In-hole gradient tensor transient electromagnetic detection method

Abstract

The invention belongs to the technical field of total-space transient electromagnetic detection in geophysical methods, and particularly relates to an in-hole gradient tensor transient electromagnetic detection method, which comprises the following steps of: simultaneously transmitting a transient electromagnetic field by using a group of pairwise orthogonal transmitting coils, and simultaneously receiving an induced electromagnetic field signal by using four groups of pairwise orthogonal receiving coils; gradient tensor calculation processing is carried out on the signals to obtain data of nine components of a direction gradient tensor. Gradient tensor data of a circle beside a hole can be received through continuous measurement of the gradient tensor transient electromagnetic probe in the direction along with the hole, and therefore stratum information is reflected. According to the measured direction gradient tensor information, the position of the anomalous body is judged, the boundary of the anomalous body is depicted, and the anisotropic characteristics of the stratum are judged. According to the method, transient electromagnetic signals are fully utilized from different angles, so that unnecessary signals are suppressed, required abnormal signals are highlighted, and boundaries and positions of abnormal bodies are depicted.

Description

Technical Field

[0001] The present invention belongs to the technical field of full-space transient electromagnetic detection in geophysical methods, and particularly relates to a borehole gradient tensor transient electromagnetic detection method. Background Technique

[0002] At present, in the full-space background, with the increase in exploration complexity and data complexity, traditional transient electromagnetic methods have deficiencies in suppressing interference, highlighting anomalies, and resolving anisotropic formations. Currently, conventional full-space transient electromagnetic is only limited to single-direction induced potential information, lacking sufficient data for analyzing the position of full-space anomalies and being unable to make a more reasonable judgment on anisotropic formations. In addition, most transient electromagnetic data processing methods mainly rely on conventional technical means such as filtering and denoising, such as Fourier transform and wavelet transform. Although these methods can suppress noise to a certain extent, their effects are often limited under complex geological conditions and it is difficult to effectively distinguish geological anomaly signals from noise signals. Conventional transient electromagnetic detection methods can only collect data of the induced electromagnetic field of one point and one component each time, unable to fully reflect the anisotropic characteristics of underground media.

[0003] In view of the above technical background, the present invention proposes a full-space borehole gradient tensor transient electromagnetic detection method that advances with the borehole. Summary of the Invention

[0004] The object of the present invention is to provide a borehole gradient tensor transient electromagnetic detection method, which uses a brand-new gradient tensor transient electromagnetic measurement system for full-space detection. The transmitting coil and receiving coil of this system are in multiple directions. The received electromagnetic signals are calculated and integrated by the proposed gradient tensor algorithm, and induced potential anisotropic gradient signals of 9 components can be obtained. These gradient tensor information can more clearly judge the orientation of the anomaly and depict the boundary of the anomaly. Moreover, continuous measurement is carried out while drilling, and a detection area in the form of a relatively large cylinder centered on the borehole can be obtained, greatly increasing the detection efficiency and obtaining a series of directional gradient data. The aim is to make full use of transient electromagnetic signals from different angles, thereby suppressing unwanted signals and highlighting the required anomaly signals, as well as depicting the boundary and position of the anomaly, etc.

[0005] The technical solution adopted by the present invention is specifically as follows:

[0006] A borehole gradient tensor transient electromagnetic detection method includes the following steps:

[0007] Step 1: Set a gradient tensor transient electromagnetic multi-axis transmitting and receiving probe. The multi-axis transmitting and receiving probe includes a group of transmitting coils Tx and four groups of receiving coils [Rx1, Rx2, Rx3, Rx4]. Each group of coils consists of two mutually orthogonal micro-coils;

[0008] Step 2: Each of the four receiving coils can receive the signals of the time derivative of the magnetic flux density at its respective position, which are respectively denoted as:

[0009] Step 3: Represent the gradient tensor G of the transient electromagnetic signal received by the gradient tensor transient electromagnetic multi-axis transmitting and receiving probe as:

[0010]

[0011] The gradient tensor represents the gradients of the three components of the time derivative of the magnetic flux density received at the actual measurement point in the x, y, and z directions respectively under the excitation of a primary tensor source, and a total of 9 components of different gradient information are obtained; where:

[0012]

[0013] Step 3: Through continuous measurement of the gradient tensor transient electromagnetic multi-axis transmitting and receiving probe in the direction along the hole, the gradient tensor data around the hole are received, thereby reflecting the formation information;

[0014] Step 4: Based on the measured directional gradient tensor information, determine the position of the abnormal body, depict the boundary of the abnormal body, and judge the anisotropic characteristics of the formation.

