A dam time-lapse monitoring method and system based on shallow transient electromagnetic method

Abstract

The application discloses a dam time-lapse monitoring method and system based on a shallow transient electromagnetic method, and the method comprises the following steps: collecting transient electromagnetic data of a dam for the first time, obtaining full-range apparent resistivity values as initial values of apparent resistivity of the dam; obtaining transient electromagnetic data of the dam in different periods, and obtaining apparent resistivity values in different periods respectively; obtaining an apparent resistivity change rate section according to the initial values of apparent resistivity and the apparent resistivity values in different periods, and judging seepage hidden dangers of the dam according to apparent resistivity change rate curve distribution and the condition that the apparent resistivity change rate values are less than 1; and the system comprises a transient electromagnetic host, a receiving and transmitting coil and a processing platform. Compared with the current time-lapse monitoring method, the application is convenient to operate, realizes effective collection of full-section data of the dam, reduces the disadvantages of a large amount of redundant information in the prior art, and also reduces the cost.

Description

Technical Field

[0001] This invention relates to the field of geophysical monitoring technology, and more specifically to a method and system for monitoring the time-lapse of dams based on shallow transient electromagnetic methods. Background Technology

[0002] High-density electrical resistivity tomography (EDT), ground-penetrating radar (GPR), natural electric field method, magnetic apparent resistivity method, and pseudo-flow field method have all been applied in seepage detection of earth-rock dams and have achieved good results. However, the above methods have low working efficiency during application, and some methods also have technical bottlenecks due to the influence of human noise. In addition, with the reinforcement of dam crest hardening, the acquisition and installation of high-density electrical resistivity tomography has also become an important issue.

[0003] Currently, reservoir dam detection mainly adopts a one-time detection mode. Due to the unclear spatial location and distribution of hidden dangers, a single detection is insufficient to accurately identify hidden danger information. On the other hand, the internal problems of the dam are a gradual deterioration process, and a single detection obviously cannot grasp the time-varying characteristics of hidden dangers inside the dam. Invention patent (publication number CN110632131A) discloses a method for monitoring seepage in channel embankment engineering. It can simultaneously acquire the geoelectric field distribution of the entire detected area at a certain point in time, and can perform multiple "time-shift" detections on the same detected area over time, realizing the dynamic monitoring of the evolution of seepage channels from non-existent to present, and from minor to severe, thereby improving the accuracy of seepage detection in channel embankment engineering. Invention patent (publication number CN108267394A) provides a seepage field monitoring system and early warning method for earth-rock dams, including an electrical resistivity tomography (OTM) instrument, a data transmission device, a host computer, and multiple electrodes installed in the dam body. The multiple electrodes are all connected to the input end of the OTM instrument, and the output end of the OTM instrument transmits data information to the host computer through the data transmission device. An early warning method for a seepage field monitoring system of an earth-rock dam includes on-site monitoring, monitoring system deployment, and terminal early warning. The monitoring system provides a full-section view of the dam's seepage characteristics, facilitating a comprehensive assessment of the dam's health status. It requires only a one-time installation, offers more flexible testing periods, and allows for real-time analysis of changes in the seepage field within the dam, enabling long-term monitoring. The monitoring system is flexible in on-site deployment, with a simple structure and easy installation. However, existing monitoring methods primarily rely on conventional apparent resistivity interpretation, which can easily miss information from the dam abutment area, resulting in incomplete monitoring coverage. Furthermore, existing monitoring methods require permanent installation on the dam, which is unnecessary during non-flood seasons or low water levels, leading to unnecessary waste of equipment and increased maintenance costs.

[0004] Therefore, how to provide a faster method and system for monitoring the time-lapse of dams is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] In view of this, the present invention provides a method and system for monitoring the time-shift of dams based on the shallow transient electromagnetic method. According to the characteristics of the transient electromagnetic method using induction coil testing, the method monitors the changes of the electromagnetic field inside the dam over time at different periods, thereby achieving rapid tracking and early warning of leakage anomalies. Compared with current time-shift monitoring methods, this method is also convenient to operate, achieves effective acquisition of full-section data of the dam, reduces the drawbacks of existing technologies that use a large amount of redundant information, and also reduces costs.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for monitoring the time-lapse of dams based on shallow transient electromagnetic methods includes the following steps:

[0008] S1. Conduct the first transient electromagnetic data acquisition of the dam. Based on the acquired transient electromagnetic data, obtain the apparent resistivity value throughout the entire process, which will be used as the initial value of the apparent resistivity of the dam.

