A low-frequency large-span electromagnetic wave CT data correction method

Abstract

The present invention discloses a low-frequency large-span electromagnetic wave CT data correction method, which comprises the following steps: Step 1, obtaining the initial emission field strength, the ground surface absorption coefficient and the absorption coefficient in the borehole: using the initial emission field strength obtained in Step 1, the distance from the emission antenna in the borehole to the hole mouth, and the distance from the receiving antenna in the borehole to the hole mouth, to obtain the field strength propagating along the borehole and the field strength propagating along the borehole, and the field strength of the corrected low-frequency electromagnetic wave propagating in a straight line at the shortest distance in the medium. The correction method disclosed by the present invention solves the problem of poor inversion effect of the measured data of the low-frequency cross-hole electromagnetic wave CT, realizes the application of the low-frequency cross-hole electromagnetic wave CT technology in a large hole distance of 20-40 m, improves the detection efficiency and reduces the detection cost.

Description

Technical Field

[0001] The invention belongs to the field of electromagnetic wave CT data processing, and in particular relates to a low-frequency and large-span electromagnetic wave CT data correction method in the field. Background Art

[0002] like Figure 1 The cross-hole electromagnetic wave CT technology shown in FIG. 1 is a technology that changes the positions of the transmitting antenna and the receiving antenna placed in two boreholes (wells), measures the electromagnetic wave receiving voltage at different positions, and then uses the ray propagation theory to obtain the resistivity profile between the two holes, such as Figure 2 As shown, this technology can be used for cave detection in urban underground space development.

[0003] The operating frequency range of cross-hole electromagnetic wave CT is 100KHz to 30MHz, but due to the abnormal data processing results of low frequency (below 1MHz), it cannot meet the engineering requirements, and the main application frequency is above 1MHz. However, the underground transmission attenuation of this frequency band is large, and the measurement distance between holes is limited to 5-10m. A large number of test pipes need to be pre-buried (generally no less than 4 pipes are pre-buried every 20 meters), and the subsequent detection volume is large, which is time-consuming and increases the cost. In this context, the development of low-frequency and large-span electromagnetic wave CT technology less than 1MHz is considered, and the detection distance can reach 20-40m, ultimately achieving the purpose of increasing detection speed, improving detection quality and reducing detection costs. However, the operating frequency below 1MHz belongs to the long-wave band and has its own unique propagation characteristics, which are very different from the propagation characteristics of high-frequency radio waves. How to pre-correct the measurement data so as to perform effective inversion imaging based on the existing ray theory has become a bottleneck for the use of low-frequency electromagnetic waves in large-span electromagnetic wave CT technology. It is urgent to propose a new data correction method to solve this problem. Summary of the invention

[0004] In view of the problem that the existing ray theory inversion cannot be directly applied to low-frequency and large-span electromagnetic wave CT technology, the present invention proposes a new large-span electromagnetic wave CT data correction method based on the propagation characteristics of low-frequency electromagnetic waves in layered media. The purpose is to solve the problem that the existing ray theory has too large an error in inverting low-frequency measurement data and cannot meet the interpretation of low-frequency cross-hole electromagnetic wave tomography data below 1 MHz.

[0005] The present invention adopts the following technical solution:

[0006] A method for correcting low-frequency and large-span electromagnetic wave CT data, the improvement of which is that it comprises the following steps:

[0007] Step 1: Calculate the initial field strength E of the transmitter 0 , ground surface absorption coefficient α d and the absorption coefficient α in the borehole h :

[0008] The transmitting antenna is placed at the opening of the borehole ZK1, and the receiving antenna is placed at the openings of the boreholes ZK2 and ZK3 adjacent to the borehole ZK1. The low-frequency electromagnetic waves emitted by the transmitting antenna are received by the receiving antenna.

[0009] The field strength received by the receiving antenna at the borehole ZK2 is E zk1zk2 ,

[0010]

[0011] Where E zk1zk2 is the distance between the openings of boreholes ZK1 and ZK2;

[0012] The field strength received by the receiving antenna at the borehole ZK3 is E zk1zk3 ,

[0013]

[0014] Where R zk1zk3 is the distance between the openings of boreholes ZK1 and ZK3;

[0015] Combine equations (1) and (2) to obtain the initial field strength E 0 and the ground surface absorption coefficient α d ;

[0016] Place the transmitting antenna in the borehole ZK1 at a distance R from the hole mouth. h The receiving antenna is placed at the hole mouth, and the low-frequency electromagnetic waves emitted by the transmitting antenna are received by the receiving antenna.

