Device and method for detecting submarine bubble-type shallow gas by surface navigation

Abstract

The present invention discloses a device and method for detecting submarine bubble-type shallow gas while navigating on the water surface. This solution is based on resistivity detection formed by the potential difference and supply current at the acquisition points to achieve two-dimensional vertical detection. It has the advantages of a large detection range, high efficiency, is not easily affected by interference factors, can determine the distribution range of the gas-containing area according to the distribution range of the relatively high-resistance area in the true resistivity profile, and can determine the qualitative change of the gas content according to the relative size of the resistivity in the relatively high-resistance area; moreover, by selecting different combinations of power supply electrodes or adjusting the combination method of measurement electrodes, it is possible to achieve precise detection of submarine bubble-type shallow gas with different volumes, burial depths, and water depths; it can meet the application requirements of scientific research and engineering with real-time requirements, and provide more favorable technical support and theoretical support for ocean engineering development and construction.

Description

Technical Field

[0001] The present invention belongs to the technical field of submarine shallow gas measurement, and particularly relates to a device and method for detecting the distribution range and gas content of bubble-type shallow gas on the seabed by surface navigation. Background Art

[0002] In recent years, the development of offshore marine engineering projects such as reclamation and cross-sea bridges has ushered in a new upsurge. There is a large-scale shallow gas widely distributed in the seabed, and its main component is methane. During the process of marine development, its dynamic changes will lead to an intensified greenhouse effect, induce marine geological disasters, and bring risks such as marine engineering construction.

[0003] Shallow gas has different occurrence forms, and the identification characteristics and movement disaster-causing mechanisms of shallow gas in different occurrence forms are different. In coarse-grained sediments, gas migrates through capillary intrusion and growth, and forms a high-pressure sandy gas storage layer with interconnected pores under the capping trap effect. However, due to the large capillary resistance of the pore throats in fine-grained sediments, gas is difficult to invade capillarily, but forms "bubbles" by splitting the sediments, thus emerging bubble-type shallow gas with characteristics different from those of the traditionally recognized interconnected pore-type shallow gas.

[0004] The bubble-type shallow gas in the formation will significantly change the acoustic properties of sediments. The propagation of sound waves and seismic waves in seawater is significantly better than that of visible light and electromagnetic waves. In the prior art, underwater acoustic and seismic detection using these two signals are the main methods for carrying out shallow gas exploration at present. Acoustic detection technologies include multi-beam sounding, side-scan sonar, and shallow profile detection, etc. Shallow seismic technologies include single-channel or multi-channel seismic, and there are also high-resolution seismic methods specially designed for underwater detection. The two detection technologies mainly determine the existence and distribution of shallow gas by analyzing the typical abnormal characteristics of the detection profile. There are limitations for the detection of bubble-type shallow gas. Because its detection principle is based on the shielding effect of shallow gas on signals, it is impossible to give the bottom boundary of the gas-bearing area and the change of gas content. In addition, the detection results are easily affected by the type and particle size of sediments, forming scattering and reflection characteristics similar to those of shallow gas. Interferences such as multiple waves and low-frequency random noise existing in the shallow sea environment, as well as problems such as severe high-frequency energy absorption and uneven lateral energy, will also interfere with image interpretation. In short, this detection technology is still mainly qualitative detection at present. Except for being able to judge the possible existence of shallow gas, it is still impossible to clearly determine the distribution of shallow gas.

[0005] In addition, almost all traditional shallow gas survey methods are aimed at connected shallow gas, and rarely are they subdivided according to the occurrence form of shallow gas in the gas-bearing area. It remains unknown what the detection effect of the survey on bubble-type shallow gas is. This has led to problems such as the inability to detect and inaccurate measurement of bubble-type shallow gas in previous shallow gas survey work. However, the seabed surface in offshore waters is mostly fine-grained sediments such as silty clay. This kind of "bubble" that has not been given enough attention exists widely and is even the main occurrence type of shallow gas in the surface layer. In view of this, it is urgent to develop targeted survey methods that can accurately describe bubble-type shallow gas and put them on the agenda to provide more favorable technical support and theoretical support for ocean engineering development and construction. Summary of the Invention

[0006] To make up for the deficiencies in the survey of bubble-type shallow gas by existing acoustic or seismic means, the present invention proposes a method for detecting bubble-type shallow gas on the seabed based on surface vessel navigation, realizing the detection of the distribution range (especially the bottom boundary range) of bubble-type shallow gas and the qualitative change of gas content in the same gas-bearing area.

