the signal will reappear at the other side of the washout along the borehole axis. Therefore, in certain illustrative methods, the washout zone is mapped (but not the enlarged diameter).

<FIG>, <FIG> are graphs showing the computed amplitude for the compressional and shear wave of the <NUM> waveforms of <FIG> in linear and log scale. The amplitude may be computed in a number of ways such as, for example, using Equation <NUM> above. <FIG> and <FIG> show the linear scale for compressional and shear wave amplitudes, respectively, while <FIG> and <FIG> show the log scale for compressional and shear wave amplitudes, respectively. As shown in <FIG>, the amplitude for both compressional and shear wave shows exponential decay, which, in certain methods herein, is approximated as a linear trend when plotted in log scale, as shown in <FIG> and <FIG>. In other embodiments, however, the decay pattern may be approximated with other trends, such as, for example, exponential, inverse power or a combination of these trends. As will be described below, deviation from this linear pattern is used to identify fractures and other discontinuities in the illustrative embodiments of methods described herein.

Now that the underlying theory of the present disclosure has been described, various illustrative acoustic tool designs will now be described. <FIG> shows an acoustic imaging tool positioned along a borehole. Acoustic tool <NUM> is positioned within a borehole <NUM> in an eccentric manner. Borehole <NUM> is filled with mud <NUM>, surrounded by formation <NUM>. Acoustic tool <NUM> includes a transmitter <NUM> and receiver array <NUM>. Transmitters <NUM> may be, for example, piezoelectric, piezoresistive or electromagnetic transmitters, and receivers <NUM> may be piezoelectric, piezoresistive or electromagnetic receivers. A discontinuity (e.g., fracture/crack/wall irregularities) <NUM> is positioned along the wall of borehole <NUM>. During operation of acoustic tool <NUM>, transmitter(s) <NUM> emits an acoustic wave 414a, which then travels along the wall of borehole <NUM>, and is refracted (as refracted waves 414b) adjacent discontinuity <NUM>, where they are received by receiver array <NUM>. Once received, the amplitudes of refracted waves 414b are computed.

With reference to <FIG>, three different cases were simulated wherein crack <NUM> was located at different positions along the wall of borehole <NUM>. The amplitude of refracted shear waves at each of the three positions (with a linch (<NUM>) standoff) are plotted in <FIG>, and <FIG>. The result for a case with no crack is also plotted in <FIG> for comparison. All three plots in <FIG> show the presence of a crack/discontinuity/wall irregularities can be identified as an increase of amplitude caused by scattering of the refracted wave. As can be seen, the increase in amplitude arrives slightly later than the actual crack location, which is because the refracted waves propagate from borehole surface to the receivers in an oblique direction, causing a shift in the location identified on the receivers.

<FIG> is a graph showing the effect of standoff for a discontinuity at the same location along the borehole wall. As can be seen, the increase of amplitude is more distinct (with higher amplitude and narrower spread between receivers) for cases with lower standoff. This allows the location of the fracture to be accurately measured, and may be favorable for LWD tool as the collar is thicker and the transmitter and receiver needs to be installed closer to the borehole compare to wireline tools. Moreover, the standoff does not vary significantly with the use of a stabilizer to minimize lateral movement of the tool.

<FIG> shows an acoustic imaging tool positioned along a borehole. Acoustic imaging tool <NUM> is similar to acoustic tool <NUM>, so like numerals refer to like elements. However, acoustic imaging tool <NUM> employs a transmitter <NUM> and at least two receivers <NUM>. In this example, <NUM> cases (i.e., shifts of receivers) were simulated with the transmitter and receivers shifted by. 12inch (<NUM>) each shift along the borehole axis. The two receivers <NUM> were spaced <NUM>. 3inch (<NUM>) apart with a standoff of linch (<NUM>) and offset of <NUM>. 8inches (<NUM>).

