Reflection acoustic logging method and device for adjacent well collision prevention

Abstract

The invention relates to a reflection acoustic logging method and device for adjacent well collision prevention. The method comprises the following steps that an original waveform obtained through orthogonal dipole acoustic logging is preprocessed; carrying out compression vibration processing on the original waveform; suppressing residual shaft direct waves in the original waveform; separating the waveform of the reflected wave to obtain an uplink reflected wave and a downlink reflected wave of a time-depth domain, and performing dynamic correction and common midpoint superposition on the uplink reflected wave and the downlink reflected wave; the uplink reflected waves and the downlink reflected waves are subjected to migration imaging, and the real position of the reflector is obtained; and calculating the form and azimuth information of the crack according to the real position of the reflector. According to the technical scheme, the reflection body which is tens of meters away from the well or even farther away from the well can be imaged through the adjacent well anti-collision reflection acoustic logging method, the resolution ratio is high, the adjacent well can be better detected, and well collision accidents are prevented.

Description

Technical Field

[0001] The invention relates to the technical field of petroleum exploration and development, and in particular to a reflected acoustic wave logging method and device for preventing collision of adjacent wells. Background Art

[0002] Oil and gas well engineering technology is a key technical means to build oil and gas channels and conduct oil production. With the improvement of my country's oil exploration and development, the method of using dense well networks to exploit unconventional oil and gas resources has been frequently used. The distance between wells has become smaller and smaller with the increase of well network density. It is particularly important to detect the location of adjacent wells to prevent collision between wells.

[0003] In recent years, the research on collision prevention of adjacent wells has achieved many research results, such as Chinese journal literature: Liu Xiushan, Su Yinao, Expression and application of the closest distance between adjacent wells [J]. China Offshore Oil and Gas Engineering, 2000(04):31-34+67-5. Tang Xueping. An accurate method for calculating well spacing [J]. Mathematics in Practice and Theory, 2012, 42(17):69-78.

[0004] Chinese patent CN109915116A discloses a method and device for preventing collision with adjacent wells during drilling detection. The method comprises the following steps: placing a magnetic beacon and a probe tube in a well being drilled, wherein one end of the magnetic beacon is connected to the drill bit at the bottom of the well being drilled, and the other end is connected to a downhole motor in the well being drilled; the probe tube is located at the upper part of the downhole motor; a circulating drilling fluid is adopted so that the magnetic beacon rotates while the probe tube does not rotate, wherein the magnetic beacon is used to generate an alternating magnetic field; environmental data of the well being drilled is collected through the probe tube, wherein the environmental data comprises: the three-axis magnetic field strength of the magnetic field at the position of the probe tube and the three-axis components of gravity acceleration; the relative position data of the well being drilled and the adjacent well being drilled are determined according to the environmental data of the well being drilled and the borehole trajectory data of the adjacent well being drilled, wherein the borehole trajectory data comprises: the well inclination angle and the well inclination azimuth angle;

[0005] The Chinese journal article "Study on Anti-collision Calculation Method of Electromagnetic Distance Measurement While Drilling in Neighboring Wells", Petroleum Drilling Technology, 2016, discloses an anti-collision calculation method of electromagnetic distance measurement while drilling in neighboring wells, which includes the following steps: considering the magnetic source as a magnetic dipole, according to the magnetic field distribution near the magnetic dipole, by deriving the calculation formula of the magnetic induction intensity after the casing is magnetized by the magnetic source and the calculation formula of the magnetic induction intensity detected by the magnetic field sensor, the calculation formula of electromagnetic distance measurement while drilling in neighboring wells can be obtained. According to the magnetic induction intensity detected by the magnetic field sensor, the distance to the neighboring well can be calculated.

