Variable gain electromagnetic wave resistivity detection method and system while drilling

Abstract

The present application belongs to the technical field of geophysical well logging, and particularly relates to a variable gain while drilling electromagnetic wave resistivity detection method and system. The method comprises: obtaining an induced current signal, extracting a real part and an imaginary part of a magnetic field strength vector, and calculating an antenna pair phase difference and a received signal amplitude; evaluating a basic gain prediction value based on a time variation rate of the antenna pair phase difference and a received signal amplitude fluctuation; obtaining a gain correction coefficient by combining the antenna pair phase difference, the received signal amplitude evaluation, and an equivalent formation impedance matching residual error; obtaining a variable gain control voltage based on the basic gain prediction value, the gain correction coefficient, and a circuit environment temperature deviation cumulative term, and adjusting a received gain multiple of a receiving circuit according to the variable gain control voltage. The present application solves the technical problem that a traditional gain control is prone to saturation at a sudden change interface of a formation, ensures that a signal is always in an optimal linear working zone, and improves the accuracy and robustness of full range resistivity measurement.

Description

Technical Field

[0001] This invention relates to the field of geophysical logging technology. More specifically, this invention relates to a variable gain electromagnetic resistivity detection method and system for drilling. Background Technology

[0002] Electromagnetic resistivity detection while drilling is an important means of evaluating the oil and gas content of formations in real time during oil and gas drilling engineering. This technology mainly relies on a receiving antenna array set on the drill collar to sense the alternating magnetic field in space and to capture the induced current signal through the receiving circuit to calculate the distribution characteristics of formation conductivity. Due to the huge differences in the physical properties of the downhole formations, from highly conductive brine layers to high resistivity oil and gas layers, the energy attenuation of electromagnetic waves in different rock layers exhibits nonlinearity and varies across multiple orders of magnitude.

[0003] In existing resistivity detection systems, signal amplification links generally employ fixed gain amplification factors or simple feedback control logic based on signal amplitude. This approach has significant limitations when dealing with abrupt changes in formation parameters. When the drill bit enters a low-resistivity layer, the electromagnetic wave signal attenuates drastically, and the low fixed gain results in an excessively small voltage amplitude output by the receiving circuit, leading to insufficient quantization accuracy of the analog-to-digital converter and difficulty in extracting weak formation characteristic signals. Conversely, when the drill bit enters a high-resistivity layer, the received signal amplitude increases dramatically, and an excessively high fixed gain can easily cause saturation distortion in the receiving circuit, resulting in irreversible loss of crucial antenna pair phase difference and amplitude information, directly affecting the reliability of subsequent formation resistivity inversion results.

[0004] The drilling environment is extremely harsh. High-frequency vibrations of the drilling tools and drilling fluid pulses generate strong random interference, which is often mixed with the actual formation interface signals. Traditional linear feedback control methods usually only adjust the received signal amplitude based on lag, and cannot make advance predictions before the formation environment undergoes nonlinear transitions. In addition, the high temperature environment downhole can cause parameter drift in the electronic components of the receiving circuit, further aggravating the instability of gain control. Frequent and lagging gain jitter not only disrupts the phase continuity in the signal processing process and produces false formation parameter jumps, but also makes it difficult to meet the robust requirements for high-precision resistivity monitoring under complex working conditions. Summary of the Invention

[0005] To address the technical problems of delayed receiver gain adjustment, signal saturation distortion, and decreased monitoring accuracy caused by environmental noise interference in drilling electromagnetic resistivity detection when dealing with drastic changes in formation properties, the present invention provides solutions in the following aspects.