[0007] The present invention is realized by adopting the following technical solutions: A device for detecting bubble-type shallow gas on the seabed based on surface vessel navigation includes a survey ship, a cable system, an acquisition host, and a power supply system. The acquisition host and the power supply system are arranged on the survey ship. The acquisition host is connected to the cable system. The power supply system provides power for establishing an electric field during the operation and measurement of the device. The survey ship is used to provide navigation power, control the navigation speed, and also provide an offshore working platform for the entire detection process;

[0008] The cable system includes a tail anchor, floating balls, a multi-channel electrode array, and a tension rope. The tail anchor is connected to the survey ship through the tension rope. The multi-channel electrode array and the floating balls are fixed on the tension rope. A plurality of floating balls are arranged at intervals to keep the cable floating on the water surface;

[0009] The multi-channel electrode array is connected to the acquisition host through a multi-channel mother-daughter plug. The multi-channel electrode array includes a plurality of power supply electrodes and a plurality of measurement electrodes. The plurality of power supply electrodes are arranged in sequence near the survey ship end, and the plurality of measurement electrodes are arranged at equal intervals in sequence after the power supply electrodes and away from the survey ship end.

[0010] Furthermore, a GPS real-time positioning system, a real-time monitoring system for navigation speed, and a single-beam water depth detection device are also arranged on the survey ship; during the navigation detection process, the GPS real-time positioning system gives the longitude and latitude coordinates of the beginning and end of the survey line for survey line positioning; the real-time monitoring system for navigation speed determines the relative distance of the measurement points based on the navigation speed and the acquisition time interval; the single-beam water depth detection device is used to measure the real-time water depth data during the navigation detection of the system.

[0011] Further, the power supply electrodes include three power supply electrodes C1, C2, and C3 arranged in sequence near the survey ship end, and the measurement electrodes include thirteen measurement electrodes P1 to P13 arranged in sequence from the power supply electrode C1. The distance between the power supply electrode C3 and the measurement electrode P1 is defined as the offset distance.

[0012] Further, the distance between C1 and C2 is h1, the distance between C2 and C3 is h2, the distance between the adjacent power supply electrode C3 and the measurement electrode P1 is h3, and the distance between adjacent measurement electrodes is h4. And h1, h2, and h3 are 15 - 25 m, and h4 is 3 - 8 m.

[0013] Further, the acquisition host includes a master control unit and a host module. The host module includes a central control unit, a data transmission unit, an acquisition unit, and a variable - frequency square - wave transmitting unit connected to the central control unit; a GPS real - time positioning system, a navigation speed real - time monitoring system, a single - beam water depth detection device, and a power supply system are all connected to the master control unit. The master control unit is connected to the data transmission unit, the acquisition unit is connected to the measurement electrodes, and the variable - frequency square - wave transmitting unit is connected to the power supply electrodes. The supply current to the power supply electrodes is denoted as I.

[0014] The present invention also proposes a method for a device based on surface - cruising detection of submarine bubble - type shallow gas, including the following steps:

[0015] Step 1: The survey ship drags the cable system to move on the sea surface and supplies power to two power supply electrodes in the multi - channel electrode system to form an electric field.

[0016] Step 2: Obtain the values of the supply current I of the two power supply electrodes and the potential values of the measurement electrodes. Every two measurement electrodes form a measurement electrode pair, and two power supply electrodes form a power supply electrode pair. Subtract the measured potential of each group of measurement electrodes to form the potential difference of the measurement electrode pair.

[0017] Step 3: Use the GPS real - time monitoring device to determine the positions of the head and tail of the survey line, and use the navigation speed real - time monitoring and the acquisition interval to determine the acquisition point positions corresponding to the potential differences of each acquisition point.

[0018] The horizontal position of the acquisition point is the center of the connection line between the power supply electrode close to the measurement electrode pair and the measurement electrode close to the power supply electrode. The depth of the acquisition point from the water surface is half of the length of the connection line between the power supply electrode close to the measurement electrode pair and the measurement electrode close to the power supply electrode.

[0019] Step 4: Calculate the apparent resistivity of the acquisition point according to the supply current I and the potential difference ΔU of the acquisition point, and construct an apparent resistivity profile according to the apparent resistivity of the acquisition point and the acquisition point position.