[0009] S2. Maintain the same acquisition parameters as during the initial transient electromagnetic data acquisition process of the dam, acquire transient electromagnetic data of the dam at different times, and obtain the apparent resistivity values ​​for different times respectively;

[0010] S3. Obtain a cross-sectional diagram of the apparent resistivity change rate based on the initial value of apparent resistivity and the apparent resistivity values ​​at different times. Determine the potential seepage hazards of the dam based on the distribution of the apparent resistivity change rate curve and the cases where the apparent resistivity change rate value is less than 1. When the potential seepage hazard of the dam is a local leakage hazard, extract the apparent resistivity value of the center point of the local leakage hazard area in the current monitoring area, and determine the seepage prevention measures to be taken based on the change of the apparent resistivity value over time.

[0011] Preferably, the specific content of S1 includes:

[0012] When the reservoir water level is at the normal storage level, a transient electromagnetic detection route is planned based on the reservoir dam information data. The first transient electromagnetic data acquisition of the dam is carried out using a point measurement method, and the position of each measuring point is marked. The induced electromotive force value corresponding to each measuring point is obtained, and the apparent resistivity value of the entire process is obtained using the full-process apparent resistivity calculation method. The full-process apparent resistivity data is stored as the initial value of the apparent resistivity of the reservoir dam. At the same time, an apparent resistivity cloud map is formed according to the horizontal position and vertical depth.

[0013] Preferably, the specific content of S2 includes:

[0014] Different monitoring periods are preset, and the induced electromotive force of the dam is obtained at different periods according to the first transient electromagnetic data acquisition method and acquisition parameters. The apparent resistivity values ​​at different periods are obtained respectively, and the apparent resistivity values ​​at different periods are stored and formed into an apparent resistivity cloud map according to the horizontal position and vertical depth.

[0015] Preferably, the different monitoring periods include three periods: before the flood season, during the flood season, and after the flood season;

[0016] Pre-flood season monitoring is used to assess the safety status of reservoir dams and ensure their safe passage through the flood season.

[0017] Monitoring during the flood season is used to determine the health status of potential hazards during the high-water season and to promptly report to management units so that emergency measures can be taken.

[0018] Post-flood season monitoring is used to investigate the changes in the seepage behavior of dams after experiencing high reservoir water levels, providing technical support for reinforcement and safety improvement.

[0019] Preferably, the specific content for determining the seepage pattern of the dam in S3 includes:

[0020] The ratio of the apparent resistivity values ​​at different times to the initial apparent resistivity value is calculated, and the Kriging interpolation algorithm is used to obtain the cross-sectional diagram of the apparent resistivity change rate.

[0021] When the apparent resistivity change rate curves on the apparent resistivity change rate cross-sectional diagram are layered or uniformly distributed, and the overall apparent resistivity change rate is less than 1, it is judged that there is a global abnormal seepage hazard in the dam.

[0022] When the apparent resistivity change rate curve shows a closed anomaly, and the apparent resistivity change rate in a local area is less than 1, it is judged that there is a potential local leakage hazard in the dam.

[0023] Preferably, when there is a potential for localized leakage in the dam, the apparent resistivity value of the center point of the potential localized leakage area is extracted. Based on the change of the apparent resistivity value over time, if the apparent resistivity value changes from high to low, it is determined that subsequent monitoring should be carried out; if the apparent resistivity value continues to decrease, and the absolute value of the difference between the apparent resistivity value at different times and the initial resistivity value divided by the initial resistivity value exceeds 10%, it is determined that seepage prevention treatment should be implemented in the area.

[0024] Preferably, specific methods for obtaining apparent resistivity values ​​through transient electromagnetic data include:

[0025] Vertical component B of the secondary field at the center point of a circular transmitting coil under uniform half-space conditions z for:

[0026]

[0027] Corresponding rate of change over time for:

[0028]

[0029] In the formula, I is the transmitting current, ρ is the resistivity of the half-space, a is the radius of the transmitting coil, μ is the permeability of the uniform half-space medium, and t is the decay time. It is the error function;

[0030] Define Z as the transient field parameter:

[0031]

[0032]

[0033] In the formula, ρ s Apparent resistivity values ​​at different times or apparent resistivity over the entire period;

[0034]

[0035]

[0036] in, For B z Kernel function;

[0037] for Kernel function;

[0038] Y(Z) and Y′(Z) are obtained through the observation of Bz, and then the Z value is obtained by solving for it, and finally the apparent resistivity value is obtained for the whole process.