[0006] The above-mentioned existing technologies still have some problems, mainly including: ① The positioning of neighboring wells adopts map projection coordinates and elevation for horizontal positioning and vertical positioning respectively. Due to the deformation of map projection and the failure to consider the curvature of the earth's ellipsoid, the positioning results have obvious errors; ② The existing well distance scanning and solution methods may miss the danger points of collision prevention of neighboring wells. The analytical method can solve this problem, but it is only applicable to spatial arc trajectories and is not applicable to other wellbore trajectory models; ③ The separation coefficient involves the radius of the wellbore trajectory error ellipsoid on the scanning vector. The center vector method and the foot line method are based on the error ellipse rather than the error ellipsoid, so the calculation is not accurate enough. Summary of the invention

[0007] The purpose of the present invention is to provide a reflection acoustic wave logging method for adjacent well collision prevention, aiming to solve the technical problem that the traditional adjacent well distance calculation method is not accurate enough.

[0008] To achieve the above object, the present invention proposes a reflection acoustic wave logging method for adjacent well collision prevention, comprising the following steps:

[0009] S1: preprocessing the original waveform obtained by orthogonal dipole acoustic logging to eliminate random noise of the original waveform;

[0010] S2: Perform compression and vibration processing on the original waveform to reduce the amplitude and duration of the borehole bending wave;

[0011] S3: suppress the wellbore direct wave remaining in the original waveform to obtain the reflected wave waveform after filtering out the wellbore direct wave;

[0012] S4: Separate the reflected wave waveform to obtain the upgoing reflected wave and the downgoing reflected wave in the time-depth domain;

[0013] S5: Perform dynamic correction and common center point superposition on the upgoing reflected wave and the downgoing reflected wave;

[0014] S6: offset and image the upgoing reflected wave and the downgoing reflected wave to obtain the real position of the reflector;

[0015] S7: Calculate the shape and orientation information of the crack according to the true position of the reflector.

[0016] Furthermore, the step S1 comprises:

[0017] Use the orthogonal dipole logging tool to log the target layer and obtain the original data;

[0018] The original data is de-gained and delayed to obtain the original waveform of the true acoustic response of the formation.

[0019] Furthermore, the step S1 further includes:

[0020] Select the frequency band below 2kHz, take discrete Fourier transform, and perform frequency domain bandpass filtering on the original waveform to eliminate the noise in the waveform. The discrete Fourier transform formula is:

[0021]

[0022] Furthermore, in step S2, an adaptive Wiener filtering method is used to perform compression vibration processing on the original waveform.

[0023] Furthermore, the formula of the adaptive Wiener filtering method is:

[0024]

[0025] Where r(i, j) is the output pixel value, s(i, j) is the selected template pixel value, μ is the mean value of the M×N pixels in the selected template, σ 2 is the mean square error of the template selected for the current processing pixel, and v is the mean square error of the noise of the entire frame image.

[0026] Furthermore, in step S3, a median filtering method is used to suppress the wellbore direct wave remaining in the original waveform.

[0027] Furthermore, the median filtering method comprises the following steps:

[0028] Sort the pixel values ​​in the L×H neighborhood around the target pixel in the waveform image;

[0029] The middle value is taken as the value of the target pixel, where L and H are the window length and window height of the median filter used respectively;

[0030] The waveform of the reflected wave is obtained by subtracting the waveform after median filtering from the original waveform.

[0031] Furthermore, in step S4, the FK filtering method is used to separate the reflected wave waveform into an uplink reflected wave and a downlink reflected wave in the time-depth domain.

[0032] Furthermore, the FK filtering method comprises the following steps:

[0033] Perform a two-dimensional Fourier transform on the reflected wave waveform, and the waveform data is converted from the time-depth domain (tx) to the frequency-wavenumber domain (FK). Assuming the full-wave data is y(x, t), the two-dimensional Fourier transform formula is:

[0034]

[0035] The signal in the negative wavenumber domain after K transformation corresponds to the downlink reflection wave in the time-depth domain;

[0036] The signal of the upgoing reflected wave in the frequency-wavenumber domain is obtained by cutting off the wave in the negative wavenumber plane, and the upgoing reflected wave in the time-depth domain is obtained by inverse FK transformation.

[0037] Furthermore, in step S5, the specific algorithm of dynamic correction is:

[0038]

[0039] Where L is the source distance, that is, the distance from the sound source to the receiver, θ is the inclination angle of the reflection interface, and v is the propagation speed of the sound wave. is the self-excitation and self-reception time, and d is the distance between the center point and the reflection point.

