Superficial sea area magnetotelluric acquisition system and method

Abstract

A magnetotelluric instrument is used for recording electromagnetic field time sequence data, shallow sea non-polarized electrodes are hung on the four sides of an electric channel collection platform on the water surface to receive electric field signals, and the electric channel collection platform is connected to the magnetotelluric instrument through a waterproof cover, a multi-core cable and a shielding wire. The three-component magnetic rod sinking to the bottom receives magnetic track signals and is connected to the magnetotelluric instrument through the magnetic rod wire, five-component collection is conducted on each measuring point, the GPS in the middle of the electric field collection platform records position information of the measuring points, and the net bag stone anchor conducts position fixing and fixed-point measurement on the whole collection system. The working reliability of the magnetotelluric acquisition system in the shallow sea area is improved, the accuracy of magnetotelluric acquisition in the shallow sea area is realized, the degree of foundation geology and engineering geology survey of a land-sea combined zone is also improved to a certain extent, and the requirements of major engineering layout planning and resource development and utilization are met.

Description

Technical Field

[0001] The present invention belongs to the technical field of electromagnetic detection methods, and particularly relates to a magnetotelluric acquisition system and method for shallow sea areas. Background Art

[0002] The magnetotelluric method is a frequency-domain electromagnetic detection method. By observing the natural alternating electromagnetic field on the earth's surface and obtaining the magnetotelluric tensor impedance, it is used to study the underground geological structure. It has the characteristics of high lateral resolution, large exploration depth, being particularly sensitive to low-resistivity bodies, simple and portable equipment, and high working efficiency, and is widely used in hydrology, engineering geology, prospecting, etc. In general magnetotelluric field work, Pb-PbCl non-polarizing electrodes are used to receive electric field signals, and induction magnetic sensors are used to receive magnetic field signals. They are connected to a magnetotelluric acquisition instrument through electric cables and magnetic cables to record magnetotelluric time series data. Five-component acquisition is performed at each measurement point (two electric field components E x 、E y and three magnetic field components H x 、H y 、H z ), with north and east corresponding to the x and y directions respectively, and z being perpendicular to the ground. GPS synchronous acquisition is used between survey stations, and the precise measurement point positions are recorded.

[0003] However, due to limitations in survey work means and difficulties in working in the intertidal zone, the survey work in the intertidal zone and shallow sea areas with a water depth of less than 10 m has not been systematically carried out, and the survey work is almost blank. Domestic and foreign marine magnetotelluric and controlled-source audio magnetotelluric work mainly focuses on areas with a water depth of more than 10 m, and mostly uses large scientific research vessels as transportation tools, with all instruments placed underwater for data acquisition. Some scholars have conducted magnetotelluric observation experiments with electromagnetic channel separation and synchronous acquisition in shallow seas, placing magnetic sensors on the coastal land and electric channels in shallow seas, which will have unpredictable effects on the final data. Since the shallow sea area is a land-sea transition zone, it is difficult for personnel and marine geological survey vessels to move, and neither land nor marine magnetotelluric work can be effectively carried out in such areas. The seamless survey of land and sea has not been achieved, and it is necessary to develop suitable technical methods to improve the degree of basic geological and engineering geological surveys in the land-sea combination zone to meet the needs of major project layout planning and resource development and utilization. Summary of the Invention

[0004] In view of this, the present invention provides a magnetotelluric acquisition system and method for shallow sea areas that can improve the degree of basic geological and engineering geological surveys in the land-sea combination zone, and establish the accuracy and effectiveness of magnetotelluric measurement in shallow sea areas according to the characteristics of seawater flow in shallow sea areas.

[0005] In a first aspect, the present invention provides a magnetotelluric acquisition system for shallow waters. The magnetotelluric acquisition system for shallow waters includes a drag-type electric field acquisition device, an underwater three-component magnetic field acquisition device, and a magnetotelluric instrument. The drag-type electric field acquisition device and the underwater three-component magnetic field acquisition device are both connected to the magnetotelluric instrument;

[0006] The drag-type electric field acquisition device includes an electric circuit acquisition platform, a GPS positioning module, a horizontal telescopic rod, a vertical telescopic rod, and a plurality of electrode stabilizing components. The horizontal telescopic rod and the vertical telescopic rod are perpendicularly arranged on the electric circuit acquisition platform. The GPS positioning module is arranged at the intersection of the horizontal telescopic rod and the vertical telescopic rod. The plurality of electrode stabilizing components are respectively arranged at both ends of the horizontal telescopic rod and both ends of the vertical telescopic rod. The electrode stabilizing component includes a floating ball, a shallow sea non-polarizing electrode, and a net bag stone weight. The shallow sea non-polarizing electrode is arranged at the end of the horizontal telescopic rod or the vertical telescopic rod. The floating ball is arranged above the shallow sea non-polarizing electrode. The net bag stone weight is arranged below the shallow sea non-polarizing electrode;

[0007] The underwater three-component magnetic field acquisition device includes a box body, three-component magnetic rods installed in the box body and perpendicular to each other in pairs, and an orientator installed on the box body. The three-component magnetic rods are used to establish a spatial coordinate system. The orientator is used to calibrate the inclination angle and azimuth angle of each magnetic rod. The magnetic rods are connected to the magnetotelluric instrument through magnetic track lines.

[0008] As a preference of the above technical solution, the drag-type electric field acquisition device further includes a multi-core cable, a shielded wire, and three net bag stone anchors. The shallow sea non-polarizing electrode corresponding to the vertical telescopic rod is connected to the multi-core cable through the shielded wire. The shallow sea non-polarizing electrode corresponding to the horizontal telescopic rod is connected to the multi-core cable. The multi-core cable is connected to the magnetotelluric instrument. The three net bag stone anchors are respectively arranged on both sides in the length direction of the multi-core cable and on one side of the vertical telescopic rod.

