A data acquisition method for cross-hole resistivity CT

Abstract

The present invention discloses a data acquisition method for cross-hole resistivity CT, using a four-electrode array AMN-B, and the running process is shown in the figure: ① The electrodes AMN are in the same borehole, and the arrangement order is A1, M1, N1 in the figure, and the electrode B is in another borehole. The distances of AM and MN are both 1: ② The electrode array starts from the top, keeps AB at the same height, and moves downward by 1 each time until N reaches the bottom; ③ Keep the AM distance unchanged, increase the MN distance by 1, and repeat step ② until MN traverses all distances; ④ The AM distance increases by 1, and the MN distance starts from 1 again, and repeats steps ②③ until AM traverses all distances; ⑤ MN is transferred to the borehole where B is located, and the AM in steps ③④ is changed to BM, and steps ②-④ are repeated; ⑥ The arrangement order of the three electrodes in step ① is changed to N3, M3, A3 in the figure, and the top in step ② is changed to the bottom, the bottom is changed to the top, and the downward is changed to the upward, and steps ①-⑤ are repeated to form the final acquisition sequence.

Description

Technical Field

[0001] The invention belongs to the technical field of cross-hole resistivity CT, and in particular relates to a data acquisition method for cross-hole resistivity CT. Background Art

[0002] Resistivity CT is widely used in underground surveys due to its high efficiency of injecting current directly into the detection area and its simple and fast data acquisition. The traditional high-density electrical method has the limitation of low resolution in deep areas, while cross-hole resistivity CT can conduct deeper underground detection. In addition, cross-hole resistivity CT has the following advantages: ① There are detection electrodes on both sides of the detection profile, which has a higher resolution for the detection area than the high-density electrical method with only one side; ② The closed environment makes the electromagnetic interference noise smaller and the signal-to-noise ratio of the collected data higher; ③ Two or more boreholes can obtain richer data, that is, more information in the detection area. Therefore, cross-hole resistivity CT has a good application and development prospect.

[0003] Cross-hole resistivity CT has a variety of observation arrays: two-electrode, three-electrode and four-electrode. Two-electrode and three-electrode arrays place one or two electrodes at infinity, but in actual applications, infinity is impossible to achieve, so two-electrode and three-electrode arrays will have systematic errors, while quadrupole arrays do not have this problem, and with a larger sensitivity distribution range, the quadrupole array observation mode has gradually become the mainstream.

[0004] There are also many types of four-electrode arrays, and three commonly used ones are cross-hole quadrupole arrays: AM-BN, AB-MN and AMN-B (A and B are excitation electrodes, M and N are detection electrodes). Research by many scholars has shown that among the three arrays, AMN-B has the best detection effect and the highest resolution of the detection area.

[0005] The AMN-B array still has a variety of running methods, and the amount of data and information in the detection area obtained by different running methods are different, and the final resolution of the detection area is also different, and the time required for inversion is also different. The traditional AMN-B running method generates a small amount of electrode sequence data but a large number of independent excitation pairs, which makes it have a lower resolution of the detection area and takes longer to invert. Summary of the invention

[0006] In view of the problems existing in the traditional data acquisition method of cross-hole resistivity CT, the present invention proposes a new data acquisition method, which is mainly aimed at the running pole mode of AMN-B. By using all possible AM ​​and MN distances, increasing the running pole mode of the electrode moving from the bottom to the top, and keeping the two excitation electrodes always at the same height in the borehole, the amount of effective data is greatly increased, the resolution of the cross-hole resistivity CT is significantly improved, and the inversion time is also greatly reduced.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A data acquisition method for cross-hole resistivity CT using a four-electrode array AMN-B, wherein the running electrode method comprises the following steps:

[0009] (1) Select electrodes A, M, and N in the same borehole and arrange them in the order of A, M, and N from top to bottom. Electrode B is in another borehole. Then select the distance between AM and MN to be 1 unit electrode distance;

[0010] (2) The electrode array starts from the top and moves downward one unit electrode distance each time until electrode N reaches the bottom, during which the positions of electrodes A and B always remain at the same height;

[0011] (3) Keeping the AM distance unchanged, the MN distance increases by 1 unit electrode distance, and repeating step (2) until MN traverses all distances;

[0012] (4) The AM distance increases by 1 unit electrode distance, and the MN distance starts again from 1 unit electrode distance, and steps (2) and (3) are repeated until AM traverses all distances;

[0013] (5) Electrodes M and N are transferred to the borehole where electrode B is located, AM in steps (3)-(4) is replaced with BM, and steps (2)-(4) are repeated;

[0014] (6) Change the order of A, M, N in the borehole in step (1) to N, M, A, change "start from the top" in step (2) to "start from the bottom", change "move down" to "move up", and change "reach the bottom" to "reach the top", repeat steps (1)-(5) to form the final acquisition sequence.

