Underground electromagnetic logging system and logging method

Abstract

The underground electromagnetic logging system comprises a measuring module, the measuring module is connected with a decoding module, and the decoding module is electrically connected with a computer; the measuring module comprises a shell, the shell is structurally a hollow cylinder, a plurality of short-distance probes, a plurality of middle-distance probes, a plurality of long-distance probes, a time sequence control module and a coding output module are sequentially arranged in the shell in the axial direction, and the coding output module is located at one end in the shell. The invention further discloses an underground electromagnetic logging method which comprises the following steps: acquiring the diameter of the casing pipe, and setting the transmitting signal voltage of the underground electromagnetic logging system; putting the measurement module into a well to obtain a sampling signal; when the remote probe detects the cable, a sampling signal is sent to the computer; the computer processes the received signals and then draws an underground cable image, and logging is completed. According to the underground electromagnetic logging system, support is effectively provided for salvage of underground instruments and fractured cables, and the efficiency of salvage of the underground cables is improved.

Description

Technical Field

[0001] This invention belongs to the field of oil well logging equipment technology, and relates to downhole electromagnetic logging systems and methods. Background Technology

[0002] In the field of oil well logging, stuck cables frequently break, necessitating their retrieval. For cable and instrument retrieval, the first step is to determine their location and distribution downhole. Current technologies mostly employ lead stamping or downhole television logging tools to accomplish this. Downhole television logging tools allow for direct visualization of downhole conditions back to the surface, primarily through downhole television. However, this technology is mainly suitable for wells with clear water; it is ineffective in wells with opaque mud. Current methods involve continuously injecting clear water while maintaining downhole pressure before or during the deployment of the downhole television, replacing the mud and clarifying the turbid water or mud before testing. However, this approach involves a massive workload, increases the risk of working under pressure, and is not applicable to all wells. For example, if the cable or instrument becomes stuck in an open hole, the tool is unusable. Therefore, the limitations of downhole television logging tools become apparent in mud wells, which are often the most common types of wells. There is currently no good method to efficiently and accurately obtain the distribution of downhole cables in murky or muddy wells, thus providing support for rapid retrieval.

[0003] In summary, existing technologies suffer from the problem of inconvenient detection of downhole cable locations in mud wells. Summary of the Invention

[0004] The purpose of this invention is to provide a downhole electromagnetic logging system that solves the problem of inconvenient downhole cable location detection in mud wells in the prior art.

[0005] Another objective of this invention is to provide a downhole electromagnetic logging method.

[0006] The technical solution adopted in this invention is a downhole electromagnetic logging system, including a measurement module connected to a decoding module, and the decoding module electrically connected to a computer. The measurement module includes a housing, which is a hollow cylinder in structure. Inside the housing, several near-range probes, several medium-range probes, several long-range probes, a timing control module, and an encoding output module are arranged sequentially along the axial direction. The near-range probes, medium-range probes, and long-range probes are all independent electromagnetic sensors. The encoding output module is located at one end of the housing and is electrically connected to the decoding module.

[0007] The invention is further characterized by:

[0008] The encoding output module is electrically connected to the timing control module, and several long-distance probes are distributed in a ring along the inner wall of the housing. The long-distance probes are electrically connected to the timing control module.

[0009] Several mid-range probes are arranged in a ring along the inner wall of the housing, and the mid-range probes are electrically connected to the timing control module.

[0010] Several close-range probes are arranged in a ring along the inner wall of the housing, and the close-range probes are electrically connected to the timing control module.

[0011] Another technical solution adopted in this invention is a downhole electromagnetic logging method, which specifically includes the following steps:

[0012] Step 1: Obtain the sleeve diameter and set the transmission signal voltage of the long-range and short-range probes according to the sleeve diameter;

[0013] Step 2: The measurement module is lowered downhole through the tubing string. The timing control module sends a signal, the long-range probe emits electromagnetic wave signals and begins sampling, the medium-range probe begins sampling, and the short-range probe emits electromagnetic wave signals and begins sampling, thus obtaining the long-range sampling signal, the medium-range sampling signal, and the short-range sampling signal.

[0014] Step 3: After the long-distance probe detects the cable, the lowering speed of the measurement module is slowed down. The long-distance sampling signal, medium-distance sampling signal, and short-distance sampling signal are sent to the computer after passing through the timing control module, encoding output module, and decoding module.