[0009] As a preference of the above technical solution, the underwater three-component magnetic field acquisition device includes a waterproof cover and a counterweight. The waterproof cover is used to accommodate the box body. The magnetic track line passes through a waterproof buckle on the waterproof cover. The counterweight is arranged at the bottom of the waterproof cover. The counterweight is used to sink the waterproof cover to the bottom of the water.

[0010] As a preference of the above technical solution, the magnetotelluric acquisition system for shallow waters further includes a hull for setting the magnetotelluric instrument. The three-component magnetic field acquisition device is respectively tied to the handle of the waterproof cover and the hull through nylon ropes. The handle is arranged at the top of the waterproof cover. The nylon ropes are used for the retraction and release of the three-component magnetic rods during the operation in the shallow sea area.

[0011] In a second aspect, the present invention further provides a magnetotelluric acquisition method for shallow waters, which is applied to the magnetotelluric acquisition system for shallow waters described above, and specifically includes the following steps:

[0012] S1: Obtain the preset parameters of the waterproof cover, multi-core cable, shielded wire, and shallow sea non-polarizable electrode, and select a weather with a force 4 wind and small waves for magnetotelluric acquisition in shallow waters;

[0013] S2: Arrange a drag-type electric field acquisition device on the sea surface: fully open two horizontal telescopic rods and two vertical telescopic rods, place the electric circuit acquisition platform on the sea surface, turn on the GPS positioning module on the electric circuit acquisition platform and set it to the track recording mode; determine the position of the corresponding shallow sea non-polarizable electrode through the GPS positioning module to determine the azimuth of the electric circuit, and lower three net bag stone anchors at different positions for fixation;

[0014] S3: Place the three-component magnetic rod at the bottom of the water: Connect the magnetic track line to the three-component magnetic rod, install it in the waterproof cover, tie the cement counterweight block, then tie the nylon rope to the handle of the waterproof cover, and place the underwater three-component magnetic field acquisition device into the sea water and sink it to the bottom;

[0015] S4: Place the magnetotelluric instrument on the hull, connect the multi-core cable, shielded wire, and magnetic track line to the magnetotelluric instrument, and then turn on the machine for data recording; after the measurement of the data at the measurement point is completed, pull the three net bag stone anchors and the three-component magnetic rod onto the hull, and quickly tow the electric circuit acquisition platform to the next measurement point position;

[0016] S5: Export the magnetic field data and the measurement data of the measurement points and perform data preprocessing. Among them, during preprocessing, first input the measurement point coordinates recorded by the GPS positioning module at the center of the electric circuit acquisition platform into the original data.

[0017] As a preference of the above technical solution, the horizontal telescopic rod and the vertical telescopic rod are carbon telescopic rods, the length of the carbon telescopic rod is 1.6 m to 20 m, and the electric circuit acquisition platform is composed of a square formed by four PVC pipes with a diameter of 10 cm and a length of 2 m.

[0018] As a preference of the above technical solution, S6 includes:

[0019] The time-varying electromagnetic field follows Maxwell's law equation and is expressed in the frequency domain as:

[0020]

[0021] Among them, represents the differential form of the law of total current, represents an arbitrary position below the surface layer, ω, μ 0 and σ respectively represent the angular frequency of time variation, the magnetic permeability of vacuum, and the conductivity, z > 0;

[0022] The horizontal components of the electric and magnetic fields measured at a specific location according to the linearity of Maxwell's equations and boundary conditions There is a linear relationship on the Earth's surface, and the corresponding expression is:

[0023] E h (r h , ω) = Z(r h , ω)H h (r h , ω) (3)

[0024] Wherein, E h and H h are the horizontal electric and magnetic fields respectively, and Z is the magnetotelluric impedance corresponding to a complex-valued 2×2 tensor;

[0025] The electromagnetic scattering theory is used to define signals and noise. The electric field E and magnetic field H at any position r in a non-uniform earth can be expressed in the form of integral equations as:

[0026] Wherein, r' represents a position different from r below the surface layer, E 0 , H 0 are the electromagnetic fields of the one-dimensional ground σ 0 (z), G E and G H represent the electric and magnetic Green tensors, dV' represents the volume derivative of the conductivity heterogeneity, and V k represents the volume of each conductivity heterogeneity, and the conductivity contrast expression with respect to σ 0 (z) is:

[0027] δσ(r) = σ(r) - σ 0 (z) (6).

[0028] As an optimization of the above technical solution, the lateral variance of δσ in formula (6) is minimized by using a regional average section, and the magnetotelluric observation array is obtained in a linear or logarithmic ratio according to the area average of the conductivity of each depth slice, and the corresponding expression is:

[0029]

[0030] Wherein, ΔS represents the area of the observation matrix, and the average conductivity profile is given by formulas (7) and (8). Formula (7) is the regional arithmetic average distribution, and formula (8) is the geometric average distribution;

[0031] The actually observed electromagnetic field contains noise, and the horizontal components of the electric and magnetic fields measured at a certain position Adopted on the Earth's surface and The corresponding expressions are:

[0032]

[0033] and

[0034] Among them, the first terms on the right sides of formulas (9) and (10) are composed of the horizontal components of the electromagnetic field and are given by formula (4) or formula (5); the second terms are derived from different terms, e m (r h , ω) represents the electric field measurement noise, and h m (r h , ω) represents the magnetic field measurement noise.