[0006] In a first aspect, the present invention provides a variable gain drilling electromagnetic resistivity detection method, comprising: acquiring an induced current signal and performing feature extraction; acquiring the antenna pair phase difference and the received signal amplitude; acquiring a basic gain prediction value based on the time change rate of the antenna pair phase difference and the received signal amplitude fluctuation; acquiring a gain correction coefficient based on the antenna pair phase difference, the received signal amplitude, and the second derivative of the received signal amplitude with respect to time; acquiring a variable gain control voltage based on the basic gain prediction value, the gain correction coefficient, the circuit ambient temperature, and the reference temperature; and adjusting the receiving gain multiple of the receiving circuit based on the variable gain control voltage.

[0007] This invention obtains the basic gain prediction value by comprehensively utilizing the time change rate of the antenna phase difference, and obtains the gain correction coefficient by combining the antenna phase difference and the received signal amplitude. Finally, the variable gain control voltage is obtained by the ambient temperature of the coupling circuit to adjust the receiving gain of the receiving circuit, thereby reducing the signal saturation phenomenon caused by abrupt changes in the formation interface and improving the linearity of signal processing during drilling exploration.

[0008] Preferably, the method for obtaining the basic gain prediction value is as follows: obtaining the time change rate of the antenna pair phase difference based on the antenna pair phase difference; obtaining the cumulative integral of the absolute difference of the received signal amplitude within the length of the integral sliding window; and obtaining the basic gain prediction value based on the time change rate of the antenna pair phase difference and the cumulative integral.

[0009] This invention obtains the basic gain prediction value based on the cumulative integration of the antenna's phase difference time change rate and the received signal amplitude. By utilizing the antenna's advanced sensing characteristic of phase difference on formation changes, it reduces the follow-up delay of gain adjustment and improves the ability to predict abrupt changes in formation interfaces.

[0010] Preferably, the predicted base gain value satisfies the expression: In the formula, for Predicted base gain at time step For sampling time index, for Phase difference between antenna pairs at any given time This is the proportional gain coefficient. Here is the stability constant. As a volatility-sensitive factor, for Receive signal amplitude at all times for Receive signal amplitude at all times The integral sliding window length, It is an exponential function with the natural constant as its base. For the time differential term, It is the integral variable.

[0011] This invention utilizes the proportional gain coefficient, stability constant, and exponential function based on the natural constant to obtain the predicted value of the basic gain. It combines the change trajectory of the phase difference of the antenna pair with the nonlinear fluctuation of the received signal amplitude, thereby reducing the impact of drastic fluctuations in signal strength on the detection circuit in complex geological environments.

[0012] Preferably, the fluctuation sensitivity factor is obtained by performing variational mode decomposition on the collected drill vibration sensor signal, extracting the residual component reflecting the high-frequency impact energy, and evaluating the variance value of the residual component as the fluctuation sensitivity factor.

[0013] This invention performs variational mode decomposition on the collected drill string vibration sensor signals to extract residual components, and evaluates the variance of the residual components as a fluctuation sensitivity factor. By extracting and suppressing mechanical vibration noise at the drilling site, it reduces the misleading effect of random interference on gain prediction results and improves the stability of control parameters.

[0014] Preferably, the step of acquiring the induced current signal and performing feature extraction to acquire the phase difference between the antenna pairs and the amplitude of the received signal includes: using a receiving antenna array to sense an alternating magnetic field in space; using a receiving circuit to perform amplitude limiting and bandpass filtering on the induced original signal to acquire the induced current signal; using a fast Fourier transform algorithm to project the induced current signal in the time domain to the frequency domain, and extracting the real part and imaginary part of the magnetic field strength vector corresponding to the fundamental frequency of the transmission; using orthogonal demodulation technology based on the real part and imaginary part of the magnetic field strength vector to calculate the amplitude ratio between the antenna pairs and the phase difference between the antenna pairs, and evaluating the amplitude ratio between the antenna pairs as the amplitude of the received signal.

[0015] This invention utilizes Fast Fourier Transform and orthogonal demodulation techniques to extract the real and imaginary parts of the magnetic field strength vector from the induced current signal, thereby obtaining the phase difference between the antenna pairs and the amplitude ratio as the amplitude of the received signal. This ensures the accuracy of the extraction of geological physical parameter features and improves the reliability of the basic data for subsequent gain adjustment.