[0015] Furthermore, the distance between AM and MN during the running process is selected from all possible distances.

[0016] Furthermore, during the running process, the positions of electrode A and electrode B are always maintained at the same height.

[0017] Furthermore, during the running electrode process, the movement of the electrode sequence is not only from top to bottom, but also from bottom to top.

[0018] Furthermore, the collected data were screened for data quality using geometric factors, with a geometric factor threshold of 1000, and data greater than 1000 were eliminated.

[0019] Compared with the existing AMN-B data collection method, the present invention has the following beneficial effects:

[0020] (1) In the present invention, the distances between AM and MN are not fixed, but all possible distances are selected. However, when three electrodes are placed in the same borehole, the arrangement order of the two detection electrodes on one side of the excitation electrode is still followed, and the array in which the excitation electrode is located between the two detection electrodes is not used (a portion of low-quality data equal to 0 will exist in the collected voltage data), thereby greatly increasing the amount of effective data, obtaining more information about the detection area, and improving the resolution of the cross-hole resistivity CT method for the detection area. When 21 electrodes are used in each borehole, the traditional running pole method takes the AM and MN distances as two unit electrode distances, and the amount of data generated is 714, while the amount of data generated by the present invention is 4516. (After eliminating low-quality data with excessive geometric factors)

[0021] (2) The positions of the excitation electrodes A and B of the present invention are always kept at the same height, which reduces the number of independent excitation pairs, so that the time of the entire inversion process is reduced while the amount of data is greatly increased. On a computer with a main frequency of 2.1 GHz, a memory of 256 GB, and 24 cores, the three-dimensional cross-hole resistivity CT of the same geoelectric model is iterated 6 times, and the traditional data acquisition method and the method of the present invention take 925 seconds and 400 seconds respectively.

[0022] (3) The present invention adds a moving mode of the electrode array starting from the bottom to the top, thereby eliminating the defect of poor resolution at the bottom of the detection area in the optimization mode of only moving the electrode in one direction from the top to the bottom. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the detection device and the running pole method of the present invention.

[0024] Figure 2 It is the cumulative sensitivity distribution diagram of the electrode sequences generated by the traditional AMN-B running method, the optimized AMN-B running method (unidirectional running method) and the optimized AMN-B running method (bidirectional running method).

[0025] Figure 3 It is a three-dimensional geoelectric model constructed to demonstrate the advantages of the present invention and the inversion results of the traditional AMN-B running pole method, the optimized AMN-B running pole method (one-way running pole) and the optimized AMN-B running pole method. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0027] The present invention proposes a data acquisition method for cross-hole resistivity CT. The electrode array used for data acquisition is a four-electrode array AMN-B. A simple schematic diagram of the running electrode mode of AMN-B is shown in FIG. Figure 1 In the figure, the electrode with "1" is arranged in an array in the first process described below, the electrode with "2" is arranged in an array in the second process described below, and the electrode with "3" is arranged in an array in the third process described below. The specific running electrode process includes the following steps:

[0028] (1) Select electrodes A, M, and N in the same borehole and arrange them in the order of A1, M1, and N1 in the figure. Electrode B is in another borehole. Then select the distance between AM and MN to be 1 unit electrode distance;

[0029] (2) The electrode array starts from the top and moves downward by one unit electrode distance each time until electrode N reaches the bottom, during which the positions of electrodes A and B always remain at the same height;

[0030] (3) Keeping the AM distance unchanged, the MN distance increases by 1 unit electrode distance, and repeating step (2) until MN traverses all distances;

[0031] (4) The AM distance increases by 1 unit electrode distance, and the MN distance starts again from 1 unit electrode distance, and steps (2) and (3) are repeated until AM traverses all distances;

[0032] At this point, the first process is completed. Next, proceed to the second process:

[0033] (5) Electrodes M and N are transferred to the borehole where electrode B is located, AM in steps (3)-(4) is replaced with BM, and steps (2)-(4) are repeated;

[0034] At this point, the second process is completed, and the third process is carried out next:

[0035] (6) Change the arrangement order of A, M, and N in the borehole in step (1) to N3, M3, and A3 in the figure, change "start from the top" in step (2) to "start from the bottom", change "move down" to "move up", and change "reach the bottom" to "reach the top", repeat steps (1) to (5) to form the final acquisition sequence.

[0036] The collected data are screened for data quality using geometric factors, with a geometric factor threshold of 1000, and data greater than 1000 are discarded.