[0035] As an optimization of the above technical solution, for the magnetotelluric array estimating impedance at each discrete observation point, a side length L of a square matrix and the number of site points N on one side are preset, and the site point spacing is Define the Nyquist wavenumber as The resolution of the wavenumber is Apply the Nyquist-Shannon sampling theorem to magnetotelluric measurements;

[0036] The impedance Z at the jth observation point obs The corresponding site point Is located on the Earth's surface, and represent the measured electromagnetic fields in formulas (9) and (10) as:

[0037]

[0038] Among them, δ represents the effect of the measurement noise that does not follow a linear relationship. Regard Z obs As an approximation of the impedance Z and is related to the horizontal electromagnetic field. The corresponding expression is:

[0039] E n (r j , ω) = Z(r j , ω)H h (r j , ω) (12)

[0040] Among them, Z(r j , ω) represents the sampled impedance at (r j , ω);

[0041] Based on the Nyquist-Shannon sampling theorem, separate the scattered field in the equation, divide formulas (4) and (5) into two parts. One part of the spatial variation can be analyzed by the given observation array, and the other part cannot be analyzed. At the position r in the observation array j =(xj , y j The horizontal electromagnetic fields E h and H h sampled at () are formally expressed as:

[0042]

[0043] and

[0044] wherein, and is a vector composed of two horizontal components at the earth's surface in formulas (4) and (5), e S and h S respectively represent the signals of the scattered horizontal electric and magnetic fields, e N and h N respectively represent the noises of the horizontal electric and magnetic fields;

[0045] In magnetotelluric sounding, the electromagnetic field without distortion effect is the regional electromagnetic field, and the regional electromagnetic field includes the primary field and the secondary field. The corresponding expression is:

[0046]

[0047] wherein, the superscript R on the left represents the regional field, and the regional impedance Z R has the following expression:

[0048]

[0049] According to the conductivity of Z R the contaminated geological noises e N and h N in the observed electromagnetic field are eliminated.

[0050] As an optimization of the above technical solution, formulas (4) and (5) are transformed into the spatial wavenumber domain of the earth's surface according to the characteristics of the electromagnetic field and the impedance in the spatial wave domain. The corresponding expression is:

[0051]

[0052]

[0053] wherein, ^ represents the spatial Fourier transform, k x and k y are the wavenumbers in the x and y directions respectively, and represent the Fourier transforms of the secondary electric and magnetic fields, The expression of is rewritten as:

[0054]

[0055] If all four elements of the impedance at a certain frequency are continuous functions over a large area of the earth's surface, and the spatial Fourier transform of each element is obtained as:

[0056]

[0057] where Z ij is the (i,j)-th element of the electromagnetic impedance;

[0058] If the electromagnetic measurements are made at discrete positions distributed in a square array, the expression for the discrete Fourier transform is:

[0059]

[0060] where n x , n y represent the number of point positions in the x and y directions respectively, l x and l y are integers between -N / 2 and N / 2; the wave numbers in the x and y directions are given by and , and have discrete values of 1 / L between -k N and k N ; l x and l y are dimensionless wave numbers, is the complex Fourier transform of Z ij , and the characteristics of the signal and noise based on the wave number dependence are

[0061] The present invention provides a magnetotelluric acquisition system and method for shallow sea areas. By using a magnetotelluric instrument to record the electromagnetic field time series data, the electric channel acquisition platform on the water surface is suspended on four sides with non-polarizable electrodes for receiving electric field signals in shallow sea areas, and is connected to the magnetotelluric instrument through a waterproof cover, a multi-core cable and a shielded wire. The three-component magnetic rod at the bottom receives the magnetic track signal and is connected to the magnetotelluric instrument through a magnetic rod wire. Five-component acquisition is carried out at each measurement point. The GPS in the middle of the electric field acquisition platform records the position information of the measurement point, and the net bag stone anchor fixes the position of the entire acquisition system and conducts fixed-point measurement, improving the working reliability of the magnetotelluric acquisition system in shallow sea areas, realizing the accuracy of magnetotelluric acquisition in shallow sea areas, and to a certain extent also improving the degree of basic geological and engineering geological surveys in the land-sea combination zone, meeting the needs of major project layout planning and resource development and utilization. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments. It should be understood that the following drawings only show certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0063] Figure 1 It is a structural block diagram of the magnetotelluric acquisition system in the shallow sea area provided by the present invention;

[0064] Figure 2 It is a structural schematic diagram of the magnetotelluric acquisition system in the shallow sea area provided by the present invention;

[0065] Figure 3 It is a structural schematic diagram of the drag-type electric field acquisition device provided by the present invention;

[0066] Figure 4 It is a structural schematic diagram of the underwater three-component magnetic field acquisition device provided by the present invention;

[0067] Figure 5 It is a flow chart of the magnetotelluric acquisition method in the shallow sea area provided by the present invention;

[0068] Figure 6 It is a magnetotelluric observation test curve graph provided by the present invention;

[0069] Figure 7 It is a schematic diagram of the electromagnetic observation array provided by the present invention.

[0070] The main element symbols are explained as follows:

[0071] 1 - Drag-type electric field acquisition device; 11 - Electric circuit acquisition platform; 12 - GPS positioning module; 13 - Horizontal telescopic rod; 14 - Vertical telescopic rod; 15 - Electrode stabilization component; 151 - Floating ball; 152 - Shallow sea non-polarizable electrode; 153 - Net bag stone weight; 16 - Multi-core cable; 17 - Shielded wire; 18 - Net bag stone anchor; 2 - Underwater three-component magnetic field acquisition device; 21 - Box body; 22 - Three-component magnetic rod; 23 - Directional device; 24 - Magnetic track wire; 25 - Nylon rope; 26 - Waterproof cover; 27 - Counterweight block; 3 - Magnetotelluric instrument. Detailed implementation manners

[0072] The following will describe in detail the embodiments of the present invention. The examples of the embodiments are shown in the drawings, where the same or similar reference numerals represent the same or similar elements or elements with the same or similar functions from beginning to end. The embodiments described below with reference to the drawings are exemplary and are only used to explain the present invention and should not be construed as limiting the present invention.