[0016] Preferably, the gain correction coefficient satisfies the expression: In the formula, for Timing gain correction factor for Receive signal amplitude at all times for Phase difference between antenna pairs at any given time To correct the scaling factor, For impedance matching factor, For spatial gradient operators, It is the hyperbolic tangent function. For modulo operation, for The second derivative of the received signal amplitude with respect to time at any given moment.

[0017] Preferably, the impedance matching factor is obtained by using a downhole ultrasonic radial scanner to obtain the roughness index of the well wall, and evaluating the reciprocal of the roughness index of the well wall after nonlinear normalization as the impedance matching factor.

[0018] Preferably, the variable gain control voltage satisfies the expression: In the formula, for Variable gain control voltage For voltage adjustment step size, for Predicted base gain at time step for Timing gain correction factor for Constant circuit ambient temperature, For reference temperature, To find the set of independent variables that maximizes the function value.

[0019] This invention utilizes the target control gain and combines it with the cumulative deviation term between the circuit ambient temperature and the reference temperature. By taking the set of independent variables that maximizes the function value, the variable gain control voltage is obtained. The negative feedback compensation mechanism is used to offset the parameter drift of electronic components caused by temperature, thereby reducing the adverse effects of the high-temperature environment downhole on the stability of gain control.

[0020] Preferably, adjusting the receiving gain of the receiving circuit includes: the controller changing the conduction state of the feedback resistor array of the programmable gain amplifier based on the variable gain control voltage, thereby adjusting the receiving gain of the receiving circuit so that the sampled induced current signal is always in the linear operating region.

[0021] Secondly, the present invention provides a variable gain drilling electromagnetic resistivity detection system, including a processor and a memory, wherein the memory stores computer program instructions, and when the computer program instructions are executed by the processor, the above-mentioned variable gain drilling electromagnetic resistivity detection method is implemented.

[0022] By adopting the above technical solution, a computer program for the variable gain drilling electromagnetic resistivity detection method is generated and stored in a memory so that it can be loaded and executed by a processor. A terminal device can then be made based on the memory and processor for convenient use.

[0023] The beneficial effects of this invention are as follows: This invention combines advanced prediction based on antenna phase difference with real-time correction logic based on equivalent formation impedance matching residuals, reducing receiver circuit saturation distortion caused by abrupt changes in formation interface during drilling exploration and improving the system's switching response speed between formations with different impedances.

[0024] This invention reduces the interference of downhole high-frequency impacts and extreme high-temperature conditions on signal detection accuracy by assessing drill string vibration interference and compensating for circuit ambient temperature deviations in real time, thereby improving the robustness of variable gain control.

[0025] This invention utilizes multi-dimensional parameters to collaboratively obtain the variable gain control voltage, ensuring that the induced current signal is in the optimal linear operating range throughout the entire range, reducing phase discontinuities during signal extraction, and improving the measurement accuracy of electromagnetic resistivity detection while drilling. Attached Figure Description

[0026] Figure 1 The flowchart of a variable gain electromagnetic resistivity detection method for drilling is illustrated in this invention. Figure 2 This diagram illustrates the response of the base gain prediction value to formation variations. Figure 3 A schematic diagram illustrating the change in gain correction coefficient at abrupt formation interfaces; Figure 4 This diagram illustrates the voltage regulation of variable gain control under high temperature conditions. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0029] This invention discloses a variable gain electromagnetic resistivity detection method for drilling, referring to... Figure 1 This includes steps S1-S4: S1. Acquire the induced current signal and perform feature extraction to obtain the phase difference of the antenna pair and the amplitude of the received signal.