[0020] Step 5: Perform inversion calculation on the apparent resistivity profile to obtain the true resistivity profile; determine the distribution range of bubble-type shallow gas and the change in gas content based on the true resistivity profile.

[0021] Further, in the said Step 2, the power supply electrodes include three power supply electrodes C1, C2, and C3 arranged in sequence from the end close to the survey ship. Select C1C3 or C2C3 as the power supply electrode pair. The measurement electrodes include thirteen measurement electrodes P1 to P13 arranged in sequence from the power supply electrode C1.

[0022] Define the distance from the starting power supply electrode closest to the hull of the survey ship to the end measurement electrode P13 as the total length of the device arrangement. The detection depth is positively correlated with the total length of the device arrangement. Specifically, select the combination of measurement electrodes according to the detection depth requirement, specifically:

[0023] If the detection depth does not exceed one-sixth of the total length of the device arrangement, then select the method of combining adjacent measurement electrodes in pairs for detection.

[0024] If the detection depth exceeds one-sixth of the total length of the device arrangement, then select the method of combining the first 12 measurement electrodes P1 - P12 with the end measurement electrode P13 in sequence for detection.

[0025] Further, in the said Step 5, during the process of inverting the apparent resistivity profile into the true resistivity profile, improve the inversion accuracy through the layered coupling inversion of seawater and sediment layer, that is, assign the seawater depth and seawater resistivity at different positions in the inversion model. The seawater depth at different positions is obtained in real time through a single-beam device during the detection process, and the seawater resistivity is obtained by taking the reciprocal of the seawater conductivity measured by a conductivity meter.

[0026] Further, in the said Step 5, when determining the distribution range of bubble-type shallow gas and the change in gas content based on the true resistivity profile:

[0027] First, determine whether there is a relatively high-resistance area in the true resistivity profile. A relatively high-resistance area refers to an area where the resistivity as a whole is higher than the adjacent positions. If there is a high-resistance area, delimit the relatively high-resistance area in the sediment layer as the distribution range of bubble-type shallow gas, and determine the relative amount of gas content based on the relative resistivity size in the relatively high-resistance area. If the resistivity is larger, the gas content is relatively more; conversely, if the resistivity is smaller, the gas content is less.

[0028] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0029] This solution is based on resistivity detection formed by the potential difference at the acquisition points and the supply current, achieving two-dimensional vertical detection. It has the advantages of a large detection range, high efficiency, being less susceptible to interference factors, and can determine the distribution range of the gas bubble area based on the distribution range of the relatively high-resistance area in the true resistivity profile, and determine the qualitative change of the gas content based on the relative magnitude of the resistivity in the relatively high-resistance area.

[0030] Moreover, by selecting different combinations of power supply electrodes or adjusting the combination method of measurement electrodes, precise detection of submarine bubble-type shallow gas with different volumes, burial depths, and water depths can be achieved; it can meet the application requirements of scientific research and engineering with real-time requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic structural diagram of the detection device described in the embodiment of the present invention;

[0032] Figure 2 is Figure 1 a schematic circuit principle diagram in;

[0033] Figure 3 It is a schematic diagram of the method for detecting the distribution range of submarine bubble-type shallow gas in the embodiment of the present invention;

[0034] Figure 4 is based on Figure 3 a true resistivity profile diagram of the gas bubble area obtained by the method;

[0035] Figure 5 It is a schematic circuit principle diagram of the acquisition unit in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] In order to more clearly understand the above-mentioned objects, features, and advantages of the present invention, the present invention will be further described below with reference to the accompanying drawings and embodiments. Many specific details are set forth in the following description in order to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0037] First of all, it should be noted that this solution emphasizes the use of a surface cruising detection technology, considering that bubble-type shallow gas is mostly located in shallow water. If an electric field is emitted underwater, the seawater-air interface will cause a serious mirror effect, generating virtual point sources, and since the distance between the power supply position and the sea surface is not fixed, this influence is difficult to eliminate. Surface cruising is not affected by the medium mirror effect, and the measured data has little interference, which is suitable for the investigation of bubble-type shallow gas.