[0073] It should be noted that when an element is referred to as "fixed to" another element, it can be directly on the other element or there can also be an intermediate element. When an element is considered to be "connected" to another element, it can be directly connected to the other element or there may be an intermediate element at the same time. On the contrary, when an element is referred to as being "directly on" another element, there is no intermediate element. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only.

[0074] In the present invention, unless otherwise clearly specified and defined, terms such as "installed", "connected", "connected to", "fixed" and the like shall be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0075] Referring to Figure 1 、 Figure 2 and Figure 3 As shown in

[0076] the present invention provides a magnetotelluric acquisition system for shallow waters, the magnetotelluric acquisition system for shallow waters includes a drag-type electric field acquisition device 1, an underwater three-component magnetic field acquisition device 2 and a magnetotelluric instrument 3, the drag-type electric field acquisition device 1 and the underwater three-component magnetic field acquisition device 2 are both connected to the magnetotelluric instrument 3;

[0077] The underwater three-component magnetic field acquisition device 2 includes a box body 21, three-component magnetic rods 22 installed in the box body 21 and perpendicular to each other in pairs, and a direction finder installed on the box body 21. The three-component magnetic rods 22 are used to establish a spatial coordinate system, the direction finder 23 is used to calibrate the dip angle and azimuth angle of each magnetic rod, and the magnetic rods are connected to the magnetotelluric instrument 3 through magnetic track lines 24.

[0078] In this embodiment, referring to Figure 4 , the drag-type electric field acquisition device 1 further includes a multi-core cable 16, a shielded wire 17, and three net bag stone anchors 18. The shallow sea non-polarizable electrodes 152 corresponding to the longitudinal telescopic rod 14 are connected to the multi-core cable 16 through the shielded wire 17. The shallow sea non-polarizable electrodes 152 corresponding to the transverse telescopic rod 13 are connected to the multi-core cable 16. The multi-core cable 16 is connected to the magnetotelluric instrument 3. The three net bag stone anchors 18 are respectively arranged on both sides of the length direction of the multi-core cable 16 and on one side of the longitudinal telescopic rod 14. The underwater three-component magnetic field acquisition device 2 includes a waterproof cover 26 and a counterweight 27. The waterproof cover 26 is used to accommodate the box body 21. The magnetic track line 24 passes through the waterproof buckle on the waterproof cover 26. The counterweight 27 is arranged at the bottom of the waterproof cover 26, and the counterweight 27 is used to sink the waterproof cover 26 to the bottom of the water. The shallow sea magnetotelluric acquisition system further includes a hull on which the magnetotelluric instrument 3 is arranged. The three-component magnetic field acquisition device 2 is respectively tied to the handle of the waterproof cover 26 and the hull through a nylon rope 25. The handle is arranged at the top of the waterproof cover 26, and the nylon rope 25 is used for the retraction and release of the three-component magnetic rods 22 during operation in the shallow sea area.

[0079] It should be noted that in order to achieve magnetotelluric measurements in the shallow sea area within a water depth of 10 meters, conduct seamless land-sea coordinated surveys, and improve the degree of basic geological and engineering geological surveys in the land-sea combination zone. According to the characteristics of seawater flow in the shallow sea area, a magnetotelluric measurement process in the shallow sea area can be initially established. The magnetotelluric instrument 3 is used to record the time series data of the electromagnetic field. The electric circuit acquisition platform 11 on the water surface is suspended on all four sides with modified shallow sea non-polarizable electrodes 152 to receive the electric field signal, and is connected to the magnetotelluric instrument 3 through a waterproof multi-core shielded cable that can withstand pulling; the three-component magnetic rods 22 on the bottom receive the magnetic track signal and are connected to the magnetotelluric instrument 3 through the magnetic track line 24 (such as Figure 2 ). Five-component acquisition is carried out at each measurement point (two electric field components E x , E y and three magnetic field components E y , H y , H z), with north and east corresponding to the x and y directions respectively, and z perpendicular to the ground. The GPS in the middle of the electric field acquisition platform records the position information of the measurement points. The net bag stone anchor 18 fixes the position of the entire acquisition system to achieve fixed-point measurement. Among them, the electric circuit acquisition platform 11 is also called the electric field acquisition platform, and the GPS positioning module 12 can be abbreviated as GPS or handheld GPS. The electric field acquisition platform is composed of four PVC pipes with a diameter of 10 cm and a length of 2 m connected by joints to form a square (as shown in Figure 3 ). During connection, ensure that the joints fit tightly with the PVC pipes to prevent water leakage, so that the platform can float.

[0080] Among them, in the middle of the electric field acquisition platform is a net woven with nylon threads. A high-precision (positioning accuracy of 0.2 m) handheld GPS is placed in the center of the net to record the precise position of the measurement point. The GPS is installed in a plastic sealed bag and tied to the top of the floating ball 151 to prevent water ingress. The four directions of the electric field acquisition platform are four carbon fiber telescopic rods with a length of 20 m (the shortest can be reduced to 1.6 m), so that the length of the electrode distance can be adjusted. The end of the telescopic rod is a shallow sea non-polarizable electrode 152. Its upper part is a floating ball 151, and its lower part is a net bag stone weight 153 of about five kilograms, which can ensure that the electrode is in a basically stable longitudinal state under a four-level wind and the corresponding tidal current and lateral water flow. The north-south shallow sea non-polarizable electrodes 152 are connected to the multi-core cable 16 through the shielded wire 17, and the east-west shallow sea non-polarizable electrodes 152 are directly connected to the multi-core cable 16. Then, the multi-core cable 16 is connected to the magnetotelluric instrument 3 on the hull. The entire electric field acquisition system is fixed by three net bag stone anchors 18, which are distributed at the head and tail ends of the multi-core cable 16 and at the south side PVC pipe of the electric field acquisition platform. The three net bag stone anchors 18 form a stable triangle.