The amplitude of the refracted shear wave for two receivers in each of the <NUM> shifts was computed and plotted in <FIG>. As can be seen, the amplitude data remains flat before a discontinuity, increases at the discontinuity location, and decrease thereafter. Hence, the increase of amplitude indicates the presence of a discontinuity. To remove the effect of standoff in amplitude variation, the difference in the response of two receivers can be used to identify the fracture location which is plotted in <FIG>, which shows the normalized amplitude difference between the two receivers based on <NUM> sets of data with transmitter and receiver shifted (standoff linch (<NUM>), offset <NUM>. 8inches (<NUM>), receiver spacing. 3inches (<NUM>), each shift. 12inches (<NUM>)). The peak value in <FIG> (i.e., shift <NUM>) corresponds to the peak value at the 1st receiver in <FIG>. The slight decrease of value in amplitude of <FIG> (at shift <NUM>) is due to the initial increase of amplitude for the second receiver at shift <NUM> of <FIG>.

In the illustrative examples described herein, the acoustic measurements may be acquired in a number of ways including axially shifting the tool along the borehole. For example, the acoustic tool may be deployed into a desired axial position along the borehole, held in place, then acoustic waves are fired and received, whereby amplitude data is calculated. The tool is then shifted to a second axial location, held in place, and more acoustic signals are acquired as desired. In other embodiments, however, the tool is shifted radially while the data is collected along the same azimuthal angle and incremental depth. For example, in LWD, the acoustic tool is connected along the drill string, which rotates in the borehole and penetrates into the formation. In wireline application, the tool is centralized in the borehole, being pulled by wireline to surface while rotating. In such examples, the acoustic tool repeatedly fires and receives ultrasonic signal when it is rotating azimuthally and shifting radially/azimuthally at the same time. The ultrasonic wave travels much faster than the rotation of the tool, so that the relative movement of the tool between time of firing and receiving is negligible. With repeated data acquisition, signal from all azimuthal angles and depths are collected. During processing, the data collected from the same azimuthal angle and incremental depth are selected for processing.

<FIG> illustrates an alternative embodiment of an acoustic imaging tool which is shifted in a radial or azimuthal manner. Acoustic imaging tool <NUM> is illustrated in a sectional fashion looking down borehole <NUM>. Transmitter <NUM> and two receivers <NUM> are positioned along the same plane around the body of tool <NUM>. During operation tool <NUM> is positioned downhole and held in place, then transmitter <NUM> fires an ultrasonic pulse 1014a toward the wall of borehole <NUM>, which then travels along the wall of borehole <NUM> in a radial manner. Receivers <NUM> then receive the refracted ultrasonic wave 1014b. After acquisition of the first refracted ultrasonic waves, tool <NUM> is shifted radially, transmitter <NUM> is fired, and more acoustic waves are acquired, whereby the amplitudes are calculated as described herein, and used to image the borehole. In other methods, the tool is constantly rotating and translating in the borehole connecting to either wireline or drill string. The acoustic tool repeatedly fires and receives signal along various azimuthal angle and depth. Finally, in either example, the data collected from the same depth and incremental azimuthal angles are selected for processing.

<FIG> is a two-dimension borehole image generated using an illustrative method of the present disclosure. The image is generated using spatial information. To generate the image, time (converted to distance) and tool rotation are combined. Therefore, the time data from the reflection is used, in addition to the speed of the medium, to covert the time to the axial distance. This conversion is performed at each angular position of the tool to obtain the azimuthal spatial info. Therefore, the 2D fracture image can be produced by using both axial and azimuthal coordinates of the fracture reflectors.

