A seismic detection method for scatterers directly in front of a tunnel face

Abstract

The present invention discloses a seismic detection method for scatterers directly in front of a tunnel heading face. First, a seismic observation system is arranged in the already excavated tunnel, and a seismic source is arranged between the heading face and the observation system to generate seismic waves. If the scatterers are from directly in front of the heading face in the collected seismic data, their scattering times have a similar time difference with the direct waves, that is, there is a linear relationship. While for the scatterers not directly in front, they show hyperbolic characteristics on each geophone. According to this characteristic, the seismic scattering waves directly in front of and not directly in front of the heading face can be quickly and effectively identified from the seismic data waveform, and the energy of the scattering waves directly in front of the heading face is enhanced by using the superposition method to ensure the accuracy of detecting the scatterers in front of the heading face. Finally, the distance between the scatterers directly in front of the heading face and the geophone closest to the heading face can be calculated, and precise imaging can be carried out, so as to accurately determine the position of the scatterers directly in front of the heading face.

Description

Technical Field

[0001] The invention relates to a method for seismic detection of a scatterer directly in front of a tunnel face, and belongs to the technical field of seismic detection. Background Art

[0002] During tunnel excavation, accurate detection of geological conditions in front of the face has always been a research difficulty. Complex geological conditions, including stratum changes, rock fractures, karst caves, etc., may affect the engineering safety during tunnel excavation. If the geological conditions in front of the face cannot be accurately obtained and construction is carried out blindly, it may cause groundwater outbursts or the shield machine to fall into the cave, which can easily cause engineering stagnation and delays, thereby increasing engineering costs. Therefore, it is very necessary to detect seismic anomalies in front of the face during tunnel excavation, which can improve construction safety and efficiency and reduce unnecessary shutdowns and adjustments caused by geological factors.

[0003] There are existing geological prediction technologies in tunnel construction technology for TBM (full-face hard rock tunnel boring machine), such as advance drilling, seismic reflection, direct current method, electromagnetic method, etc. All of the above methods can achieve a certain advance detection effect, but due to the interference of complex construction environment and limited observation aperture, the accuracy of the above methods still needs to be improved. Among them, the seismic reflection method has the advantage of high-resolution imaging. The basic principle of the advance detection technology of seismic waves is: using the inversion and scattering waveforms generated by seismic waves at the wave impedance interface, using a highly sensitive signal detector to receive the reflected seismic signal, and performing wave impedance imaging in front of the face. For seismic signals with low signal-to-noise ratio, how to quickly and effectively identify the scatterers directly in front of the face and carry out precise imaging is one of the technical problems that the industry urgently needs to solve. Summary of the invention

[0004] In view of the problems existing in the above-mentioned prior art, the present invention provides a seismic detection method for scatterers directly in front of the tunnel face, which can quickly and effectively identify the scattered waves generated by the scatterers directly in front of the tunnel face, and carry out precise imaging, so as to accurately determine the position of the scatterers directly in front of the tunnel face.

[0005] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a seismic detection method for scatterers directly in front of a tunnel face, the specific steps are:

[0006] Step 1: In the tunnel excavated behind the tunnel face, at least one row of geophones are arranged on the tunnel wall along the tunnel direction, and each geophone is connected to a seismic acquisition instrument to form a seismic observation system; an active seismic source is arranged between the tunnel face and the geophone to stimulate and generate seismic waves; the seismic observation system continuously collects seismic data;

[0007] Step 2: According to the seismic data collected in Step 1, first calculate the arrival times of the scattered waves and direct waves generated by the scatterers directly in front of the heading face received by each geophone, then obtain the time difference of the direct waves at different geophones, and correct the direct wave data of each geophone to the self-excitation and self-reception stacking profile. Then, the scattered waves generated by the scatterers directly in front of the heading face will also be corrected to the horizontal direction accordingly.

[0008] Step 3: Perform directional filtering on the corrected profile, retain the reflection axes with horizontal characteristics, reduce the influence of scattered waves in other directions, and finally carry out common reflection point (CRP) stacking to form a stacked profile.