[0081] Refer to again Figure 4 , the three-component magnetic rod 22 in the shallow sea is composed of three magnetic rods with the same parameters installed perpendicular to each other in a plastic box 21. The coordinate system x is defined as north, y as east, and z perpendicular to the ground. The attitude (dip angle and azimuth angle) of the magnetic rod under the sea water is measured by the orientator 23 installed on the top of the box 21. The measured magnetic field can be calibrated to a certain direction (such as due north and due east) through the measured dip angle and azimuth angle. The box 21 of the three-component magnetic rod 22 is installed in a bell-shaped fiberglass waterproof cover 26, which can meet the requirements of waterproof and pressure resistance for working within 20 meters of water depth. A magnetic track line 24 passes through the waterproof buckle on the top of the waterproof cover 26 and connects the three magnetic rods to the magnetotelluric instrument 3. The two ends of the nylon rope 25 are respectively tied to the handle of the waterproof cover 26 and the ship, which is convenient for the collection and release of the three-component magnetic rod 22 during work in the shallow sea area. A cement counterweight 27 is tied to the bottom of the waterproof cover 26 to make the hollow waterproof cover 26 sink to the bottom of the water.

[0082] It should be understood that by using the magnetotelluric instrument 3 to record the electromagnetic field time series data, the electric circuit acquisition platform 11 on the water surface is suspended on all four sides and receives the electric field signal through the shallow sea non-polarizing electrodes 152. It is connected to the magnetotelluric instrument 3 through the waterproof cover 26, the multi-core cable 16 and the shielded wire 17. The three-component magnetic rod 22 at the bottom receives the magnetic track signal and is connected to the magnetotelluric instrument 3 through the magnetic track wire 24. Five-component acquisition is carried out at each measuring point. The GPS in the middle of the electric field acquisition platform records the position information of the measuring point. The net bag stone anchor 18 fixes the position of the entire acquisition system and conducts fixed-point measurement, improving the working reliability of the magnetotelluric acquisition system in the shallow sea area, realizing the accuracy of magnetotelluric acquisition in the shallow sea area, and to a certain extent also improving the degree of basic geology and engineering geology surveys in the land-sea combination zone, meeting the needs of major project layout planning and resource development and utilization.

[0083] Refer to Figure 5 , the present invention also provides a magnetotelluric acquisition method for the shallow sea area, which is applied to the above-mentioned magnetotelluric acquisition system for the shallow sea area, and specifically includes the following steps:

[0084] S1: Obtain the preset parameters of the waterproof cover, multi-core cable, shielded wire and shallow sea non-polarizing electrodes, and select the weather with level 4 wind and small waves for magnetotelluric acquisition in the shallow sea;

[0085] S2: Arrange the drag-type electric field acquisition device on the sea surface: fully open the two horizontal telescopic rods and the two vertical telescopic rods, place the electric circuit acquisition platform on the sea surface, turn on the GPS positioning module on the electric circuit acquisition platform and set it to the track recording mode; determine the positions of the corresponding shallow sea non-polarizing electrodes through the GPS positioning module to determine the azimuth of the electric circuit, and lower the net bag stone anchors at three different positions for fixation;

[0086] S3: Place the three-component magnetic rod at the bottom of the water: Connect the magnetic track wire to the three-component magnetic rod, install it in the waterproof cover, tie the cement counterweight block, then tie the nylon rope to the handle of the waterproof cover, and put the underwater three-component magnetic field acquisition device into the sea water and sink it to the bottom;

[0087] S4: Place the magnetotelluric instrument on the hull, connect the multi-core cable, shielded wire and magnetic track wire to the magnetotelluric instrument, and then turn on the machine to record data; after the measurement of the measuring point data is completed, pull the three net bag stone anchors and the three-component magnetic rod onto the hull, and quickly tow the electric circuit acquisition platform to the next measuring point position;

[0088] S5: Export the magnetic field data and the measurement data of the measuring point and perform data preprocessing. Among them, when preprocessing, first input the measuring point coordinates recorded by the GPS positioning module in the center of the electric circuit acquisition platform into the original data.

[0089] In this embodiment, the horizontal telescopic rod and the vertical telescopic rod are carbon telescopic rods. The length of the carbon telescopic rod is 1.6 m to 20 m. The electric circuit acquisition platform consists of a square formed by four PVC pipes with a diameter of 10 cm and a length of 2 m. The process of magnetotelluric acquisition in shallow waters is as follows: Referring to the industry standards of the People's Republic of China for geology and mineral resources, "Technical Regulations for Audio Magnetotelluric Method of Natural Field (DZ / T 0305 - 2017)" and "Technical Regulations for Magnetotelluric Sounding Method (DZ / T 0173 - 2022)", the technical process of magnetotelluric acquisition in shallow waters is as follows: (1) According to the acquisition parameters, prefabricate waterproof multi-core shielded cables, shallow sea non-polarizable electrodes, etc.; (2) Select a day with a force 4 wind and small waves for magnetotelluric acquisition in shallow waters; (3) Arrange the electric field acquisition system on the sea surface. Fully open the four telescopic rods, place the electric field acquisition platform on the sea surface, and at the same time turn on the GPS on the platform and set it to the track recording mode. Determine the position of the corresponding non-polarizable electrode through a high-precision handheld GPS, thus determining the azimuth of the electric circuit, and lower three net bag stone anchors at different positions for fixation to prevent the entire acquisition system from drifting; (4) Place the three-component magnetic rod at the bottom of the water. Connect the magnetic track line to the three-component magnetic rod, install it in a waterproof cover, tie the cement counterweight block, and then use a nylon rope to tie the handle of the waterproof cover and then place the whole into the sea water and sink to the bottom; (5) Place the magnetotelluric instrument on the ship, connect the multi-core shielded cable and the magnetic track line to the instrument, and turn on the machine to record data; (6) After measuring the data at the measuring point, pull the three net bag stone anchors and the three-component magnetic rod onto the ship, and you can tow the electric field acquisition platform to quickly go to the next measuring point position; (7) After completing the work, export the data and perform data preprocessing. When preprocessing, first input the measuring point coordinates recorded by the GPS at the center of the electric field acquisition platform into the original data, and other processing processes are basically the same as those of land magnetotellurics.

