Exploration system based on reflected wave signals, exploration vessel group, processing unit, control unit, exploration method and program

Abstract

This invention relates to a survey system based on reflected wave signals for searching for objects in aquatic bodies of water. Conventional seismic streamer methods have problems such as the difficulty of deploying and retrieving long cables, low flexibility in arrangement, and degradation of signal quality due to traction noise. This invention comprises a transmitting boat that emits survey waves and a plurality of receiving boats, each having a receiver. These receiving boats are controlled in a survey formation while physically unconnected, and survey data is generated by integrating the received reflected wave signals based on positional and time information. This allows for flexible changes in the arrangement of receivers to be linear or planar, and enables the efficient acquisition of high-quality survey data with a high signal-to-noise ratio in diverse environments, including shallow seas and narrow bodies of water.

Description

Technical Field

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[0001] The present invention relates to a survey system, a group of survey boats, a processing device, a control device, a survey method, and a program based on reflected wave signals for surveying objects in water areas. More specifically, the present invention relates to a technique for controlling a plurality of receiving boats in a survey formation in a physically unconnected state, receiving reflected wave signals by receivers independently supported by each receiving boat, and generating survey data based on the reflected wave signals. The present invention can be used in a wide range of water area survey fields such as marine geological surveys, undersea resource surveys, surveys of underwater structures, and environmental surveys.

Background Art

[0002] Conventionally, in seismic surveys and submarine geological surveys in the ocean, a method has been widely used in which a long cable-shaped receiving device called a seismic streamer is towed from a survey ship to receive reflected waves of acoustic pulses. The seismic streamer has a structure in which a number of receivers such as hydrophones are arranged inside a flexible pipe, and the total length can reach several kilometers to more than ten kilometers. For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2008-506928) discloses a technique for applying a coating to prevent biological adhesion to the casing of a seismic streamer, and a streamer having a scale of about 5 to 6.4 cm in diameter and about 10 to 15 km in length is described.

[0003] However, the conventional seismic streamer method has problems such as requiring a great deal of time and labor for the deployment and recovery of a long cable, the receivers being limited to a one-dimensional linear arrangement, the signal-to-noise ratio being reduced due to hydrodynamic noise associated with towing, it being difficult to operate in shallow waters and narrow water areas, and the arrangement pattern of the receivers not being able to be dynamically changed during the survey.

Prior Art Documents

Patent Documents

[0004] The following patent documents exist as prior art documents.

[0005] [Patent Document 1] Special Publication No. 2008-506928 [Overview of the project] [Problems that the invention aims to solve]

[0006] This invention has been made in view of the problems of the prior art described above, and aims to provide an exploration system that can receive reflected wave signals using multiple independent wave receivers without using long streamer cables, and generate high-quality exploration data. Furthermore, this invention aims to provide an exploration system that can dynamically change the arrangement pattern of wave receivers and flexibly respond to various exploration conditions.

[0007] Furthermore, the present invention aims to provide a survey system that reduces noise during the reception period and achieves a high signal-to-noise ratio.

[0008] In addition, the present invention aims to provide an exploration system that can be efficiently operated even in restricted water areas such as shallow seas, narrow water areas, and harbor areas. Furthermore, the present invention aims to provide a group of exploration vessels that are easy to deploy and retrieve, and can flexibly respond to changes in the exploration line and turning. Moreover, the present invention aims to provide a processing and control device that can appropriately integrate reflected wave signals from dispersed receivers and generate exploration data equivalent to or better than that of a continuous streamer cable. [Means for solving the problem]

[0009] To solve the above problems, a first aspect of the present invention provides a survey system based on reflected wave signals for surveying objects in a body of water, comprising: an oscillating boat having an oscillating unit that emits survey waves; a plurality of receiving boats, each having a receiver that receives reflected waves corresponding to the survey waves emitted by the oscillating unit of the oscillating boat; a control unit that controls the plurality of receiving boats into a survey formation in a physically disconnected state; and a processing unit that generates survey data based on the reflected wave signals received by the receivers.

[0010] In the exploration system of the present invention, the processing unit can integrate the reflected wave signals based on the positional and time information of the plurality of wave-receiving boats. This makes it possible to reconstruct reflected wave signals obtained from multiple wave-receiving boats that are not physically connected as spatiotemporally consistent exploration data. Each wave-receiving boat is equipped with a positioning device such as a Global Navigation Satellite System (GNSS) receiver and a high-precision clock, and accurately records the time and location at which the receiver receives the reflected wave, thereby providing reference information for signal integration.

[0011] Furthermore, the processing unit can generate the exploration data so that the receivers, which are distributed across the multiple wave-receiving boats, function as a wave-receiving channel sequence equivalent to that which would occur if they were physically arranged in a continuous sequence. In other words, by integrating the signals from the individually distributed receivers as if they were signals from a group of receivers arranged on a single continuous streamer cable, it becomes possible to generate an exploration data format compatible with conventional streamer systems.

[0012] In the exploration system of the present invention, the survey formation can be a linear formation in which the plurality of wave-receiving boats are arranged in a substantially linear manner along the survey line. The linear formation realizes a one-dimensional arrangement similar to conventional seismic streamers, and can eliminate constraints caused by physical connections while maintaining compatibility with existing analysis methods and data processing pipelines. Each wave-receiving boat navigates independently, but maintains a substantially linear arrangement through formation control by the control unit.

[0013] Furthermore, the survey formation can also be a planar formation that includes at least one of a grid, staggered, fan, or arc shape. A planar formation enables a two-dimensional receiver arrangement, allowing for three-dimensional exploration of the target structure and high-precision imaging through reception of reflected waves from multiple directions. A grid formation is suitable for uniform spatial sampling, a staggered formation efficiently achieves denser sampling than a grid formation, and a fan or arc formation can improve sensitivity to reflected waves from a specific direction.

[0014] In the exploration system of the present invention, the exploration wave can be an elastic wave. The elastic wave propagates through water and is reflected at the seabed and the subsurface geological boundary, generating a reflected wave signal suitable for exploring subsurface structures. As the elastic wave source, known oscillation sources such as air guns, underwater sparkers, boomers, and chirp sonars can be used, and the oscillation wave is emitted by being mounted on or towed by an oscillating boat. The frequency band of the elastic wave is appropriately selected according to the purpose of exploration and the target depth.

[0015] In the exploration system of the present invention, the control unit can control the wave-receiving boat to a lower noise state than outside the wave-receiving period during the wave-receiving period after the wave-receiving period has emanated. The wave-receiving period refers to the period from when the wave-receiving boat emits an exploration wave until the reflected wave from an object such as the seabed reaches the wave-receiving device. By minimizing the noise emitted by the wave-receiving boat during this period, the signal-to-noise ratio of the reflected wave signal can be significantly improved.

[0016] The low-noise state may include at least one of the following: reduced propulsion output, cessation of propulsion, transition to a drifting state, or minimal attitude control to maintain the distance between boats. Specifically, by reducing or cessating the output of the wave-receiving boat's propulsion engine during the wave-receiving period, and temporarily putting the wave-receiving boat into a drifting state, mechanical vibrations and hydrodynamic noise caused by the propulsion engine can be significantly reduced. In addition, by performing minimal attitude control only when the distance between boats fluctuates beyond an acceptable range, noise can be suppressed while preventing a significant breakdown of the formation.

[0017] In the exploration system of the present invention, the control unit can move the wave-receiving boat to a transition formation different from the survey formation when turning or switching survey lines. In conventional streamer systems, a great deal of time was required to handle the long cables when turning, but in the present invention, since each wave-receiving boat can navigate independently, each wave-receiving boat can individually follow the optimal path when turning and efficiently reach the starting position of the next survey line. The transition formation is set considering safety and efficiency when turning.

[0018] In the exploration system of the present invention, the wave receiver can be fixed to the hull of the wave-receiving boat, suspended from the wave-receiving boat, or held near the wave-receiving boat by a short holding member. The hull fixing method is the simplest configuration and is achieved by attaching the wave receiver to the keel of the wave-receiving boat. The suspension method involves suspending the wave receiver to a predetermined depth from the bottom of the wave-receiving boat by a cable or the like, and has the advantage of reducing the influence of surface wave noise. The short holding member method is a method of holding the wave receiver near the hull of the wave-receiving boat via a rigid holding member.