[0009] Step 4: Finally, based on the velocity measured by formation testing, by stacking the arrival times of the scattered waves of each geophone, the distance between the scatterer directly in front of the heading face and the geophone closest to the heading face can be calculated, and then the position detection of the scatterer directly in front of the heading face can be realized.

[0010] Step 5: If there are scattered waves at multiple different times on the stacked profile, it indicates that there are multiple scatterers in front of the heading face. Then, repeat Steps 2 to 4 for each scattered wave respectively, so as to calculate the distances between each scatterer and the geophone closest to the heading face, and then realize the position detection of different scatterers in front of the heading face.

[0011] Furthermore, in Step 1, two rows of geophones are arranged in the tunnel, and active seismic sources are arranged on the tunnel wall between each row of geophones and the heading face. The two active seismic sources are alternately excited. The two rows of geophones respectively receive the seismic data generated by each seismic source, and respectively calculate the positions of the scatterers directly in front of the heading face. Finally, the average value of the two is taken as the final position of the scatterer directly in front of the heading face. In this way, when the detection distance is much larger than the tunnel diameter, the positions of the scatterers directly in front of the heading face detected by the two rows can corroborate and verify each other, and finally a more accurate position can be obtained.

[0012] Furthermore, in Step 1, the active seismic source generates multiple vibration waves by means of multiple excitation and stacking, so as to improve the signal-to-noise ratio of the received seismic data.

[0013] Furthermore, Step 2 is specifically as follows:

[0014] ①. The collected seismic data includes direct waves and scattered waves. From the perspective of equivalent rays, the direct wave satisfies the following equation:

[0015]

[0016] Among them, i represents the serial number of each geophone away from the seismic source in turn, t i is the arrival time of the direct wave transmitted to the i-th geophone, v1 represents the equivalent velocity of the direct wave between the seismic source and the first geophone, v iIt represents the equivalent velocity of the direct wave between the seismic source and the remaining geophones. Δx represents the geophone spacing, and offset represents the distance from the seismic source to the first geophone.

[0017] ②. The arrival time equation of the scattered wave of any scatterer in the three-dimensional space before and after this row of geophones in space at the geophones is as follows:

[0018]

[0019] Where represents the arrival time after the seismic source is excited and transmitted to the i-th geophone through the scatterer. represents the equivalent velocity of the path from the scatterer to each geophone. Δx represents the geophone spacing, t0 represents the time from the excitation of the seismic source to the scatterer, h represents the perpendicular distance between the scatterer and the plane formed by the array, and x0 represents the distance between the intersection point formed by the perpendicular of the scatterer and the extension line of this row of geophones and the first geophone. The plus or minus sign in the formula is related to the relative position relationship between the perpendicular intersection point and the first geophone.

[0020] ③. Comparing the above scattered wave equations, if the anomaly is located on the forward extension line of the observation system array, that is, directly in front of the tunnel face, that is, h approaches 0, then t i degenerates to

[0021]

[0022] Where represents the equivalent velocity in front of the tunnel face. At this time, x0 represents the distance between the scatterer and the first geophone. Based on the above formula, for the scatterer directly in front of the tunnel face, due to linear propagation, its scattering time has a similar time difference with the direct wave, that is, there is a linear relationship. For the scatterer not directly in front, it shows hyperbolic characteristics at each geophone. According to this characteristic, the scattered wave data generated by the scatterer directly in front of the tunnel face can be extracted from the seismic data waveform.

[0023] ④. Subtracting the arrival time of the scattered wave generated by the scatterer directly in front of the tunnel face from the arrival time of the direct wave, we get:

[0024]

[0025] It can be concluded from the calculation results that for the scattered wave in front of the tunnel face, the reception times at different geophones are exactly the same.