[0040] Furthermore, in step S5, the common center point superposition is to add the gathers of the common center points of the uplink reflected waves and the downlink reflected waves after the dynamic correction.

[0041] Furthermore, the step S6 includes: using a diffraction scanning offset superposition imaging method to return the upgoing reflected wave and the downgoing reflected wave to the reflection point on the reflection interface where the reflected wave is generated, so as to obtain the real position of the reflector.

[0042] Furthermore, the diffraction scanning migration imaging algorithm comprises the following steps:

[0043] The radial plane perpendicular to the well axis is divided into several grids, and the position of each grid is represented by (x i , z i ) to determine;

[0044] When the logging instrument is measuring at position A, the arrival time of each grid point corresponding to the logging instrument at a fixed source distance is

[0045]

[0046] Where v is the speed of sound wave propagation, x i Indicates depth, z i Indicates the position from the well axis, z T represents the location of the sound source, z R Indicates the location of the receiver.

[0047] Furthermore, in step S7, the morphology and orientation information of the fracture includes fracture orientation, inclination, off-well position, and extension length.

[0048] The present invention also provides a reflected acoustic wave logging device for preventing collision with adjacent wells, comprising:

[0049] The acoustic ranging module is used to perform well logging through orthogonal dipole acoustic waves to obtain the original waveform;

[0050] Calculation module; used for preprocessing the original waveform; performing pressure-vibration processing on the original waveform; suppressing the direct wave of the wellbore remaining in the original waveform; separating the reflected wave waveform to obtain the upgoing reflected wave and the downgoing reflected wave in the time-depth domain; performing dynamic correction and common center point superposition on the upgoing reflected wave and the downgoing reflected wave; offset imaging the upgoing reflected wave and the downgoing reflected wave to obtain the real position of the reflector; calculating the morphology and orientation information of the fracture according to the real position of the reflector.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] The reflected acoustic wave logging method for preventing collisions with adjacent wells of the technical solution includes the following steps: S1: pre-processing the original waveform obtained by orthogonal dipole acoustic wave logging to eliminate the random noise of the original waveform; S2: performing pressure-vibration processing on the pre-processed original waveform to reduce the amplitude and duration of the wellbore bending wave; S3: suppressing the wellbore direct wave remaining in the original waveform to obtain the reflected wave waveform after filtering out the wellbore direct wave; S4: separating the reflected wave waveform to obtain the upgoing reflected wave and the downgoing reflected wave in the time-depth domain; S5: performing dynamic correction and common center point superposition on the upgoing reflected wave and the downgoing reflected wave; S6: offset imaging the upgoing reflected wave and the downgoing reflected wave to obtain the real position of the reflector; S7: calculating the morphology and orientation information of the fracture according to the real position of the reflector, wherein the morphology and orientation information of the fracture include the fracture azimuth, inclination, position from the wellbore, and extension length. This technical solution can image reflectors tens of meters or even farther outside the well through the reflected acoustic wave logging method for preventing collision with adjacent wells, and has high resolution, which can better detect adjacent wells and prevent the occurrence of well collision accidents. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0054] Figure 1 It is the remote detection imaging diagram of the mathematical model of the adjacent wells;

[0055] Figure 2 This is an example diagram of adjacent well detection. DETAILED DESCRIPTION

[0056] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0057] It should be noted that if the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative position relationship, movement status, etc. between the components in a certain specific posture. If the specific posture changes, the directional indication will also change accordingly.