[0019] In the exploration system of the present invention, at least one of the wave-receiving boats can support a short multi-channel array in which multiple wave receivers are arranged along a short streamer cable. The short multi-channel array has multiple wave receivers arranged on a cable that is significantly shorter (e.g., several meters to tens of meters) than conventional long streamer cables, and can increase the number of wave-receiving channels per boat. This makes it possible to ensure a sufficient spatial sampling density while suppressing the total number of wave-receiving boats.

[0020] In the exploration system of the present invention, the control unit can change the distance between the boats, the shape of the survey formation, or the selection of wave-receiving boats according to at least one of the following: target depth, direction of the target structure, required resolution, sea conditions or obstacle distribution, oscillation conditions, or wave reception conditions. For example, the distance between the boats is reduced when exploring shallow targets with high resolution, and increased when exploring deep targets over a wide area. Furthermore, the distance between the boats is increased to ensure a safety margin when sea conditions deteriorate, and the formation is changed to avoid obstacles when obstacles are present. In addition, by re-selecting wave-receiving boats to participate in the exploration from among multiple wave-receiving boats according to the wave reception conditions, data quality can be maintained and operational efficiency can be improved.

[0021] In the exploration system of the present invention, the processing unit can perform interpolation or regularization processing to correct the irregularities in the positions of the plurality of wave-receiving boats. Although physically unconnected wave-receiving boats may be displaced from their ideal positions due to disturbances such as ocean currents and wind, the processing unit can estimate the signal value at the wave-receiving point on a virtual regular arrangement based on the actual position information of each wave-receiving boat by interpolation processing. Furthermore, regularization processing can suppress artifacts in the exploration data caused by irregularities in position.

[0022] A group of exploration vessels based on reflected wave signals according to a second aspect of the present invention includes an oscillating vessel having an oscillating unit and a plurality of wave-receiving vessels, each of which independently supports at least one wave receiver, and the plurality of wave-receiving vessels are configured to form an exploration formation in a physically unconnected state. This group of exploration vessels provides a new exploration platform that replaces the conventional combination of a large research vessel and a long streamer, and the flexibility and efficiency of the survey are greatly improved by the fact that each wave-receiving vessel has the ability to navigate independently.

[0023] The aforementioned wave-receiving boats can be further configured to maintain the survey formation while in a low-noise state during the wave-receiving period after wave emission, and to form a transition formation different from the survey formation when turning or switching survey lines. This makes it possible to achieve both high-quality data acquisition during exploration and efficient movement between survey lines. The low-noise state during the wave-receiving period and the transition formation when switching survey lines are switched autonomously by each wave-receiving boat based on commands from the control unit.

[0024] A processing device according to a third aspect of the present invention is a processing device used for exploration based on reflected wave signals, comprising: an input unit that acquires reflected wave signals received by receivers independently supported by a plurality of wave receiving boats, the wave receiving boats being distributed in a physically disconnected state; and a processing unit that integrates the reflected wave signals and generates exploration data. This processing device provides a processing function that spatiotemporally integrates signals from a plurality of distributed receivers and generates exploration data equivalent to that of a conventional continuous streamer method.

[0025] The control device according to the fourth aspect of the present invention is a control device used for exploration based on a reflected wave signal, comprising a formation control unit that controls a plurality of wave receiving boats into an investigation formation in a physically unconnected state, and a low noise control unit that controls at least the wave receiving boats into a low noise state during a wave receiving period after the exploration wave is oscillated. The formation control unit monitors the positions of each wave receiving boat and transmits a navigation command to each wave receiving boat so that the target investigation formation is maintained. The low noise control unit performs output control of the propulsion engine of the wave receiving boat in synchronization with the oscillation timing of the exploration wave.

Advantages of the Invention

[0026] According to the present invention, by controlling a plurality of wave receiving boats into an investigation formation in a physically unconnected state, various problems caused by a conventional long streamer cable can be solved, and flexible and high-quality underwater exploration can be realized. Specifically, the arrangement pattern of the receivers can be freely changed from a linear formation to a planar formation, and an optimal receiver arrangement according to the exploration conditions can be dynamically realized. In addition, by low noise control during the wave receiving period, it is possible to significantly improve the reduction in the signal-to-noise ratio caused by towing noise.

[0027] Furthermore, according to the present invention, since each wave receiving boat can navigate independently, deployment and recovery are easy, and efficient exploration is possible even in environments with restrictions such as shallow seas and narrow waters. By forming a transition formation during turning or when switching the survey line, the survey efficiency can be significantly improved. In addition, by interpolation processing and regularization processing by the processing unit, irregularities in the positions of the wave receiving boats can be corrected, and high-quality exploration data can be stably generated.

Brief Description of the Drawings

[0028] [Figure 1] It is a diagram schematically showing the overall configuration of an exploration system according to a first embodiment of the present invention. [Figure 2] It is a plan view showing an example of a linear formation in the first embodiment. [Figure 3]This is a plan view showing an example of a planar formation in the first embodiment, where (a) is a grid formation, (b) is a staggered formation, (c) is a fan formation, and (d) is an arc formation. [Figure 4] This figure shows the timing chart for the oscillation of the probe wave and the reception of the reflected wave. [Figure 5] This diagram shows a flowchart of the signal integration process in the processing unit. [Figure 6] This is a plan view showing an example of a transition formation during turning or switching survey lines. [Figure 7] This diagram shows variations in the mounting method of the receiver, with (a) being the hull-fixed method, (b) being the suspension method, and (c) being the short-length holding member method. [Figure 8] This figure shows an example configuration of a short-length multi-channel array. [Figure 9] This figure shows an example of dynamic formation changes by the control unit. [Figure 10] This is a diagram showing an example of a communication architecture configuration. [Figure 11] This diagram shows examples of formation selection according to the exploration scenario, with (a) representing high-resolution exploration in shallow waters, (b) representing wide-area exploration in deep waters, (c) representing exploration of harbors and narrow water areas, and (d) representing exploration around structures. [Figure 12] This diagram shows the decision-making flow for re-selecting the roles of the wave-receiving boat and the wave-sending boat. [Figure 13] This diagram shows the selection flow for the data transfer method. [Figure 14] This is a flowchart showing the operational cycle of the fleet. [Modes for carrying out the invention]

[0029] Embodiments of the present invention will be described in detail below with reference to the drawings. Note that the following embodiments are not limiting to the present invention, and modifications can be made as appropriate within the scope of the technical concept of the present invention. Furthermore, the same or corresponding components are denoted by the same reference numerals in each drawing, and redundant explanations may be omitted. The following embodiments will primarily describe seismic exploration in the ocean as an example, but the present invention is also applicable to exploration in inland waters such as lakes, rivers, and reservoirs.

[0030] Figure 1 is a schematic diagram showing the overall configuration of the exploration system 1 according to the first embodiment of the present invention. The exploration system 1 comprises an oscillating boat 10 having an oscillating unit 11 that emits exploration waves, a plurality of wave-receiving boats 20a to 20n each having a wave receiver 21 that receives reflected waves, a control unit 30 that controls the plurality of wave-receiving boats 20a to 20n in an exploration formation while they are not physically connected, and a processing unit 40 that generates exploration data based on the reflected wave signals received by the wave receivers 21. The oscillating boat 10 and the wave-receiving boats 20a to 20n are ships or unmanned vessels capable of navigating the water and are not physically connected to each other by cables or the like.

[0031] The oscillating boat 10 is equipped with or towed as an oscillating unit 11, which includes an elastic wave source such as an air gun, underwater sparker, boomer, or chirp sonar. The oscillating unit 11 emits exploration waves at predetermined timings based on an oscillation command from the control unit 30. The exploration waves are elastic waves that propagate through the water and are reflected at the seabed and the geological boundary below the seabed. In addition to the function of emitting exploration waves, the oscillating boat 10 has an autonomous navigation function or a remote control function and navigates along a predetermined survey line according to the survey plan. The oscillating boat 10 is equipped with a GNSS receiver to measure its own position with high precision.