[0039] Preferably, the rate of change of apparent resistivity η:

[0040]

[0041] In the formula, ρ t (x,z) refers to the apparent resistivity values ​​monitored at different times; ρ0(x,z) refers to the initial apparent resistivity value, where x is the x-coordinate of the recording point and z is the depth coordinate of the recording point.

[0042] A dam time-lapse monitoring system based on shallow transient electromagnetic method includes: a transient electromagnetic host, a transceiver coil, and a processing platform;

[0043] The transient electromagnetic host includes a parameter acquisition unit and a command acquisition unit;

[0044] The parameter acquisition unit is used to acquire sampling parameters, which include transmission frequency, sampling frequency, number of superpositions and / or number of measurement channels;

[0045] The command acquisition unit is used to acquire sampling commands and control the transient electromagnetic host to transmit current pulses to the transceiver coil according to the commands. The transceiver coil acquires transient electromagnetic data and sends it to the transient electromagnetic host.

[0046] The processing platform is used to call the transient electromagnetic data collected by the transient electromagnetic host, convert the transient electromagnetic data into apparent resistivity, and obtain the apparent resistivity change rate profile based on the initial value of apparent resistivity and the apparent resistivity values ​​at different times. Based on the distribution of the apparent resistivity change rate curve and the cases where the apparent resistivity change rate value is less than 1, the platform can determine the potential seepage hazards of the dam.

[0047] As can be seen from the above technical solution, compared with the prior art, the present invention discloses a method and system for monitoring the time-lapse of dams based on the shallow transient electromagnetic method, which includes the following beneficial effects:

[0048] 1. This invention utilizes the transient electromagnetic method to detect reservoir dams, which has the advantages of fast detection speed and wide coverage, and can realize the detailed exploration of potential leakage hazards in the entire dam.

[0049] 2. Compared with existing apparent resistivity time-shift monitoring technology, this invention eliminates the need to install instruments and equipment at the dam site for extended periods, thereby reducing instrument costs and unnecessary power consumption;

[0050] 3. This invention detects the apparent resistivity characteristics of reservoir safety at different critical periods, and monitors relative time-shifted apparent resistivity, thus avoiding the need to process massive amounts of redundant monitoring data and improving data utilization.

[0051] 4. This invention adopts a modular design for data acquisition and processing, enabling rapid setting of data acquisition parameters and efficient display of detection results. Attached Figure Description

[0052] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0053] Figure 1 A flowchart of a dam time-lapse monitoring method based on shallow transient electromagnetic method provided by the present invention;

[0054] Figure 2 The apparent resistivity cloud map of two detections provided in Embodiment 1 of the present invention;

[0055] Figure 3 This is an apparent resistivity cloud map obtained from multiple probes as provided in Embodiment 2 of the present invention;

[0056] Figure 4 The apparent resistivity change rate at different times is provided in Embodiment 2 of the present invention. Detailed Implementation

[0057] 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 embodiments of the present invention, and not all embodiments. 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.

[0058] This invention discloses a method for monitoring the time-lapse of dams based on shallow transient electromagnetic methods, such as... Figure 1 As shown, it includes the following steps:

[0059] S1. Conduct the first transient electromagnetic data acquisition of the dam. Based on the acquired transient electromagnetic data, obtain the apparent resistivity value throughout the entire process, which will be used as the initial value of the apparent resistivity of the dam.

[0060] S2. Maintain the same acquisition parameters as during the initial transient electromagnetic data acquisition process of the dam, acquire transient electromagnetic data of the dam at different times, and obtain the apparent resistivity values ​​for different times respectively;

[0061] S3. Obtain a cross-sectional diagram of the apparent resistivity change rate based on the initial value of apparent resistivity and the apparent resistivity values ​​at different times. Determine the potential seepage hazards of the dam based on the distribution of the apparent resistivity change rate curve and the cases where the apparent resistivity change rate value is less than 1. When the potential seepage hazard of the dam is a local leakage hazard, extract the apparent resistivity value of the center point of the local leakage hazard area in the current monitoring area, and determine the seepage prevention measures to be taken based on the change of the apparent resistivity value over time.