[0017] The field strength received by the receiving antenna at the borehole ZK1 is E h1 ,

[0018]

[0019] The known E 0 、E h1 and R h Substituting into formula (3), we can obtain the absorption coefficient α in the borehole h ;

[0020] Step 2, based on the following formula

[0021]

[0022] Where α is the absorption coefficient of the low-frequency electromagnetic wave propagation path, and R is the length of the low-frequency electromagnetic wave propagation path;

[0023] Use the E obtained in step 1 0 , α h and the distance R from the transmitting antenna to the hole in borehole ZK1 h1, the distance R from the receiving antenna to the hole in borehole ZK2 h2 , calculate the field strength E propagating along the borehole ZK1 h1 and the field strength E propagating along borehole ZK2 h2 ; Use E obtained in step 1 0 , α d The distance R between the openings of holes ZK1 and ZK2 g , calculate the field strength E propagating along the ground surface g , the total field strength E received by the receiving antenna in borehole ZK2 总 Subtract E h1 、E h2 and E g , that is, the corrected low-frequency electromagnetic wave propagates in a straight line in the medium with the shortest distance. d .

[0024] The beneficial effects of the present invention are:

[0025] The correction method disclosed in the present invention solves the problem of poor inversion effect of low-frequency cross-hole electromagnetic wave CT measurement data, realizes the application of low-frequency cross-hole electromagnetic wave CT technology in large hole spacing of 20-40m, improves detection efficiency, and reduces detection costs. This correction method has been tested many times in the construction of Beijing Subway Administrative Sub-Center Station, Zhengzhou Subway Dongdajie Station, and Wuhan Metro Line, and has achieved good results. The application of this correction method will have great significance for improving the efficiency of urban underground space development and controlling construction quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of the principle of cross-hole electromagnetic wave CT measurement;

[0027] Figure 2 It is a schematic diagram of the response principle of cross-hole electromagnetic wave CT to an abnormal body;

[0028] Figure 3 is the ground surface absorption coefficient α d Schematic diagram of the calculation principle;

[0029] Figure 4 It is a schematic diagram of the radio wave propagation of low-frequency and large-span electromagnetic wave CT. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail 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.

[0031] Example 1: This example discloses a method for correcting data of low-frequency and large-span electromagnetic wave CT detection. Based on the propagation characteristics of low-frequency electromagnetic waves in layered media, the surface absorption coefficient and the hole absorption coefficient are calculated respectively. The method is applicable to the correction of low-frequency detection data below 1 MHz, and includes the following steps:

[0032] Step 1: Calculate the initial field strength E of the transmitter 0 , ground surface absorption coefficient α d and the absorption coefficient α in the borehole h :

[0033] like Figure 3 As shown, the transmitting antenna is placed at the hole mouth of the borehole ZK1, and the receiving antenna is placed at the hole mouths of the boreholes ZK2 and ZK3 adjacent to the borehole ZK1. The low-frequency electromagnetic waves emitted by the transmitting antenna are received by the receiving antenna. At this time, it can be considered that the low-frequency electromagnetic waves are all propagated along the ground surface. The measurement data of the receiving antenna represents the attenuation of the ground surface wave, and the surface absorption coefficient remains unchanged within a certain distance.

[0034] The field strength received by the receiving antenna at the borehole ZK2 is E zk1zk2 ,

[0035]

[0036] Where R zk1zk2 is the distance between the openings of boreholes ZK1 and ZK2;

[0037] The field strength received by the receiving antenna at the borehole ZK3 is R zk1zk3 ,

[0038]

[0039] Where R zk1zk3 is the distance between the openings of boreholes ZK1 and ZK3;

[0040] Combine equations (1) and (2) to obtain the initial field strength E 0 and the ground surface absorption coefficient α d ;

[0041] Place the transmitting antenna in the borehole ZK1 at a distance R from the hole mouth. h The receiving antenna is placed at the hole mouth, and the low-frequency electromagnetic waves emitted by the transmitting antenna are received by the receiving antenna.