[0030] It should be noted that in order to evaluate the equivalent formation impedance and obtain the original physical parameters, this invention requires real-time capture of the induced electromotive force sequence obtained by the receiving antenna array; when the drilling electromagnetic wave propagates in formations with different resistivity, the attenuation of its received signal amplitude and the phase difference shift characteristics of the antenna pair directly reflect the conductivity of the rock formation. By extracting the frequency domain feature components, basic data can be provided for subsequent gain adjustment.

[0031] Specifically, the present invention acquires induced current signals; uses a receiving antenna array set on the drill collar to sense alternating magnetic fields in space, and uses a receiving circuit to perform basic amplitude limiting and bandpass filtering; and uses a standard fast Fourier transform algorithm to project the induced current signal in the time domain to the frequency domain, locks the fundamental frequency of transmission, and extracts the real part and the imaginary part of the magnetic field strength vector.

[0032] Furthermore, the present invention utilizes orthogonal demodulation technology to calculate the amplitude ratio and phase difference between antenna pairs; evaluates the amplitude ratio between antenna pairs as the received signal amplitude, and uses the phase difference between antenna pairs and the received signal amplitude as the basic inputs of variable gain control logic.

[0033] S2. Obtain the basic gain prediction value based on the time change rate of the phase difference of the antenna pair and the amplitude fluctuation of the received signal.

[0034] It should be noted that the time-varying rate of change of the phase difference of the antenna pair in the stratum is closely related to the changing trend of the stratum conductivity, and the trajectory of the phase difference change of the antenna pair usually exhibits a nonlinear turning characteristic before the attenuation of the received signal amplitude. If the gain is adjusted only based on the received signal amplitude at the current moment, it will cause the control system to have a follow-up delay on the time axis, and it will be unable to respond to the sudden changes in the stratum interface in time. To this end, the present invention introduces an advance prediction factor based on the time-varying rate of change of the phase difference of the antenna pair into the gain control loop to obtain the basic gain prediction value, so as to compensate for the drastic fluctuations in the received signal amplitude in advance.

[0035] Specifically, the present invention calculates the time change rate of the phase difference between the antenna pairs; and obtains the basic gain prediction value by combining the amplitude fluctuation of the received signal.

[0036] The predicted base gain satisfies the following expression:

[0037] In the formula, express Predicted base gain value at time step; Indicates the sampling time index; express Phase difference between antenna pairs at any given time; Indicates the proportional gain coefficient; Represents the stability constant; Indicates volatility sensitivity factor; express The amplitude of the received signal at any given time; express The amplitude of the received signal at any given time; Indicates the length of the integral sliding window; Represents an exponential function with the natural constant as its base; Represents the time differential term; This represents the integral variable.

[0038] In the formula, the predicted base gain value A larger value indicates a higher amplification factor required by the receiving channel as predicted by this invention, meaning the drill bit is about to enter a high-resistivity formation or the attenuation of the received signal amplitude is intensifying; the square of the time change rate of the antenna with respect to the phase difference is... An increase in the value indicates a drastic change in the formation environment, leading to a decrease in the predicted base gain. Synchronous elevation to enhance signal acquisition capability; fluctuations in received signal amplitude A larger value indicates that the integral sliding window length... The increase in the amplitude fluctuation of the received signal within the range amplifies the predicted value of the base gain through the exponential function. Nonlinear growth occurs.

[0039] It should be added that this invention obtains the fluctuation sensitivity factor. The method involves performing variational mode decomposition on the collected drill string vibration sensor signals, extracting the residual component reflecting the high-frequency impact energy, and evaluating the variance of this residual component as a fluctuation sensitivity factor. .

[0040] It should be further noted that the proportional gain coefficient in this invention... Set to 1.2; this value typically ranges from 0.8 to 1.5; if the proportional gain coefficient... If the proportional gain coefficient is set too low, the system's prediction strength for abrupt changes in the formation interface will be insufficient, causing weak signals to be masked by noise; if the proportional gain coefficient is too low... If the value is set too high, the gain prediction will be too sensitive and easily affected by drill string vibration, causing the receiving circuit to switch frequently. This invention uses 1.2 as an empirical value, which can maintain the smoothness of gain adjustment while ensuring the signal detection depth.

