A towed AMT detection device and detection method for coastal tidal flats

Abstract

The present invention discloses a towed AMT detection device and detection method for coastal tidal flats, which relate to the technical field of geological exploration for tidal flats. The detection device includes an amphibious towed air cushion vessel, a magnetotelluric instrument, a waterproof magnetic sensor, and an electric signal acquisition chain. The electric signal acquisition chain is composed of a plurality of waterproof non-polarized electrodes connected in series. The magnetotelluric instrument is arranged on the amphibious towed air cushion vessel, and the waterproof magnetic sensor and the electric signal acquisition chain are respectively connected to the magnetotelluric instrument. The present invention also provides a detection method using the above-mentioned detection device. The waterproof non-polarized electrodes and waterproof magnetic sensors used in the present invention avoid the influence of seawater corrosion on the stability of the electric and magnetic receiving signals. The electric signal acquisition chain solves the difficulty of low measurement efficiency within the limited window time of low tide due to the inability to move quickly. The apparent resistivity calculated by the underwater electric field and the water surface magnetic field of the present invention is corrected to the same interface, which solves the problem that conventional methods cannot be implemented in water areas of tidal flats.

Description

Technical Field

[0001] The present invention relates to the technical field of geological exploration of tidal flats, and in particular to a towed AMT detection device and a detection method for coastal tidal flats. Background Art

[0002] Tidal flats generally refer to the tidal zone between the high and low tide lines of coastal areas. In well-developed coastal areas, these can be over 10 km wide, with minimal slopes, typically only around 0.2‰, and are primarily composed of fine-grained sediments ranging from fine silt to clay. Broadly speaking, tidal flats also include undeveloped supratidal zones and underwater shallows that remain barely visible at low tide. Due to the unique surface conditions, complex surface structures, and significant tidal influences of tidal flats, geological data collection and construction are difficult and time-consuming. Furthermore, neither purely land-based nor purely offshore exploration techniques can resolve the geological challenges of tidal flats. Therefore, geological data collection in tidal flats has always been a challenge, resulting in insufficient geological surveys in these areas and a lack of coordinated management of land and sea geological surveys.

[0003] Currently, commonly used techniques for tidal flat water exploration include multi-beam bathymetry, single-beam bathymetry, side-by-side sonar scanning, shallow profiling, and single-channel seismic surveys. These survey the topography and geomorphology of the seafloor, the distribution of sediment types and strata, shallow structures, faults, potential geological hazards such as shallow gas formations, sand waves, landslides, collapses, and uplift, as well as the distribution characteristics of paleo-river channels and their sedimentary environments. Remote sensing is used to study the spatiotemporal changes in tidal flat shorelines, the evolution of erosion and deposition, underwater topography changes, nearshore suspended sediment information, and geological environmental information related to the tidal flats. Geophysical work in the waterless areas of tidal flats at low tide is minimal; instead, shallow seismic work is primarily focused on investigating geological hazards such as submarine landslides and environmental pollution, and DC electrical methods are used to characterize water content in strata deeper than 20 meters. For example, in the mudflat area on the south side of Hangzhou Bay in Zhejiang Province, the Jiangsu Institute of Geological Exploration and Technology and other units conducted experiments on various exploration methods for the construction of a cross-sea bridge. They used a combination of gravity measurement, shallow seismic and shallow layer profile measurement methods to restore the sedimentary characteristics of shallow strata, the distribution characteristics of shallow structures and faults, and used shallow seismic methods to identify the distribution status of shallow gas that is a hazard to engineering construction. These works are basically concentrated in coastal land areas or shallow water areas near coastal waters. Due to method limitations, the detection depth is relatively shallow and the underground geological information obtained is limited.

[0004] The significant electrical differences between water-free strata and water-bearing strata of varying degrees of mineralization in tidal flats create the fundamental conditions for AMT exploration in these areas. AMT's advantages, including deep exploration depth, high identification of low-resistivity layers, and low cost, allow it to detect deeper geological information in thickly covered coastal areas. Limited by the geomorphological characteristics of tidal flats, AMT exploration in coastal zones has been conducted only in reclaimed areas adjacent to these areas using land-based AMT instruments and acquisition methods, targeting transgressions, paleo-river channels, and geothermal sources. Previous experience and achievements indicate that ground-based AMT in reclaimed areas does not significantly differ from traditional ground-based AMT in terms of equipment, construction techniques, or processing and interpretation techniques, making it unsuitable for tidal flats. In the early days, tidal flats were primarily areas for land reclamation and aquaculture, lacking the need for major engineering projects. This limited the development of relevant geological survey methods and technologies for these areas. Amphibious work platforms, transmitter deployment, waterproof and corrosion-resistant equipment, and efficient acquisition methods and technologies for AMT exploration in tidal flats are largely unavailable.

[0005] Therefore, there is an urgent need to study an AMT detection device and detection method that can quickly and efficiently collect data in coastal tidal flat areas. Summary of the Invention

[0006] In order to solve the above technical problems, the present invention discloses a towed AMT detection device and detection method for coastal tidal flats. This detection method can not only identify geological information such as underground structure and aquifer distribution, but also improve the construction efficiency of conventional rolling arrangement AMT detection methods, save data collection construction time in tidal flats, and is particularly suitable for tidal flats that are severely affected by tidal time.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A first aspect of the present invention provides a towed AMT detection device for coastal tidal flat areas.

[0009] In an optional embodiment, a towed AMT detection device for coastal tidal flats includes an amphibious towing air cushion vessel, a magnetotelluric instrument, a waterproof magnetic sensor and an electrical signal acquisition chain. The electrical signal acquisition chain is composed of multiple waterproof non-polarized electrodes connected in series. The magnetotelluric instrument is arranged on the amphibious towing air cushion vessel, and the waterproof magnetic sensor and the electrical signal acquisition chain are respectively connected to the magnetotelluric instrument.