[0090] Optionally, referring to Figure 7 , S6 includes: The time-varying electromagnetic field follows Maxwell's law equations and is expressed in the frequency domain as:

[0091]

[0092] where represents the differential form of Ampere's law, represents an arbitrary position below the surface layer, ω, μ 0 and σ represent the angular frequency of time variation, the magnetic permeability of vacuum, and the conductivity respectively, z > 0;

[0093] According to Maxwell's equations and the linearity of the boundary conditions, the horizontal components of the electric and magnetic fields measured at a specific position have a linear relationship on the Earth's surface, and the corresponding expression is:

[0094] E h (r h, ω) = Z(r h , ω)H h (r h , ω) (3)

[0095] where E h and H h are the horizontal electric and magnetic fields respectively, and Z is the magnetotelluric impedance corresponding to a complex-valued 2×2 tensor;

[0096] Using electromagnetic scattering theory to define signals and noise, the electric field E and magnetic field H at any position r in a non-uniform earth can be expressed in the form of integral equations as:

[0097] where r' represents a position different from r below the surface layer, E 0 , H 0 are the electromagnetic fields of the one-dimensional ground σ 0 (z), G E and G H represent the electric and magnetic Green's tensors, dV' represents the volume derivative of the conductivity heterogeneity, and V k represents the volume of each conductivity heterogeneity, and the conductivity contrast expression with respect to σ 0 (z) is:

[0098] δσ(r) = σ(r) - σ 0 (z) (6).

[0099] Specifically, the lateral variance of δσ in formula (6) is minimized by using a regional average profile, and the magnetotelluric observation array is obtained in linear or logarithmic scale based on the area average of the conductivity of each depth slice, and the corresponding expression is:

[0100]

[0101] or

[0102] where ΔS represents the area of the observation matrix, the average conductivity profile is given by formulas (7) and (8), formula (7) is the regional arithmetic mean distribution, formula (8) is the geometric mean distribution, and is approximated to σ 0 ;

[0103] The actually observed electromagnetic fields contain noise, and the horizontal components of the electric and magnetic fields measured at a certain position at the earth's surface using and The corresponding expressions are:

[0104]

[0105] and

[0106] Among them, the first terms on the right sides of formulas (9) and (10) are composed of the horizontal components of the electromagnetic field and are given by formula (4) or formula (5); the second terms are derived from different terms, e m (r h , ω) represents the electric field measurement noise, and h m (r h , ω) represents the magnetic field measurement noise.

[0107] In this embodiment, for the magnetotelluric array estimating impedance at each discrete observation point, a side length L of a square matrix and the number of site points N on one side are preset, and the site point spacing is Define the Nyquist wavenumber as The resolution of the wavenumber is Apply the Nyquist-Shannon sampling theorem to magnetotelluric measurements;

[0108] The impedance Z at the jth observation point obs The corresponding site point is located on the Earth's surface, and represent the measured electromagnetic fields in formulas (9) and (10) as:

[0109]

[0110] Among them, δ represents the effect of the measurement noise that does not follow a linear relationship. Consider Z obs as an approximation of the impedance Z and related to the horizontal electromagnetic field, and the corresponding expression is:

[0111] E h (r j , ω) = Z(r j , ω)H h (t j , ω) (12)

[0112] Among them, Z(r j , ω) represents the sampled impedance at (r j , ω);

[0113] Based on the Nyquist-Shannon sampling theorem, separate the scattered field in the equation, divide formulas (4) and (5) into two parts. One part of the spatial variation can be analyzed by the given observation array, and the other part cannot be analyzed. At the position r j = (x j , y j ) where the horizontal electromagnetic fields E h and H h are sampled and formally represented as:

[0114]

[0115] and

[0116] wherein, and is a vector composed of two horizontal components at the earth's surface in formulas (4) and (5), e S and h S respectively represent the signals of the scattered horizontal electric field and magnetic field, e N and h N respectively represent the noises of the horizontal electric field and magnetic field;

[0117] In magnetotelluric sounding, the electromagnetic field without distortion effect is the regional electromagnetic field, and the regional electromagnetic field includes the primary field and the secondary field, and the corresponding expressions are:

[0118]

[0119] wherein, the superscript R on the left represents the regional field, and the regional impedance Z R has the expression:

[0120]

[0121] According to the conductivity of Z R eliminate the contaminated geological noises e N and h N .

[0122] Specifically, according to the characteristics of the electromagnetic field and the impedance in the spatial wave domain, convert formulas (4) and (5) to the spatial wavenumber domain at the earth's surface, and the corresponding expressions are:

[0123]

[0124] wherein, ^ represents the spatial Fourier transform, k x and k y are the wavenumbers in the x and y directions respectively, and represent the Fourier transforms of the secondary electric field and magnetic field, has the expression rewritten as:

[0125]

[0126] If all four elements of the impedance at a certain frequency are continuous functions within a large area of the earth's surface, and the spatial Fourier transforms of each element are obtained as:

[0127]

[0128] wherein, Z ijis the (i,j)-th element of the electromagnetic impedance;

[0129] If the electromagnetic measurements are made at discrete positions distributed in a square array, the expression for the discrete Fourier transform is:

[0130]

[0131] where n x and n y represent the number of point positions in the x and y directions respectively, l x and l y are integers between -N / 2 and N / 2; the wave numbers in the x and y directions are given by and and have discrete values of 1 / L between -k N and k N ; l x and l y are dimensionless wave numbers, is the complex Fourier transform of Z ij and the characteristics of the signal and noise based on the wave number dependence are