[0041] For example, Figure 2This is a schematic diagram illustrating the response of the base gain prediction value to changes in the formation. The diagram shows the correlation between the base gain prediction value and the phase difference of the antenna pair during drilling. When the drilling encounters a formation interface that causes a sharp deflection of the antenna pair phase difference, such as at 3 seconds and 7 seconds, the base gain prediction value calculated based on the phase change rate can respond and produce a significant increase. This indicates that the present invention can predict the gain adjustment requirement in advance before the signal amplitude undergoes substantial attenuation or saturation, ensuring the integrity of subsequent signal acquisition.

[0042] S3. Obtain the gain correction coefficient based on the phase difference between the antenna pairs, the amplitude of the received signal, and the second derivative of the amplitude of the received signal with respect to time.

[0043] It should be noted that although the basic gain prediction can adapt to most formation gradients, when dealing with thin oil and gas interfaces, electromagnetic waves will produce a strong boundary reflection effect, causing the received signal amplitude to exhibit non-steady-state fluctuations. If the mismatch of equivalent formation impedance is not corrected, the resistivity curve will show false peaks or distortions at the boundary. To this end, this invention introduces a nonlinear correction term based on the equivalent formation impedance matching residual to constrain the basic gain prediction in real time, thereby improving the detection accuracy of complex thin layers.

[0044] Specifically, the present invention uses the phase difference of the antenna and the amplitude of the received signal to evaluate the equivalent ground impedance; evaluates the matching residual between the equivalent ground impedance and the ideal model, thereby obtaining the gain correction coefficient.

[0045] The gain correction coefficient satisfies the following expression:

[0046] In the formula, express Timing gain correction factor; express The amplitude of the received signal at any given time; express Phase difference between antenna pairs at any given time; Indicates the correction scaling factor; Indicates the impedance matching factor; Represents the spatial gradient operator; Represents the hyperbolic tangent function; This represents the modulo operation; This represents the second derivative of the received signal amplitude with respect to time.

[0047] In the formula, the gain correction coefficient A larger value indicates a higher strength of gain correction and compensation in this invention, which means more severe reflection interference at the formation boundary; the spatial gradient term modulus value is... The increase in the value reflects a strong impedance mismatch phenomenon generated by the electromagnetic wave at the geological interface, which leads to the increase in the gain correction factor. The amplitude increases; the second derivative of the received signal amplitude with respect to time is... When the value deviates from zero, it indicates that the signal has entered the unsteady transition region. The gain correction coefficient is guided by the saturation constraint of the hyperbolic tangent function. It generates directional compensatory adjustment.

[0048] It should be added that the method of obtaining the impedance matching factor in this invention is as follows: the roughness index of the well wall is obtained by using a downhole ultrasonic radial scanner, and the reciprocal of the roughness index of the well wall after nonlinear normalization is evaluated as the impedance matching factor.

[0049] It should be further noted that the scaling factor in this invention is corrected. Set to 0.4; this value typically ranges from 0.2 to 0.8; if the scaling factor is adjusted... If the setting is too small, the suppression effect on fluctuations caused by formation boundary reflections will be insignificant, leading to spurious fluctuations in the resistivity curve; if the scaling factor is adjusted... Setting the value too high will weaken the true stratigraphic characteristics due to overcompensation; this invention uses 0.4 as an empirical value, which can effectively suppress the boundary reflection effect while ensuring the accuracy of stratigraphic resistivity inversion.