[0032] Each wave-receiving boat 20a to 20n is equipped with a wave receiver 21, which includes underwater acoustic sensors such as hydrophones, acceleration sensors, and pressure sensors. The wave receiver 21 receives reflected waves generated when the exploration waves emitted from the oscillating boat 10 are reflected off the seabed, etc., and converts them into electrical signals. In addition to the wave receiver 21, each wave-receiving boat 20a to 20n is equipped with a GNSS receiver 22, a high-precision clock 23, a data recording device 24, a communication device 25, and a propulsion device 26. The GNSS receiver 22 measures the position of the wave-receiving boat with high precision, and the high-precision clock 23 accurately records the time of reception of the reflected waves.

[0033] The communication device 25 communicates wirelessly with the control unit 30 and other wave-receiving boats, transmitting and receiving position information, time information, formation control commands, and reflected wave signal data. The communication method can include wireless LAN, LTE, satellite communication, or a proprietary wireless communication protocol, and is selected appropriately depending on the amount of data and communication distance. The propulsion device 26 is an electric motor-driven screw, water jet, or thruster, and autonomously navigates the wave-receiving boat. The output of the propulsion device 26 is controlled based on commands from the control unit 30, contributing to maintaining the survey formation and achieving a low-noise state.

[0034] The control unit 30 is a computer system that comprehensively controls the entire exploration system 1 and is mounted on the oscillating boat 10 or a separately provided control vessel. The control unit 30 has a formation control function, an oscillation control function, and a low-noise control function. The formation control function monitors the position of each wave-receiving boat 20a to 20n in real time and transmits navigation commands to each wave-receiving boat so that the target survey formation is maintained. The oscillation control function commands the oscillation unit 11 of the oscillating boat 10 to set the oscillation timing. The low-noise control function commands the wave-receiving boats to transition to a low-noise state in synchronization with the oscillation timing.

[0035] The processing unit 40 is a computer system that collects reflected wave signals received by the receivers 21 of multiple wave-receiving boats 20a to 20n, processes them in an integrated manner, and generates exploration data. The processing unit 40 may be implemented on the same computer system as the control unit 30, or it may be configured as a separate processing unit. The processing unit 40 stores the reflected wave signals received from each wave-receiving boat in association with corresponding position information and time information, and performs signal integration processing based on this information. The detailed processing contents of the processing unit 40 will be described later.

[0036] Figure 2 is a plan view showing an example of a linear formation in the first embodiment. The linear formation is a formation in which multiple wave-receiving boats 20a to 20n are arranged in a substantially linear manner along the survey line L, realizing a one-dimensional wave-receiving arrangement similar to that of conventional seismic streamers. The oscillating boat 10 navigates near the leading edge of the survey line L, and the wave-receiving boats 20a to 20n are arranged in a substantially linear manner behind the oscillating boat 10, maintaining a constant distance d between boats. The distance d between boats is set according to the purpose of the exploration and the required spatial resolution, and is typically in the range of tens to hundreds of meters.

[0037] In a linear formation, each wave-receiving boat 20a to 20n navigates with a predetermined position on the survey line L as its target, and maintains the distance d between boats and the lateral deviation from the survey line L within a certain range based on commands from the control unit 30. If a wave-receiving boat deviates from its target position due to disturbances such as ocean currents or wind, the control unit 30 detects the amount of deviation and sends a correction command, and the wave-receiving boat controls its propulsion system 26 to return to the target position. However, in order to prioritize low noise conditions during the wave-receiving period, the output of the propulsion system 26 may be limited to a minimum even if the deviation is large.

[0038] Figure 3 is a plan view showing an example of a planar formation in the first embodiment. The planar formation is a formation in which multiple wave-receiving boats 20a to 20n are arranged two-dimensionally, and includes various patterns such as grid, staggered, fan-shaped, or arc-shaped. Figure 3(a) shows a grid formation, in which the wave-receiving boats are regularly arranged in the direction of the survey line and in directions perpendicular thereto. Figure 3(b) shows a staggered formation, in which wave-receiving boats in adjacent rows are offset by half a pitch. Compared to the grid formation, the staggered formation can achieve denser spatial sampling with the same number of wave-receiving boats.

[0039] Figure 3(c) shows a fan-shaped formation, in which wave-receiving boats are arranged in a fan shape with the oscillating boat 10 as the apex of the fan. The fan-shaped formation is suitable for receiving reflected waves from the oscillation point over a wide angular range and is excellent for detecting the inclination of the reflective surface and changes in lateral structure. Figure 3(d) shows an arc-shaped formation, in which wave-receiving boats are arranged on a circular arc with a predetermined radius of curvature. The arc-shaped formation is suitable for focused reception of reflected waves directed toward a specific focal point and exhibits high detection sensitivity when the target structure is known. These planar formations can be appropriately selected or combined depending on the purpose of the exploration.

[0040] Figure 4 is a diagram showing the timing chart for the oscillation of exploration waves and the reception of reflected waves. The oscillation unit 11 of the oscillation boat 10 oscillates exploration waves at time t0 based on a command from the control unit 30. The exploration waves propagate through the water and are reflected at the seabed surface and the geological boundary surface below the seabed. The reflected waves reach and are received by the receivers 21 of each wave-receiving boat 20a to 20n. The arrival time of the reflected waves depends on the path length from the oscillation point through the reflection point to the reception point and the speed of sound. The reception period Tr is defined as the period from time t0 until the reflected wave from the furthest reflective surface reaches the furthest wave-receiving boat.

[0041] Before the start of the wave-receiving period Tr, the control unit 30 transmits a command to each wave-receiving boat 20a to 20n to transition to a low-noise state. Upon receiving the command, each wave-receiving boat reduces or stops the output of its propulsion system 26 and transitions to a low-noise state. During the wave-receiving period Tr, the wave-receiving boats are essentially in a drifting state to minimize noise caused by their propulsion systems. However, if the distance between boats fluctuates beyond an acceptable range or for safety reasons, only minimal attitude control is permitted. After the end of the wave-receiving period Tr, the control unit 30 transmits a command to each wave-receiving boat to return to the normal state, and the wave-receiving boats restart their propulsion systems 26 and readjust their formation in preparation for the next oscillation.

[0042] Figure 5 is a flowchart showing the signal integration process in the processing unit 40. First, the processing unit 40 acquires reflected wave signal data, position information, and time information from each wave receiver 20a to 20n (step S1). Next, the processing unit 40 converts the actual position coordinates of each receiver to a reference coordinate system and synchronizes the time axis of each reflected wave signal based on the time information (step S2). Through this synchronization process, the signals from all receivers are aligned to a common time reference.

[0043] Next, the processing unit 40 performs interpolation processing to correct the irregularities in the position of each receiver (step S3). In the interpolation processing, the signal value at the receiving point on a virtual regular arrangement is estimated based on the actual position coordinates of each receiver. Known interpolation methods such as linear interpolation, spline interpolation, kriging, and inverse distance weighting can be used. In addition, by using regularization processing in combination, the occurrence of artifacts associated with interpolation can be suppressed. As for regularization processing, methods such as Tikhonov regularization and LASSO regularization can be applied.

[0044] Next, the processing unit 40 performs standard seismic exploration data processing such as common depth point (CDP) superposition and migration processing using the interpolated signal data (step S4). The processing unit 40 generates exploration data so that the distributed receivers function as a receiving channel sequence equivalent to if they were physically arranged in a continuous sequence. That is, the processing unit 40 reconstructs the signals from receivers distributed across multiple receivers as signals from a receiving channel sequence on a virtual continuous streamer, and generates exploration data such as exploration cross-sectional images (step S5).

[0045] Figure 6 is a plan view showing an example of a transition formation during turning or switching survey lines. After the exploration on survey line L1 is completed, when moving to the next survey line L2, the multiple wave-receiving boats 20a to 20n transition from a survey formation (e.g., a linear formation) to a transition formation. The transition formation is a formation designed so that each wave-receiving boat can reach the starting position of the next survey line L2 efficiently and safely. Examples include a dispersed turning formation in which each wave-receiving boat individually follows the shortest path, and a collective turning formation in which all wave-receiving boats turn while maintaining a constant formation.