[0132] In this embodiment, the Fourier transform of the electromagnetic impedance is introduced to relate the observed spatial distribution of the electromagnetic impedance to its spectrum in the spatial wave number domain and to examine the sampling effect (signal-noise relationship) of the multi-time impedance. As Figure 6 shown, magnetotelluric acquisition experiments were carried out on the coast of DQ Island, CD County, YT City, SD Province. Each telescopic rod was extended to 20 meters, so the electrode distances in the north-south and east-west directions were each 40 meters. The three-component magnetic rods were sunk to the seabed at a water depth of about 10 meters. The observation duration was 20 minutes. The test results showed that for the apparent resistivity and phase observed in the shallow sea area, except for the "dead band" (where the natural field signal is very weak near 1 - 4 kHz) being affected by noise and the curve frequency points jumping, the data at the other frequency points were good, the curve was continuous and smooth, belonging to the normal magnetotelluric data curve, indicating that the magnetotelluric acquisition system in the shallow sea area is efficient and reliable.

[0133] In all examples shown and described here, any specific value should be interpreted as merely exemplary and not as a limitation. Therefore, other examples of the exemplary embodiments may have different values.

[0134] It should be noted that: Similar reference numerals and letters denote similar items in the following figures. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0135] The above-described embodiments merely represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation to the scope of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention.

Claims

1. A shallow sea magnetotelluric acquisition system, characterized in that: The shallow sea area magnetotelluric acquisition system comprises a towed electric field acquisition device, an underwater three-component magnetic field acquisition device and a magnetotelluric instrument, wherein the towed electric field acquisition device and the underwater three-component magnetic field acquisition device are both connected to the magnetotelluric instrument; The towed electric field acquisition device comprises an electric field acquisition platform, a GPS positioning module, a transverse telescopic rod, a longitudinal telescopic rod and a plurality of electrode stabilizing components. The transverse telescopic rod and the longitudinal telescopic rod are vertically arranged on the electric field acquisition platform. The GPS positioning module is arranged at the intersection of the transverse telescopic rod and the longitudinal telescopic rod. The plurality of electrode stabilizing components are respectively arranged at both ends of the transverse telescopic rod and the longitudinal telescopic rod. The electrode stabilizing components comprise a buoy, a shallow-sea non-polarizing electrode and a net bag stone sinker. The shallow-sea non-polarizing electrode is arranged at the end of the transverse telescopic rod or the longitudinal telescopic rod. The buoy is arranged above the shallow-sea non-polarizing electrode, and the net bag stone sinker is arranged below the shallow-sea non-polarizing electrode. The underwater three-component magnetic field acquisition device includes a box, three-component magnetic bars installed in the box and perpendicular to each other, and an orienter installed on the box. The three-component magnetic bars are used to establish a spatial coordinate system, and the orienter is used to calibrate the inclination and azimuth of each magnetic bar. The magnetic bars are connected to the magnetotelluric instrument through magnetic track lines.

2. The shallow sea magnetotelluric acquisition system according to claim 1, characterized in that: The towed electric field acquisition device also includes a multi-core cable, a shielded wire and three net bag stone anchors. The shallow-water non-polarized electrode corresponding to the longitudinal telescopic rod is connected to the multi-core cable through the shielded wire, the shallow-water non-polarized electrode corresponding to the transverse telescopic rod is connected to the multi-core cable, the multi-core cable is connected to the magnetotelluric instrument, and the three net bag stone anchors are respectively arranged on both sides of the length direction of the multi-core cable and on one side of the longitudinal telescopic rod.

3. The shallow sea magnetotelluric acquisition system according to claim 1, characterized in that: The underwater three-component magnetic field acquisition device includes a waterproof cover and a counterweight block. The waterproof cover is used to accommodate the box body. The magnetic track line passes through the waterproof buckle on the waterproof cover. The counterweight block is arranged at the bottom of the waterproof cover. The counterweight block is used to make the waterproof cover sink to the bottom of the water.

4. The shallow sea magnetotelluric acquisition system according to claim 3 is characterized in that: The shallow sea magnetotelluric acquisition system also includes a hull on which the magnetotelluric instrument is installed. The three-component magnetic field acquisition device is respectively tied to the handle of the waterproof cover and the hull through nylon ropes. The handle is arranged on the top of the waterproof cover. The nylon rope is used for retracting and releasing the three-component magnetic rod when working in shallow sea areas.

5. A shallow sea magnetotelluric acquisition method, characterized in that: The shallow sea magnetotelluric acquisition system according to any one of claims 1 to 4 specifically comprises the following steps: S1: Obtain the preset parameters of waterproof cover, multi-core cable, shielded wire and shallow sea non-polarized electrode, and select the weather with level 4 wind and small waves for shallow sea magnetotelluric acquisition; S2: Arrange a towed electric field collection device on the sea surface: fully open two transverse telescopic rods and two longitudinal telescopic rods, place an electric field collection platform on the sea surface, turn on the GPS positioning module on the electric field collection platform and set it to a track recording mode; determine the position of the corresponding shallow sea non-polarizable electrode through the GPS positioning module to determine the direction of the electric field, and put down three net bags and stone anchors at different positions for fixing; S3: Put the three-component magnetic bar into the water bottom: connect the magnetic track line with the three-component magnetic bar, put it into the waterproof cover, tie the cement counterweight block, tie the nylon rope to the handle of the waterproof cover, put the underwater three-component magnetic field acquisition device into the sea water and sink it to the bottom; S4: Place the magnetotelluric instrument on the hull, connect the multi-core cable, shielded wire and magnetic track wire to the magnetotelluric instrument, and then start the instrument to record data; after the data measurement of the measuring point is completed, pull three net bag stone anchors and three-component magnetic rods onto the hull, and drag the track acquisition platform to the next measuring point quickly; S5: Export the magnetic field data and the measurement data of the measuring points and perform data preprocessing, wherein during the preprocessing, the coordinates of the measuring points recorded by the GPS positioning module at the center of the electric track acquisition platform are first input into the original data.