[0050] For example, Figure 3 This diagram illustrates the change in gain correction coefficient at a ground transition interface, demonstrating the compensating effect of the gain correction coefficient when the received signal amplitude undergoes a nonlinear jump. At moments of rapid amplitude change, such as an amplitude increase at 3 seconds and a sudden drop at 7 seconds, the gain correction coefficient exhibits a significant response, indicating that this invention effectively identifies the ground impedance mismatch boundary and provides timely correction to suppress distortion at the boundary.

[0051] S4. Obtain the variable gain control voltage based on the basic gain prediction value, gain correction coefficient, circuit ambient temperature, and reference temperature, and adjust the receiving gain multiple of the receiving circuit based on the variable gain control voltage.

[0052] It should be noted that the calculated predicted value and correction coefficient must be converted into control actions of the receiving circuit in order to achieve a substantial expansion of the dynamic range of resistivity detection. Due to the severe temperature drift in the downhole environment, the parameters of the electronic components in the receiving circuit will change, causing system deviations in gain control. To address this, this invention combines the basic gain prediction value, gain correction coefficient, and temperature compensation term to obtain a variable gain control voltage, ensuring that the sampled induced current signal is always in the linear operating range.

[0053] Specifically, the present invention integrates the predicted base gain value and the gain correction coefficient to obtain the target control gain; and uses the target control gain to update the controlled voltage of the receiving circuit channel to obtain the variable gain control voltage.

[0054] The variable gain control voltage satisfies the following expression:

[0055] In the formula, express Variable gain control voltage; Indicates the voltage adjustment step size; express Predicted base gain value at time step; express Timing gain correction factor; express Constant circuit ambient temperature; Indicates the reference temperature; The operator represents the set of independent variables that maximizes the function value.

[0056] In the formula, the variable gain control voltage A larger value indicates a higher control level output to the controlled amplifier, meaning the receiving circuit will perform a greater amplification; the total gain requirement is... The larger the sum of the values, the more effective the variable gain control voltage becomes through the optimization operator. Synchronous increase to maintain signal dynamic range; circuit ambient temperature deviation accumulation term, i.e. A larger value indicates an increase in circuit temperature drift, which in turn reduces the variable gain control voltage. The output strength indicates that the present invention compensates for the spontaneous drift of hardware gain caused by high temperature environment through negative feedback.

[0057] Furthermore, by utilizing the acquired variable gain control voltage, when the dynamic range of the real-time induced current signal exceeds the linear threshold, the present invention changes the conduction state of the feedback resistor array of the programmable gain amplifier through the controller, thereby realizing the real-time physical adjustment of the receiving gain multiple in the variable gain drilling electromagnetic wave resistivity detection system.

[0058] Furthermore, this operation ensures that the induced current signal collected by the receiving antenna array does not undergo saturation distortion in high resistivity strata and does not lose weak features in low resistivity strata, thereby improving the measurement accuracy of resistivity across the entire range.

[0059] For example, Figure 4This diagram illustrates the adjustment of the variable gain control voltage under high-temperature conditions, reflecting the adaptive adjustment trend of the variable gain control voltage as the circuit ambient temperature continuously increases. Although the base prediction value fluctuates with the formation, the variable gain control voltage generally exhibits a compensation characteristic that is inversely related to temperature drift. This indicates that the present invention effectively reduces component parameter drift caused by high temperatures, ensuring the robustness of the receiver circuit gain control under complex thermal environments.

[0060] This invention also discloses a variable gain drilling electromagnetic resistivity detection system, including a processor and a memory. The memory stores computer program instructions, which, when executed by the processor, implement a variable gain drilling electromagnetic resistivity detection method according to the present invention.

[0061] The system also includes other components well known to those skilled in the art, such as communication buses and communication interfaces, the settings and functions of which are known in the art and will not be described in detail here.

