A method for measuring transient electromagnetic field in wells using a galvanic dipole source

Abstract

The present invention relates to a method for measuring transient electromagnetic (TEM) measurements of ground-to-borehole data using an electric dipole source. The method comprises: sequentially arranging multiple transmitting points at different depths in a borehole; placing two transmitting electrodes of the transmitting electric dipole source at the first transmitting point in the borehole; connecting a transmitter to a probe in the borehole, and feeding a current of a predetermined waveform and magnitude from the borehole wall to the earth through the transmitting electrode of the transmitting dipole source; arranging receiving points on the surface, measuring the tangential magnetic field component of the line connecting the borehole and the receiving points, and obtaining the transient electromagnetic attenuation response of the earth; varying the depth of the probe in the borehole to obtain the transient electromagnetic attenuation response of the transmitting source at different depths until all designed transmitting points have been measured; and calculating the apparent resistivity of the earth based on the maximum moments of the transient electromagnetic attenuation responses of the different transmitting points based on electric dipole transient electromagnetic theory. The apparent resistivity of the earth is obtained by differentiation, and this is used as an initial model for conducting earth resistivity inversion. The present invention enables rapid and high-resolution measurement of earth resistivity.

Description

Technical Field

[0001] The invention belongs to the technical field of geophysical exploration, and in particular relates to a method for measuring transient electromagnetic field with a galvanic dipole source. Background Art

[0002] Transient electromagnetic (TEM) is a time-domain electromagnetic exploration method. Its basic principle is to establish a steady magnetic field in the detection area using a grounded or ungrounded source. The source is then momentarily shut off. According to the law of electromagnetic induction, a time-varying secondary field is induced in the underground medium. By observing and recording the induced transient electromagnetic response, the electrical distribution of the underground medium can be further inferred. Traditional TEM exploration typically uses a ground-based TEM observation system, where both the transmitter and receiver are deployed on the surface. Because TEM is an induction-based exploration method, its vertical resolution is low, making it difficult to detect significant anomalies at depth. To improve the method's vertical resolution, drilling is often combined with extending the observation device deeper into the ground for exploration, enabling high-resolution detection of deep or near-well ore bodies.

[0003] In the prior art, there are two main methods for transient electromagnetic (TEM) measurements using boreholes. The first involves deploying a transmitter on the surface and inserting a receiver through a borehole deep underground for measurement. This is known as the ground-to-hole transient electromagnetic (TEM) method. A disadvantage of this method is that the transmitter position remains fixed during a single measurement, and the excited state of the underground medium remains constant. If the ore body and the transmitter are poorly coupled, the detection effect will inevitably be poor, or even result in missed detections due to blind spots. Therefore, multiple large transmitters are often required to ensure good coupling between the underground geological body and the transmitters, which generally requires extensive field work and high costs. The second method integrates both the transmitter and receiver into a downhole probe, which is then simultaneously lowered into the ground through a borehole for measurement. This is known as the "in-hole transient electromagnetic (TEM) method." A disadvantage of this method is that, due to the extremely limited space within the downhole probe, integrating both the transmitter and receiver compresses each other's space, making it difficult to maintain the transmission magnetic moment and receiving area of ​​the observation device due to space constraints. In addition, due to space limitations, the distance between the transmitting and receiving devices is often close, and the mutual inductance effect is strong, which will cause strong distortion of the normal transient electromagnetic field. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for measuring transient electromagnetic field in the well using an electric dipole source. Under the premise of achieving good coupling between the transmitting source and the ore body, the deployment of multiple large transmitting sources is avoided, the workload and cost of field work are reduced, and sufficient transmitting power and receiving area are ensured. The distortion of the transient electromagnetic field caused by the mutual inductance effect is eliminated, thereby realizing high-resolution electrical detection of deep or well-near ore bodies.

[0005] The present invention provides a method for measuring transient electromagnetic field in a well using a galvanic dipole source, comprising the following steps:

[0006] Step 1: According to the work design and accuracy requirements, multiple launch points are arranged in sequence at different depths in the borehole;

[0007] Step 2: Place the two transmitting electrodes of the transmitting galvanic source at the first transmitting point in the borehole through the well probe, and make the transmitting electrodes close to the well wall to ensure that the electrodes and the well wall are in a state of mutual communication;

[0008] Step 3: Connect the transmitter to the probe in the well, turn on the transmitter, and feed a current of a set waveform and magnitude from the well wall to the ground through the transmitting electrode of the transmitting dipole source according to the working design;