[0058] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of technical features indicated. Therefore, the features limited to "first" and "second" may explicitly or implicitly include at least one of the features. In addition, if "and / or" or "and / or" appears in the full text, its meaning includes three parallel solutions. Taking "A and / or B" as an example, it includes solution A, solution B, or solutions that satisfy both A and B. In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the ability of ordinary technicians in this field to implement. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0059] In one embodiment of the reflected acoustic wave logging method for preventing collision with adjacent wells of the present technical solution, the following steps are included:

[0060] Step S1: Use an orthogonal dipole logging instrument to log the target layer, remove gain and delay corrections on the obtained logging data, and obtain the true acoustic response of the formation after preprocessing. During the logging process, due to the influence of factors such as wellbore collapse and collision between the instrument and the wellbore, the acoustic waveform data will contain noise. The main frequency of the dipole bending wave signal used in the present invention is 4 to 6 kHz, while the frequency of the noise is generally below 2 kHz. Therefore, the Fourier transform method can be adopted to perform frequency domain bandpass filtering on the original signal to eliminate the noise in the waveform. The collected digital signal needs to be discrete Fourier transformed, and the specific algorithm is:

[0061]

[0062] Step S2: After the processing of step 1, the low-frequency random noise in the original waveform can be effectively eliminated. However, the present invention utilizes the wave propagated into the formation and reflected back by the interface with strong impedance difference such as the adjacent well, and the wellbore direct bending wave detected by the intersection instrument, which has a weak amplitude. In order to better separate the reflected wave from the original waveform, it is necessary to perform pressure vibration processing on the wellbore direct wave to reduce the amplitude and duration of the wellbore bending wave. To this end, it is necessary to perform adaptive Wiener filtering on the pre-processed waveform. The specific algorithm is:

[0063]

[0064] Where r(i, j) is the output pixel value, s(i, j) is the selected template pixel value, μ is the mean value of the M×N pixels in the selected template, σ 2 is the mean square error of the template selected for the current processing pixel, and v is the noise mean square error of the entire frame image. The selected template is generally a depth segment in the preprocessed waveform with good waveform acquisition quality and unobvious dispersion.

[0065] Step S3: After the pressure vibration, the amplitude of the borehole bending wave is reduced, and the oscillation period is shortened. The median filtering method is adopted to remove the direct wave of the borehole after the pressure vibration. The specific algorithm is: sort the pixel values ​​in the L×H neighborhood around the target pixel point in the waveform image, and take the middle value as the value of the target pixel, where L and H are the window length and window height of the median filter used respectively. Then, the waveform of the reflected wave is obtained by subtracting the waveform after the median filter from the original waveform.

[0066] Step S4: After the above processing, the remaining signal in the waveform still contains the upgoing reflected wave and the downgoing reflected wave. This is because the relative position of the logging instrument and the reflector is different during the detection process. The upgoing and downgoing waves can be separated by using the FK filtering method. The specific algorithm is to perform a two-dimensional Fourier transform on the waveform data y(x, t), and then the waveform data is converted from the time-depth domain (tx) to the frequency-wavenumber domain (FK). Suppose the full wave data is y(x, t), and the two-dimensional Fourier transform formula is

[0067]

[0068] The signal in the negative wavenumber domain after FK transformation corresponds to the downlink reflected wave in the time-depth domain. After cutting off the wave in the negative wavenumber plane, the signal of the uplink reflected wave in the frequency-wavenumber domain is obtained. The uplink reflected wave in the time-depth domain can be obtained by inverse FK transformation.

[0069] Step S5: For the uplink and downlink waves separated in step 4, since the distances between each waveform receiver and the sound source are different, the arrival time of the same signal and the same phase waveform on different receivers is different. The dynamic correction is used to eliminate the influence of the source distance, which is beneficial to the subsequent signal processing. The specific algorithm of dynamic correction is:

[0070]

[0071] Where L is the source distance, that is, the distance from the sound source to the receiver, θ is the inclination angle of the reflection interface, and v is the propagation speed of the sound wave. is the self-excitation and self-reception time, and d is the distance between the center point and the reflection point.

[0072] Common-center point superposition is to add the common-center point gathers after dynamic correction. When the reflected waves are leveled to the same arrival time after dynamic correction, the reflected waves are multiplied due to the same phase, which suppresses interference noise, improves the signal-to-noise ratio, and is beneficial to subsequent signal processing.