[0010] Optionally, the waterproof non-polarizable electrode is placed horizontally and includes a waterproof cover, a waterproof screw, a PVC tank body, a PbCl2 main body and a multi-core dragging cable. Waterproof covers and waterproof screws are provided at both ends of the PVC tank body. The PVC tank body is evenly provided with a number of openings. The PbCl2 main body is arranged inside the PVC tank body and is grounded to the measuring point through the openings. A threading tube is provided in the center of the PVC tank body. The multi-core dragging cable passes through the threading tube and is connected to the PbCl2 main body for transmitting the collected signal and performing the dragging function.

[0011] Optionally, the waterproof magnetic sensor includes an inner cavity formed by a magnetic sensor body and a magnetic sensor shell, the magnetic sensor body is installed in the inner cavity, and the top and bottom ends of the magnetic sensor shell are respectively installed with an upper waterproof cover and a lower waterproof cover, and the upper waterproof cover and the lower waterproof cover are fixed with water-proof parts. A cable waterproof head is installed at one end of the magnetic sensor shell, and a cable waterproof screw is provided on the cable waterproof head. The magnetic sensor body includes a connected coil, a magnetic core and a circuit, and an insulating potting glue is filled between the connected coil, the magnetic core and the circuit to improve the waterproof performance of the magnetic sensor body, and its function is to convert the magnetic field into an electrical signal and output it to the signal processing circuit; the magnetic core is the core component of the sensor, usually made of soft magnetic materials, such as ferrite, soda-lime glass, etc.; the function of the magnetic core is to enhance the magnetic field strength and improve the sensitivity of the sensor; the signal processing circuit is the core component of the sensor, and its function is to amplify, filter, linearize and other processes the output electrical signal to facilitate subsequent data processing and control.

[0012] Optionally, the water-isolating member is a rubber gasket and an O-ring, which is used to isolate the upper waterproof cover and the lower waterproof cover from water.

[0013] Optionally, the distance between each two waterproof non-polarizable electrodes is 25 to 50 m.

[0014] A second aspect of the present invention provides a detection method using the above-mentioned AMT detection device.

[0015] In an optional embodiment, a detection method using the above-mentioned AMT detection device includes the following steps:

[0016] Step S1, deploying the above-mentioned towed AMT detection device in the coastal mudflat area;

[0017] Step S2: planning a survey line on the surface of the tidal flat area, deploying a collection station in a waterless area of ​​the tidal flat area near the survey line, and collecting tensor magnetic signals;

[0018] Step S3: In the area of ​​the mudflat where there is no water or the water depth is less than 2m, measurement is performed using a fixed-point observation method using a towed station;

[0019] Step S4: After the data of one arrangement is collected, the amphibious towing hovercraft moves forward by an arrangement distance, collects the data of the arrangement, and records and stores it;

[0020] Step S5: Repeat steps S3 and S4 until the test reaches the end of the exploration line or the water depth in the mudflat area is greater than 2m;

[0021] Step S6: Calculate the magnetic field components of the two mutually orthogonal directions collected synchronously to the magnetic field components at any angle, and calculate the apparent resistivity and impedance phase by tensor rotation; wherein, the electromagnetic separation measurement method is used in the water area of ​​the tidal flat, and the apparent resistivity calculated by the electric field and the magnetic field is corrected to the same interface;

[0022] Step S7: After completing data acquisition for a survey line, the apparent resistivity and impedance phase of the corrected measurement point data in the water area are calculated, and then inverted together with the measurement point data in the water-free area to form an apparent resistivity inversion profile.

[0023] Optionally, in step S3, in an area of ​​the mudflat where there is no water and the water depth is less than 2m, a towed station fixed-point observation method is used for measurement, specifically including: adjusting the position of the amphibious towing hovercraft so that the detection device forms a straight line and is parallel to the exploration line; using a magnetotelluric instrument and an electrical signal acquisition chain to perform scalar observation and acquisition of electrical signals, towing the first electrode of the electrical signal acquisition chain to the starting point of the exploration line, recording the azimuth of the tow cable arranged during measurement, collecting the arrangement data and recording and storing it; if the arrangement is located in a water area, water depth data needs to be recorded.

[0024] Optionally, in step S6, the electromagnetic separation measurement method comprises the following steps: placing an electric signal acquisition chain underwater, and placing a waterproof magnetic sensor on shore or in a waterless area of ​​a mudflat to collect seabed electric field signals and magnetic field signals respectively.

[0025] Optionally, in step S6, the step of correcting the apparent resistivity calculated from the electric field and the magnetic field to the same interface includes:

[0026] In a horizontal layered one-dimensional medium, the wave equation in any layer of the medium is:

[0027]

[0028] The general solution of formula (1) is:

[0029]

[0030] Formula (2) is the expression of electromagnetic field intensity in layered medium, where E x is the electric field strength, H y is the magnetic field intensity, k is the complex wave number, j is a complex unit, ω = 2πf is the angular frequency, μ = 4π × 10 -7 H / m, σ i is the conductivity of the i-th layer medium; z is the vertical depth, z i-1 ≤z i ≤z i+1 , A i 、B i is the integration constant between each layer;

[0031] For the top and bottom fields of the first layer of seawater, z = 0 and z = h1, respectively, they are substituted into equation (2) to obtain:

[0032]

[0033] Where, is the top electric field strength of seawater, A i 、B i is the integration constant between each layer, Z i is the characteristic impedance of the i-th layer medium, is the top magnetic field strength of seawater;

[0034]