[0015] Preferably, the first receiving coil Rx1 is located at the actual measurement point position, and the second receiving coil Rx2, the third receiving coil Rx3, and the fourth receiving coil Rx4 are respectively located in the positive x direction, positive y direction, and positive z direction of the first receiving coil Rx1, and the distances between the second receiving coil Rx2, the third receiving coil Rx3, and the fourth receiving coil Rx4 and the actual measurement point position are all d.

[0016] Preferably, the distance between the transmitting coil and the receiving coil is D, and the distance D can be appropriately adjusted.

[0017] Preferably, the transmitting coil Tx is composed of three mutually orthogonal coils to synchronously transmit transient electromagnetic pulses, and the four receiving coils [Rx1, Rx2, Rx3, Rx4] are composed of three mutually orthogonal coils, and the gradient components are collected in real time through spatial difference.

[0018] Preferably, a system verification is carried out on the established underground abnormal body model. The gradient tensor transient electromagnetic multi-axis transmitting and receiving probe combination is measured from top to bottom in the hole, and the process of continuously exciting and receiving the probe from top to bottom in the hole is equivalent to exciting and receiving at multiple measurement points at different depths.

[0019] Preferably, in the underground anomaly model, a low-resistivity anomaly body with a size of 80m × 80m × 80m is set under the homogeneous medium model; the background resistivity is 100Ω·m, and the resistivity of the anomaly body is 1Ω·m. One measurement point is taken every 10m from -15m to -395m in the borehole, and a total of 39 measurement points are taken.

[0020] The technical effects achieved by the present invention are as follows:

[0021] In the present invention, multi-component data acquisition is adopted to enhance the information dimension. The present invention is a full-component gradient tensor measurement: four groups of orthogonal receiving coils are used to synchronously collect electromagnetic signals, and combined with the gradient tensor algorithm for calculation and integration to obtain the induced potential gradient data of 9 components. Compared with the traditional single-component detection, the amount of information is increased by an order of magnitude, and the anisotropic characteristics of the underground medium can be comprehensively characterized.

[0022] The spatial orientation, geometric boundary and extension trend of the anomaly body can be accurately judged through the directional information of the gradient tensor, significantly improving the reliability of geological interpretation.

[0023] In the present invention, full-space efficient detection breaks through the traditional limitations: the probe continuously excites and receives during the drilling process, forming a cylindrical detection area centered on the borehole, with a radius of dozens to hundreds of meters, realizing the full-space efficient coverage of "drilling while detecting", greatly improving the exploration efficiency; due to the self-canceling effect of the gradient operation on the uniform background field, the gradient tensor signal has a natural suppression ability for near-field interference and is applicable to the full-space complex environment under strong noise background.

[0024] In the present invention, high resolution and anti-interference ability: the gradient tensor algorithm can effectively suppress the background field and common-mode noise through spatial difference operation, such as effectively suppressing the interference of the borehole metal casing, highlighting the local anomaly signal, and significantly improving the signal-to-noise ratio. By analyzing the difference and symmetry characteristics of each component of the gradient tensor, the anisotropic and isotropic media of the formation can be distinguished, such as the fracture development direction and the occurrence of rock strata, providing a quantitative basis for geological structure analysis.

[0025] In the present invention, coil array design: the transmitting end adopts a three-axis coil group with two-by-two orthogonality, and the receiving end adopts a redundant configuration of four groups of orthogonal coils. Combined with the synchronous transmit-receive timing control technology, it ensures the high-precision synchronous acquisition of multi-component data.