[0009] Step 4: Deploy receiving points on the surface and measure the tangential magnetic field component of the line connecting the borehole and the receiving point during the transmission off period to obtain the transient electromagnetic attenuation response of the earth;

[0010] Step 5: Change the depth of the probe in the well and repeat steps 3 to 4 to obtain the transient electromagnetic attenuation response corresponding to the emission source at different depths until all the designed emission points are measured;

[0011] Step 6: Based on the electric dipole transient electromagnetic theory, calculate the apparent resistivity of the earth according to the maximum moment of the transient electromagnetic attenuation response at different emission points, obtain the earth's apparent resistivity by differentiation, and use this as the initial model to carry out earth resistivity inversion.

[0012] Furthermore, the step 6 includes:

[0013] The emitter in the well is regarded as a vertical electric dipole, and the apparent resistivity of the earth is calculated according to the maximum moment of transient electromagnetic attenuation response at different emission points. For the i-th emission point T i , the maximum moment of the corresponding response is recorded as t i , the depth is recorded as Z i , then the apparent resistivity of the earth is Calculated by the following formula:

[0014]

[0015] Where μ represents the earth's magnetic permeability, r is the horizontal distance from the receiving point to the borehole, and b is an empirical coefficient. The earth's apparent resistivity is obtained by differentiation: The calculation formula is:

[0016]

[0017] Earth's apparent resistivity As an initial model, the earth resistivity inversion is carried out.

[0018] The above scheme, using the dipole-source well-to-ground transient electromagnetic measurement method, enables rapid, high-resolution measurements of earth resistivity by moving the transmitting dipole source within the well. Furthermore, the measurement method of transmitting in the well and receiving on the surface effectively avoids the deployment of large transmitting devices and eliminates mutual inductance distortion. It offers advantages such as simple construction, low labor costs, high measurement reliability, and fast processing and interpretation.

[0019] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the layout of launch points in a well in one embodiment of the present invention;

[0021] Figure 2 This is a schematic diagram of a well probe in one embodiment of the present invention;

[0022] Figure 3 A schematic diagram of transmitting a square wave in one embodiment of the present invention;

[0023] Figure 4 Schematic diagram of measuring the tangential component of the magnetic field in one embodiment of the present invention. DETAILED DESCRIPTION

[0024] The following embodiments of the present invention are described in further detail with reference to the accompanying drawings and examples. The following examples are used to illustrate the present invention but are not intended to limit the scope of the present invention.

[0025] This embodiment provides a method for measuring transient electromagnetic field with a galvanic dipole source. The basic steps include:

[0026] S1. According to the work design and accuracy requirements, multiple launch points are arranged in sequence at different depths in the borehole;

[0027] S2. Place the two transmitting electrodes of the transmitting galvanic source at the first transmitting point in the borehole through the well probe, and make the transmitting electrodes close to the well wall to ensure that the electrodes and the well wall are in a state of mutual communication;

[0028] S3. Connect the transmitter to the probe in the well, turn on the transmitter, and feed a current with a specific waveform and a certain magnitude from the well wall to the ground through the transmitting electrode of the transmitting dipole source according to the working design;

[0029] S4. Deploy receiving points on the surface and measure the tangential magnetic field component of the line connecting the borehole and the receiving point during the off period of transmission to obtain the transient electromagnetic attenuation response of the earth;

[0030] S5. Change the depth of the probe in the well and repeat S3-S4 to obtain the transient electromagnetic attenuation response corresponding to the emission source at different depths until all the designed emission points are measured;

[0031] S6. Based on the electric dipole transient electromagnetic theory, the apparent resistivity of the earth is calculated according to the maximum moment of the transient electromagnetic attenuation response at different emission points. The earth's apparent resistivity is obtained by differentiation and used as the initial model to carry out earth resistivity inversion.

[0032] The present invention is further described in detail below with reference to a specific embodiment.