[0046] In a dispersed turning formation, each wave-receiving boat turns by following its own optimal path while maintaining a safe distance from the others. This method minimizes turning time, but the path planning and collision avoidance control for each wave-receiving boat become complex. In a group turning formation, all wave-receiving boats turn together while maintaining a predetermined turning radius and turning speed. This method has a simpler path planning and is safer, but the area of ​​water required for turning is larger. The control unit 30 selects the optimal transition formation considering the size of the water area, the presence or absence of obstacles, sea conditions, etc.

[0047] Figure 7 shows a modified example of the mounting method for the wave receiver 21. Figure 7(a) shows a hull-mounted method in which the wave receiver 21 is fixed to the keel of the hull of the wave-receiving boat 20. This method has a simple structure and high positioning accuracy because the position of the wave receiver directly corresponds to the position of the wave-receiving boat. Figure 7(b) shows a suspension method in which the wave receiver 21 is suspended from the bottom of the wave-receiving boat 20 by a cable 50. The length of the cable 50 is usually several meters to more than ten meters, allowing the wave receiver 21 to be placed at a depth where the influence of surface waves is minimal.

[0048] Figure 7(c) shows a short-length holding member system in which the wave receiver 21 is held near the wave-receiving boat 20 by a short-length holding member 60. The short-length holding member 60 is a rigid or semi-rigid rod-shaped or tubular member that protrudes laterally or downward from the hull of the wave-receiving boat 20 to hold the wave receiver 21. The length of the short-length holding member 60 is usually 1 meter to several meters, and by separating the wave receiver 21 from the hull at an appropriate distance, the transmission of hull vibrations to the wave receiver 21 is suppressed. These mounting methods are appropriately selected according to the exploration conditions and the specifications of the wave-receiving boat.

[0049] Figure 8 shows an example configuration of a short multi-channel array 70. The short multi-channel array 70 consists of multiple receivers 21a to 21m arranged along a short streamer cable 71 and is supported by a wave-receiving boat 20. The total length of the short streamer cable 71 is significantly shorter than the total length of conventional earthquake streamers (several kilometers to tens of kilometers), for example, about 5 to 100 meters. The short multi-channel array 70 is used either towed from the stern of the wave-receiving boat 20 or suspended from the bottom of the boat.

[0050] The number of receivers 21a to 21m arranged in the short-length multi-channel array 70 ranges from, for example, 4 to 48 channels, with channel spacing ranging from, for example, 1 meter to several meters. By using the short-length multi-channel array 70, the number of receiving channels per boat can be increased, making it possible to secure sufficient spatial sampling density with a small number of receiving boats. Furthermore, because it is short, it is easy to handle and does not have the difficulties of deployment and retrieval that conventional long-length streamers have. It can also be operated safely in shallow waters and waters with obstacles.

[0051] Figure 9 shows an example of dynamic formation changes by the control unit 30. The control unit 30 can change the distance between boats, the shape of the survey formation, or the selection of wave-receiving boats according to at least one of the following: target depth, bearing of target structure, required resolution, sea conditions or obstacle distribution, oscillation conditions, or wave-receiving conditions. Figure 9(a) shows a narrow formation when searching a shallow target with high resolution, and the distance between wave-receiving boats is reduced compared to normal. Figure 9(b) shows a wide formation when searching a deep target over a wide area, and the distance between wave-receiving boats is increased.

[0052] The control unit 30 can dynamically change the formation based on data acquired in real time during the exploration. For example, if the processing unit 40 evaluates the quality of the reflected wave signal in real time and the signal-to-noise ratio falls below a threshold, it requests the control unit 30 to change the formation. Based on this request, the control unit 30 adjusts the distance between the boats and changes the formation. It also has a function to reconfigure the formation as optimally as possible with the remaining wave-receiving boats if one of the wave-receiving boats fails or loses communication. Furthermore, the control unit 30 can also change the selection of wave-receiving boats to participate in the exploration. For example, if the data quality of a particular wave-receiving boat deteriorates, it can temporarily exclude that boat from the exploration and deploy a standby backup boat.

[0053] Next, a second embodiment will be described. The exploration system according to the second embodiment has a configuration in addition to the configuration of the first embodiment, with multiple oscillator boats 10 provided. By using multiple oscillator boats, the oscillation interval of exploration waves can be shortened, and the efficiency of the survey can be improved. Furthermore, by using reflected waves from multiple oscillation points, the number of overlaps at common reflection points can be increased, and the quality of the exploration data can be improved. The oscillation timing of the multiple oscillator boats is adjusted by the control unit 30, and they are controlled so that their oscillation waves do not interfere with each other.

[0054] In the second embodiment, multiple oscillator boats 10a and 10b are positioned at different locations along the survey line and emit probe waves alternately or with a predetermined delay time. Receiving boats 20a to 20n each receive reflected waves corresponding to the probe waves from each oscillator boat, and the processing unit 40 identifies and processes the reflected wave signals corresponding to each oscillator boat individually. Identification of the oscillator boats is performed by differences in oscillation timing, differences in the frequency characteristics of the emitted waves, or correlation processing of encoded oscillation signals. This makes it possible to simultaneously acquire reflected wave data for multiple oscillation points with a single receiver array.

[0055] Next, a third embodiment will be described. The exploration system according to the third embodiment has a configuration that uses an unmanned surface vehicle (USV) as a wave-receiving vessel. A USV is a small unmanned vessel that can be remotely controlled or navigate autonomously and can navigate on the water without carrying any personnel. By using a USV as a wave-receiving vessel, advantages such as reduced labor costs, long-term continuous operation, and safe operation in dangerous sea areas can be obtained. Each USV is an autonomous wave-receiving platform that integrates a wave receiver, GNSS receiver, high-precision clock, data recording device, communication device, and propulsion device.

[0056] In the third embodiment, each USV autonomously navigates based on commands from the control unit 30 and forms a survey formation. The autonomous navigation algorithm of the USV includes a waypoint navigation function based on GNSS positioning information, a collision avoidance function with other USVs, and a sea condition adaptation function. The control unit 30 is installed on a control vessel or a land-based control station and comprehensively controls each USV via wireless communication. The dimensions of the USV are, for example, about 1 to 5 meters in length, and are significantly smaller than conventional survey vessels, making them suitable for operation in shallow waters and narrow waters.

[0057] Next, a fourth embodiment will be described. The fourth embodiment relates to another method of signal integration processing in the processing unit 40. In the fourth embodiment, the processing unit 40 integrates reflected wave signals from distributed receivers based on compressed sensing theory. Compressed sensing is a technique that reconstructs the original signal with high accuracy from fewer samples than the Nyquist criterion when the signal is sparse in a certain conversion region, and is applicable even when the spacing between receivers does not meet the spatial Nyquist criterion.

[0058] In the fourth embodiment, the processing unit 40 acquires the reflected wave signals from each receiver as observation vectors and assumes that the reflection coefficient distribution on a discretized grid of the area to be explored is a sparse signal. The processing unit 40 estimates a high-resolution reflection coefficient distribution using data from a limited number of irregularly arranged receivers by solving an L1 norm minimization problem or a similar optimization problem. This method makes it possible to generate exploration data that maintains spatial resolution even when the number of wave receivers is limited.

[0059] Next, a fifth embodiment will be described. The fifth embodiment relates to the details of formation control in the control unit 30. The formation control unit of the control unit 30 has a discrete control loop that updates the position of each wave-receiving boat at discrete time steps. In each control step, the formation control unit obtains the current position of all wave-receiving boats, calculates the deviation of each wave-receiving boat from its target position in the target formation, and transmits a navigation command to each wave-receiving boat to minimize the deviation. The navigation command is given as a pair of target heading and target speed, and each wave-receiving boat controls its propulsion system to follow the command.

[0060] The formation control unit in the fifth embodiment further includes a predictive control function. The predictive control function predicts the future position of each wave-receiving boat based on ocean current prediction models, wind direction and speed predictions, and motion models of each wave-receiving boat, and generates advance navigation commands based on the predicted positions. This reduces the delay in the formation's response to disturbances and enables more stable formation maintenance. The formation control unit also has a collision avoidance function between wave-receiving boats, and if it determines that the distance between wave-receiving boats falls below a safety limit, it commands avoidance action with the highest priority.