[0015] Step 4: The computer processes the received signals and draws a picture of the downhole cable. After the drawing is completed, the measurement module is taken out of the well, and the logging is completed.

[0016] Another feature of the technical solution of this invention is that:

[0017] In step one, the range of the transmission signal voltage of the long-range probe is 5V-9V, and the range of the transmission signal voltage of the short-range probe is 4.5V-5.5V. The larger the diameter of the sleeve, the larger the transmission signal voltage.

[0018] Step two specifically includes the following steps:

[0019] 2.1. The measurement module is lowered downhole through the tubing string, and the timing control module emits a square wave signal with an amplitude of 5V-9V;

[0020] 2.2 The long-distance probe emits electromagnetic wave signals and begins sampling;

[0021] After the long-distance probe emits electromagnetic wave signals, it starts sampling periodically to obtain long-distance sampling signals;

[0022] 2.3. The mid-range probe begins sampling;

[0023] The mid-range probe starts sampling periodically after the long-range probe emits electromagnetic wave signals, thus obtaining the mid-range sampling signal.

[0024] 2.4 The near-field probe begins emitting electromagnetic wave signals and starts sampling;

[0025] After the far-distance probe emits an electromagnetic wave signal, the near-range probe emits an electromagnetic wave signal and starts sampling periodically to obtain the near-range sampling signal.

[0026] In step three, the timing control module amplifies the signal; in step three, the encoding output module encodes the amplified signal; and in step three, the decoding module decodes the encoded signal.

[0027] The beneficial effects of this invention are as follows: By adopting the principle that electromagnetic wave propagation is unaffected by mud compared to camera image acquisition, multiple array probes are set up. After measuring multiple sets of data by measuring the strength and amplitude relationship of the transmitted and received electromagnetic waves, the measurement information is analyzed to obtain the distribution status of broken cables in the well. This allows the downhole electromagnetic logging system to measure the cable distribution data in the mud, which is more efficient and safer. It effectively provides support for the retrieval of downhole instruments and broken cables, and improves the efficiency of downhole cable retrieval. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the downhole electromagnetic logging system in this invention;

[0029] Figure 2 This is a schematic diagram of the measurement module in the downhole electromagnetic logging system of this invention;

[0030] Figure 3 This is a schematic diagram illustrating the working principle of the measurement module in the downhole electromagnetic logging system of this invention;

[0031] Figure 4 This is a sampling timing diagram of the long-distance probe in the downhole electromagnetic logging method in the embodiments of the present invention;

[0032] Figure 5 This is a sampling timing diagram of the medium-range probe in the downhole electromagnetic logging method in the embodiments of the present invention;

[0033] Figure 6 This is a sampling timing diagram of the close-range probe in the downhole electromagnetic logging method in the embodiments of the present invention;

[0034] Figure 7 This is a schematic diagram of the cable detection principle in the downhole electromagnetic logging method of the present invention.

[0035] Figure 8This is a schematic diagram of the detection results in the downhole electromagnetic logging method in an embodiment of the present invention.

[0036] In the diagram, 1. Near-range probe; 2. Medium-range probe; 3. Long-range probe; 4. Timing control module; 5. Encoding output module; 6. Housing; 7. Decoding module; 8. Computer; 9. Measurement module. Detailed Implementation

[0037] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0038] Downhole electromagnetic logging systems, such as Figure 1 As shown, it includes a measurement module 9, which is connected to a decoding module 7, and the decoding module 7 is electrically connected to a computer 8.

[0039] like Figure 2 As shown, the measurement module 9 includes a housing 6, which is a hollow cylinder. Inside the housing 6, several near-range probes 1, several medium-range probes 2, several long-range probes 3, a timing control module 4, and an encoding output module 5 are arranged sequentially along the axial direction. The near-range probes 1, medium-range probes 2, and long-range probes 3 are all independent electromagnetic sensors. The encoding output module 5 is located at one end of the housing 6 and is electrically connected to the decoding module 7.

[0040] The encoding output module 5 is electrically connected to the timing control module 4. Several long-range probes 3 are arranged in a ring along the inner wall of the housing 6, and are electrically connected to the timing control module 4. Several medium-range probes 2 are arranged in a ring along the inner wall of the housing 6, and are electrically connected to the timing control module 4. Several short-range probes 1 are arranged in a ring along the inner wall of the housing 6, and are electrically connected to the timing control module 4.