[0033] S1. For the vertical borehole ZK001 with a total depth of 1000m, 101 emission points are arranged in the borehole from shallow to deep at a spacing of 10m, namely T1, T2, ..., T101 (see Appendix Figure 1 );

[0034] S2, through the well probe S1, place the two transmitting electrodes A and B of the transmitting galvanic source at the first transmitting point T1 in the borehole, and make the transmitting electrodes close to the well wall to ensure that the electrodes and the well wall are in a state of mutual communication, which is used to supply power to the earth (see Appendix Figure 2 );

[0035] S3, connect the transmitter Tx to the well probe S1, turn on the transmitter Tx, and feed a square wave with a duty cycle of 1:1 from the well wall to the ground through the transmitting electrodes A and B of the transmitting dipole source. The current intensity is I (see Appendix Figure 3 );

[0036] S4. Place a receiving point R1 on the surface at a distance r from the borehole. During the off period of transmission, measure the tangential (φ direction) magnetic field component H of the horizontal line connecting the borehole and the receiving point. φ , obtain the transient electromagnetic attenuation response of the earth (Appendix Figure 4 );

[0037] S5. Move the probe in the well to the next emission point and repeat S3-S4 to obtain the transient electromagnetic attenuation response corresponding to the emission source at different depths until all the designed emission points T1, T2, ..., T101 are measured;

[0038] S6. Consider the emission source in the well as a vertical electric dipole, and calculate the apparent resistivity of the earth according to the maximum moment of transient electromagnetic attenuation response at different emission points. For the i-th emission point T i , the maximum moment of the corresponding response is recorded as t i , the depth is recorded as Z i , then the apparent resistivity of the earth is It can be calculated by the following formula:

[0039]

[0040] Where μ represents the earth's magnetic permeability, r is the horizontal distance from the receiving point to the borehole, and b is an empirical coefficient. The calculation formula is:

[0041]

[0042] Using this as the initial model, the earth resistivity inversion is carried out.

[0043] This galvanic dipole source well-to-ground transient electromagnetic measurement method uses galvanic dipole source emission. The transient electromagnetic emission source is placed in the borehole through a downhole probe, and the emission is carried out underground. The tangential component of the magnetic field is observed on the surface, and the apparent resistivity is defined and inverted based on the maximum moment of the transient response. The specific technical effects include the following:

[0044] 1) The present invention places the emission source in the borehole, and ensures coupling with the ore body around the well by moving the emission source in the well. There is no need to arrange a large emission frame on the ground, which reduces the cost of field work.

[0045] 2) The present invention adopts a working device with separate transmission and reception. The transmitting source is placed in the borehole through a downhole probe, and the receiving point is arranged on the ground. There is a large distance between the two, which can greatly weaken the mutual induction effect of the transient electromagnetic observation device and effectively solve the response distortion problem caused by the mutual induction effect of the in-hole transient electromagnetic method.

[0046] 3) The present invention adopts the form of a device that transmits in the well and receives on the ground, which ensures the layout space of the transmitting and receiving devices, easily realizes high-power transmission and high-sensitivity reception, and the transmitting device is close to the underground ore body, which can fully stimulate the response of the ore body, which is conducive to realizing large-depth and high-resolution detection.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for measuring transient electromagnetic field in a well using a galvanic dipole source, characterized in that: The steps include: Step 1: According to the work design and accuracy requirements, multiple launch points are arranged in sequence at different depths in the borehole; Step 2: Place the two transmitting electrodes of the transmitting galvanic source at the first transmitting point in the borehole through the well probe, and make the transmitting electrodes close to the well wall to ensure that the electrodes and the well wall are in a state of mutual communication; Step 3: Connect the transmitter to the probe in the well, turn on the transmitter, and feed a current of a set waveform and magnitude from the well wall to the ground through the transmitting electrode of the transmitting dipole source according to the working design; Step 4: Deploy receiving points on the surface and measure the tangential magnetic field component of the line connecting the borehole and the receiving point during the transmission off period to obtain the transient electromagnetic attenuation response of the earth; Step 5: Change the depth of the probe in the well and repeat steps 3 to 4 to obtain the transient electromagnetic attenuation response corresponding to the emission source at different depths until all the designed emission points are measured; Step 6: Based on the electric dipole transient electromagnetic theory, calculate the apparent resistivity of the earth according to the maximum moment of the transient electromagnetic attenuation response at different emission points. Obtain the earth's apparent resistivity by differentiation, and use this as the initial model to carry out earth resistivity inversion, including: The emitter in the well is regarded as a vertical electric dipole, and the apparent resistivity of the earth is calculated according to the maximum moment of transient electromagnetic attenuation response at different emission points. For the i-th emission point T i , the maximum moment of the corresponding response is recorded as t i , the depth is recorded as Z i , then the apparent resistivity of the earth is Calculated by the following formula: Where μ represents the earth's magnetic permeability, r is the horizontal distance from the receiving point to the borehole, and b is an empirical coefficient. The earth's apparent resistivity is obtained by differentiation: The calculation formula is: Earth's apparent resistivity As an initial model, the earth resistivity inversion is carried out.

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