[0061] Next, a sixth embodiment will be described. The sixth embodiment relates to a configuration that includes a boat that combines the functions of both an oscillating boat and a wave-receiving boat. That is, at least one of the multiple boats is configured as an oscillating / wave-receiving boat equipped with both an oscillating unit that emits exploration waves and a wave-receiving unit that receives reflected waves. By using an oscillating / wave-receiving boat, a variety of exploration geometries can be realized, including arrangements in which the oscillating point and the wave-receiving point are the same or close to each other, and the acquisition of zero-offset data and the like can be made easier.

[0062] In the sixth embodiment, the dual-purpose wave-oscillating and receiving boat can receive both reflected waves from the exploration waves emitted by its own boat and reflected waves from exploration waves emitted by other wave-oscillating boats. The processing unit 40 identifies the source of each reflected wave signal and processes them according to their source. Reflected waves from the own boat's oscillation are used as zero offset or short-range offset data, while reflected waves from other boats' oscillations are used as long-range offset data. By combining these, high-quality exploration data covering a wide offset range can be obtained.

[0063] Next, the details of the communication method in the exploration system of the present invention will be described. It is desirable that communication between each wave-receiving boat and the control unit 30 be configured as a low-latency communication channel for real-time control and a high-bandwidth communication channel for data transfer. The low-latency communication channel is used for transmitting information requiring real-time performance, such as formation control commands, oscillation timing synchronization signals, and low-noise state transition commands, and utilizes, for example, a proprietary wireless protocol or cellular communication such as LTE. The high-bandwidth communication channel is used for transferring reflected wave signal data, and utilizes broadband wireless communication such as Wi-Fi.

[0064] Furthermore, the transfer of reflected wave signal data does not necessarily have to be done in real time. It is also possible to temporarily store the data in the data recording device 24 of each wave-receiving boat and transfer it all at once after the survey is completed or during the wave-receiving period. If real-time transfer is performed, the amount of communication bandwidth used can be reduced by performing data compression processing on the wave-receiving boat side. It is also possible to adopt a distributed processing method in which data is pre-processed in a distributed manner among the wave-receiving boats, and only the processing results are transferred to the control unit 30 or processing unit 40.

[0065] Next, a detailed embodiment of the communication architecture in the exploration system of the present invention will be described with reference to Figure 10. Figure 10 is a diagram showing an example of the configuration of the communication architecture. In this embodiment, communication is classified into three layers. The first layer is the inter-boat communication layer C1 between each wave-receiving boat 20a to 20n and the control unit 30, and is used for transmitting and receiving real-time control data such as formation control commands, oscillation timing synchronization signals, and low-noise state transition commands. The second layer is the exploration data transfer layer C2 from each wave-receiving boat 20a to 20n to the control ship or processing unit 40, and is used for transferring reflected wave signal data. The third layer is the wide-area communication layer C3 between the control ship and the land base station or remote monitoring center, and is used for remote monitoring of the exploration status, land transfer of exploration data, and commands to update the survey plan.

[0066] Because the inter-boat communication layer C1 requires low latency and high reliability, it can utilize proprietary wireless communication protocols, LTE / 5G cellular communication, or LPWAN (Low Power Wide Area Network) communication such as LoRa®. Although the amount of control data is relatively small (for example, tens to hundreds of bytes per message), the oscillation timing synchronization signal requires a delay of tens of milliseconds or less. For this reason, it is desirable that the communication channel for control data be secured as a dedicated channel independent of other data transfers. Furthermore, the inter-boat communication layer C1 can also configure mesh-type communication between each wave-receiving boat, enabling multi-hop communication to transmit control data via relay boats to wave-receiving boats that are far away and for which direct communication with the control vessel is difficult.

[0067] Because the exploration data transfer layer C2 requires large-capacity data transfer, it can utilize wireless LAN (IEEE 802.11ac / ax compliant), 5G NR, or dedicated broadband wireless communication. Reflected wave signal data is generated in amounts ranging from several hundred kilobytes to several megabytes per wave-receiving cycle from each wave-receiving vessel. For real-time transfer, it is effective to reduce the amount of data transferred by performing data compression or edge pre-processing on the wave-receiving vessel side. Edge pre-processing includes bandpass filtering, decimation, and the assignment of quality flags. By performing these pre-processing steps on the onboard computer of each wave-receiving vessel, it is possible to reduce the amount of data transferred to about one-tenth to one-fifth of the original amount.

[0068] The exploration data transfer layer C2 can also employ a store-and-forward method. In the store-and-forward method, each wave-receiving vessel temporarily stores reflected wave signal data in the data recording device 24 and transfers it all at once during the formation readjustment period between wave-receiving periods. This method has the advantage of being able to eliminate communication traffic during wave-receiving periods and completely eliminate the influence of electromagnetic interference caused by communication on the wave receivers. The transfer order of the stored data is controlled based on priority, with high-quality reflected wave data being transferred preferentially. If the communication quality is degraded, low-priority data can be omitted from transfer and will be physically recovered when the vessel is recovered.

[0069] Wide-area communication layer C3 uses satellite communication (VSAT, Iridium, Starlink, etc.) or LTE / 5G cellular communication between the control vessel and the land-based base station. Cellular communication is available for coastal surveys, but satellite communication is required for offshore surveys. Through wide-area communication layer C3, it is possible to transmit some of the survey data or preview data to the land-based analysis center in near real-time, allowing remote experts to monitor the progress of the survey and dynamically modify the survey plan.

[0070] Next, an example of formation selection according to the exploration scene will be described with reference to Figure 11. Figure 11 is a diagram showing typical formation configurations for each exploration scene, with (a) showing high-resolution exploration in shallow waters, (b) showing wide-area exploration in deep waters, (c) showing harbor and narrow water area exploration, and (d) showing structure orbital exploration. The control unit 30 has a formation selection function that automatically selects or proposes an appropriate formation pattern based on inputs such as exploration scene information, target depth, water area shape, and obstacle information.

[0071] In the shallow-water high-resolution exploration scene shown in Figure 11(a), the water depth is relatively shallow (e.g., 50m or less), and it is required to grasp the fine structure near the seabed with high resolution. In this scene, a dense linear formation with a narrow distance d between boats is adopted. Specifically, the distance d between boats is set to 5m to 30m, and 10 to 20 wave-receiving boats are densely arranged behind the oscillating boat. To achieve high resolution, a high-frequency chirp sonar (e.g., 1kHz to 10kHz) is used in the oscillating unit 11, and it oscillates repeatedly at short oscillation intervals. Because the reception period is short, the duration of maintaining a low-noise state is also short, enabling efficient sweeping of the survey line.

[0072] In the deep-sea wide-area exploration scene shown in Figure 11(b), the water depth is great (e.g., 500m or more), and it is necessary to understand the geological structure beneath a wide area of ​​the seabed. In this scene, a diffuse planar formation with a wide distance d between boats is adopted. Specifically, the distance d between boats is set to 100m to 500m, and 20 to 50 or more wave-receiving boats are arranged in a grid or staggered pattern in two dimensions. A high-power low-frequency air gun (e.g., 10Hz to 200Hz) is used in the oscillating unit 11 to ensure a long reception period and receive reflected waves from the depths. The planar formation allows for the coverage of a wide swath width with a single sweep of the survey line, efficiently acquiring three-dimensional geological structure data.

[0073] In the harbor / narrow water area exploration scene shown in Figure 11(c), the width of the water area is limited, and exploration is required in an environment where obstacles (quays, moored vessels, buoys, etc.) are present. In this scene, a reduced linear formation or trapezoidal formation is adopted using a small number of wave-receiving boats (e.g., 3 to 8 boats). The trapezoidal formation is a formation in which wave-receiving boats are shifted in the direction of the survey line while also being dispersed laterally, and has characteristics intermediate between linear and planar formations. The distance d between boats is set from 3 to 20 meters depending on the constraints of the water area, and the formation is changed in real time based on the position information of obstacles.

[0074] In the structure orbital exploration scene shown in Figure 11(d), it is required to explore around structures such as bridge piers, monopiles, and submarine pipelines. In this scene, an arc-shaped formation or an orbital formation centered on the structure is adopted. In the orbital formation, the oscillating boat 10 and wave-receiving boats 20a to 20n are arranged to surround the structure, and reflected wave data from multiple directions is acquired while orbiting the structure. Based on the position and shape information of the structure, the control unit 30 generates a path that allows each wave-receiving boat to safely orbit the structure while maintaining a constant distance from it.