[0035] Where k is the complex wave number, σ1 is the conductivity of the first layer of medium, is the bottom electric field strength of seawater, is the bottom magnetic field strength of seawater, h1 is the depth of seawater, A i 、B i is the integral constant between each layer, Z1 is the characteristic impedance of the first layer medium, i.e. seawater,

[0036] After adding equation (3) and equation (4) respectively, solve them together to eliminate A i 、B i ,get:

[0037]

[0038] Similarly, subtract the two equations separately and solve them together to eliminate A. i 、B i ,get:

[0039]

[0040] For the water-beach area, the magnetic field data uses the measurement data of the water-free area. According to the relative uniformity of the magnetotelluric field, the magnetic field is regarded as the water surface magnetic field, that is, Then there is Therefore, both sides of equations (5) and (6) are divided by Then add the two sides together to get the relationship equation between electromagnetic separation data and water surface electromagnetic synchronization, as follows:

[0041] Z ′ =Z 0 cosh(k1h1)-Z1sinh(k1h1)(7)

[0042] Z 0 =(Z ′ +Z1sinh(k1h1)) / cosh(k1h1)(8)

[0043] Similarly, divide both sides of equations (5) and (6) by Then add the two sides together to get the relationship equation between electromagnetic separation data and bottom electromagnetic synchronization, as follows:

[0044] Z ′ =Z 1 Z1 / [Z1 cosh(k1h1)+Z 1 sinh(k1h1)](9)

[0045] Z 1 =Z ′ Z1 cosh(k1h1) / [Z1-Z ′ sinh(k1h1)](10)

[0046] Where k is the complex wave number, j is a complex unit, ω = 2πf is the angular frequency, μ = 4π × 10 -7 H / m, σ1 is the conductivity of the first layer medium; is the characteristic impedance of the first layer medium; Z 0 To correct for water surface impedance; Z 1 To correct for the bottom impedance; Z ′ is the measured electromagnetic separation impedance;

[0047] The data of the measurement points in the water area are corrected to the water surface or bottom through equations (7), (8) and (9), (10).

[0048] Optionally, in step S6, the relationships among the apparent resistivity, impedance phase and impedance are:

[0049]

[0050] Where, ρ a is the apparent resistivity, P h is the impedance phase, ω=2πf is the angular frequency, μ=4π×10 -7 H / m, Z is impedance.

[0051] The beneficial effects of the present invention are:

[0052] 1. The present invention's towed AMT detection method for coastal tidal flats is particularly suitable for coastal tidal flats, which are constrained by geological exploration equipment and tidal time. It maximizes the flat surface terrain of tidal flats and uses a towing method to collect AMT data. The present invention employs waterproof, non-polarized electrodes, waterproof magnetic sensors, and an electrical signal acquisition chain to address the issue of seawater corrosion affecting the stability of electrical and magnetic signals. The electrical signal acquisition chain addresses the difficulty of rapid movement of personnel in tidal flats, resulting in low measurement efficiency during the limited low tide window. Compared with conventional methods and instruments, this method boasts waterproof and corrosion-resistant instruments and equipment, high data acquisition efficiency, and authentic and valid data.

[0053] 2. The detection method of the present invention does not need to be dragged strictly in the designed direction during the measurement process. It only needs to record the azimuth of the towline arranged during the measurement, calculate the magnetic field component to any angle by collecting the magnetic field components in two mutually orthogonal directions, and finally calculate the correct apparent resistivity value by tensor rotation, thereby improving work efficiency. The present invention uses the apparent resistivity calculated by the bottom electric field and the water surface magnetic field to correct it to the same interface (water surface or bottom), and convert it into the apparent resistivity and impedance phase of the water surface or bottom, which solves the problem that the conventional AMT detection method cannot be implemented in the area due to the inability to synchronously observe the electric field and magnetic field data at the same level in the water area.

[0054] 3. The towed AMT detection method for coastal tidal flats designed by the present invention can achieve the application effects of conventional methods in terms of detection depth, accuracy and resolution, and can provide real and reliable data support for the fine division of underground stratigraphic structure in tidal flats. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 Schematic diagram of a waterproof non-polarizable electrode structure according to an embodiment of the present invention, wherein (a) is a cross-sectional view and (b) is a cross-sectional view;

[0056] Figure 2 1 is a schematic structural diagram of a waterproof magnetic sensor according to an embodiment of the present invention;

[0057] Figure 3 1 is a schematic diagram of the structure of an electrical signal acquisition chain according to an embodiment of the present invention;

[0058] Figure 4 Schematic diagram of a towed AMT detection device for a tidal flat area according to an embodiment of the present invention;

[0059] Figure 5 This is a schematic diagram of electromagnetic wave propagation in layered geology under seawater coverage conditions, showing an embodiment of the present invention;

[0060] Figure 6This is a comparison of the workload that can be completed in a single day using a conventional method and a drag-and-drop method, as shown in one embodiment of the present invention;

[0061] Figure 7 1 is a graph comparing apparent resistivity and impedance phase using scalar and tensor impedance calculation methods according to an embodiment of the present invention, wherein (a) is a graph comparing apparent resistivity, and (b) is a graph comparing impedance phase.

[0062] Figure 8 1 is a comparison diagram of calibration curves of apparent resistivity and impedance phase converted to water surface or water bottom and actual measurement results, shown in one embodiment of the present invention, wherein (a) is a comparison diagram of calibration curves of apparent resistivity and actual measurement results, and (b) is a comparison diagram of calibration curves of impedance phase and actual measurement results;

[0063] Figure 9 Schematic diagram of fixed measurement and drag-type measurement points according to an embodiment of the present invention;

[0064] Figure 10 1 is a comparison diagram of conventional measurement and towed measurement inversion results according to an embodiment of the present invention, wherein (a) is a conventional measurement inversion result diagram, and (b) is a towed measurement inversion result diagram.