[0073] Step S6: After the above processing, the waveform, noise and the bending wave signal directly reaching the wellbore have been effectively eliminated, and the reflected wave has also separated the up and down waves and has been greatly enhanced. Through the time-depth conversion algorithm, the waveform signal can be matched with the position of the adjacent well to obtain the borehole trajectory map of the adjacent well. The present invention adopts the time-depth conversion algorithm as the diffraction scanning migration imaging algorithm. First, the radial direction plane perpendicular to the well axis is divided into several grids, and the position of each grid is represented by (x i , z i ). When the logging instrument is measuring at position A, the arrival time of each grid point corresponding to the logging instrument at a fixed source distance is:

[0074]

[0075] Where v is the speed of sound wave propagation, x i Indicates depth, z i Indicates the position from the well axis, z T represents the location of the sound source, z R Indicates the location of the receiver.

[0076] Step S7: Calculate the shape and orientation information of the fracture according to the real position of the reflector. The shape and orientation information of the fracture includes the fracture orientation, inclination, position from the wellbore, and extension length.

[0077] This technical solution can image reflectors tens of meters or even farther outside the well through the reflected acoustic wave logging method for preventing collision with adjacent wells, and has high resolution, which can better detect adjacent wells and prevent the occurrence of well collision accidents.

[0078] The technical solution also proposes a reflection acoustic wave logging device for preventing collision with adjacent wells, comprising:

[0079] The acoustic ranging module is used to perform well logging through orthogonal dipole acoustic waves to obtain the original waveform;

[0080] Calculation module; used for preprocessing the original waveform; performing pressure-vibration processing on the original waveform; suppressing the direct wave of the wellbore remaining in the original waveform; separating the reflected wave waveform to obtain the upgoing reflected wave and the downgoing reflected wave in the time-depth domain; performing dynamic correction and common center point superposition on the upgoing reflected wave and the downgoing reflected wave; offset imaging the upgoing reflected wave and the downgoing reflected wave to obtain the real position of the reflector; calculating the morphology and orientation information of the fracture according to the real position of the reflector.

[0081] The above descriptions are only optional embodiments of the present invention, and are not intended to limit the patent scope of the present invention. All equivalent structural changes made using the contents of the present invention's specification and drawings, or directly / indirectly applied in other related technical fields, are included in the patent protection scope of the present invention.

Claims

1. A reflection acoustic wave logging method for preventing collision with adjacent wells. It is characterized in that The steps include: S1: preprocessing the original waveform obtained by orthogonal dipole acoustic logging to eliminate random noise of the original waveform; S2: Perform compression and vibration processing on the original waveform to reduce the amplitude and duration of the borehole bending wave; S3: suppress the residual wellbore direct wave in the original waveform to obtain the reflected wave waveform after filtering out the wellbore direct wave; S4: Separate the reflected wave waveform to obtain the upgoing reflected wave and the downgoing reflected wave in the time-depth domain; S5: Perform dynamic correction and common center point superposition on the upgoing reflected wave and the downgoing reflected wave; S6: offset and image the upgoing reflected wave and the downgoing reflected wave to obtain the real position of the reflector; S7: Calculate the shape and orientation information of the crack according to the true position of the reflector.

2. The reflected acoustic wave logging method for preventing collision with adjacent wells according to claim 1, It is characterized in that The step S1 comprises: Use the orthogonal dipole logging tool to log the target layer and obtain the original data; The original data is de-gained and delayed to obtain the original waveform of the true acoustic response of the formation.

3. The reflected acoustic wave logging method for preventing collision with adjacent wells according to claim 2, It is characterized in that The step S1 further comprises: Select the frequency band below 2kHz, take discrete Fourier transform, and perform frequency domain bandpass filtering on the original waveform to eliminate the noise in the waveform. The discrete Fourier transform formula is:

4. The reflected acoustic wave logging method for preventing collision with adjacent wells according to claim 1, It is characterized in that In step S2, an adaptive Wiener filtering method is used to perform compression vibration processing on the original waveform.

5. The reflected acoustic wave logging method for preventing collision with adjacent wells according to claim 4, It is characterized in that The formula of the adaptive Wiener filtering method is: Where r(i, j) is the output pixel value, s(i, j) is the selected template pixel value, μ is the mean value of the M×N pixels in the selected template, σ 2 is the mean square error of the template selected for the current processing pixel, and v is the mean square error of the noise of the entire frame image.