[0075] The formation selection function of the control unit 30 also responds to transitions in the exploration scene. For example, when extending the survey line from the coastal area to the offshore area, a transition occurs from a shallow-water high-resolution exploration scene to a deep-sea wide-area exploration scene as the water depth changes. At this time, the control unit 30 detects the transition based on the water depth data and gradually changes the distance d between boats and the formation shape. During the transition period, in order to maintain the continuity of the wave receiver placement between adjacent shots, a gradual transition control is performed, in which the position of the wave receiver is gradually changed over multiple shot cycles.

[0076] Next, an example of the re-selection of the roles of the oscillating boat and the wave-receiving boat will be described with reference to Figure 12. Figure 12 is a diagram showing the determination flow for the re-selection of the roles of the oscillating boat and the wave-receiving boat. In this example, the control unit 30 continuously monitors the status information and external information of each boat, and dynamically changes the roles of the oscillating boat and the wave-receiving boat when predetermined re-selection conditions are met. In the flow of Figure 12, first, the control unit 30 executes a monitoring step S11 in which it obtains status information from each boat and external information from an external system. Next, the control unit 30 executes a determination step S12 in which it determines whether the re-selection conditions are met based on the acquired information, and returns to the monitoring step S11 if the re-selection conditions are not met. If the re-selection conditions are met, the control unit 30 executes a classification step S13 in which it classifies whether the met condition is a boat state trigger or an external information trigger. Re-selection conditions are classified into two types: boat state triggers and external information triggers.

[0077] The boat status triggers are re-selection conditions based on changes in the status of each boat itself. Specifically, these include (i) when the battery level of the oscillating boat falls below a predetermined threshold (e.g., 20%), (ii) when an abnormality is detected in the oscillating unit 11 of the oscillating boat (e.g., decreased oscillation output, distortion of the oscillation waveform), (iii) when an abnormality is detected in the receiver 21 of the receiving boat (e.g., decreased sensitivity, channel loss), (iv) when an abnormality is detected in the propulsion system 26 of any of the boats, and (v) when the communication quality of any of the boats falls below a predetermined threshold. The control unit 30 detects these status changes based on status information transmitted periodically from each boat.

[0078] External information triggers are re-selection conditions based on changes in the exploration environment. Specifically, these include (i) when changes in weather information make it difficult for a vessel at a particular location to continue operations, (ii) when approaching vessels are detected by AIS or radar information and a particular vessel needs to evacuate, (iii) when a command to change the exploration plan is received from the control station or a shore expert, and (iv) when localized deterioration of sea conditions is detected by sea condition sensors.

[0079] When a boat status trigger or external information trigger is identified, the control unit 30 performs a reconfiguration determination step S14 to evaluate the need for a role change or replacement deployment. If a role change is necessary, the control unit 30 performs a selection step S15 to select a new oscillating or wave-receiving boat by evaluating the battery level, onboard equipment health, current position, distance traveled, and communication quality of candidate boats. Next, the control unit 30 performs a switching step S16 to perform a role switch or retreat / replacement deployment at the end of the shot cycle, and then proceeds to a return step S17 to continue the exploration with the updated formation and division of roles. The process when the re-selection conditions are met will be explained in detail. If the battery level of the oscillating boat is low, the control unit 30 selects the boat with the most battery remaining and the shortest distance to the oscillating position in the current formation from among the wave-receiving boats configured as oscillating and wave-receiving boats as the new oscillating boat candidate. The selected boat moves to the oscillating position during the next shot cycle, and the original oscillating boat is converted to a wave-receiving boat or standby boat. It is desirable that this switching be completed within a single shot cycle so as not to disrupt the continuity of the exploration data.

[0080] If a wave-receiving boat malfunctions, the control unit 30 temporarily removes the wave-receiving boat from the exploration and deploys a replacement boat from the standby boat pool. The standby boat pool consists of reserve boats that are not directly participating in the exploration but are waiting near the exploration area, and includes boats that can both transmit and receive waves. The control unit 30 moves the replacement boat to the formation position that the removed wave-receiving boat was in, and restores the formation. Until the replacement boat is deployed, the arrangement of the remaining wave-receiving boats is temporarily re-optimized, and the missing wave-receiving point data is filled in by interpolation processing by the processing unit 40.

[0081] In the event of an external information trigger, such as the approach of another vessel, the control unit 30 evaluates the interference between the predicted route of the other vessel and the exploration area of ​​the fleet. If interference is predicted, it temporarily evacuates the wave-receiving boats located in the relevant area. The evacuated wave-receiving boats either return to their original positions after the other vessel has passed, or the formation is reconfigured. If any wave-receiving boats are unable to participate in the exploration due to evacuation, the control unit 30 automatically decides, based on the data quality estimated by the processing unit 40, whether to deploy a replacement boat from the standby boat pool or continue the exploration with only the remaining wave-receiving boats.

[0082] Next, an example of the selection of a data transfer method will be described with reference to Figure 13. Figure 13 is a diagram showing the data transfer method selection flow. The control unit 30 automatically selects either a real-time transfer method, a store-and-forward method, or a hybrid method according to the communication environment, data volume, and the requirement for immediacy of the search. In the flow of Figure 13, first, the control unit 30 executes a communication status acquisition step S21 to acquire the communication bandwidth, communication delay, line stability, remaining storage capacity, and required immediacy. Next, the control unit 30 executes a requirement evaluation step S22 to evaluate the required level of real-time performance and the total data volume. Subsequently, the control unit 30 executes a first determination step S23 to determine whether real-time transfer is possible based on the acquired communication status and requirement evaluation results, and if possible, proceeds to a selection step S24 to select a real-time transfer method. If real-time transfer is difficult, the control unit 30 executes a second determination step S25 to determine whether immediate transfer of only quality indicator data is necessary, and if necessary, proceeds to a selection step S26 to select a hybrid method, or if not, proceeds to a selection step S27 to select a store-and-forward method. Once a method is selected, the control unit 30 distributes the selection result to each wave-receiving boat and processing unit 40, sets the transfer parameters, and executes application step S28 to start data transfer. The real-time transfer method is a method that transfers data sequentially in parallel with wave reception and is suitable when the processing unit 40 evaluates data quality in real time and performs feedback control of exploration parameters. The store-and-forward method is a method that temporarily stores data and transfers it all at once and is suitable when communication bandwidth is limited. The hybrid method is a method that transfers only quality indicator data in real time and transfers all waveform data using the store-and-forward method, and offers an excellent balance between communication bandwidth and immediacy.

[0083] Next, an example of the operation cycle of the boat group will be described with reference to Figure 14. Figure 14 is a flowchart of the operation cycle of the boat group. One operation cycle consists of five steps: (S31) formation formation step, (S32) exploration execution step, (S33) formation transition step, (S34) state evaluation step, and (S35) continuation determination step. The following describes each step in accordance with the flowchart in Figure 14. In the formation formation step S31, the control unit 30 determines the target formation based on the exploration scene information and commands each wave-receiving boat to the target position.

[0084] In the exploration execution step S32, the transmitting boat 10 moves along the survey line and repeatedly emits exploration waves, while the wave receiving boats receive the reflected waves. Low-noise control is applied in each oscillation and reception cycle. During the exploration execution step S32, the control unit 30 continuously monitors the status of each boat and determines whether the aforementioned re-selection conditions are met. If the re-selection conditions are met, role re-selection is performed after the completion of the shot cycle.

[0085] In the formation transition step S33, when the survey line is switched, the wave-receiving boat group transitions to a transition formation and moves to the starting position of the next survey line. If the store-and-forward method is adopted, the batch transfer of accumulated data is performed in this step. The status evaluation step S34 is performed after the completion of a predetermined number of survey line explorations, and the battery level, equipment health, communication quality, etc. of each boat are comprehensively evaluated. Boats whose battery level falls below the threshold are replaced by boats in the standby boat pool and return to the mother ship or charging station. In the subsequent continuation determination step S35, it is determined whether or not to continue operations based on the results of the status evaluation. If it is possible to continue operations, the process returns to the formation formation step S31 of the next operation cycle; if it is difficult to continue, the boat group transitions to recovery or maintenance mode.