[0041] The decoding module 7 transmits command information to the encoding output module 5 and parses the data information transmitted by the encoding output module 5 before sending it to the computer 8. The encoding output module 5 encodes the data for transmission. The timing control module 4 controls the timing and amplitude of the transmitted signal and amplifies the received sampled signal before transmitting it to the encoding output module 5. The long-range probe 3 transmits and receives electromagnetic waves over long distances. The medium-range probe 2 receives electromagnetic waves emitted by the long-range probe 3. The short-range probe 1 transmits and receives electromagnetic waves at close range. The setting of the short-range probe 1 allows for more refined data acquisition. During use, not only can the cooperation of the long-range probe 3 and the medium-range probe 2 achieve efficient and simple detection, roughly determining the cable distribution and quickly completing cable retrieval, but also, when more refined detection is required, the cooperation of the short-range probe 1 can detect more precise electromagnetic data, thus providing support for downhole condition detection and analysis.

[0042] The downhole electromagnetic logging method specifically includes the following steps:

[0043] Step 1: Obtain the sleeve diameter and set the transmission signal voltage of the long-range probe 3 and the short-range probe 1 according to the sleeve diameter;

[0044] The range of the transmission signal voltage of the long-range probe 3 is 5V-9V, and the range of the transmission signal voltage of the short-range probe 1 is 4.5V-5.5V. The larger the diameter of the sleeve, the larger the transmission signal voltage.

[0045] Step 2: Lower the measurement module 9 downhole through the tubing string, such as... Figure 3 As shown, the timing control module 4 sends a signal, the long-range probe 3 emits an electromagnetic wave signal and starts sampling, the medium-range probe 2 starts sampling, and the short-range probe 1 emits an electromagnetic wave signal and starts sampling, thus obtaining the long-range sampling signal, the medium-range sampling signal, and the short-range sampling signal;

[0046] 2.1. The measurement module 9 is lowered downhole through the tubing string, and the timing control module 4 emits a square wave signal with an amplitude of 5V-9V.

[0047] 2.2 The long-distance probe 3 emits electromagnetic wave signals and begins sampling;

[0048] After the long-distance probe 3 emits an electromagnetic wave signal, it starts sampling periodically to obtain a long-distance sampling signal;

[0049] 2.3. Mid-range probe 2 begins sampling;

[0050] After the long-range probe 3 emits an electromagnetic wave signal, the medium-range probe 2 starts sampling periodically to obtain the medium-range sampling signal;

[0051] 2.4. The near-field probe 1 begins emitting electromagnetic wave signals and starts sampling;

[0052] After the far-distance probe 3 emits an electromagnetic wave signal, the near-distance probe 1 emits an electromagnetic wave signal and starts sampling periodically to obtain the near-distance sampling signal.

[0053] Step 3: When the long-distance probe 3 detects the cable, the lowering speed of the measurement module 9 is slowed down. The long-distance sampling signal, the medium-distance sampling signal, and the short-distance sampling signal are sent to the computer 8 through the timing control module 4, the encoding output module 5, and the decoding module 7.

[0054] The timing control module 4 amplifies the signal; in step three, the encoding output module 5 encodes the amplified signal; and in step three, the decoding module 7 decodes the encoded signal.

[0055] Step 4: The computer 8 processes the received signal and draws a picture of the downhole cable. After the drawing is completed, the measurement module 9 is taken out of the well, and the logging is completed.

[0056] Example 1

[0057] like Figure 1-2 As shown, this embodiment proposes a downhole electromagnetic logging system, including a measurement module 9, which is connected to a decoding module 7, and the decoding module 7 is electrically connected to a computer 8. The measurement module 9 includes a housing 6, which is structurally a hollow cylinder. Inside the housing 6, six near-range probes 1, six mid-range probes 2, six far-range probes 3, a timing control module 4, and an encoding output module 5 are arranged sequentially along the axial direction. The near-range probes 1, mid-range probes 2, and far-range probes 3 are all independent electromagnetic sensors. The encoding output module 5 is located at one end of the housing 6 and is electrically connected to the decoding module 7. The encoding output module 5 is also electrically connected to the timing control module 4. The six far-range probes 3 are arranged in a ring along the inner wall of the housing 6 and are electrically connected to the timing control module 4. The six mid-range probes 2 are also arranged in a ring along the inner wall of the housing 6 and are electrically connected to the timing control module 4. The six near-range probes 1 are also arranged in a ring along the inner wall of the housing 6 and are electrically connected to the timing control module 4.