Claims

1. A method for detecting electromagnetic resistivity while drilling with variable gain, characterized in that, include: Acquire the induced current signal and perform feature extraction to obtain the antenna pair phase difference and the received signal amplitude; The predicted base gain value is obtained based on the antenna's temporal rate of change of phase difference and the amplitude fluctuation of the received signal. Gain correction coefficients are obtained based on the antenna pair phase difference, received signal amplitude, and the second derivative of received signal amplitude with respect to time. The variable gain control voltage is obtained based on the base gain prediction value, gain correction coefficient, circuit ambient temperature, and reference temperature. The receiving gain of the receiving circuit is adjusted based on the variable gain control voltage. The method for obtaining the basic gain prediction value is as follows: The time rate of change of the phase difference of the antenna pair is obtained based on the phase difference of the antenna pair. The cumulative integral of the absolute difference in the amplitude of the received signal within the length of the integral sliding window is obtained, and the basic gain prediction value is obtained based on the antenna's time rate of change of phase difference and the cumulative integral. The predicted base gain value satisfies the expression: ； In the formula, for Predicted base gain at time step For sampling time index, for Phase difference between antenna pairs at any given time This is the proportional gain coefficient. Here is the stability constant. As a volatility-sensitive factor, for Receive signal amplitude at all times for Receive signal amplitude at all times The integral sliding window length, It is an exponential function with the natural constant as its base. For the time differential term, For integration variables; The gain correction coefficient satisfies the expression: ； In the formula, for Timing gain correction factor for Receive signal amplitude at all times for Phase difference between antenna pairs at any given time To correct the scaling factor, For impedance matching factor, For spatial gradient operators, It is the hyperbolic tangent function. For modulo operation, for The second derivative of the received signal amplitude with respect to time at any given moment; The variable gain control voltage satisfies the following expression: ； In the formula, for Variable gain control voltage For voltage adjustment step size, for Predicted base gain at time step for Timing gain correction factor for Constant circuit ambient temperature, For reference temperature, To find the set of independent variables that maximizes the function value.

2. The variable gain electromagnetic resistivity detection method for drilling according to claim 1, characterized in that, The method for obtaining the fluctuation sensitivity factor is as follows: Variational mode decomposition was performed on the collected drill vibration sensor signals to extract the residual components that reflect high-frequency impact energy. The variance of the residual components was then evaluated as a fluctuation sensitivity factor.

3. The variable gain electromagnetic resistivity detection method for drilling according to claim 1, characterized in that, The process of acquiring the induced current signal and performing feature extraction to obtain the antenna pair phase difference and received signal amplitude includes: An alternating magnetic field in space is induced using a receiving antenna array. The induced raw signal is then subjected to amplitude limiting and bandpass filtering by a receiving circuit to obtain an induced current signal. A fast Fourier transform algorithm is used to project the induced current signal in the time domain to the frequency domain, and the real and imaginary parts of the magnetic field strength vector corresponding to the fundamental frequency of the transmission are extracted. Based on the real and imaginary parts of the magnetic field strength vector, orthogonal demodulation technology is used to calculate the amplitude ratio and phase difference between the antenna pairs. The amplitude ratio between the antenna pairs is then evaluated as the amplitude of the received signal.

4. The variable gain electromagnetic resistivity detection method for drilling according to claim 1, characterized in that, The impedance matching factor is obtained as follows: The roughness index of the wellbore is obtained using a downhole ultrasonic radial scanner. The reciprocal of the roughness index after nonlinear normalization is evaluated as the impedance matching factor.

5. The variable gain electromagnetic resistivity detection method for drilling according to claim 1, characterized in that, The adjustment of the receiving gain factor of the receiving circuit includes: The controller adjusts the receiving gain of the receiving circuit by changing the conduction state of the feedback resistor array of the programmable gain amplifier based on the variable gain control voltage, so that the sampled induced current signal is always in the linear operating region.

6. A variable gain electromagnetic resistivity detection system for drilling, characterized in that, include: A processor and a memory, wherein the memory stores computer program instructions that, when executed by the processor, implement a variable gain drilling electromagnetic resistivity detection method according to any one of claims 1-5.

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