[0086] Next, the power supply method in the exploration system of the present invention will be described. Each wave-receiving boat is basically powered by an onboard battery, but in order to achieve long-term continuous operation, renewable energy power generation devices such as solar power generation panels, wave power generation devices, or wind power generation devices can be installed as auxiliary power. A rotational operation method in which the boat returns to the mother ship or charging station for recharging between surveys can also be adopted. Low-noise control that stops the propulsion system during wave reception contributes to reducing power consumption and extends the cruising range of the wave-receiving boat.

[0087] Next, we will describe examples of applications of the exploration system of the present invention. The first application example is seabed resource exploration. In the exploration of seabed resources such as oil and natural gas, it is necessary to understand the geological structure beneath the seabed over a wide area with high precision. The exploration system of the present invention can efficiently acquire three-dimensional exploration data over a wide area using a planar formation, and can achieve exploration quality equivalent to or better than conventional multiple streamer methods with a more flexible operating configuration. In particular, the ability to dynamically change the receiver arrangement to achieve the optimal arrangement according to the exploration target is advantageous.

[0088] A second application example is seabed topography surveys in coastal and harbor areas. Coastal and harbor areas are regions where detailed seabed topography information is required in relation to ship navigation, submarine cable laying, and construction of offshore structures. However, these waters are shallow and have many obstacles, making safe operation difficult with conventional long-length streamer methods. The exploration system of the present invention can be operated using a small wave-receiving boat with a short array or single wave receiver, making it suitable for exploration in shallow and narrow water areas.

[0089] A third application example is the inspection of underwater structures. The exploration system of the present invention can be used for non-destructive inspection of the condition of underwater structures such as bridge piers, dam bodies, and subsea pipelines. In this case, the number of wave-receiving boats is relatively small (for example, around 3 to 10 boats), and by arranging them to surround the structure, reflected wave data from multiple directions can be acquired, making it possible to detect damage and deterioration inside the structure with high accuracy. Dynamic changes to the wave-receiving arrangement allow for the optimal arrangement according to the shape and size of the object being inspected.

[0090] The fourth application example is environmental surveys. The exploration system of the present invention can be used in bottom sediment surveys, sediment distribution surveys, and aquatic ecosystem surveys in lakes and rivers. Inland waters generally have limited area, making it difficult to apply conventional streamer methods. The exploration system using a small wave-receiving boat of the present invention is suitable for environmental surveys because it enables efficient exploration even in limited water areas and minimizes environmental impact through low-noise control during the wave-receiving period.

[0091] The positioning accuracy in the exploration system of the present invention is further explained below. The GNSS receiver 22 on each wave-receiving boat can achieve centimeter-order positioning accuracy by using either the RTK method or the PPP method. When using the RTK method, a reference station is installed on the transmitting boat 10 or the control vessel, and correction information is wirelessly transmitted to each wave-receiving boat. When using the PPP method, high-precision positioning is performed independently using precise orbital and clock information from satellites. High-precision positioning directly affects the accuracy of the integrated processing of reflected wave signals, and is therefore an important elemental technology in the present invention.

[0092] Furthermore, details regarding the high-precision clocks 23 on each wave-receiving vessel are provided. The high-precision clocks 23 are crystal oscillators, rubidium oscillators, or GNSS-synchronous clocks, minimizing time discrepancies among all wave-receiving vessels. The time synchronization accuracy depends on the frequency band of the solicitation waves and the required spatial resolution, but typically a synchronization accuracy on the order of microseconds is required. By periodically calibrating each high-precision clock based on the time signal from the GNSS receiver 22, time drift can be suppressed even during long-term operation.

[0093] In the exploration system of the present invention, the number of wave-receiving boats is appropriately set according to the purpose of exploration and the required exploration quality. In general seabed geological surveys, for example, 5 to 50 wave-receiving boats are used. In large-scale oil exploration, it is possible to mobilize 100 or more wave-receiving boats. While increasing the number of wave-receiving boats improves spatial sampling density and the quality of exploration data, it also increases the complexity of formation control and operational costs. Therefore, selecting the optimal number of boats according to the purpose of exploration is important.

[0094] Next, the exploration method of the present invention will be described in detail. The exploration method of the present invention first includes the step of preparing an oscillating boat having an oscillating unit that emits exploration waves. As described above, the oscillating boat is a ship or unmanned vessel equipped with an elastic wave source such as an air gun, and is deployed in the survey area. Next, a step is performed to control a plurality of wave-receiving boats into a survey formation in a physically unconnected state. The control unit commands each wave-receiving boat to set a target position, and the wave-receiving boats form a survey formation by autonomous navigation. After the formation is stable, the step of emitting exploration waves from the oscillating boat is performed.

[0095] After the exploration wave is emitted, the reflected wave is received by receivers on multiple wave-receiving boats. As mentioned above, the wave-receiving boats are controlled to a low-noise state during the reception period, enabling high signal-to-noise ratio reception of the reflected wave. After the reception of the reflected wave is complete, a process is performed to generate exploration data based on the reflected wave signals received by the receivers. The processing unit integrates the reflected wave signals from each wave-receiving boat based on positional and time information to generate exploration data such as exploration cross-sectional images. These processes are repeatedly performed as the emitting boat moves along the survey line.

[0096] Furthermore, the present invention can also be implemented as a program that enables a computer to realize the functions of the above-described exploration system. Specifically, a program is provided that causes a computer to function as a formation control means for controlling the plurality of wave-receiving boats into an exploration formation while they are physically disconnected, and as a processing means for generating exploration data based on the reflected wave signals received by the wave receivers. This program can be stored and distributed on a computer-readable recording medium, or it can be provided in a form that can be downloaded via a communication network.

[0097] The safety management in the exploration system of the present invention will now be described. The control unit 30 has safety management functions including a status monitoring function for each wave-receiving boat, a collision avoidance function, an emergency recovery function, and an autonomous return function in the event of communication loss. The status monitoring function monitors the battery level, propulsion system operating status, and communication quality of each wave-receiving boat in real time and issues a warning if an abnormality is detected. The collision avoidance function constantly monitors the distance between wave-receiving boats and automatically commands avoidance action if the distance falls below a safe distance. The emergency recovery function transmits a return command to all wave-receiving boats in the event of an emergency such as rough weather.

[0098] The autonomous return function in the event of communication loss is a function that initiates autonomous navigation toward a pre-set return point if the wave-receiving boat loses communication with the control unit 30 for a certain period of time or longer. This function prevents the wave-receiving boat from being lost even if a communication failure occurs. In addition, each wave-receiving boat is equipped with an AIS (Automatic Identification System) transponder and lighting equipment to ensure visibility to other vessels, thereby reducing the risk of collision with other ships. These safety management functions are particularly important when using unmanned surface vessels as wave-receiving boats.

[0099] Next, a modified version of the processing unit of the present invention will be described. The processing unit 40 can also be built in a cloud computing environment. In this case, reflected wave signal data from each wave-receiving boat is transmitted to a cloud server via satellite communication or cellular communication, and the processing unit 40 on the cloud server performs signal integration processing and exploration data generation. By utilizing a cloud environment, high-speed processing using large-scale computing resources becomes possible, and experts located far from the survey site can also check and analyze the exploration data in real time.

[0100] Furthermore, the processing unit 40 can also have a function to improve the quality of reflected wave signals using machine learning models. Specifically, this includes a function to remove residual noise such as propulsion engine noise of the wave-receiving boat using a deep learning-based noise reduction model, and a function to compensate for spatial gaps in the receiver using a signal interpolation model. These machine learning models are trained in advance using a large amount of training data and incorporated into the processing unit 40. By utilizing machine learning, it becomes possible to remove complex noise patterns and perform high-precision signal interpolation, which was difficult with conventional signal processing methods.