[0065] Figure numerals: 1. multi-core dragging cable, 2. waterproof screw, 3. PVC tank body, 4. PbCl2 main body, 5. threading tube, 6. waterproof cover, 7. lower waterproof cover, 8. sensor housing, 9. magnetic sensor main body, 10. upper waterproof cover, 11. cable waterproof head, 12. cable waterproof screw. DETAILED DESCRIPTION

[0066] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0067] Example 1

[0068] A towed AMT detection device in coastal mudflats, such as Figure 3 and 4As shown, the system includes an amphibious towed air cushion vessel, a Phoenix magnetotelluric instrument, waterproof magnetic sensors, and an electrical signal acquisition chain. The electrical signal acquisition chain is composed of multiple waterproof non-polarized electrodes connected in series. The Phoenix magnetotelluric instrument is mounted on the amphibious towed air cushion vessel, and the waterproof magnetic sensors and electrical signal acquisition chain are connected to the Phoenix magnetotelluric instrument. The waterproof non-polarized electrodes, influenced by gravity, can be tightly coupled to the bottom surface of both dry and wet areas of the mudflat, saving the time required for manual electrode burial and electrode spacing measurement. The waterproof magnetic sensors are difficult to position horizontally and precisely in a single direction underwater. Therefore, a separate electromagnetic signal data acquisition method is employed: a magnetic signal acquisition station is deployed in a dry area of ​​the mudflat near the survey line, and a towed electrical signal acquisition station is deployed along the survey line. The acquired data is then used to calculate accurate apparent resistivity and impedance phase through tensor rotation and underwater magnetic field correction. This data is then used for inversion calculations to form an apparent resistivity inversion profile.

[0069] Conventional standing non-polarized electrodes are not waterproof and their performance is unstable under the highly corrosive conditions of high seawater content in mudflats. At the same time, due to the large bottom opening, the PbCl2 body is rapidly lost when immersed in seawater. Therefore, improving its waterproof and anti-corrosion performance is crucial for the collection of electrical signals. Conventional non-polarized electrodes are composed of terminal blocks, PVC tanks, and built-in PbCl2 bodies. In this optional embodiment, the waterproof non-polarized electrode, such as Figure 1 As shown, it is placed horizontally and includes a waterproof cover 6, a waterproof screw 2, a PVC tank body 3, a PbCl2 main body 4 and a multi-core towing cable 1. Waterproof covers 6 and waterproof screws 2 are provided at both ends of the PVC tank body 3. The PVC tank body 3 is evenly distributed with a number of openings. The PbCl2 main body 4 is arranged inside the PVC tank body 3 and is grounded to the measuring point through the openings. Compared with the large-area grounding contact at the bottom of the conventional non-polarized electrode, the openings are evenly distributed around the PVC tank body 3, greatly reducing the loss caused by seawater immersion corrosion; a threading tube 5 is provided in the center of the PVC tank body 3, and the multi-core towing cable 1 passes through the threading tube 5 and is connected to the PbCl2 main body 4 for transmitting the collected signal and assuming the towing function; due to the passage of the multi-core towing cable 1, both ends of the waterproof non-polarized electrode are waterproofly sealed with waterproof covers 6 and waterproof screws 2. When the cable is towing, both the waterproofness of the cable in the threading tube 5 and the towing performance of the towing cable are guaranteed.

[0070] Conventional magnetic sensors are designed for land use, and their top and bottom are not waterproof structures. Signal output connectors and cables do not have waterproof structures. When used in mudflat areas, seawater entering the sensor may cause its function to fail, affecting data collection. Figure 2As shown, the waterproof magnetic sensor of this embodiment includes an inner cavity formed by a magnetic sensor body 9 and a magnetic sensor housing 8, and the magnetic sensor body 9 is installed in the inner cavity. The top and bottom ends of the magnetic sensor housing 8 are respectively installed with an upper waterproof cover 10 and a lower waterproof cover 7, and the upper waterproof cover 10 and the lower waterproof cover 7 are fixed with rubber gaskets and O-rings, and double-layer protection is used to make the upper waterproof cover 10 and the lower waterproof cover 7 waterproof; one end of the magnetic sensor housing 8 is installed with a cable waterproof head 11, and the cable waterproof head 11 is provided with a cable waterproof screw 12; the magnetic sensor body 9 includes a connected coil, a magnetic core and a circuit, and an insulating potting glue is filled between the coil, the magnetic core and the circuit to improve the waterproof performance of the magnetic sensor body 9. Even if the waterproof performance of the upper waterproof cover 10 and the lower waterproof cover 7 is attenuated due to aging of the rubber gasket and the O-ring, the internal magnetic sensor body 9 can still work normally.

[0071] Example 2

[0072] A detection method using the AMT detection device described in Example 1 includes the following steps:

[0073] Step S1, deploying the above-mentioned towed AMT detection device in the coastal mudflat area;

[0074] like Figure 4 As shown, the signal acquisition chain consists of multiple waterproof, non-polarizable electrodes connected in series. The distance between each pair of electrodes, or the electrode spacing, is typically 25 to 50 meters. This spacing is adjusted based on the electrical signal characteristics of the construction site; this example uses a 25-meter spacing. The signal acquisition chain collects signals by connecting to a Phoenix magnetotelluric instrument deployed on an amphibious towed hovercraft. The towing distance is typically 10 to 15 meters; this example uses a 10-meter towing distance.