[0026] Generally speaking, through the multi-component acquisition and full-space dynamic detection of the gradient tensor transient electromagnetic technology, the present invention can effectively solve the problems of low information dimension, weak anti-interference ability and insufficient anisotropy resolution of the traditional transient electromagnetic method, realizing high-precision, high-efficiency and strong adaptability full-space geological detection, and providing a breakthrough technical means for resource exploration and engineering safety under complex geological conditions. Description of the Drawings

[0027] Figure 1 Schematic diagram of the gradient tensor transient electromagnetic multi-axis transmitting and receiving probe of the present invention;

[0028] Figure 2 Schematic diagram of the underground anomaly model of the present invention;

[0029] Figure 3 Gradient response curve of the present invention;

[0030] Figure 4 Profile curves of the xx gradient component and yy gradient component of the present invention. Detailed implementation manners

[0031] In order to make the objectives and advantages of the present invention clearer and more understandable, the present invention will be specifically described below in conjunction with embodiments. It should be understood that the following text is only used to describe one or several specific implementation manners of the present invention, and does not strictly limit the specific scope of protection claimed by the present invention.

[0032] As Figures 1-4 shown, the objective of the present invention is to propose a borehole gradient tensor transient electromagnetic detection method, which can highlight the orientation and boundary of the anomaly, the nature of the anomaly, and can reflect the anisotropic characteristics of the formation. The probe continuously excites and receives during the process of forward detection while drilling, and a series of geological gradient tensor information can be obtained, and the medium information of the area around the borehole can be obtained, greatly improving the detection efficiency. Moreover, the gradient information can fully highlight the anomaly, improve the efficiency of subsequent data processing and interpretation, and adapt to the detection in complex full-space environments. Specifically, it includes the following steps:

[0033] Step 1: As Figure 1 shown, set the gradient tensor transient electromagnetic multi-axis transmitting and receiving probe. The multi-axis transmitting and receiving probe includes a set of transmitting coils Tx and four sets of receiving coils [Rx1, Rx2, Rx3, Rx4], and each set of coils is composed of two micro-coils orthogonal to each other;

[0034] Step 2: Each of the four receiving coils can receive the signal of the time derivative of the magnetic flux density at its respective position, which are respectively denoted as:

[0035] Step 3: Represent the gradient tensor G of the transient electromagnetic signal received by the gradient tensor transient electromagnetic multi-axis transmitting and receiving probe as:

[0036]

[0037] The gradient tensor represents the gradients of the three components of the time derivative of the magnetic flux density received at the actual measurement point in the x, y, and z directions respectively under the excitation of a single tensor source, and a total of 9 components of different gradient information are obtained; where:

[0038]

[0039] This method realizes the localization of abnormal bodies by analyzing the response characteristics of the spatial gradient tensor components of transient electromagnetic fields. The gradient tensor information can highlight local abnormal signals. By analyzing the obtained gradient tensor, various properties of underground media and abnormal bodies can be obtained;

[0040] Step 3: Through continuous measurement of the gradient tensor transient electromagnetic multi-axis transmitting and receiving probe in the direction along the hole, the gradient tensor data around the hole are received, thereby reflecting the formation information;

[0041] Step 4: Based on the measured directional gradient tensor information, the position of the abnormal body is judged, the boundary of the abnormal body is characterized, and the anisotropic characteristics of the formation are judged.

[0042] Preferably, as Figure 1 shown, the first receiving coil Rx1 is located at the actual measurement point position, the second receiving coil Rx2, the third receiving coil Rx3, and the fourth receiving coil Rx4 are respectively located in the positive x direction, positive y direction, and positive z direction of the first receiving coil Rx1, and the distances between the second receiving coil Rx2, the third receiving coil Rx3, and the fourth receiving coil Rx4 and the actual measurement point position are all d; the distance between the transmitting coil and the receiving coil is D, and the distance D can be adjusted appropriately; the transmitting coil Tx is composed of three mutually orthogonal coils to synchronously transmit transient electromagnetic pulses, and the four groups of receiving coils [Rx1, Rx2, Rx3, Rx4] are composed of three mutually orthogonal coils, and the gradient components are collected in real time through spatial difference.

[0043] In actual use, the gradient tensor transient electromagnetic multi-axis transmitting and receiving probe moves continuously with the advancement of the borehole, and combines with the synchronous triggering technology to realize dynamic excitation and reception. After the acquisition is completed, through the analysis of the amplitude, phase and directional characteristics of each component, the three-dimensional conductivity distribution around the hole is reconstructed, and the azimuth, boundary and anisotropic parameters of the abnormal body are accurately identified. The system adopts an anti-interference shielding design, adapts to the borehole environment, and realizes efficient detection in the whole space.