In <FIG>, a sample image is shown which was plotted based on simulation result of <NUM>. 5inch (<NUM>) standoff of <FIG>. The fracture is located at <NUM>. 78inches (<NUM>) from the first receiver, and the fracture is assumed to be unchanged along the perpendicular direction. The amplitude of the fracture image was taken as the difference between the amplitude of refracted shear wave for the fracture case and the amplitude of normal decay trend (case without a fracture), at the portion where the amplitude increases above the normal decay trend. In this example, the amplitude of the image is related to the width and depth of the fracture. As shown in <FIG>, the amplitude is higher for fractures with larger widths and depths. In certain other illustrative methods, the location of the fracture image can also be determined by back propagating the corresponding receiver location to the borehole, according to the oblique angle at which the refracted waves propagate (as shown in <FIG>). The angle is the critical angle of the refracted wave which can be calculated from the ratio between the refracted wave velocity and fluid velocity.

In summary, the amplitude of refracted waves are sensitive to borehole fractures and other discontinuities. In the illustrative embodiment of <FIG>, an array receiver covering at least about. 5inch (<NUM>) with good receiver spacing is utilized to acquire the refracted signals, thus negating the need for multiple shifts along the borehole. A discontinuity may be identified by a local increase of amplitude using one set of array receiver data without shifting the transmitter and receivers. As such, acoustic imaging tool <NUM> allows less sampling along the borehole axial direction. With the use of acoustic tools <NUM> or <NUM>, however, a less number of receivers is required, but more sampling along the borehole axial and radial directions is needed to provide sufficient resolution.

Now various applications of the present disclosure will be described. <FIG> is a diagram of an illustrative well system 1200a. Well system 1200a includes a logging system <NUM> and a subterranean region <NUM> beneath the ground surface <NUM>. A well system can include additional or different features that are not shown in <FIG>. For example, the well system 1200a may include additional drilling system components, wireline logging system components, etc..

The subterranean region <NUM> can include all or part of one or more subterranean formations or zones. The subterranean region <NUM> shown in <FIG> includes multiple subsurface layers <NUM> and a wellbore <NUM> penetrating the subsurface layers <NUM>. The subsurface layers <NUM> can include sedimentary layers, rock layers, sand layers, or combinations of these other types of subsurface layers. One or more of the subsurface layers can contain brine, oil, gas, etc. Although wellbore <NUM> shown in <FIG> is a vertical wellbore, the logging system <NUM> can be implemented in other wellbore orientations. For example, the logging system <NUM> may be adapted for horizontal wellbores, slant wellbores, curved wellbores, vertical wellbores, or combinations of these.

The illustrative logging system <NUM> includes an acoustic imaging tool <NUM>, surface equipment <NUM>, and a tool controller <NUM>. In the example shown in <FIG>, acoustic imaging tool <NUM> is a downhole acoustic imaging tool that operates while disposed in wellbore <NUM>, as described herein. The example surface equipment <NUM> shown in <FIG> may operate at or above the surface <NUM>, for example, near a well head <NUM> of wellbore <NUM>, to position acoustic imaging tool <NUM> and optionally other downhole equipment or other components of the well system 1200a. Tool controller <NUM> may be operable to control surface equipment and to receive and analyze logging and imaging data from the acoustic imaging tool <NUM>. Logging system <NUM> can include additional or different components or features, and such may be arranged and operated as represented in <FIG> or in another suitable manner.

In some instances, all or part of tool controller <NUM> can be implemented as a component of, or can be integrated with one or more components of, the surface equipment <NUM>, the acoustic imaging tool <NUM>, or both to implement the methods described herein. In some cases, tool controller <NUM> can be implemented as one or more discrete computing system structures separate from surface equipment <NUM> and acoustic imaging tool <NUM>. In some implementations (not illustrated), controller <NUM> may be located entirely within acoustic imaging tool <NUM>, and controller <NUM> and acoustic imaging tool <NUM> can operate concurrently while disposed in wellbore <NUM>. Although tool controller <NUM> is shown above surface <NUM> in the example shown in <FIG>, all or part of the tool controller <NUM> may reside below surface <NUM>, for example, at or near the location of the acoustic imaging tool <NUM>.