[0062] To further implement the above technical solution, the specific content of S1 includes:

[0063] When the reservoir water level is at the normal storage level, a transient electromagnetic detection route is planned based on the reservoir dam information data. The first transient electromagnetic data acquisition of the dam is carried out using a point measurement method, and the position of each measuring point is marked. The induced electromotive force value corresponding to each measuring point is obtained, and the apparent resistivity value of the entire process is obtained using the full-process apparent resistivity calculation method. The full-process apparent resistivity data is stored as the initial value of the apparent resistivity of the reservoir dam. At the same time, an apparent resistivity cloud map is formed according to the horizontal position and vertical depth.

[0064] It should be noted that:

[0065] The information data on reservoir dams can include data such as the dam type, structure, and extent of the dam.

[0066] The distance between measuring points is generally 0.5m or 1m.

[0067] To further implement the above technical solution, the specific content of S2 includes:

[0068] Different monitoring periods are preset, and the induced electromotive force of the dam is obtained at different periods according to the first transient electromagnetic data acquisition method and acquisition parameters. The apparent resistivity values ​​at different periods are obtained respectively, and the apparent resistivity values ​​at different periods are stored and formed into an apparent resistivity cloud map according to the horizontal position and vertical depth.

[0069] To further implement the above technical solutions, the different monitoring periods include three periods: before the flood season, during the flood season, and after the flood season.

[0070] Pre-flood season monitoring is used to assess the safety status of reservoir dams and ensure their safe passage through the flood season.

[0071] Monitoring during the flood season is used to determine the health status of potential hazards during the high-water season and to promptly report to management units so that emergency measures can be taken.

[0072] Post-flood season monitoring is used to investigate the changes in the seepage behavior of dams after experiencing high reservoir water levels, providing technical support for reinforcement and safety improvement.

[0073] To further implement the above technical solution, the specific content of determining the seepage pattern of the dam in S3 includes:

[0074] The ratio of the apparent resistivity values ​​at different times to the initial apparent resistivity value is calculated, and the Kriging interpolation algorithm is used to obtain the cross-sectional diagram of the apparent resistivity change rate.

[0075] When the apparent resistivity change rate curves on the apparent resistivity change rate cross-sectional diagram are layered or uniformly distributed, and the overall apparent resistivity change rate is less than 1, it is judged that there is a global abnormal seepage hazard in the dam.

[0076] When the apparent resistivity change rate curve shows a closed anomaly, and the apparent resistivity change rate in a local area is less than 1, it is judged that there is a potential local leakage hazard in the dam.

[0077] To further implement the above technical solution, when there is a potential for localized leakage in the dam, the apparent resistivity value of the center point of the potential localized leakage area is extracted. Based on the change of the apparent resistivity value over time, when the apparent resistivity value changes from high to low, it is determined that subsequent monitoring should be carried out; when the apparent resistivity value continues to decrease and the decrease difference exceeds 10% of the initial value of the apparent resistivity, it is determined that seepage prevention treatment should be carried out in the area.

[0078]

[0079] In the formula, The apparent resistivity refers to the rate of change of apparent resistivity, ρ0(x,z) refers to the initial apparent resistivity, ρ t (x,z) refers to the apparent resistivity at different times.

[0080] To further implement the above technical solution, specific methods for obtaining apparent resistivity values ​​through transient electromagnetic data include:

[0081] Vertical component B of the secondary field at the center point of a circular transmitting coil under uniform half-space conditions z for:

[0082]

[0083] Corresponding rate of change over time for:

[0084]

[0085] In the formula, I is the transmitting current, ρ is the resistivity of the half-space, a is the radius of the transmitting coil, μ is the permeability of the uniform half-space medium, and t is the decay time. It is the error function;

[0086] Define Z as the transient field parameter:

[0087]

[0088]

[0089] In the formula, ρ s The above formula represents the apparent resistivity values ​​at different times or the apparent resistivity over the entire period. In other words, the above formula is a general formula that can be used to calculate the apparent resistivity at different times and the apparent resistivity over the entire period.

[0090]

[0091]

[0092] in, For B z Kernel function;

[0093] for Kernel function;

[0094] Y(Z) and Y′(Z) are obtained through the observation of Bz, and then the Z value is obtained by solving for it, and finally the apparent resistivity value is obtained for the whole process.

[0095] To further implement the above technical solution, the apparent resistivity change rate η:

[0096]

[0097] In the formula, ρ t (x,z) refers to the apparent resistivity values ​​monitored at different times; ρ0(x,z) refers to the initial apparent resistivity value, where x is the x-coordinate of the recording point and z is the depth coordinate of the recording point.