[0037] Compared with the traditional AMN-B running pole method, the difference of the optimized AMN-B running pole method proposed in the present invention is that: ① The distance between AM and MN in the present invention is not fixed, but all possible distances are selected, which greatly increases the amount of effective data and obtains more information about the detection area; ② The positions of the excitation electrodes A and B are always kept at the same height, reducing the number of independent excitation pairs, so that the amount of data is greatly increased while the time consumption of the entire inversion process is reduced. ③ The movement mode of the electrode sequence moving from the bottom to the top is added, eliminating the defect of poor resolution at the bottom of the detection area of ​​the optimized AMN-B method of unidirectional running pole.

[0038] Cumulative sensitivity is the sum of the absolute values ​​of the sensitivities of all four-electrode arrays included in the electrode sequence, and is a parameter that reflects the detection capability of the entire electrode sequence for the detection area. In order to compare the detection capabilities of the electrode sequences generated by the three running electrode methods, the cumulative sensitivity distribution diagrams of the electrode sequences generated by the three running electrode methods were made, as shown in Figure 2. Figure 2 As shown. The figure shows the cumulative sensitivity distribution on two sections of the three-dimensional detection area. The four black lines represent four detection boreholes with a borehole spacing of 8m. The black dots on the lines represent electrodes, 21 in each borehole, and the unit electrode spacing is 1m. The grid is divided into cubic blocks with a side length equal to the unit electrode spacing. It can be seen that the cumulative sensitivity value of the traditional AMN-B is much smaller than that of the optimized AMN-B. In terms of distribution range, only the optimized AMN-B is symmetrical in the longitudinal direction, while the cumulative sensitivity values ​​of the traditional AMN-B and the optimized AMN-B (one-way running pole) are larger in the upper half of the borehole and smaller in the lower half, especially the optimized AMN-B (one-way running pole), which drops to a very small value at the bottom of the detection area.

[0039] In order to prove the beneficial effects of the present invention, multiple three-dimensional geoelectric models were constructed for verification. Figure 3 The inversion results of a typical model are shown. The background resistivity of the model is 100Ω.m. There are two high-resistance anomalies with a size of 2×2×2m and a resistivity of 500Ω.m in the middle of the top and bottom of the detection area (top view). The figure only shows the XY section of the location of the anomaly in the three-dimensional detection area, which is displayed in a grid format to make the location of the anomaly in the inversion image more intuitive. Figure 3As shown, the optimized AMN-B running pole method has a better detection effect than the traditional running pole method, especially in the inversion value, which is much improved. This corresponds to the fact that the cumulative sensitivity value of the optimized AMN-B running pole method is much larger than that of the traditional running pole method. However, because the location of the anomaly is far away from the electrode, the sensitivity is low, and the smooth constraint is applied, so there is a large gap with the actual value of 500 of the anomaly. The optimized AMN-B running pole method (one-way running pole) can well distinguish the anomaly at the top of the detection area, but basically cannot distinguish the anomaly at the bottom, which also corresponds to the fact that its cumulative sensitivity value is very small at the bottom of the detection area. The optimized AMN-B running pole method has the best inversion effect, which proves the beneficial effect of the present invention in improving the resolution of cross-hole resistivity CT.

[0040] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A data acquisition method for cross-hole resistivity CT, characterized in that: The four-electrode array AMN-B was used for acquisition. The AMN-B running method included the following steps: (1) Select electrodes A, M, and N in the same borehole and arrange them in the order of A, M, and N from top to bottom. Electrode B is in another borehole. Then select the distance between AM and MN to be 1 unit electrode distance. (2) The electrode array starts from the top and moves downward one unit electrode distance each time until electrode N reaches the bottom. During this time, the positions of electrodes A and B always remain at the same height. (3) Keep the AM distance unchanged, increase the MN distance by 1 unit electrode distance, and repeat step (2) until MN traverses all distances; (4) The AM distance increases by one unit electrode distance, and the MN distance starts again from 1 unit electrode distance. Steps (2) and (3) are repeated until AM traverses all distances. (5) Electrodes M and N are transferred to the borehole where electrode B is located, AM in steps (3)-(4) is replaced with BM, and steps (2)-(4) are repeated; (6) Change the arrangement order of electrodes A, M, N in the borehole in step (1) to N, M, A, change "start from the top" in step (2) to "start from the bottom", change "move down" to "move up", and change "reach the bottom" to "reach the top", and repeat steps (1)-(5) to form the final acquisition sequence.

2. The data collection method according to claim 1, characterized in that: The collected data are screened for data quality using geometric factors, with a geometric factor threshold of 1000, and data greater than 1000 are discarded.

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