Well system 1200a can include communication or telemetry equipment that provides a communication link <NUM> between tool controller <NUM>, acoustic imaging tool <NUM>, and optionally other components of the logging system <NUM>. For example, the components of logging system <NUM> can each include one or more transceivers or similar apparatus for wired or wireless data communication among the various components. The logging system <NUM> can include systems and apparatus for wireline telemetry, wired pipe telemetry, mud pulse telemetry, acoustic telemetry, electromagnetic telemetry, or a combination of these other types of telemetry. In some cases, acoustic imaging tool <NUM> receives commands, status signals, or other types of information from tool controller <NUM> or another source. In some cases, tool controller <NUM> receives logging data, status signals, or other types of information from acoustic imaging tool <NUM> or another source.

Logging operations can be performed in connection with various types of downhole operations at various stages in the lifetime of a well system. Structural attributes and components of surface equipment <NUM> and acoustic imaging tool <NUM> can be adapted for various types of logging operations. For example, logging may be performed during drilling operations, during wireline logging operations, or in other contexts. As such, surface equipment <NUM> and acoustic imaging tool <NUM> may include, or may operate in connection with drilling equipment, wireline logging equipment, or other equipment for other types of operations.

In some examples, logging operations are performed during wireline logging operations. <FIG> shows an exemplary well system 1200b that includes acoustic imaging tool <NUM> in a wireline logging environment. In some example wireline logging operations, surface equipment <NUM> includes a platform above surface <NUM> that is equipped with a derrick <NUM> or a winch <NUM> that supports a conveyance <NUM> that extends into wellbore <NUM>. Wireline logging operations can be performed, for example, after a drilling string is removed from wellbore <NUM> to allow acoustic imaging tool <NUM> to be lowered by wireline or logging cable into the wellbore <NUM>.

As shown, for example, in <FIG>, acoustic imaging tool <NUM> can be suspended in wellbore <NUM> by a conveyance <NUM>, which may be a coiled tubing, wireline cable, or another structure that connects the tool to a surface control unit or other components of surface equipment <NUM>. In some implementations, acoustic imaging tool <NUM> is lowered to the bottom of a region of interest and subsequently pulled upward (e.g., at a substantially constant speed) through the region of interest.

In some examples, logging operations are performed during drilling operations. <FIG> shows an exemplary well system 1200c that includes acoustic imaging tool <NUM> in an LWD environment. Drilling is commonly carried out using drill pipes connected together to form a drill string <NUM> that is lowered through a rotary table into wellbore <NUM>. In some cases, a drilling rig <NUM> at surface <NUM> supports drill string <NUM>, as drill string <NUM> is operated to drill a wellbore penetrating subterranean region <NUM>. Drill string <NUM> may include, for example, a kelly, drill pipe, a bottom hole assembly, and other components. The bottom hole assembly may include drill collars, drill bits, acoustic imaging tool <NUM>, and other components.

Acoustic imaging tool <NUM> can be deployed in the wellbore <NUM> on jointed drill pipe, hardwired drill pipe, or other deployment hardware. In some implementations, acoustic imaging tool <NUM> collects data during drilling operations as it moves downward through the region of interest during drilling operations, as described herein. In some implementations, acoustic imaging tool <NUM> collects data while the drilling string <NUM> is moving, for example, while it is being run in or tripped out of wellbore <NUM>. In other embodiments, however, acoustic imaging tool <NUM> is held in place, acoustic measurements are acquired, the tool is shifted, more signals are acquired, etc., as described herein.

In some implementations, acoustic imaging tool <NUM> collects data at discrete logging points in the wellbore <NUM>. For example, acoustic imaging tool <NUM> can move upward or downward incrementally (or radially) to each logging point at a series of depths in wellbore <NUM>. At each logging point, instruments in acoustic imaging tool <NUM> perform measurements within the wellbore. The measurement data can be communicated to tool controller <NUM> for storage, processing, and analysis. Such data may be gathered and analyzed during drilling operations (e.g., during LWD operations), during wireline logging operations, or during other types of activities.