[0098] A dam time-lapse monitoring system based on shallow transient electromagnetic method includes: a transient electromagnetic host, a transceiver coil, and a processing platform;

[0099] The transient electromagnetic host includes a parameter acquisition unit and a command acquisition unit;

[0100] The parameter acquisition unit is used to acquire sampling parameters, which include transmission frequency, sampling frequency, number of superpositions and / or number of measurement channels;

[0101] The command acquisition unit is used to acquire sampling commands and control the transient electromagnetic host to transmit current pulses to the transceiver coil according to the commands. The transceiver coil acquires transient electromagnetic data and sends it to the transient electromagnetic host.

[0102] The processing platform is used to call the transient electromagnetic data collected by the transient electromagnetic host, convert the transient electromagnetic data into apparent resistivity, and obtain the apparent resistivity change rate profile based on the initial value of apparent resistivity and the apparent resistivity values ​​at different times. Based on the distribution of the apparent resistivity change rate curve and the cases where the apparent resistivity change rate value is less than 1, the platform can determine the potential seepage hazards of the dam.

[0103] It should be noted that:

[0104] When the dam height is less than 15m, the transmission frequency is set to 25Hz; when the dam height is 15-30m, the transmission frequency is set to 12.5Hz.

[0105] When the dam height is less than 15m, the transmission frequency is set to 1.25Hz; when the dam height is 15-30m, the sampling frequency is set to 0.5MHz and 0.25MHz.

[0106] When the dam height is within 15m, the number of superpositions is set to 512; when the dam height is 15-30m, the number of superpositions is set to 1024.

[0107] In addition, the measurement channels are set to 120.

[0108] The transceiver coil can be a zero-flux induction coil, which is a cylindrical coil with a diameter of 1m and a thickness of 20cm.

[0109] The processing platform is also used to import the collected data and perform preprocessing such as parameter modification, distortion removal, data smoothing, and interference correction. The processing platform also includes a preprocessing storage module. Once the preprocessing is set up for the first time, the processing platform can directly load the initially set preprocessing when monitoring data for multiple times to achieve batch preprocessing of the data.

[0110] The above content will be further explained in detail below through specific embodiments:

[0111] Example 1:

[0112] The catchment area above the dam site of Reservoir A is 1.36 km², the main channel length is approximately 1.3 km, the total reservoir capacity is 1.167 million m³, and the normal reservoir capacity is 1.01 million m³. 3 The reservoir covers an area of ​​2,200 mu and is a small (1) type reservoir mainly for agricultural irrigation, combined with flood control, water supply, and aquaculture. The dam is an earth core dam with a top elevation of 300.40m, a top length of 86m, a maximum height of 25.7m, and a top width of 4.0m. The surface is paved with square environmental protection bricks, and a cement mortar and gravel cushion layer is laid underneath. Transient electromagnetic detection was performed twice before and after grouting. On February 4, 2021, the transient electromagnetic emission frequency was 12.5Hz, the sampling frequency was 0.5MHz, the number of superpositions was 1024, the measurement channel was 120, and the measurement point distance was 1m. On June 23, 2021, the transient electromagnetic emission frequency was 12.5Hz, the sampling frequency was 0.5MHz, the number of superpositions was 1024, the measurement channel was 120, and the measurement point distance was 1m.

[0113] Figure 2 (a) is a voltage profile of the first detection of the initial measurement channel 50. It can be seen from the figure that there are multiple high potential signals on the measurement line, which may indicate that there are multiple high conductor points in the dam. The range of high potential signals is relatively wide in the 5-15m and 75-85m sections of the measurement line. Figure 2 (b) The x-axis is opposite to that of the first detection, but the anomalies at the corresponding locations are consistent. During the grouting process, seepage was found in the contact zone and bedrock of the right bank of the dam. Comparing the results of the two detections, the high potential on the right bank has decreased, indicating that the grouting has been effective.

[0114] Example 2:

[0115] Reservoir B has a catchment area of ​​1.8 square kilometers, a main stream length of 2 kilometers, and a total storage capacity of 350,000 cubic meters. 3 The reservoir has a normal capacity of 300,000 m3 and is a small (1) type reservoir mainly used for irrigation, combined with aquaculture, domestic water use and comprehensive utilization. It irrigates 1,230 mu of farmland. The reservoir also has the function of reducing flood peak flow and mitigating flood disasters in downstream villages and farmland. There are about 4,000 people in 6 administrative villages downstream of the reservoir, as well as National Highway 330 and Jinliwen Railway.