[0044] Preferably, as Figure 2 shown, the established underground abnormal body model is used for system verification. The gradient tensor transient electromagnetic multi-axis transmitting and receiving probe combination measures in the hole from top to bottom. The process of continuously exciting and receiving the probe from top to bottom in the hole is equivalent to exciting and receiving at multiple measurement points at different depths;

[0045] In the underground abnormal body model, a low-resistance abnormal body with a size of 80m×80m×80m is set under the homogeneous medium model; the background resistivity is 100Ω·m, the resistivity of the abnormal body is 1Ω·m, and a measurement point is taken every 10m within the range of -15m to -395m from top to bottom in the hole, and a total of 39 measurement points are taken.

[0046] Figure 3 shows the comparison curves of the gradient tensor components between the homogeneous full-space model and the model containing a low-resistivity anomaly when the measuring point is at -45 m underground. The experimental data show that, as Figure 3 shown in a, under the condition of a homogeneous isotropic medium, all gradient components of the time derivative of the magnetic flux density show a highly consistent exponential decay law. Among them, the zz component has the largest initial amplitude due to the influence of the main magnetic field direction, and the other components show a convergent characteristic during the decay stage. When there is a low-resistivity anomaly in the y-axis direction of the borehole, as Figure 3 shown in b, the response curves of all gradient components are significantly distorted. Among them, the zy component has the largest relative change rate, and the zx component has the smallest change amplitude. This anisotropic characteristic indicates that the gradient tensor invariant has the ability to characterize the anomaly orientation, and the specific orientation discrimination criterion needs to be systematically verified by constructing a multi-parameter model.

[0047] To further verify the ability of the method to identify the boundary of the anomaly, Figure 4 shows the xx and yy gradient component profile curves along the borehole axis (-15 m to -395 m). The experimental data show that in the depth range of 130 - 210 m, corresponding to the upper and lower interfaces of the anomaly, both components show abnormal responses: both the xx component and the yy component show abnormal amplitudes, and the change amplitude of the yy component is larger than that of the xx component. Combining the consistency of the attenuation characteristics of the two components in the homogeneous medium, it can be inferred that the gradient change in the y direction is mainly affected by the electrical boundary of the anomaly, and this characteristic provides a tensor invariant criterion for the three-dimensional anomaly spatial positioning. It should be noted that there is a complex non-linear relationship between the anisotropy degree of the gradient component response and the geometric parameters and electrical differences of the anomaly, and a quantitative interpretation model needs to be established through tensor decomposition and multi-dimensional parameter inversion.

[0048] In the present invention, multi-component data acquisition improves the information dimension. The present invention is a full-component gradient tensor measurement: four groups of orthogonal receiving coils are used to synchronously collect electromagnetic signals, and combined with the gradient tensor algorithm for calculation and integration to obtain the induced potential gradient data of 9 components. Compared with the traditional single-component detection, the amount of information is increased by an order of magnitude, and the anisotropic characteristics of the underground medium can be comprehensively characterized.

[0049] The spatial orientation, geometric boundary and extension trend of the anomaly can be accurately judged through the directional information of the gradient tensor, significantly improving the reliability of geological interpretation.

[0050] In the present invention, full-space efficient detection breaks through the traditional limitations: the probe continuously excites and receives during the borehole process, forming a cylindrical detection area centered on the borehole. The radius can reach dozens to hundreds of meters, realizing the full-space efficient coverage of "drilling while exploring", and greatly improving the exploration efficiency.

[0051] Due to the self-canceling effect of the gradient operation on the uniform background field, the gradient tensor signal has a natural suppression ability for near-field interference and is applicable to the full-space complex environment under a strong noise background.

[0052] In the present invention, for high resolution and anti-interference ability: Through spatial difference operation, the gradient tensor algorithm can effectively suppress the background field and common-mode noise. For example, it can effectively suppress the interference of the metal casing in the borehole, highlight the local abnormal signal, and significantly improve the signal-to-noise ratio. By analyzing the difference and symmetry characteristics of each component of the gradient tensor, the anisotropic and isotropic media of the formation can be distinguished, such as the fracture development direction and the occurrence of rock strata, providing a quantitative basis for geological structure analysis.