[0058] Example 2

[0059] like Figure 1-3 As shown, this embodiment proposes a downhole electromagnetic logging method, which specifically includes the following steps:

[0060] Step 1: Obtain the sleeve diameter and set the transmission signal voltage of the long-range probe 3 and the short-range probe 1 according to the sleeve diameter;

[0061] The range of the transmission signal voltage of the long-range probe 3 is 7V, and the range of the transmission signal voltage of the short-range probe 1 is 5V. The larger the diameter of the sleeve, the larger the transmission signal voltage.

[0062] Step 2: The measurement module 9 is lowered downhole through the tubing string. The timing control module 4 sends a signal, the long-range probe 3 emits an electromagnetic wave signal and starts sampling, the medium-range probe 2 starts sampling, and the short-range probe 1 emits an electromagnetic wave signal and starts sampling, thus obtaining the long-range sampling signal, the medium-range sampling signal and the short-range sampling signal.

[0063] 2.1. The measurement module 9 is lowered downhole through the tubing string, and the timing control module 4 emits a square wave signal with an amplitude of 7V and a period of 30ms.

[0064] 2.2 The long-distance probe 3 emits electromagnetic wave signals and begins sampling;

[0065] like Figure 4As shown, after the long-distance probe 3 emits an electromagnetic wave signal, it starts sampling by taking 10 samples at an interval of 0.5ms and then taking 20 samples at an interval of 1.25ms as a sampling cycle to obtain the long-distance sampling signal.

[0066] 2.3. Mid-range probe 2 begins sampling;

[0067] like Figure 5 As shown, after the long-range probe 3 emits an electromagnetic wave signal, the medium-range probe 2 starts sampling at a time of 8 times with an interval of 2ms to obtain the medium-range sampling signal.

[0068] 2.4. The near-field probe 1 begins emitting electromagnetic wave signals and starts sampling;

[0069] like Figure 6 As shown, after the far-field probe 3 emits an electromagnetic wave signal, the near-field probe 1 emits an electromagnetic wave signal and samples at 8 times with an interval of 2ms to obtain the near-field sampling signal.

[0070] Step 3: When the long-distance probe 3 detects the cable, the lowering speed of the measurement module 9 is slowed down. The long-distance sampling signal, the medium-distance sampling signal, and the short-distance sampling signal are sent to the computer 8 through the timing control module 4, the encoding output module 5, and the decoding module 7.

[0071] Step 4: The computer 8 processes the received signal and draws a picture of the downhole cable. After the drawing is completed, the measurement module 9 is taken out of the well, and the logging is completed.

[0072] Example 3

[0073] like Figure 1-3 As shown, this embodiment proposes a downhole electromagnetic logging method, which specifically includes the following steps:

[0074] Step 1: Obtain the sleeve diameter and set the transmission signal voltage of the long-range probe 3 and the short-range probe 1 according to the sleeve diameter;

[0075] The range of the transmission signal voltage of the long-range probe 3 is 5V, and the range of the transmission signal voltage of the short-range probe 1 is 4.5V. The larger the diameter of the sleeve, the larger the transmission signal voltage.

[0076] Step 2: The measurement module 9 is lowered downhole through the tubing string. The timing control module 4 sends a signal, the long-range probe 3 emits an electromagnetic wave signal and starts sampling, the medium-range probe 2 starts sampling, and the short-range probe 1 emits an electromagnetic wave signal and starts sampling, thus obtaining the long-range sampling signal, the medium-range sampling signal and the short-range sampling signal.

[0077] 2.1. The measurement module 9 is lowered downhole through the tubing string, and the timing control module 4 emits a square wave signal with an amplitude of 5V and a period of 30ms.

[0078] 2.2 The long-distance probe 3 emits electromagnetic wave signals and begins sampling;

[0079] like Figure 4 As shown, after the long-distance probe 3 emits an electromagnetic wave signal, it starts sampling by taking 10 samples at an interval of 0.5ms and then taking 20 samples at an interval of 1.25ms as a sampling cycle to obtain the long-distance sampling signal.