[0116] The reservoir's key engineering works consist of a dam, spillway, and culvert. The dam is a clay-core dam with a crest elevation of 183.75 meters, a foundation elevation of 163.10 meters, a maximum dam height of 24.65 meters, a crest length of 126 meters, and a crest width of 4.5 meters. The spillway is a side weir type.

[0117] To investigate the seepage patterns inside the dam, transient electromagnetic method (TEM) detection was conducted on the dam for the first time on November 1, 2021. The measurement point spacing was 1m, the transient electromagnetic emission frequency was 12.5Hz, the sampling frequency was 0.25MHz, the number of superpositions was 1024, and 120 measurement channels were used. The apparent resistivity of the entire measurement path was calculated using a data processing platform. Figure 3 As shown in (a). On May 23, 2022 and August 24, 2022, the reservoir dam was probed again using the same parameters, and the preprocessing storage module, apparent resistivity calculation module, and time-shift monitoring module of the initial data processing were retrieved to obtain the apparent resistivity at different times. Figure 3 (b) Figure 3 (c) and cross-sectional diagram of apparent resistivity change rate ( Figure 4 (a) Figure 4 (b)).

[0118] from Figure 4 As can be seen from the above, the apparent resistivity change rate cross-section is relatively uniform, with the overall apparent resistivity change rate being less than 1, indicating an abnormal seepage phenomenon inside the dam. Furthermore, the apparent resistivity change rate curves at 59m horizontally and -10m depth, and at 92m horizontally and -10m depth, are closed loops. The resistivity values ​​for these two locations are shown in the table below:

[0119] Table 1. Apparent Resistivity Overview

[0120]

[0121]

[0122] When the first measurement was taken on November 1, 2021, the apparent resistivity of the two measuring points was 42 Ω·m and 57 Ω·m, respectively. 10% of the two values ​​were 8.4 Ω·m and 11.4 Ω·m. As can be seen from Table 1, the minimum difference in apparent resistivity between the two measuring points was 15 Ω·m, indicating that the dam was at risk of leakage.

[0123] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0124] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for monitoring the time-lapse of dams based on shallow transient electromagnetic methods, characterized in that, Includes the following steps: S1. Conduct the first transient electromagnetic data acquisition of the dam. Based on the acquired transient electromagnetic data, obtain the apparent resistivity value throughout the entire process, which will be used as the initial value of the apparent resistivity of the dam. S2. Maintain the same acquisition parameters as during the initial transient electromagnetic data acquisition process of the dam, acquire transient electromagnetic data of the dam at different times, and obtain the apparent resistivity values ​​for different times respectively; S3. Calculate the ratio between the apparent resistivity values ​​at different times and the initial apparent resistivity value, and use the Kriging interpolation algorithm to obtain the cross-sectional diagram of the apparent resistivity change rate. When the apparent resistivity change rate curves on the apparent resistivity change rate cross-sectional diagram are layered or uniformly distributed, and the overall apparent resistivity change rate is less than 1, it is judged that there is a global abnormal seepage hazard in the dam. When the apparent resistivity change rate curve shows a closed anomaly, and the apparent resistivity change rate in a local area is less than 1, it is judged that there is a potential local leakage in the dam. When the dam's seepage hazard is a localized leakage hazard, the apparent resistivity value of the center point of the localized leakage hazard area within the current monitoring area is extracted. Based on the change of the apparent resistivity value over time, when the apparent resistivity value changes from high to low, it is determined that subsequent monitoring should be carried out; when the apparent resistivity value continues to decrease, and when the absolute value of the difference between the apparent resistivity value at different times and the initial resistivity value divided by the initial resistivity value exceeds 10%, it is determined that seepage prevention treatment should be implemented in the area.

2. The dam time-lapse monitoring method based on shallow transient electromagnetic method according to claim 1, characterized in that, The specific content of S1 includes: When the reservoir water level is at the normal storage level, a transient electromagnetic detection route is planned based on the reservoir dam information data. The first transient electromagnetic data acquisition of the dam is carried out using a point measurement method, and the position of each measuring point is marked. The induced electromotive force value corresponding to each measuring point is obtained, and the apparent resistivity value of the entire process is obtained using the full-process apparent resistivity calculation method. The full-process apparent resistivity data is stored as the initial value of the apparent resistivity of the reservoir dam. At the same time, an apparent resistivity cloud map is formed according to the horizontal position and vertical depth. The range and distribution of the hidden danger area are preliminarily determined through the cloud map.