[0075] Step S2: planning a survey line on the surface of the tidal flat area, deploying a collection station in a waterless area of ​​the tidal flat area near the survey line, and collecting tensor magnetic signals;

[0076] like Figure 7 As shown in the figure, in a certain area of ​​the North Sea, the exploration line can cross the land area, the dry mudflat area, and the water area of ​​the mudflat (water depth less than 2m), and can be laid out in a linear, mesh, or free mesh manner. In the later stage, depending on the needs of resistivity and impedance phase calculation, a collection station is deployed near the dry mudflat area of ​​the exploration line to synchronously collect tensor magnetic signals, such as Figure 4 As shown in the figure, the collection station in the dry area of ​​the mudflat area needs to use a waterproof magnetic sensor to connect to the Phoenix magnetotelluric instrument. If the location of the collection station is submerged by seawater during high and low tides, it can be freely moved near the survey line to the dry area of ​​the mudflat area to cooperate with the towed electrical signal collection work until all the measurement points on the survey line are completed.

[0077] Step S3: In the area of ​​the mudflat where there is no water or the water depth is less than 2m, measurement is performed using a fixed-point observation method using a towed station;

[0078] Specifically, the process involves adjusting the position of the amphibious towing hovercraft so that the detection device forms a straight line and is parallel to the survey line; using a Phoenix magnetotelluric instrument and an electrical signal acquisition chain to perform scalar observation and acquisition of electrical signals; towing the first electrode of the electrical signal acquisition chain to the starting point of the survey line; recording the azimuth of the towline arrangement during measurement; and collecting and storing the arrangement data; and recording water depth data if the arrangement is located in a water area.

[0079] At the beginning of each acquisition, the starting point of the survey line is set. During high tide, the starting point of the survey line is set at the seaward end, and during low tide, the starting point is located at the shoreward end to ensure the maximum data acquisition time window. The first electrode of the towed electrical signal acquisition chain is located at the starting point of the survey line and does not need to be strictly parallel to the survey line direction during the measurement process.

[0080] like Figure 4 As shown in the figure, after the magnetic signal acquisition stations were deployed in the dry areas of the mudflat, the amphibious towing hovercraft was positioned so that its towed electrical signal acquisition chain was parallel to the survey line. Data acquisition was performed using scalar electric field observation. The electrical signals collected by the signal acquisition chain were transmitted via the multi-core wires within the multi-core towing cable to the Phoenix magnetotelluric instrument on the amphibious towing hovercraft, where they were recorded and stored on the Phoenix magnetotelluric instrument's built-in memory card. The azimuth of the towed cables during the measurement was also recorded.

[0081] Step S4: After the data of one arrangement is collected, the amphibious towing hovercraft moves forward by an arrangement distance, collects the data of the arrangement, and records and stores it;

[0082] During data collection, the amphibious towing hovercraft must shut down and drop anchor. After the observation window has expired, the observation data for the current arrangement is saved. The amphibious towing hovercraft then drives the signal acquisition chain forward by one arrangement distance. During the towing process, the signal acquisition chain should be roughly parallel to the direction of the survey line until the first electrode of the signal acquisition chain is at the starting point of the arrangement. At this point, data collection begins, and the instrument is monitored for proper operation. After the observation window has expired, the observation data for the current arrangement is saved, and the towline azimuth of the arrangement is recorded.

[0083] Step S5: Repeat steps S3 and S4 until the test reaches the end of the exploration line or the water depth in the mudflat area is greater than 2m;

[0084] The arrangement of a survey line cannot be completed within one high and low tide cycle, and must be carried out in the next high and low tide cycle until the measurement work of the entire survey line is completed.

[0085] Step S6: Calculate the magnetic field components of the two mutually orthogonal directions collected synchronously to the magnetic field components at any angle, and calculate the apparent resistivity and impedance phase by tensor rotation; wherein, the electromagnetic separation measurement method is used in the water area of ​​the tidal flat, and the apparent resistivity calculated by the electric field and the magnetic field is corrected to the same interface;

[0086] Compared to conventional land-based observations, this embodiment utilizes scalar measurements. Magnetic signal acquisition stations in the mudflat area still need to simultaneously collect magnetic field components in two orthogonal directions to facilitate impedance calculation using a tensor method, improving data quality. Tensor observations at magnetic signal acquisition stations can calculate magnetic field information in any direction. By recording the streamer azimuth during measurement, the correct resistivity value can be calculated using a tensor rotation method.

[0087] The electromagnetic separation measurement method involves placing an electrical signal acquisition chain underwater and waterproof magnetic sensors onshore or in dry areas of mudflats to separately collect seafloor electric and magnetic field signals. The magnetic field in the magnetotelluric field is relatively uniform across the region, allowing the magnetic field near the survey line to replace the magnetic field at the underwater measuring point for electromagnetic signal separation. However, the apparent resistivity calculated using the underwater electric field and surface magnetic field must be corrected to the same interface (water surface or bottom) and converted into the apparent resistivity and impedance phase at the water surface or bottom before it can be combined with measurements in water or onshore for inversion interpretation; surface measuring points are not affected.

[0088] The problem of electromagnetic signal separation and collection is that the observation positions are not on the same interface, the electric field amplitude becomes smaller due to the attenuation of the water body, and the phase lags. The apparent resistivity calculated directly using the measured data is relatively small in the high-frequency band. Therefore, the measured data needs to be corrected before use.