[0038] Embodiment 1. A surface cruising device for detecting submarine bubble-type shallow gas, as Figure 1 and Figure 2 shown, Figure 1This is a schematic diagram of the device structure, Figure 2 and this is its circuit schematic diagram; the device includes a survey ship 100, a cable system 200, an acquisition host and a power supply system 300. The survey ship 100 is used to provide the navigation power and control the navigation speed. In addition, it also provides an offshore working platform for the entire detection process. The hull size of the survey ship is relatively flexible, and common vessels such as speedboats and fishing boats can be used as survey ships. The acquisition host and the power supply system 300 are arranged on the survey ship 100. The acquisition host is connected to the cable system 200. The power supply system provides power for establishing an electric field during the operation and measurement of the device, and a conventional 60A 12V lithium battery or lead-acid battery can be selected to complete this function. The acquisition host is the overall control of the system, used for issuing detection instructions, selecting detection parameters, data acquisition, data processing, data storage, etc. In addition, Figure 1 in [Figure], 500 is the seabed surface, 600 is the sediment layer, and 700 is the shallow gas.

[0039] A GPS real-time positioning system, a navigation speed real-time monitoring system, and a single-beam water depth detection device 400 are also arranged on the survey ship 100; during the navigation detection process, the longitude and latitude coordinates of the start and end of the survey line are given by the GPS real-time positioning system for survey line positioning; the navigation speed real-time monitoring system determines the relative distance of the measurement points based on the navigation speed and the acquisition time interval; the single-beam water depth detection device 400 is used to measure the real-time water depth data during the navigation detection of the system.

[0040] Continue to refer to Figure 1 , the cable system 200 includes a tail anchor 23, a float 24, a multi-channel electrode system 25, and a tension rope 26. The tail anchor 23 is connected to the hull of the survey ship 100 through the tension rope 26. The multi-channel electrode system 25 and the float 24 are bound to the tension rope 26. It can be seen from the figure that multiple floats 24 are arranged at intervals to keep the cable floating on the water surface; the tail anchor 23 provides a reaction force opposite to the navigation direction during the measurement to straighten the cable system and reduce the measurement error caused by the electrode position deviation. The multi-channel electrode system 25 is connected to the acquisition host through a multi-channel sub-plug. Multiple power supply electrodes are distributed at the near-ship end of the multi-channel electrode system 25. In this embodiment, three power supply electrodes C1, C2, and C3 (C1C3, C2C3 are power supply electrode pairs) are preferably selected. The interval distance C1C2 is h1, C2C3 is h2. After a distance of h3 from the C3 electrode, multiple measurement electrodes are equally spaced. In this embodiment, 13 measurement electrodes P1-P13 are preferably selected. The distance between adjacent measurement electrodes is h4. The distance between the power supply electrode C3 and the measurement electrode P1 is called the offset distance, specifically h3. The distances from P1 to P13 to the survey ship increase in sequence. During a single power supply process, the potential is collected in parallel. In this embodiment, h1, h2, and h3 are 15-25m, preferably 20m, and h4 is 3-8m, preferably 5m.

[0041] In addition, as Figure 2As shown in the figure, the acquisition host includes a master control machine and a host module. The host module includes a central control unit, a data transmission unit, a data acquisition unit, and a variable-frequency square-wave transmitting unit connected to the central control unit; the GPS real-time positioning system, the navigation speed real-time monitoring system, the single-beam bathymetric sounding device 400, and the power supply system are all connected to the master control machine. The master control machine is connected to the data transmission unit, the data acquisition unit is connected to the measurement electrode, and the variable-frequency square-wave transmitting unit is connected to the power supply electrode.

[0042] In this embodiment, the host module uses the variable-frequency square-wave transmitting unit to emit current. The system emission method is as follows: after receiving the emission instruction from the master control CPU, it emits power according to the set frequency, duty cycle, current value, and duration, and supplies power to the water body through the power supply electrode. In addition, in order to cooperate with the data acquisition unit to pick up weak signals, the variable-frequency square-wave transmitting unit can add a pseudo-random signal for the data acquisition unit to extract effective signals. The application of this technology effectively solves the problem of receiving weak signals in the low-resistivity environment of seawater. In addition, the variable-frequency square-wave transmitting unit also feeds back the supply current I to the central control unit after AD-converting the signal through a 24-bit analog-to-digital converter.