3. The dam time-lapse monitoring method based on shallow transient electromagnetic method according to claim 1, characterized in that, The specific content of S2 includes: By pre-setting different time periods and according to the first transient electromagnetic data acquisition method and acquisition parameters of the dam, the induced electromotive force of the dam at different time periods is obtained, and the apparent resistivity values ​​of different time periods are obtained respectively. The apparent resistivity values ​​of different time periods are stored and formed into an apparent resistivity cloud map according to the horizontal position and vertical depth.

4. The dam time-lapse monitoring method based on shallow transient electromagnetic method according to claim 1, characterized in that, The different monitoring periods include three periods: before the flood season, during the flood season, and after the flood season; Pre-flood season monitoring is used to assess the safety status of reservoir dams and ensure their safe passage through the flood season. Monitoring during the flood season is used to determine the health status of potential hazards during the high-water season and to promptly report to management units so that emergency measures can be taken. Post-flood season monitoring is used to investigate the changes in the seepage behavior of dams after experiencing high reservoir water levels, providing technical support for reinforcement and safety improvement.

5. A dam time-lapse monitoring method based on shallow transient electromagnetic method according to claim 1, characterized in that, Specific methods for obtaining apparent resistivity values ​​using transient electromagnetic data include: Vertical component B of the secondary field at the center point of a circular transmitting coil under uniform half-space conditions z for: Corresponding rate of change over time for: In the formula, I is the emission current. ρ The resistivity of half-space a The radius of the transmitting coil, μ Let t be the permeability of a uniform half-space medium and t be the decay time, defined as: Error function: Define Z as the transient field parameter: In the formula, Apparent resistivity values ​​at different times or apparent resistivity over the entire period; in, For B z Kernel function; for Kernel function; , The apparent resistivity is obtained by observing the value of Bz, then solving for the value of Z, and finally obtaining the value of Z over the entire range.

6. The dam time-lapse monitoring method based on shallow transient electromagnetic method according to claim 1, characterized in that, Apparent resistivity change rate : In the formula, This refers to the apparent resistivity values ​​monitored at different times; This refers to the initial value of the apparent resistivity. x This refers to the x-coordinate of the recording point. z This refers to the depth coordinates of the recorded point.

7. A dam time-lapse monitoring system based on shallow transient electromagnetic method, characterized in that, include: Transient electromagnetic host, transceiver coils, and processing platform; The transient electromagnetic host includes a parameter acquisition unit and a command acquisition unit; The parameter acquisition unit is used to acquire sampling parameters, which include transmission frequency, sampling frequency, number of superpositions and / or number of measurement channels; The command acquisition unit is used to acquire sampling commands and control the transient electromagnetic host to transmit current pulses to the transceiver coil according to the commands. The transceiver coil acquires transient electromagnetic data and sends it to the transient electromagnetic host. The processing platform is used to call the transient electromagnetic data collected by the transient electromagnetic host, convert the transient electromagnetic data into apparent resistivity, calculate the ratio of the apparent resistivity values ​​at different times to the initial value of apparent resistivity, and use the Kriging interpolation algorithm to obtain the cross-sectional diagram of the rate of change of apparent resistivity. When the apparent resistivity change rate curves on the apparent resistivity change rate cross-sectional diagram are layered or uniformly distributed, and the overall apparent resistivity change rate is less than 1, it is judged that there is a global abnormal seepage hazard in the dam. When the apparent resistivity change rate curve shows a closed anomaly, and the apparent resistivity change rate in a local area is less than 1, it is judged that there is a potential local leakage hazard in the dam; When the dam's seepage hazard is a localized leakage hazard, the apparent resistivity value of the center point of the localized leakage hazard area within the current monitoring area is extracted. Based on the change of the apparent resistivity value over time, when the apparent resistivity value changes from high to low, it is determined that subsequent monitoring should be carried out; when the apparent resistivity value continues to decrease, and when the absolute value of the difference between the apparent resistivity value at different times and the initial resistivity value divided by the initial resistivity value exceeds 10%, it is determined that seepage prevention treatment should be implemented in the area.

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