6. The shallow sea magnetotelluric acquisition method according to claim 5, characterized in that: The transverse telescopic rod and the longitudinal telescopic rod are carbon telescopic rods, the length of the carbon telescopic rod is 1.6m to 20m, and the electric channel collection platform includes a square composed of four PVC pipes with a diameter of 10cm and a length of 2m.

7. The shallow sea magnetotelluric acquisition method according to claim 5, characterized in that S6 include: The time-varying electromagnetic field follows Maxwell's law equations, which are expressed in the frequency domain as: in, Represents the differential form of the full current law, represents any position below the surface, ω, μ0, and σ represent the time-varying angular frequency, the magnetic permeability, and the electrical conductivity of the vacuum, respectively, z>0; The horizontal components of the electric and magnetic fields measured at a specific location according to the linearization of Maxwell's equations and boundary conditions There is a linear relationship on the surface of the earth, and the corresponding expression is: E h (r h ,ω)=Z(r h ,ω)H h (r h ,oh) (3) Among them, E h and H h are the horizontal electric and magnetic fields, respectively, and Z is a complex-valued 2 × 2 tensor corresponding to the magnetotelluric impedance; Using electromagnetic scattering theory to define signals and noise, the electric field E and magnetic field H at any position r in the non-uniform earth can be expressed in the form of integral equations: Where r' represents the position below the surface different from r, E0 and H0 are the electromagnetic fields of the one-dimensional ground σ0(z), and G E and G H represents the electric and magnetic Green's tensor, dV' represents the volume derivative of the conductivity heterogeneity, V k The volume representing each conductivity heterogeneity has a conductivity contrast expression relative to σ0(z) as: δσ(r)=σ(r)-σ0(z) (6).

8. The shallow sea magnetotelluric acquisition method according to claim 7, characterized in that: The lateral variance of δσ in formula (6) is minimized by using the regional average profile. The magnetotelluric observation array is obtained based on the area average of the conductivity of each depth slice in a linear or logarithmic scale. The corresponding expression is: or Where ΔS represents the area of ​​the observation matrix, and the average conductivity profile is given by formulas (7) and (8). Formula (7) is the regional arithmetic mean distribution, and formula (8) is the geometric mean distribution; The electromagnetic fields actually observed contain noise, the horizontal components of the electric and magnetic fields measured at a certain location On the Earth's surface and The corresponding expression is: and The first term on the right side of formula (9) and formula (10) consists of the horizontal component of the electromagnetic field and is given by formula (4) or formula (5); the second term is derived from a different term, e m (r h ,ω) represents the electric field measurement noise, h m (r h ,ω) represents the magnetic field measurement noise.

9. The shallow sea magnetotelluric acquisition method according to claim 8, characterized in that: The magnetotelluric array that estimates the impedance at each discrete observation point presupposes a square matrix with a side length L and a number of points N on one side, and a point spacing of The Nyquist wave number is defined as The wave number resolution is Applying the Nyquist-Shannon sampling theorem to magnetotelluric measurements; The impedance Z of the jth observation point obs Corresponding points Located on the surface of the Earth, the electromagnetic field measured in equations (9) and (10) is expressed as: Among them, δ represents the effect of measurement noise that does not follow a linear relationship, and Z obs It is regarded as an approximate value of the impedance Z and is related to the horizontal electromagnetic field. The corresponding expression is: E h (r j ,ω)=Z(r j ,ω)H h (r j ,oh) (12) Among them, Z(r j ,ω) means in (r j ,ω) at the sampling impedance; Based on the Nyquist-Shannon sampling theorem, the scattered field in the equation is separated into two parts, one of which can be resolved by the given observation array, and the other cannot be resolved. j =(x j ,y j ) horizontal electromagnetic field E sampled at h and H h It can be formalized as: and in, and is a vector consisting of the two horizontal components at the surface in equations (4) and (5), e S and h S The signals representing the scattered horizontal electric and magnetic fields, e N and h N represent the noise of horizontal electric and magnetic fields respectively; The electromagnetic field without distortion in magnetotelluric measurement is the regional electromagnetic field, which includes the primary field and the secondary field. The corresponding expression is: The superscript R on the left represents the region field, and the region impedance Z R The expression is: According to Z R The conductivity of the electrode eliminates the geological noise that contaminates the electromagnetic field. N and h N .

10. The shallow sea magnetotelluric acquisition method according to claim 9, characterized in that: According to the characteristics of the electromagnetic field and the impedance in the spatial wave domain, formulas (4) and (5) are converted to the spatial wave number domain of the surface, and the corresponding expressions are: Among them, ^ represents the spatial Fourier transform, k x and k y are the wave numbers in the x and y directions, and represents the Fourier transform of the secondary electric and magnetic fields, The expression is rewritten as: If all four elements of the impedance at a certain frequency are continuous functions over a large area of ​​the surface, and the spatial Fourier transform of each element is obtained as: Among them, Z ij is the (i,j)th element of electromagnetic impedance; If electromagnetic measurements are made at discrete locations distributed in a square array, the expression for the discrete Fourier transform is: Among them, n x 、n y Respectively represent the number of points in the x and y directions, l x and l y is an integer between -N / 2 and N / 2; the wave numbers in the x and y directions are given by and Given, and with -k N to k N Discrete values ​​between 1 / L; l x and l y is the dimensionless wave number, It's Z ij The complex Fourier transform of , based on the wave number dependence, characterizes the signal and noise as

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