[0043] Reference Figure 5 , the data acquisition unit of the host module mainly consists of 13 independent high-precision 24-bit analog-to-digital converters and a unit control CPU. The system acquisition method is as follows: the 13-channel potential parallel acquisition unit is connected to the cable measurement electrode and consists of 13 independent high-precision 24-bit analog-to-digital converters and a unit control CPU. The weak potential signal detected by the measurement electrode is transmitted to the high-impedance input circuit of the data acquisition unit through the cable, filtered and amplified, and then sent to the analog-to-digital converter for numerical acquisition. The acquired time-domain signal is decomposed and arranged and frequency-domain analyzed in the CPU to extract the effective signal and upload it to the central control unit of the host and receive its instructions. The analog-to-digital converter used for acquisition can select the ADS1256 model of Texas Instruments, USA. The sampling accuracy is 24 bits, and it can identify potential signals of about 0.027 μV. After being amplified and filtered by a factor of ten, the potential acquisition accuracy of the instrument can reach 0.27 μV.

[0044] Of course, the selection of the above specific quantity and distance is preferred, but it is not limited to the above structure, and a smaller or larger number of measurement electrodes can also be used.

[0045] Embodiment 2: A device for detecting submarine bubble-type shallow gas by surface navigation based on the device proposed in Embodiment 1. In this embodiment, a method for detecting submarine bubble-type shallow gas by surface navigation is proposed; in order to better understand the solution of the present invention, first, the theoretical basis of the present invention is described:

[0046] The presence of bubble-type shallow gas will change the material composition of sediments. However, since the bubble volume far exceeds the pore space, the applicant's research shows that bubble-type shallow gas will still increase the resistivity of sediments and has unique resistivity change characteristics. The resistivity law of sediments containing bubble-type shallow gas is described as follows:

[0047] The overall law includes two stages: In the first stage, when the gas content of bubble-type shallow gas (the proportion of gas volume in the total volume of gas-containing sediments) is less than the critical value, the resistivity does not change with the increase in gas content; in the second stage, when the gas content exceeds the critical value, the resistivity increases with the increase in gas content. Specifically, for the characteristics of the first stage, the critical gas content (critical value) is between 1% and 6% (6% is an extreme case, generally between 1% and 3%). The critical gas content is related to the bubble size. The smaller the bubble size, the higher the critical gas content. When the gas content exceeds the critical gas content, the resistivity change is mainly related to the gas content and is positively correlated. Although it is also affected by the bubble size distribution, the influence is small and the difference in bubble size only leads to different rates of increase in the resistivity of sediments with the increase in gas content.

[0048] It can be seen that based on the resistivity change characteristics of gas-containing sediments, the resistivity distribution of sediments can be obtained through the surface towed resistivity imaging technology, and the distribution range of gas-containing areas and the qualitative change of gas content can be interpreted based on the resistivity distribution characteristics of sediments.

[0049] As Figure 3 shown, the method for detecting the distribution range and gas content of bubble-type shallow gas on the seabed by surface towed includes the following steps:

[0050] Step 1: The survey ship drags the cable system 200 to move on the sea surface, uses the power supply unit of the main engine to supply power to two power supply electrodes (power supply electrode pair) in the multi-channel electrode system to form an electric field, and obtains the value of the power supply current I supplied by the power supply unit to the two power supply electrodes and the potential value of the measuring electrode;

[0051] Step 2: Subtract the potential measured by each group of measuring electrode pairs to form the potential difference measured by each group of measuring electrode pairs. Taking P1P13 as an example, subtract the potential of P13 from the potential of P1 to obtain the measured potential difference of the P1P13 measuring electrode pair. Twelve acquisition point potential differences are obtained by combining P1 to P12 with P13 respectively, or twelve acquisition point potential differences are obtained by combining adjacent measuring electrodes, that is, P1P2, P2P3...P12P13;

[0052] Step 3: Use the GPS real-time monitoring device to determine the positions of the head and tail of the survey line, and use the real-time monitoring of the ship speed and the acquisition interval to determine the positions of the acquisition points corresponding to the potential differences of each acquisition point;

[0053] Specifically, the positions of the power supply electrode pair and the measurement electrode pair for each collection are determined by using real-time ship speed monitoring and collection intervals; the horizontal position of the collection point is the center of the line connecting the power supply electrode close to the measurement electrode pair and the measurement electrode close to the power supply electrode, and the depth of the collection point from the water surface is half of the length of the line connecting the power supply electrode close to the measurement electrode pair and the measurement electrode close to the power supply electrode. This method is for physical field detection. The resistivity measured at the collection point is affected by the change of the overall physical field, and its location is only the position where the physical field is most sensitive to the change of the resistivity of the medium;