[0042] The field strength received by the receiving antenna at the borehole ZK1 is E h1 ,

[0043]

[0044] The known E 0 、E h1 and Rh Substituting into formula (3), we can obtain the absorption coefficient α in the borehole h ;

[0045] Step 2: The propagation path of the long-span electromagnetic wave CT based on the propagation characteristics of low-frequency electromagnetic waves in layered media is as follows: In addition to the shortest distance in the medium, a part of the energy is transmitted to the interface of the medium, propagates along the interface as a surface wave, and then is transmitted underground to the receiving antenna (such as Figure 4 Therefore, if this part of energy is not eliminated, the correctness of the final data inversion will be affected and the formation information cannot be truly reflected.

[0046] The field strength of the shortest distance propagation in the medium is recorded as E d , the field strength propagating along the borehole ZK1 is recorded as E h1 , the field strength propagating along borehole ZK2 is recorded as E h2 , the field strength propagating along the ground surface is recorded as E g , then the total received intensity E 总 for:

[0047] E 总 =E d +E h1 +E g +E h2

[0048] Based on the following formula

[0049]

[0050] Where α is the absorption coefficient of the low-frequency electromagnetic wave propagation path, and R is the length of the low-frequency electromagnetic wave propagation path;

[0051] Use the E obtained in step 1 0 , α h (The absorption coefficients of the adjacent boreholes ZK1 and ZK2 can be considered to be the same), the distance R from the transmitting antenna to the hole in the borehole ZK1 h1 , the distance R from the receiving antenna to the hole in borehole ZK2 h2 , calculate the field strength E propagating along the borehole ZK1 h1 and the field strength E propagating along borehole ZK2 h2 ; Use E obtained in step 1 0 , α d The distance R between the openings of holes ZK1 and ZK2 g , calculate the field strength E propagating along the ground surface g , the total field strength E received by the receiving antenna in borehole ZK2 总 Subtract E h1 、E h2 and Eg , that is, the corrected low-frequency electromagnetic wave propagates in a straight line in the medium with the shortest distance. d .

Claims

1. A low-frequency large-span electromagnetic wave CT data correction method, characterized in that, it includes the following steps: Step 1, obtain the initial emission field strength E 0 , the ground surface absorption coefficient α d and the absorption coefficient α in the borehole h : Place the transmitting antenna at the orifice of borehole ZK1, and place receiving antennas at the orifices of boreholes ZK2 and ZK3 adjacent to borehole ZK1. The low-frequency electromagnetic waves emitted by the transmitting antenna are received by the receiving antennas. The field strength received by the receiving antenna at the orifice of borehole ZK2 is E zk1zk2 , where R zk1zk2 is the distance between the orifices of boreholes ZK1 and ZK2; The field strength received by the receiving antenna at the orifice of borehole ZK3 is E zk1zk3 , where R zk1zk3 is the distance between the orifices of boreholes ZK1 and ZK3; Solve the simultaneous equations (1) and (2) to obtain the initial emission field strength E 0 and the ground surface absorption coefficient α d ; Place the transmitting antenna at a distance R from the hole opening in borehole ZK1 h and place the receiving antenna at the hole opening. The low-frequency electromagnetic wave emitted by the transmitting antenna is received by the receiving antenna. The field strength received by the receiving antenna at the orifice of borehole ZK1 is E h1 , Substitute the known E 0 , E h1 and R h into Equation (3), and the absorption coefficient α h in the borehole can be obtained; Step 2, based on the following formula where α is the absorption coefficient of the propagation path of the low-frequency electromagnetic wave, and R is the length of the propagation path of the low-frequency electromagnetic wave; The E obtained in Step 1 0 , α h and the distance R from the transmitting antenna in borehole ZK1 to the hole mouth h1 , the distance R from the receiving antenna in borehole ZK2 to the hole mouth h2 , calculate the field strength E propagating along borehole ZK1 h1 and the field strength E propagating along borehole ZK2 h2 ; use the E obtained in Step 1 0 , α d and the distance R between the hole mouths of boreholes ZK1 and ZK2 g , calculate the field strength E propagating along the ground surface g , subtract E 总 , E h1 , E h2 and E g from the total field strength E received by the receiving antenna in borehole ZK2, and the field strength E of the low-frequency electromagnetic wave propagating in a straight line at the shortest distance in the medium after correction can be obtained d .

Images