[0026] Based on the above principle, for the collected seismic data, only by relying on the time difference of the direct wave on different geophones, the arrival time difference of the direct wave on different geophones can be obtained, and the direct wave data of each geophone can be corrected to the self-excitation and self-reception stacking profile. Then, the scattered waves generated by the scatterers directly in front of the tunnel face will also be correspondingly corrected to the horizontal direction, while the scattered waves in other directions in space will not be corrected to the horizontal direction.

[0027] Further, the third step is replaced by the following method: for the corrected profile, directly perform horizontal stacking to form a stacked profile, which can suppress the scattered waves in other directions to a certain extent; if a trailing phenomenon occurs in this case, it is determined that the scattered wave is not generated by the scatterer directly in front of the tunnel face, and the scattered wave data is deleted.

[0028] Further, in the fourth step, the distance between the scatterer directly in front of the tunnel face and the geophone closest to the tunnel face, that is, the distance from the first geophone, is calculated. The specific formula is:

[0029]

[0030] In the formula, offset represents the distance between the seismic source and the first geophone. represents the equivalent velocity in front of the tunnel face, x0 represents the distance between the scatterer and the first geophone. represents the arrival time after the seismic source is excited and transmitted to the i-th geophone through the scatterer, t i is the arrival time of the direct wave transmitted to the i-th geophone. Due to the linear relationship, the same x0 can be obtained for any set of geophones.

[0031] In the above calculation, the velocity parameter can be selected as the P-wave or S-wave, and the time difference calculated by this method is the relative time difference. The difference can be taken at the maximum moment of the waveform, so as to effectively reduce the error of the first arrival time reading and improve the calculation efficiency and accuracy.

[0032] Further, the fifth step also includes calculating the waveform amplitude information after geometric spreading compensation (the seismic amplitude attenuation in three-dimensional space is inversely proportional to the propagation distance) while detecting the positions of different scatterers in front of the tunnel face, so as to describe the scattering coefficient intensity and further serve as the basis for determining the scatterer attributes.

[0033] Compared with the prior art, the present invention has the following advantages:

[0034] 1. The present invention is convenient to deploy. Only at least one row of geophones needs to be deployed in the already excavated tunnel to form a seismic observation system, and only several active excitations need to be carried out at the same seismic source position to generate seismic waves, so as to realize the detection of the position of the scatterer (i.e., the abnormal body) directly in front of the tunnel face. Not only the construction is simple, but also the signal-to-noise ratio of the data can be effectively improved.

[0035] 2. The inventors of the present invention have found through research that for the scatterers from the front of the heading face, due to linear propagation, the scattering time has a similar time difference with the direct wave, that is, there is a linear relationship; however, for the scatterers not directly in front, they exhibit hyperbolic characteristics on each geophone; according to this characteristic, the seismic scattering waves in front of and not in front of the heading face can be distinguished from the seismic data waveform, and the energy of the scattering waves in front of the heading face is enhanced by superposition, ensuring the accuracy of detecting the scatterers in front of the heading face.

[0036] 3. In the present invention, the maximum values of the waveforms are selected for difference calculation of relative time difference, and the corresponding formation velocity is used to quickly estimate the position of the scatterer, thereby effectively reducing the error in reading the first arrival time and improving the calculation efficiency and accuracy. Description of the Drawings

[0037] Figure 1 It is the layout and waveform data diagram of the present invention. Detailed Embodiments

[0038] The present invention will be further described below.

[0039] As Figure 1 shown, the specific steps of the present invention are as follows:

[0040] Step 1: In the already excavated tunnel behind the heading face, at least one row of geophones is arranged on the tunnel wall along the tunnel alignment, and each geophone is connected to a seismic acquisition instrument to form a seismic observation system; an active seismic source is set between the heading face and the geophones to generate seismic waves; the seismic observation system continuously acquires seismic data. Specifically: two rows of geophones are arranged in the tunnel, and active seismic sources are arranged on the tunnel walls between each row of geophones and the heading face. The two active seismic sources are alternately excited. The two rows of geophones respectively receive the seismic data generated by each seismic source, and respectively calculate the positions of the scatterers in front of the heading face. Finally, the average value of the two is taken as the final position of the scatterers in front of the heading face. In this way, when the detection distance is much larger than the tunnel diameter, the positions of the scatterers in front of the heading face detected by the two rows can corroborate and verify each other, and finally a more accurate position is obtained; the active seismic source generates multiple vibration waves by means of multiple excitation superposition, thereby improving the signal-to-noise ratio of the received seismic data.