[0054] Step 4: Calculate the apparent resistivity of the collection point according to the supply current I and the potential difference ΔU at the collection point, and construct an apparent resistivity profile according to the apparent resistivity of the collection point and the position of the collection point;

[0055] Taking the power supply electrode pair as C1C2 and the measurement electrode pair as P1P13 as an example, the calculation formula for the apparent resistivity ρ is:

[0056]

[0057] where C1P1 is the distance from C1 to P1, C1P13 is the distance from C1 to P13, C2P1 is the distance from C2 to P1, and C2P13 is the distance from C2 to P13.

[0058] Step 5: Perform inversion calculation on the apparent resistivity profile to obtain the true resistivity profile; determine the distribution range of bubble-type shallow gas and the change of gas content according to the true resistivity profile.

[0059] Specifically, in step 2, considering that the setting of the pole pitch affects the detection depth of the system. The detection depth of the surface towed resistivity method is related to the distribution of the electric field sensitivity. The higher the electric field sensitivity at different positions of the sediment layer 600, the better the detection effect. When the electric field sensitivity is zero, it indicates that detection is completely impossible. Based on the electric field sensitivity results studied by the applicant, it is considered that the detection depth is first positively correlated with the total length of the device arrangement (the total length of the device arrangement is the distance from the starting power supply electrode closest to the hull of the survey ship to the end measurement electrode P13). However, since the positions of the electrodes in the multi-channel electrode system are fixed, it is difficult to flexibly select the measurement electrode positions for different working conditions in actual work. In this embodiment, there are three power supply electrodes, and C1C3 or C2C3 can be selected as the power supply electrode pair. When C2C3 is selected as the power supply electrode pair, the distribution range of the electric field sensitivity and the detection depth are both smaller than those of C1C3. For the same measurement electrode positions, since the measurement electrodes are combined in pairs to form measurement electrode pairs to calculate the potential difference, the combination method of the measurement electrodes will have a greater impact on the detection depth. The effective detection depth of the first 12 measurement electrodes combined with the end measurement electrode P13 in sequence is about one-third of the total length of the device arrangement; the effective detection depth of each adjacent pair of measurement electrodes is about one-sixth of the total length of the device arrangement. In the actual application process, the combination of the power supply electrodes and the combination of the measurement electrodes can be selected according to the detection depth requirements.

[0060] For example, taking C1C3 as the power supply electrode pair as an example, the total length of the device arrangement is 120 m, the seawater is a m, and the required detection depth is within b m below the seabed. If a plus b does not exceed 20, that is, does not exceed one-sixth of the total length of the device arrangement, then the combination method of adjacent measurement electrodes in pairs is selected. At this time, selecting a combination method with a larger detection depth is likely to cause the detection result to be interfered by the change in the resistivity of the sediment below the target depth; if the value of a plus b is between 20 - 40, that is, exceeds one-sixth to one-third of the total length of the device arrangement, then the method of combining the first 12 measurement electrodes with the end measurement electrode P13 in sequence is selected.

[0061] In addition, in step 5, during the process of inverting the apparent resistivity profile into the true resistivity profile, the inversion accuracy is improved by the layered coupling inversion of seawater and sediment layer. Specifically, different seawater depths and seawater resistivities are given in the inversion model. The different seawater depths are obtained in real time by a single-beam device during the detection process, and the seawater resistivity is obtained by taking the reciprocal of the seawater conductivity measured by a conductivity meter. The inversion calculation is realized by using existing commercial software, such as RES2Dinv or EarthImager 2D.

[0062] The process of interpreting the distribution range of the gas bubble area and the change in gas content from the true resistivity profile is as follows:

[0063] First, determine whether there is a relatively high-resistance area in the true resistivity profile. A relatively high-resistance area refers to an area where the resistivity is generally higher than adjacent positions. If there is a high-resistance area, delimit the high-resistance area in the sediment layer 600 as the distribution range of bubble-type shallow gas. Determine the relative amount of gas content based on the relative resistivity magnitude in the relatively high-resistance area. If the resistivity is larger, the gas content is relatively more; conversely, if the resistivity is smaller, the gas content is less.