[0080] 2.3. Mid-range probe 2 begins sampling;

[0081] like Figure 5 As shown, after the long-range probe 3 emits an electromagnetic wave signal, the medium-range probe 2 starts sampling at a time of 8 times with an interval of 2ms to obtain the medium-range sampling signal.

[0082] 2.4. The near-field probe 1 begins emitting electromagnetic wave signals and starts sampling;

[0083] like Figure 6 As shown, after the far-field probe 3 emits an electromagnetic wave signal, the near-field probe 1 emits an electromagnetic wave signal and samples at 8 times with an interval of 2ms to obtain the near-field sampling signal.

[0084] Step 3: When the long-distance probe 3 detects the cable, the lowering speed of the measurement module 9 is slowed down. The long-distance sampling signal, the medium-distance sampling signal, and the short-distance sampling signal are sent to the computer 8 through the timing control module 4, the encoding output module 5, and the decoding module 7.

[0085] Step 4: The computer 8 processes the received signal and draws a picture of the downhole cable. After the drawing is completed, the measurement module 9 is taken out of the well, and the logging is completed.

[0086] Example 4

[0087] like Figure 1-2 As shown, this embodiment proposes a downhole electromagnetic logging method, which specifically includes the following steps:

[0088] Step 1: Obtain the sleeve diameter and set the transmission signal voltage of the long-range probe 3 and the short-range probe 1 according to the sleeve diameter;

[0089] The range of the transmission signal voltage of the long-range probe 3 is 9V, and the range of the transmission signal voltage of the short-range probe 1 is 5.5V. The larger the diameter of the sleeve, the larger the transmission signal voltage.

[0090] Step 2: The measurement module 9 is lowered downhole through the tubing string. The timing control module 4 sends a signal, the long-range probe 3 emits an electromagnetic wave signal and starts sampling, the medium-range probe 2 starts sampling, and the short-range probe 1 emits an electromagnetic wave signal and starts sampling, thus obtaining the long-range sampling signal, the medium-range sampling signal and the short-range sampling signal.

[0091] 2.1. The measurement module 9 is lowered downhole through the tubing string, and the timing control module 4 emits a square wave signal with an amplitude of 9V and a period of 30ms.

[0092] 2.2 The long-distance probe 3 emits electromagnetic wave signals and begins sampling;

[0093] like Figure 4 As shown, after the long-distance probe 3 emits an electromagnetic wave signal, it starts sampling by taking 10 samples at an interval of 0.5ms and then taking 20 samples at an interval of 1.25ms as a sampling cycle to obtain the long-distance sampling signal.

[0094] 2.3. Mid-range probe 2 begins sampling;

[0095] like Figure 5 As shown, after the long-range probe 3 emits an electromagnetic wave signal, the medium-range probe 2 starts sampling at a time of 8 times with an interval of 2ms to obtain the medium-range sampling signal.

[0096] 2.4. The near-field probe 1 begins emitting electromagnetic wave signals and starts sampling;

[0097] like Figure 6 As shown, after the far-field probe 3 emits an electromagnetic wave signal, the near-field probe 1 emits an electromagnetic wave signal and samples at 8 times with an interval of 2ms to obtain the near-field sampling signal.

[0098] Step 3: When the long-distance probe 3 detects the cable, the lowering speed of the measurement module 9 is slowed down. The long-distance sampling signal, the medium-distance sampling signal, and the short-distance sampling signal are sent to the computer 8 through the timing control module 4, the encoding output module 5, and the decoding module 7.

[0099] The timing control module 4 amplifies the signal; in step three, the encoding output module 5 encodes the amplified signal; and in step three, the decoding module 7 decodes the encoded signal.

[0100] Step 4: The computer 8 processes the received signal and draws a picture of the downhole cable. After the drawing is completed, the measurement module 9 is taken out of the well, and the logging is completed.

[0101] like Figure 7The figure shows the relationship between the distance h between the cable and the measurement module 9 and the amplitude of the electromagnetic wave signal during the test. Here, h represents the distance between the cable and the measurement module 9, V represents the amplitude of the received electromagnetic wave signal, B represents the curve without a medium, and B' represents the curve with a medium such as a cable. As can be seen from the figure, the distance between the measurement module 9 and the cable is inversely proportional to the output signal of the measurement module 9. The closer the distance, the stronger the output signal amplitude.