[0053] In the present invention, for the coil array design: The transmitting end adopts a three-axis coil group with two-by-two orthogonality, and the receiving end adopts a redundant configuration of four groups of orthogonal coils. Combining with the synchronous transmit-receive timing control technology, it ensures the high-precision synchronous acquisition of multi-component data.

[0054] Generally speaking, through the multi-component acquisition and full-space dynamic detection of the gradient tensor transient electromagnetic technology, the present invention can effectively solve the problems of low information dimension, weak anti-interference ability, and insufficient anisotropy resolution in the traditional transient electromagnetic method, realizing high-precision, high-efficiency, and strong-adaptability full-space geological exploration, and providing a breakthrough technical means for resource exploration and engineering safety under complex geological conditions.

[0055] The above 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. The structures, devices, and operation methods not specifically described and explained in the present invention are implemented according to the conventional means in the art without special description and limitation.

Claims

1. A gradient tensor transient electromagnetic detection method in a borehole, characterized in that: Including the following steps: Step 1: Set up a gradient tensor transient electromagnetic multi-axis transmitting and receiving probe. The multi-axis transmitting and receiving probe includes a set of transmitting coils Tx and four sets of receiving coils [Rx1, Rx2, Rx3, Rx4]. Each set of coils is composed of miniature coils orthogonal to each other in pairs; Step 2: Each of the four receiving coils can receive the signal of the time derivative of the magnetic flux density at its respective position, which are respectively denoted as: Step 3: Represent the gradient tensor G of the transient electromagnetic signal received by the gradient tensor transient electromagnetic multi-axis transmitting and receiving probe as: The gradient tensor represents the gradients of the three components of the time derivative of the magnetic flux density received at the actual measurement point in the x, y, and z directions respectively under the excitation of a single tensor source, and a total of 9 components of different gradient information are obtained; where: Step 3: Through continuous measurement of the gradient tensor transient electromagnetic multi-axis transmitting and receiving probe in the direction along the hole, receive the gradient tensor data around the hole for one week, thereby reflecting the formation information; Step 4: Based on the measured directional gradient tensor information, determine the position of the abnormal body, depict the boundary of the abnormal body, and determine the anisotropic characteristics of the formation.

2. The method for detecting gradient tensor transient electromagnetic in boreholes according to claim 1, wherein: The first receiving coil Rx1 is located at the actual measurement point position. The second receiving coil Rx2, the third receiving coil Rx3, and the fourth receiving coil Rx4 are respectively located in the positive x direction, positive y direction, and positive z direction of the first receiving coil Rx1, and the distances between the second receiving coil Rx2, the third receiving coil Rx3, and the fourth receiving coil Rx4 and the actual measurement point position are all d.

3. A method for gradient tensor transient electromagnetic detection in a borehole according to claim 1, characterized in that: The distance between the transmitting coil and the receiving coil is D, and the distance D can be adjusted appropriately.

4. A method for gradient tensor transient electromagnetic detection in a borehole according to claim 1, characterized in that: The transmitting coil Tx is composed of a three-axis coil orthogonal to each other in pairs to synchronously transmit transient electromagnetic pulses, and the four sets of receiving coils [Rx1, Rx2, Rx3, Rx4] are composed of a three-axis coil orthogonal to each other in pairs, and the gradient components are collected in real time through spatial difference.

5. A method for gradient tensor transient electromagnetic detection in boreholes according to claim 1, characterized in that: Carry out systematic verification on the established underground abnormal body model. The gradient tensor transient electromagnetic multi-axis transmitting and receiving probe combination is measured from top to bottom in the hole. The process of continuously exciting and receiving the probe from top to bottom in the hole is equivalent to exciting and receiving at multiple measurement points at different depths.

6. The method for gradient tensor transient electromagnetic detection in boreholes according to claim 5, characterized in that: In the underground abnormal body model, a low-resistance abnormal body with a size of 80m×80m×80m is set under a homogeneous medium model; the background resistivity is 100Ω·m, the resistivity of the abnormal body is 1Ω·m, and a measurement point is taken every 10m from -15m to -395m from top to bottom in the hole, and a total of 39 measurement points are taken.

Images