[0089] like Figure 5 As shown in FIG. 1 , a schematic diagram of electromagnetic wave propagation in layered geological structures under seawater cover in a mudflat area is shown. The steps of correcting the apparent resistivity calculated from the electric field and the magnetic field to the same interface include:

[0090] Under the full-space one-dimensional layered magnetotelluric theory, in a horizontal layered one-dimensional medium, the wave equation in any layer of the medium is:

[0091]

[0092] The general solution of formula (1) is:

[0093]

[0094] Formula (2) is the expression of electromagnetic field intensity in layered medium, where E x is the electric field strength, H y is the magnetic field intensity, k is the complex wave number, j is a complex unit, ω = 2πf is the angular frequency, μ = 4π × 10 -7 H / m, σ i is the conductivity of the i-th layer medium; z is the vertical depth, z i-1 ≤z i ≤z i+1 , A i 、B i is the integration constant between each layer;

[0095] For the top and bottom fields of the first layer of seawater, z = 0 and z = h1, respectively, they are substituted into equation (2) to obtain:

[0096]

[0097] Where, is the top electric field strength of seawater, A i 、B i is the integration constant between each layer, Z i is the characteristic impedance of the i-th layer medium, is the top magnetic field strength of seawater;

[0098]

[0099] Where k is the complex wave number, σ1 is the conductivity of the first layer of medium, is the bottom electric field strength of seawater, is the bottom magnetic field strength of seawater, h1 is the depth of seawater, A i 、B i is the integral constant between each layer, Z1 is the characteristic impedance of the first layer medium, i.e. seawater,

[0100] After adding equation (3) and equation (4) respectively, solve them together to eliminate A i 、B i ,get:

[0101]

[0102] Similarly, subtract the two equations separately and solve them together to eliminate A. i 、B i ,get:

[0103]

[0104] For the water-beach area, the magnetic field data uses the measurement data of the water-free area. According to the relative uniformity of the magnetotelluric field, the magnetic field is regarded as the water surface magnetic field, that is, Then there is Therefore, both sides of equations (5) and (6) are divided by Then add the two sides together to get the relationship equation between electromagnetic separation data and water surface electromagnetic synchronization, as follows:

[0105] Z ′ =Z 0 cosh(k1h1)-Z1sinh(k1h1)(7)

[0106] Z 0 =(Z ′ +Z1sinh(k1h1)) / cosh(k1h1)(8)

[0107] Similarly, divide both sides of equations (5) and (6) by Then add the two sides together to get the relationship equation between electromagnetic separation data and bottom electromagnetic synchronization, as follows:

[0108] Z ′ =Z 1 Z1 / [Z1 cosh(k1h1)+Z 1 sinh(k1h1)](9)

[0109] Z 1 =Z ′ Z1 cosh(k1h1) / [Z1-Z ′ sinh(k1h1)](10)

[0110] Where k is the complex wave number, j is a complex unit, ω = 2πf is the angular frequency, μ = 4π × 10 -7 H / m, σ1 is the conductivity of the first layer medium; is the characteristic impedance of the first layer medium; Z 0 To correct for water surface impedance; Z 1 To correct for the bottom impedance; Z ′ is the measured electromagnetic separation impedance;

[0111] The data of the measurement points in the water area are corrected to the water surface or the bottom through equations (7), (8) and (9) and (10). It can be seen from the equations that the correction of the electromagnetic separation measurement data to the bottom or the surface is only related to the seawater thickness and seawater conductivity.

[0112] In the steps of calculating apparent resistivity and impedance phase, the relationships among apparent resistivity, impedance phase and impedance are:

[0113]

[0114] Where, ρ a is the apparent resistivity, P his the impedance phase, ω=2πf is the angular frequency, μ=4π×10 -7 H / m, Z is the impedance. When AMT surveying mudflats, single-point data can be approximately regarded as a one-dimensional layered medium. The above formula can be used to correct the measured electromagnetic separation data to the water bottom or surface.

[0115] Step S7: After completing data acquisition for a survey line, the apparent resistivity and impedance phase of the corrected water-filled area measurement point data are calculated, and then inverted together with the water-free area measurement point data to form an apparent resistivity inversion profile. This embodiment will compare and analyze the towed AMT detection method (hereinafter referred to as the "towed method") and the conventional AMT detection method (hereinafter referred to as the "conventional method") in the mudflat area from three aspects: data acquisition efficiency, data validity, and inversion results:

[0116] (1) Comparison of data collection efficiency

[0117] Conventional methods require complex instruments and equipment with limited waterproof performance, which is mainly reflected in the large number of non-polarizable electrodes, electrical signal receiving cables and acquisition hosts. The plugs of the instruments and equipment that need to be connected are numerous and complex. According to statistics, an average of 20 non-polarizable electrodes, 20 electrical signal receiving cables and 5 acquisition hosts are invested in data collection work every day, and construction is impossible in water areas of the mudflats. The data collection method of the towing method only requires an amphibious towing air cushion craft, a towing cable with 6 waterproof non-polarizable electrodes and a 6-channel signal acquisition host system to complete the field AMT data collection tasks in water and non-water areas of the mudflats. Figure 4 The diagram below shows a schematic diagram of a towed AMT detection device for mudflat areas. Therefore, when conducting AMT data collection work in mudflat areas, the towed type requires fewer instruments and equipment, and the data collection method is simple, which can save a lot of time for data reception and recording, greatly improving data collection efficiency.