Tool controller <NUM> can receive and analyze the measurement data from acoustic imaging tool <NUM> to detect and characterize fluid flow, provide images of the wellbore and other objects within the wellbore, such as fractures, sand, stuck pipe, scale, and characterize the casing inner wall, its dimensions, and the presence or absence of features along the casing wall, as described herein.

In addition to wireline or LWD applications, the embodiments disclosed herein may be deployed via slickline, coil tubing, measurement-while-drilling ("MWD") or other downhole tubular assemblies. Regardless of the application selected, the acoustic tools are coupled to processing circuitry (e.g., controllers) that act as a data acquisition and/or processing system to analyze amplitude data and perform the methods described herein. Although not shown, the processing circuitry may include at least one processor, a non-transitory, computer-readable storage (also referred to herein as a "computer-program product"), transceiver/network communication module, optional I/O devices, and an optional display (e.g., user interface), all interconnected via a system bus. In one embodiment, the network communication module is a network interface card ("NIC") and communicates using the Ethernet protocol. In other embodiment, the network communication module may be another type of communication interface such as a fiber optic interface and may communicate using a number of different communication protocols. Software instructions executable by the processor for implementing software instructions in accordance with the illustrative methods described herein, may be stored in storage or some other computer-readable medium.

The processing circuitry may be connected to one or more public (e.g., the Internet) and/or private networks via one or more appropriate network connections. It will also be recognized that the software instructions may also be loaded into storage from a CD-ROM or other appropriate storage media via wired or wireless methods.

Moreover, those ordinarily skilled in the art will appreciate that embodiments of the disclosure may be practiced with a variety of computer-system configurations, including hand-held devices, multiprocessor systems, microprocessor-based or programmable-consumer electronics, minicomputers, mainframe computers, and the like. Any number of computer-systems and computer networks are acceptable for use with the present disclosure. Embodiments of the disclosure may be practiced in distributed-computing environments where tasks are performed by remote-processing devices that are linked through a communications network. In a distributed-computing environment, program modules may be located in both local and remote computer-storage media including memory storage devices. The present disclosure may therefore, be implemented in connection with various hardware, software or a combination thereof in a computer system or other processing system. Subject to network reliability, the imaging techniques described herein may be performed in real-time to update production, enhance oil recovery ("EOR") operations, and/or other operations.

<FIG> is a flow chart of a method for imaging a borehole using an acoustic tool. At block <NUM> of method <NUM>, the acoustic imaging tool is deployed downhole along a borehole to a desired axial position. At block <NUM>, ultrasonic acoustic impulses are transmitted toward the borehole wall, where they are refracted and received by one or more tool receivers at block <NUM>. At block <NUM>, processing circuitry calculates the amplitude of the rerated ultrasonic waves to generate the borehole images as described herein at block <NUM>. Thereafter, the borehole images may be applied to plan, conduct, analyze, or otherwise enhance any variety of downhole related applications (e.g., fracture identification, structural dip analysis, stress analysis, borehole stability and breakout analysis, borehole profiling and calculation of cement volume).

Claim 1:
An acoustic borehole imaging method, comprising:
positioning an acoustic imaging tool (<NUM>, <NUM>, <NUM>, <NUM>) along a borehole;
transmitting an ultrasonic pulse toward a borehole wall using a pitch-catch technique, wherein the ultrasonic pulse interacts with the borehole wall to produce a refracted ultrasonic wave;
receiving the refracted ultrasonic wave using multiple receivers (<NUM>, <NUM>, <NUM>);
calculating an amplitude of the refracted ultrasonic wave; and
generating an image of the borehole using the amplitude of the refracted ultrasonic wave,
wherein the image is generated by calculating an amplitude decay of the refracted ultrasonic wave, the amplitude decay being caused by an interaction between the ultrasonic pulse and a material discontinuity in the borehole wall that results in the refracted ultrasonic wave.