[0041] Step 2: According to the seismic data collected in Step 1, first calculate the arrival times of the scattering waves and the direct waves generated by the scatterers in front of the heading face received by each geophone, and then obtain the time differences of the direct waves on different geophones. The direct wave data of each geophone is corrected to the self-excitation self-reception superposition profile, and the scattering waves generated by the scatterers in front of the heading face will also be correspondingly corrected to the horizontal direction. Specifically:

[0042] ①. The collected seismic data includes direct waves and scattered waves. From the perspective of equivalent rays, the direct wave satisfies the following equation:

[0043]

[0044] where i represents the sequentially increasing number between each geophone and the seismic source, t i is the arrival time of the direct wave at the i-th geophone, v1 represents the equivalent velocity of the direct wave between the seismic source and the first geophone, v i represents the equivalent velocity of the direct wave between the seismic source and the remaining geophones, Δx represents the geophone spacing, and offset represents the distance between the seismic source and the first geophone;

[0045] ②. The arrival time equation of the scattered wave of any scatterer in the three-dimensional space in front of and behind this row of geophones is:

[0046]

[0047] where, represents the arrival time of the wave after being scattered by the scatterer and reaching the i-th geophone, represents the equivalent velocity of the path from the scatterer to each geophone, Δx represents the geophone spacing, t0 represents the time from the seismic source excitation to reaching the scatterer, h represents the perpendicular distance between the scatterer and the plane formed by the array, and x0 represents the distance between the intersection point formed by the perpendicular of the scatterer and the extension line of this row of geophones and the first geophone; the plus or minus sign in the formula is related to the relative position relationship between the perpendicular intersection point and the first geophone;

[0048] ③. Comparing the above scattered wave equation, if the anomaly is located on the forward extension line of the observation system array, that is, directly in front of the tunnel face (such as Figure 1 anomaly B in), that is, h approaches 0, then degenerates to

[0049]

[0050] where, represents the equivalent velocity in front of the tunnel face; at this time, x0 represents the distance between the scatterer and the first geophone; based on the above formula, for the scatterer (i.e., anomaly B) directly in front of the tunnel face, due to linear propagation, its scattering time has a similar time difference with the direct wave, that is, there is a linear relationship; while for the scatterer not directly in front, it shows a hyperbolic characteristic on each geophone; according to this characteristic, the scattered wave data generated by the scatterer directly in front of the tunnel face can be extracted from the seismic data waveform;

[0051] ④. Subtracting the arrival time of the scattered wave generated by the scatterer directly in front of the tunnel face from the arrival time of the direct wave, we get:

[0052]

[0053] It can be concluded from the calculation results that for the scattered waves directly in front of the tunnel face, the reception times at different geophones are exactly the same;

[0054] Based on the above principle, for the collected seismic data, only by relying on the time difference of the direct wave at different geophones, obtaining the arrival time difference of the direct wave at different geophones, and correcting the direct wave data of each geophone to the self-excitation and self-reception stacking profile, the scattered waves generated by the scatterer directly in front of the tunnel face will also be correspondingly corrected to the horizontal direction, while the scattered waves in other directions in space will not be corrected to the horizontal direction (such as Figure 1 the waveform data of anomaly A in

[0055] Step 3: Perform directional filtering on the corrected profile, retain the reflection axes with horizontal characteristics, reduce the influence of scattered waves in other directions, and finally carry out common reflection point (CRP) stacking to form a stacked profile; or for the corrected profile, directly carry out horizontal stacking to form a stacked profile, which can suppress the scattered waves in other directions to a certain extent; if a tailing phenomenon occurs in this case, it is determined that the scattered wave is not generated by the scatterer directly in front of the tunnel face, and the scattered wave data is deleted.