[0064] Figure 4 shows the true resistivity profile obtained by Figure 3 the method described above. In Figure 4 , the relatively high-resistance area in the true resistivity profile is mainly located in the range of the abscissa from 59 to 110 m, the top boundary is located at a depth of 7 m, and the bottom boundary is located at a depth of 17 m (mainly within the dashed box). According to this profile, the distribution range of bubble-type shallow gas in the detection area can be determined. In summary, the present invention realizes surface-sailing in-situ detection of submarine bubble-type shallow gas. The detection process is simple, fast, accurate, and low-cost, and can be carried out in real time and repeatedly.

[0065] The above is only a preferred embodiment of the present invention, and it is not intended to limit the present invention in other forms. Any person skilled in the art may use the disclosed technical content to make changes or modifications into equivalent embodiments with equivalent changes and apply them to other fields. However, as long as it does not depart from the technical solution content of the present invention, any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for detecting submarine bubble-type shallow gas by surface navigation, characterized in that, It includes the following steps: Step 1: The survey ship drags the cable system (200) to move on the sea surface, and supplies power to two power supply electrodes in the multi-channel electrode system to form an electric field; Step 2: Obtain the value of the supply current I of the two power supply electrodes and the potential values of the measurement electrodes. Every two measurement electrodes form a group to form a measurement electrode pair, and the two power supply electrodes form a power supply electrode pair. Subtract the measured potentials of each group of measurement electrodes to form the potential difference of this group of measurement electrode pairs; The power supply electrodes include three power supply electrodes C1, C2, and C3 arranged in sequence near the end of the survey ship. Select C1C3 or C2C3 as the power supply electrode pair. The measurement electrodes include thirteen measurement electrodes P1 to P13 arranged in sequence from the power supply electrode C1. Define the distance from the starting power supply electrode closest to the hull of the survey ship to the end measurement electrode P13 as the total length of the device arrangement. The detection depth is positively correlated with the total length of the device arrangement. Specifically, select the combination of measurement electrodes according to the detection depth requirement, specifically: If the detection depth does not exceed one-sixth of the total length of the device arrangement, the method of combining adjacent measurement electrodes in pairs is used for detection; If the detection depth exceeds one-sixth of the total length of the device arrangement, the method of combining the first 12 measurement electrodes P1-P12 with the end measurement electrode P13 in sequence is used for detection; Step 3: Use the GPS real-time monitoring device to determine the positions of the head and tail of the survey line, and use the ship speed real-time monitoring and the acquisition interval to determine the acquisition point position corresponding to the potential difference of each acquisition point; The horizontal position of the acquisition point is the center of the connection line between the power supply electrode close to the measurement electrode pair and the measurement electrode close to the power supply electrode. The depth of the acquisition point from the water surface is half of the length of the connection line between the power supply electrode close to the measurement electrode pair and the measurement electrode close to the power supply electrode; Step 4: Calculate the apparent resistivity of the acquisition point according to the supply current I and the potential difference ΔU of the acquisition point, and construct an apparent resistivity profile according to the apparent resistivity of the acquisition point and the acquisition point position; Step 5: Perform inversion calculation on the apparent resistivity profile to obtain the true resistivity profile; determine the distribution range of bubble-type shallow gas and the change of gas content according to the true resistivity profile.

2. The method for detecting submarine bubble-type shallow gas by a surface-sailing detector according to claim 1, wherein: In step 5, during the process of inverting the apparent resistivity profile into the true resistivity profile, the inversion accuracy is improved by the layered coupling inversion of seawater and sediment layer, that is, the seawater depth and seawater resistivity at different positions are given in the inversion model. The seawater depth at different positions is obtained in real time by the single-beam device during the detection process, and the seawater resistivity is obtained by taking the reciprocal of the seawater conductivity measured by the conductivity meter.

3. The method for detecting submarine bubble-type shallow gas by a surface cruising detector according to claim 1, wherein: In step 5, when determining the distribution range of bubble-type shallow gas and the change of gas content according to the true resistivity profile: First, determine whether there is a relatively high-resistance area in the true resistivity profile. The relatively high-resistance area refers to the area where the resistivity as a whole is higher than the adjacent position; if there is a high-resistance area, delimit the relatively high-resistance area in the sediment layer as the distribution range of bubble-type shallow gas, and determine the relative amount of gas content according to the relative resistivity size in the relatively high-resistance area. If the resistivity is large, the gas content is relatively large; on the contrary, if the resistivity is small, the gas content is small.

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