6. The reflected acoustic wave logging method for preventing collision with adjacent wells according to claim 1, It is characterized in that In step S3, a median filter method is used to suppress the wellbore direct wave remaining in the original waveform.

7. The reflected acoustic wave logging method for preventing collision with adjacent wells according to claim 6, It is characterized in that The median filtering method comprises the following steps: Sort the pixel values ​​in the L×H neighborhood around the target pixel in the waveform image; The middle value is taken as the value of the target pixel, where L and H are the window length and window height of the median filter used respectively; Subtract the median filtered waveform from the original waveform to obtain the reflected wave waveform.

8. The reflected acoustic wave logging method for preventing collision with adjacent wells according to claim 1, It is characterized in that In step S4, the FK filtering method is used to separate the reflected wave waveform into an uplink reflected wave and a downlink reflected wave in the time-depth domain.

9. The reflected acoustic wave logging method for preventing collision with adjacent wells according to claim 8, It is characterized in that The FK filtering method comprises the following steps: Perform a two-dimensional Fourier transform on the reflected wave waveform, and convert the waveform data from the time-depth domain (tx) to the frequency-wavenumber domain (FK). Assume that the full-wave data is y(x, t), and the two-dimensional Fourier transform formula is: The signal in the negative wavenumber domain after K transformation corresponds to the downlink reflection wave in the time-depth domain; The signal of the upgoing reflected wave in the frequency-wavenumber domain is obtained by cutting off the wave in the negative wavenumber plane, and the upgoing reflected wave in the time-depth domain is obtained by inverse FK transformation.

10. The reflected acoustic wave logging method for preventing collision with adjacent wells according to claim 1, It is characterized in that In step S5, the specific algorithm of dynamic correction is: Where L is the source distance, that is, the distance from the sound source to the receiver, θ is the inclination angle of the reflection interface, and v is the propagation speed of the sound wave. is the self-excitation and self-reception time, and d is the distance between the center point and the reflection point.

11. The reflected acoustic wave logging method for preventing collision with adjacent wells according to claim 10, It is characterized in that In step S5, the common center point superposition is to add the common center point gathers of the uplink reflected wave and the downlink reflected wave after the dynamic correction.

12. The reflected acoustic wave logging method for adjacent well collision prevention according to claim 1, It is characterized in that The step S6 includes: using a diffraction scanning offset superposition imaging method to return the upgoing reflected wave and the downgoing reflected wave to the reflection point on the reflection interface where the reflected wave is generated, so as to obtain the real position of the reflector.

13. The reflected acoustic wave logging method for preventing collision with adjacent wells according to claim 12, It is characterized in that The diffraction scanning migration imaging algorithm comprises the following steps: The radial plane perpendicular to the well axis is divided into several grids, and the position of each grid is represented by (x i , z i ) to determine; When the logging instrument is measuring at position A, the arrival time of each grid point corresponding to the logging instrument at a fixed source distance is Where v is the speed of sound wave propagation, x i Indicates depth, z i Indicates the position from the well axis, z T represents the location of the sound source, z R Indicates the location of the receiver.

14. The reflected acoustic wave logging method for preventing collision with adjacent wells according to claim 1, It is characterized in that In step S7, the morphology and orientation information of the fracture includes fracture orientation, inclination, off-well position, and extension length.

15. A reflected acoustic wave logging device for preventing collision with adjacent wells. It is characterized in that include: The acoustic ranging module is used to perform well logging through orthogonal dipole acoustic waves to obtain the original waveform; Computation module; Used to pre-process the original waveform; The original waveform is subjected to compression-vibration processing; the residual direct wave from the wellbore in the original waveform is suppressed; the reflected wave waveform is separated to obtain the upgoing reflected wave and the downgoing reflected wave in the time-depth domain; the upgoing reflected wave and the downgoing reflected wave are subjected to dynamic correction and common center point superposition; the upgoing reflected wave and the downgoing reflected wave are offset and imaged to obtain the true position of the reflector; the morphology and orientation information of the fracture are calculated based on the true position of the reflector.

Images