[0056] Step 4: Finally, according to the velocity of the formation test, by using the arrival times of the scattered waves at each geophone after stacking, the distance between the scatterer directly in front of the tunnel face and the geophone closest to the tunnel face, that is, the distance from the first geophone, can be calculated, thereby realizing the detection of the position of the scatterer directly in front of the tunnel face. The specific formula is:

[0057]

[0058] In the formula, offset represents the distance between the source and the first geophone, represents the equivalent velocity in front of the tunnel face, x0 represents the distance between the scatterer and the first geophone, represents the arrival time after the source excitation and passing through the scatterer to the i-th geophone, t i is the arrival time of the direct wave to the i-th geophone. Due to the linear relationship, the same x0 can be obtained for any set of geophones.

[0059] In the above calculation, the velocity parameter can be selected as the P-wave or S-wave, and the time difference calculated by this method is the relative time difference. The difference can be taken at the maximum moment of the waveform, so as to effectively reduce the error in reading the first arrival time and improve the calculation efficiency and accuracy.

[0060] Step Five: If there are multiple scattered waves at different times on the stacked section, indicating that there are multiple scatterers in front of the heading face, then Steps Two to Four are respectively repeated for each scattered wave, so as to calculate the distances between each scatterer and the geophone closest to the heading face, and further realize the detection of the positions of different scatterers in front of the heading face; in addition, while realizing the detection of the positions of different scatterers in front of the heading face, calculate the waveform amplitude information after geometric spreading compensation (the seismic amplitude attenuation in three-dimensional space is inversely proportional to the propagation distance), which is used to describe the scatter coefficient intensity, and further serve as the basis for determining the properties of the scatterers.

[0061] The above description is only the preferred embodiment of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for seismic detection of scatterers directly in front of a tunnel face, characterized in that: The specific steps are: Step 1: In the tunnel excavated behind the tunnel face, at least one row of geophones are arranged on the tunnel wall along the tunnel direction, and each geophone is connected to a seismic acquisition instrument to form a seismic observation system; an active seismic source is arranged between the tunnel face and the geophone to stimulate and generate seismic waves; the seismic observation system continuously collects seismic data; Step 2: Based on the seismic data collected in step 1, first calculate the arrival time of the scattered wave and the direct wave generated by the scatterer in front of the tunnel face at each geophone, then obtain the arrival time difference of the direct wave at different geophones, correct the direct wave data of each geophone to the self-excited and self-received superposition profile, and the scattered wave generated by the scatterer in front of the tunnel face will also be corrected to the horizontal direction accordingly; Step 3: Directionally filter the corrected profile to retain the reflection axis with horizontal characteristics, reduce the influence of scattered waves in other directions, and finally perform co-directional stacking to form a stacked profile; Step 4. Finally, according to the velocity of the formation test, by superimposing the arrival time of the scattered waves of each detector, the distance between the scatterer in front of the tunnel face and the detector closest to the tunnel face can be calculated, thereby realizing the position detection of the scatterer in front of the tunnel face; Step 5: If there are multiple scattered waves at different times on the superimposed section, it means that there are multiple scatterers in front of the tunnel face. Then, steps 2 to 4 are repeated for each scattered wave to calculate the distance between each scatterer and the detector closest to the tunnel face, thereby realizing the position detection of different scatterers in front of the tunnel face.

2. The seismic detection method for scatterers directly in front of the tunnel face according to claim 1 is characterized in that: In the step 1, two rows of geophones are arranged in the tunnel, and active seismic sources are arranged on the tunnel wall between each row of geophones and the tunnel face. The two active seismic sources are excited alternately, and the two rows of geophones respectively receive the seismic data generated by each seismic source, and respectively calculate the position of the scatterer directly in front of the tunnel face, and finally take the average value of the two as the final position of the scatterer directly in front of the tunnel face.