[0118] Figure 6The results of a five-day analysis of the conventional and drag-and-drop methods revealed that the conventional method achieved a maximum of 11 measurement points, with a minimum of 8 points, for an average of 9.4 points. The drag-and-drop method, on the other hand, achieved a maximum of 24 points, with a minimum of 20 points, for an average of 21.4 points, doubling work efficiency. This is primarily due to the significant labor and time required to deploy, connect, disassemble, and recover instrument equipment before and during data collection, during the "relocation" process, and after data collection. This includes burying and recovering non-polarizable electrodes, retracting and deploying receiving cables, and connecting and disassembling the acquisition host. This work consumes approximately 25% of the AMT data collection time in the mudflat area, leaving the remaining 75% for data reception and recording. Considering the conventional method's use of non-polarized electrodes and receiving cables without waterproofing, which can cause equipment to malfunction in the high-water and high-salinity environments of mudflats, as well as the difficulty of moving personnel in areas with water and mud, the time spent on data reception and recording will be reduced to no more than 40%. The narrow window for effective data collection is one of the main factors hindering the progress of project implementation. In contrast, the drag-and-drop method is simple and easy to use. After completing data collection for one arrangement, you can move to the next arrangement, reducing the time consumed by the difficulty of moving personnel due to the seawater and silt in the mudflats, as well as the malfunction of equipment due to the water and high-salinity environments. During the mudflat data collection process, only the time spent on "moving" the equipment accounts for 5% of the total working time, so 95% of the total working time can be used for AMT data collection, greatly improving data collection efficiency and significantly increasing the amount of work that can be completed in a single day.

[0119] Because the daily high and low tide intervals in tidal flats are short, no more than 5 hours a day, it is very difficult to carry out AMT exploration work in tidal flats. How to ensure the quality of data collection and complete the workload of the task within the limited time of each day, that is, to improve the efficiency of AMT data collection in tidal flats, is the key factor.

[0120] (2) Data validity comparison

[0121] Conventional methods use tensor observation and impedance calculation, while the drag-and-drop method uses tensor rotation to calculate impedance after observing the electrical signal as a scalar and the magnetic signal as a tensor. Therefore, the drag-and-drop method calculates the correct impedance using tensor rotation. Figure 9 This is the measured point map of the BH1 line in the Beihai Beach area using the conventional method and the dragging method. Figure 7 for Figure 9The following is a comparative analysis of the apparent resistivity and phase after tensor calculation using the conventional method and the drag-and-drop method at one point in the survey line. It can be seen from the figure that the apparent resistivity and phase obtained by the two methods are basically consistent, with errors occurring only at individual frequency points.

[0122] Figure 8 Comparing the calibration curves for apparent resistivity and phase corrected to the water surface or bottom with the measured results shows that the apparent resistivity and impedance phase curves corrected to the bottom using the dragging method are similar to those obtained using the conventional method. Therefore, for different implementation areas, calibrating data collected from measurement points in the water-filled areas of the tidal flats can yield data that is more consistent with conventional methods.

[0123] (3) Comparison of inversion results

[0124] Figure 9 This is the measured point map of the BH1 line in the Beihai Beach area using the conventional method and the dragging method. Figure 10 The comparison chart of the inversion results of the two methods is shown in Figure 2. Figure 10 (a) is the inversion profile using the conventional method. Figure 10 (b) is a profile inverted using the drag-and-drag method. The inversion results show clear electrical characteristics of the subsurface in the mudflat area, with distinct resistivity stratification from top to bottom. Based on resistivity differences, the area is roughly divided into three layers, excluding a very thin, saline, loose sedimentary sand layer at the surface. The first layer is approximately 20 meters thick with a resistivity of approximately 1 Ω·m, inferred to be a Quaternary marine deposit composed primarily of silty clay and clayey silt, serving as the primary aquiclude. The second layer is approximately 10 meters thick with a resistivity of approximately 0.3 Ω·m, inferred to be a marine deposit of sand and fine sand containing seawater, representing a saline layer. A large area of ​​high resistivity, with resistivities exceeding 30 Ω·m, is observed at a depth of approximately 30 meters. This is inferred to be a Tertiary stratum composed primarily of interbedded clay, silty clay, and gravel. Both methods provide excellent representation of both shallow, loose sedimentary layers and deep bedrock strata, with the thicknesses of the individual layers being largely consistent, consistent with known geological information. Comparison of the inversion results further validates the effectiveness of the drag-and-drag method.

[0125] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the scope of protection of the present invention.

Claims

1. A detection method for a towed AMT detection device in a coastal tidal flat area, characterized in that: The steps include: Step S1: deploying a towed AMT detection device in a coastal mudflat area; Step S2: planning a survey line on the surface of the tidal flat area, deploying a collection station in a waterless area of ​​the tidal flat area near the survey line, and collecting tensor magnetic signals; Step S3: In the area of ​​the mudflat where there is no water or the water depth is less than 2m, measurement is performed using a fixed-point observation method using a towed station; Step S4: After the data of one arrangement is collected, the amphibious towing hovercraft moves forward by an arrangement distance, collects the data of the arrangement, and records and stores it; Step S5: Repeat steps S3 and S4 until the test reaches the end of the exploration line or the water depth in the mudflat area is greater than 2m; Step S6: Calculate the magnetic field components of the two mutually orthogonal directions collected synchronously to the magnetic field components at any angle, and calculate the apparent resistivity and impedance phase by tensor rotation; wherein, the electromagnetic separation measurement method is used in the water area of ​​the tidal flat, and the apparent resistivity calculated by the electric field and the magnetic field is corrected to the same interface; Step S7: After completing data acquisition for a survey line, the apparent resistivity and impedance phase of the corrected water-containing area measurement point data are calculated, and then inverted together with the water-free area measurement point data to form an apparent resistivity inversion profile; The towed AMT detection device for coastal mudflats includes an amphibious towing air cushion vessel, a magnetotelluric instrument, a waterproof magnetic sensor, and an electrical signal acquisition chain. The electrical signal acquisition chain is composed of multiple waterproof non-polarized electrodes connected in series. The magnetotelluric instrument is arranged on the amphibious towing air cushion vessel, and the waterproof magnetic sensor and the electrical signal acquisition chain are respectively connected to the magnetotelluric instrument. The waterproof non-polarizable electrode is placed horizontally and includes a waterproof cap, a waterproof screw, a PVC tank, a PbCl2 main body, and a multi-core dragging cable. The waterproof cap and waterproof screw are provided at both ends of the PVC tank. The PVC tank is evenly provided with a plurality of openings. The PbCl2 main body is provided inside the PVC tank and is grounded to the measuring point through the openings. A threading tube is provided in the center of the PVC tank. The multi-core dragging cable passes through the threading tube and is connected to the PbCl2 main body for transmitting the collected signal and performing the dragging function. The waterproof magnetic sensor includes an inner cavity formed by a magnetic sensor body and a magnetic sensor housing. The magnetic sensor body is installed in the inner cavity. The top and bottom ends of the magnetic sensor housing are respectively installed with an upper waterproof cover and a lower waterproof cover. The upper waterproof cover and the lower waterproof cover are fixed with a water barrier. One end of the magnetic sensor housing is installed with a cable waterproof head. The cable waterproof head is provided with a cable waterproof screw. The magnetic sensor body includes a connected coil, a magnetic core and a circuit. The coil, the magnetic core and the circuit are filled with insulating potting glue to improve the waterproof performance of the magnetic sensor body. The water-isolating member is a rubber gasket and an O-ring, which is used to isolate the upper waterproof cover from the lower waterproof cover; The distance between each two waterproof non-polarized electrodes is 25 to 50 meters.