[0102] like Figure 8 The image shown is a result of detection in a cable fracture well. A cable distribution map was detected at depths of 3672m-3684m. Analysis clearly shows the cable's distribution and positional relationships downhole, providing strong evidence for cable retrieval. This method effectively solves the problem of cable detection in wells with silt. This invention is also applicable to the detection of cables or other ferromagnetic materials in both tubular and non-tubular wellbore environments.

Claims

1. A downhole electromagnetic logging system, characterized in that, The system includes a measurement module (9), which is connected to a decoding module (7), and the decoding module (7) is electrically connected to a computer (8). The measurement module (9) includes a housing (6), which is a hollow cylinder. Several near-range probes (1), several medium-range probes (2), several long-range probes (3), a timing control module (4), and an encoding output module (5) are arranged sequentially along the axial direction inside the housing (6). The near-range probes (1), medium-range probes (2), and long-range probes (3) are all independent electromagnetic sensors. The encoding output module (5) is located at one end of the housing (6) and is electrically connected to the decoding module (7).

2. The downhole electromagnetic logging system according to claim 1, characterized in that, The encoding output module (5) is electrically connected to the timing control module (4), and several of the long-distance probes (3) are distributed in a ring along the inner wall of the housing (6). The long-distance probes (3) are electrically connected to the timing control module (4).

3. The downhole electromagnetic logging system according to claim 2, characterized in that, Several mid-range probes (2) are arranged in a ring along the inner wall of the housing (6), and the mid-range probes (2) are electrically connected to the timing control module (4).

4. The downhole electromagnetic logging system according to claim 3, characterized in that, Several of the aforementioned close-range probes (1) are arranged in a ring along the inner wall of the housing (6), and the close-range probes (1) are electrically connected to the timing control module (4).

5. A downhole electromagnetic logging method, characterized in that, Specifically, the following steps are included: Step 1: Obtain the diameter of the sleeve and set the transmission signal voltage of the long-distance probe (3) and the short-distance probe (1) according to the diameter of the sleeve; Step 2: The measurement module (9) is placed downhole through the tubing. The timing control module (4) sends a signal. The long-range probe (3) emits an electromagnetic wave signal and starts sampling. The medium-range probe (2) starts sampling. The short-range probe (1) emits an electromagnetic wave signal and starts sampling, thus obtaining the long-range sampling signal, the medium-range sampling signal and the short-range sampling signal. Step 3: When the long-distance probe (3) detects the cable, the lowering speed of the measurement module (9) is slowed down, and the long-distance sampling signal, medium-distance sampling signal and short-distance sampling signal are sent to the computer (8) through the timing control module (4), the encoding output module (5) and the decoding module (7); Step 4: The computer (8) processes the received signal and draws a picture of the downhole cable. After the drawing is completed, the measurement module (9) is taken out from the well, and the logging is completed.

6. The downhole electromagnetic logging method according to claim 5, characterized in that, In step one, the range of the transmission signal voltage of the long-distance probe (3) is 5V-9V, and the range of the transmission signal voltage of the short-distance probe (1) is 4.5V-5.5V. The larger the diameter of the sleeve, the larger the transmission signal voltage.

7. The downhole electromagnetic logging method according to claim 6, characterized in that, Step two specifically includes the following steps: 2.

1. The measurement module (9) is placed downhole through the tubing, and the timing control module (4) emits a square wave signal with an amplitude of 5V-9V; 2.

2. The long-distance probe (3) emits electromagnetic wave signals and begins sampling; After the long-distance probe (3) emits electromagnetic wave signals, it starts sampling periodically to obtain long-distance sampling signals. 2.

3. The mid-range probe (2) begins sampling; The mid-range probe (2) starts sampling periodically after the long-range probe (3) emits electromagnetic wave signals to obtain mid-range sampling signals; 2.

4. The near-field probe (1) begins to emit electromagnetic wave signals and begins sampling; The near-field probe (1) emits an electromagnetic wave signal after the far-field probe (3) emits an electromagnetic wave signal, and starts sampling periodically to obtain a near-field sampling signal.

8. The downhole electromagnetic logging method according to claim 7, characterized in that, In step three, the timing control module (4) amplifies the signal; in step three, the encoding output module (5) encodes the amplified signal; and in step three, the decoding module (7) decodes the encoded signal.

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