3. The seismic detection method for scatterers directly in front of the tunnel face according to claim 1 is characterized in that: In the step 1, the active seismic source generates multiple vibration waves by means of multiple excitation superposition, thereby improving the signal-to-noise ratio of received seismic data.

4. The method for seismic detection of scatterers directly in front of a tunnel face according to claim 1, characterized in that: The step 2 is specifically as follows: ① The collected seismic data include direct waves and scattered waves. From the perspective of equivalent rays, the direct wave satisfies the following equation: Among them, i represents the number of each detector away from the source, t i is the arrival time of the direct wave to the i-th geophone, v1 is the equivalent velocity of the direct wave between the source and the first geophone, and v i represents the direct wave equivalent velocity between the source and the remaining geophones, Δx represents the distance between the geophones, and offset represents the distance between the source and the first geophone; ② The arrival time equation of the scattered wave of any scatterer in the three-dimensional space before and after the row of detectors is: in, It represents the time when the earthquake source is excited and transmitted to the i-th detector through the scatterer. represents the equivalent velocity of the path from the scatterer to each detector, Δx represents the distance between the detectors, t0 represents the time from the source excitation to the scatterer, h represents the vertical distance between the scatterer and the plane of arrangement, and x0 represents the distance between the intersection of the vertical line of the scatterer and the extension line of the row of detectors and the first detector; the positive and negative signs in the formula are related to the relative position relationship between the intersection of the vertical line and the first detector; ③. Compared with the above scattering wave equation, if the anomaly is located on the front extension line of the observation system, that is, directly in front of the tunnel face, that is, h tends to 0, then Degenerates into in, represents the equivalent velocity in front of the tunnel face; at this time, x0 represents the distance between the scatterer and the first detector; based on the above formula, the scatterer from the front of the tunnel face has a similar time difference with the direct wave due to linear propagation, that is, there is a linear relationship; while the scatterer located not in front shows a hyperbolic feature on each detector; based on this feature, the scattered wave data generated by the scatterer in front of the tunnel face can be extracted from the seismic data waveform; ④. Subtract the arrival time of the scattered wave generated by the scatterer in front of the tunnel face from the arrival time of the direct wave to obtain: From the calculation results, it can be concluded that for the scattered waves directly in front of the tunnel face, the receiving time at different detectors is exactly the same; Based on the above principle, the collected seismic data only needs to obtain the time difference of the direct wave on different detectors according to the time difference of the direct wave on different detectors, and correct the direct wave data of each detector to the self-excited and self-received superposition profile. Then, the scattered wave generated by the scatterer located directly in front of the tunnel face will also be corrected to the horizontal direction accordingly.

5. The seismic detection method for scatterers directly in front of a tunnel face according to claim 1, characterized in that: The step three is replaced by the following method: for the corrected profile, horizontal superposition is directly carried out to form a superimposed profile, which can suppress the scattered waves in other directions to a certain extent; if a tailing phenomenon occurs in this case, it is determined to be a scattered wave generated by a scattering body not directly in front of the tunnel face, and the scattered wave data is deleted.

6. The method for seismic detection of scatterers directly in front of a tunnel face according to claim 1, characterized in that: In step 4, the distance between the scatterer in front of the tunnel face and the detector closest to the tunnel face, that is, the distance to the first detector, is calculated. The specific formula is: In the formula, offset represents the distance between the earthquake source and the first geophone. represents the equivalent velocity in front of the tunnel face, x0 represents the distance between the scatterer and the first detector, It is the time when the earthquake source is excited and transmitted to the ith detector through the scatterer, t i is the arrival time of the direct wave at the i-th detector. Due to the linear relationship, any group of detectors can obtain the same x0.

7. The method for seismic detection of scatterers directly in front of a tunnel face according to claim 1, characterized in that: The step five also includes calculating waveform amplitude information after geometric diffusion compensation to describe the scattering coefficient intensity while realizing position detection of different scatterers in front of the tunnel face.

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