2. The detection method of a towed AMT detection device in a coastal tidal flat area according to claim 1, characterized in that: In step S3, in the area of ​​the mudflat where there is no water and the water depth is less than 2m, the measurement is carried out using a towed station fixed-point observation method, which specifically includes: adjusting the position of the amphibious towing air cushion vehicle so that the detection device forms a straight line and is parallel to the exploration line; using the magnetotelluric instrument and the electric signal acquisition chain to perform scalar observation and acquisition of electric signals, towing the first electrode of the electric signal acquisition chain to the starting point of the exploration line, recording the azimuth of the tow cable arranged during measurement, collecting the arrangement data and recording and storing it; if the arrangement is located in a water area, it is necessary to record the water depth data.

3. The detection method of a towed AMT detection device in a coastal tidal flat area according to claim 1, characterized in that: In step S6, the electromagnetic separation measurement method comprises the following steps: placing an electric signal acquisition chain underwater, and placing a waterproof magnetic sensor on the shore or in a waterless area of ​​a mudflat to collect seabed electric field signals and magnetic field signals respectively.

4. The detection method of a towed AMT detection device in a coastal tidal flat area according to claim 1, characterized in that: In step S6, the step of correcting the apparent resistivity calculated by the electric field and the magnetic field to the same interface includes: In a horizontal layered one-dimensional medium, the wave equation in any layer of the medium is: The general solution of formula (1) is: Formula (2) is the expression of electromagnetic field intensity in layered medium, where E x is the electric field strength, H y is the magnetic field intensity, k is the complex wave number, j is a complex unit, ω = 2πf is the angular frequency, μ = 4π × 10 -7 H / m, σ i is the conductivity of the i-th layer medium; z is the vertical depth, z i-1 ≤z i ≤z i+1 , A i 、B i is the integration constant between each layer; For the top and bottom fields of the first layer of seawater, z = 0 and z = h1, respectively, they are substituted into equation (2) to obtain: Where, is the top electric field strength of seawater, A i 、B i is the integration constant between each layer, Z i is the characteristic impedance of the i-th layer medium, is the top magnetic field strength of seawater; Where k is the complex wave number, σ1 is the conductivity of the first layer of medium, is the bottom electric field strength of seawater, is the bottom magnetic field strength of seawater, h1 is the depth of seawater, A i 、B i is the integral constant between each layer, Z1 is the characteristic impedance of the first layer medium, i.e. seawater, After adding equation (3) and equation (4) respectively, solve them together to eliminate A i 、B i ,get: Similarly, subtract the two equations separately and solve them together to eliminate A. i 、B i ,get: For the water-beach area, the magnetic field data uses the measurement data of the water-free area. According to the relative uniformity of the magnetotelluric field, the magnetic field is regarded as the water surface magnetic field, that is, Then there is Therefore, both sides of equations (5) and (6) are divided by Then add the two sides together to get the relationship equation between electromagnetic separation data and water surface electromagnetic synchronization, as follows: Z′=Z 0 cosh(k1h1)-Z1sinh(k1h1) (7) With 0 =(Z′+Z1sinh(k1h1)) / cosh(k1h1) (8) Similarly, divide both sides of equations (5) and (6) by Then add the two sides together to get the relationship equation between electromagnetic separation data and bottom electromagnetic synchronization, as follows: Z′=Z 1 Z1 / [Z1cosh(k1h1)+Z 1 birth(k1h1)] (9) Z 1 =Z′Z1cosh(k1h1) / [Z1-Z′sinh(k1h1)] (10) Where k is the complex wave number, j is a complex unit, ω = 2πf is the angular frequency, μ = 4π × 10 -7 H / m, σ1 is the conductivity of the first layer medium; is the characteristic impedance of the first layer medium; Z 0 To correct for water surface impedance; Z 1 is the impedance corrected to the bottom of the water; Z′ is the measured electromagnetic separation impedance; The data of the measurement points in the water area are corrected to the water surface or bottom through equations (7), (8) and (9), (10).

5. The detection method of a towed AMT detection device in a coastal tidal flat area according to claim 1, characterized in that: In step S6, the relationships among the apparent resistivity, impedance phase and impedance are: Where, ρ a is the apparent resistivity, P h is the impedance phase, ω=2πf is the angular frequency, μ=4π×10 -7 H / m, Z is impedance.

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