[0101] The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the technical idea of ​​the present invention. For example, the roles of the oscillating boat and the wave-receiving boat are not fixed, and each boat can be configured to function as either an oscillating boat or a wave-receiving boat depending on the situation. In this case, the control unit 30 can re-select the roles of the oscillating boat and the wave-receiving boat according to information from the outside or the state of either the oscillating boat or the wave-receiving boat. Here, information from the outside includes, for example, weather information, sea condition information, navigation information of other vessels, command information from the control station, information on changes in the survey plan, etc. Also, the state of the boat includes, for example, battery level, operating status of the propulsion system, soundness of the wave receiver, communication quality, malfunction status of the oscillating unit, etc. Specifically, if the battery level of a certain oscillating boat decreases, a wave-receiving boat with sufficient battery level can be re-selected as the new oscillating boat, and the original oscillating boat can be converted to a wave-receiving boat. Furthermore, if an abnormality is detected in the receiver of a specific wave-receiving boat, that boat can be excluded from the survey, and a backup boat can be selected as the new wave-receiving boat, or its role can be swapped with another boat that does not have a wave-receiving abnormality. In addition, by dynamically reconfiguring the combination of boats participating in the survey and the role of each boat based on external information such as changes in sea conditions and the appearance of obstacles, the continuity of the survey and the quality of the data can be maintained. Moreover, it is possible to use an underwater drone (AUV) as a wave-receiving boat instead of a surface boat, in which case the wave-receiving equipment can be directly placed at the desired depth underwater, which is expected to further reduce the impact of surface wave noise.

[0102] Furthermore, the exploration system of the present invention is not limited to seismic exploration using elastic waves, but can also be applied to other exploration methods such as underwater radar exploration using electromagnetic waves and multibeam bathymetry using acoustic waves. In this case, the oscillator is the source of the corresponding exploration wave (electromagnetic wave or acoustic wave), and the receiver is the receiving sensor for the corresponding reflected wave. The essential feature of the present invention lies in controlling multiple wave-receiving boats in an exploration formation while they are physically disconnected, making it widely applicable regardless of the type of exploration wave.

[0103] The following provides supplementary information regarding the data format in the exploration system of the present invention. It is desirable that the exploration data generated by the processing unit 40 be output in industry standard formats such as SEG-Y and SEG-D. This ensures compatibility with existing seismic exploration data analysis software, and allows the data acquired by the exploration system of the present invention to be directly incorporated into conventional analysis workflows. After integrating the signals from the distributed receivers, the processing unit 40 adds attribute information such as channel number and shot number on a virtual continuous streamer and converts it to a standard format.

[0104] Furthermore, the exploration system of the present invention is also suitable for time-lapse exploration, which involves conducting multiple explorations at different times and comparing the results. In conventional streamer methods, it was difficult to accurately reproduce the position of the receiver in surveys conducted at different times. However, in the present invention, the position of the wave-receiving boat can be controlled with high precision based on GNSS positioning, making it possible to achieve the same receiver arrangement with high reproducibility. Time-lapse exploration is used for monitoring fluid displacement in oil fields, monitoring CO2 injection in underground reservoirs, and the like.

[0105] The weather resistance and environmental resistance of the exploration system of the present invention will be described. Each wave-receiving boat must have a structure that can withstand marine environments such as waves, wind, and currents. The hull of the wave-receiving boat is made of lightweight and corrosion-resistant materials such as fiber-reinforced plastic (FRP) and aluminum alloy. The wave receiver and electronic equipment are housed in a waterproof and saltwater-resistant enclosure and are designed to maintain their function even when submerged. In addition, a ballast mechanism and an automatic attitude control mechanism may be installed to ensure the stability of the wave-receiving boat.

[0106] An example of the operation procedure for the exploration system of the present invention will be described in more detail. First, the mother ship that has arrived at the survey area launches a wave-receiving boat and several wave-receiving boats. After launching, each wave-receiving boat establishes communication with the control unit 30 and reports its own position. After confirming that all wave-receiving boats have established communication, the control unit 30 transmits a command to form a survey formation. Each wave-receiving boat moves to a predetermined position according to the command and forms a survey formation. After it is confirmed that the formation is stable, the control unit 30 commands the start of the survey, and the first survey wave is emitted from the wave-receiving boat.

[0107] During the exploration, the control unit 30 simultaneously performs navigation control of the oscillating boat, formation maintenance control of the wave-receiving boats, oscillation timing control, and low-noise state control. Each oscillation and wave-receiving cycle typically lasts from a few seconds to several tens of seconds and is repeated as the oscillating boat moves along the survey line. Once the exploration of one survey line is complete, the control unit 30 commands a transition to a transition formation, and the wave-receiving boat group efficiently moves to the starting position of the next survey line. This series of operations is repeated for all survey lines specified in the survey plan.

[0108] After the survey is completed, the control unit 30 sends a command to all wave-receiving boats to return to the mother ship, and the wave-receiving boats are recovered by the mother ship in sequence. Data from the data recording device 24 is retrieved from the recovered wave-receiving boats, and full-scale data processing is performed by the processing unit 40. If real-time data transfer is being performed during the exploration, preliminary data processing may have already been completed before recovery. The final exploration data is used for interpretation by geologists and geophysicists.

[0109] The calibration method for the exploration system of the present invention will now be described. It is desirable that the receivers of each wave-receiving boat be periodically calibrated. Calibration methods include a direct calibration method, which involves oscillating a known acoustic signal and measuring the response of each receiver, and a mutual calibration method, which involves simultaneously receiving the same reflected wave signal with multiple receivers and detecting the difference in their responses. The mutual calibration method can be implemented by temporarily positioning the wave-receiving boats in close proximity during exploration and has the advantage of not requiring additional calibration equipment. The calibration results are transmitted to the processing unit 40, and the sensitivity differences of each receiver are corrected during signal integration processing.

[0110] Another aspect of low-noise control in the control device of the present invention will now be described. In addition to controlling the propulsion system of the wave-receiving boat, the low-noise control unit can also command the stopping or low-power operation of auxiliary equipment (pumps, cooling fans, etc.) mounted on the wave-receiving boat. Furthermore, by incorporating an active vibration control device that actively suppresses hull vibrations of the wave-receiving boat and activating it in a low-noise state, it is possible to reduce hull vibrations caused by external disturbances. Active vibration control is realized by canceling vibrations detected by an acceleration sensor with a vibration damping actuator.

[0111] Another signal processing method in the processing apparatus of the present invention will now be described. The processing unit 40 can also perform beamforming. Beamforming is a process that selectively enhances signals from a specific direction by adding signals from multiple receivers with appropriate delays and weights. By beamforming using receivers arranged two-dimensionally in a planar formation, a receiver beam with three-dimensional directivity is formed, making it possible to detect reflected waves from a specific direction with high sensitivity. The direction of beamforming can be dynamically changed to achieve optimal detection sensitivity according to the direction of the object being explored.

[0112] This section describes data quality control in the exploration system of the present invention. The processing unit 40 has a real-time quality evaluation function for reflected wave signals from each wave-receiving boat as a data quality control function. Quality evaluation indicators include the signal-to-noise ratio, abnormalities in the frequency spectrum, and detection of abnormal amplitude values. If a problem is detected in the data from a particular wave-receiving boat as a result of the quality evaluation, the processing unit 40 flags the data and performs appropriate weighting in the integrated processing. Furthermore, if a serious quality degradation is detected, the processing unit 40 can also instruct the wave-receiving boat to be inspected or replaced via the control unit 30.

[0113] The scalability of the exploration system of the present invention will now be described. The exploration system of the present invention can be easily scaled up in terms of exploration scale by adding or removing wave-receiving boats. Flexible operation is possible, such as using a small number of wave-receiving boats (e.g., 3 to 5 boats) for small-scale preliminary surveys and mobilizing a large number of wave-receiving boats (e.g., 50 to 100 boats) for full-scale wide-area surveys. It is desirable that the control unit 30 has the function of automatically recognizing the number of connected wave-receiving boats and automatically determining or proposing the optimal survey formation according to the configuration of available wave-receiving boats. This scalability is a major advantage that is not present in conventional fixed streamer systems.

[0114] This section describes the fault tolerance of the exploration system of the present invention. In conventional streamer systems, if one part of the cable is damaged, all wave-receiving channels beyond the damaged part may become non-functional. In contrast, in the present invention, since each wave-receiving boat operates independently, even if one wave-receiving boat fails, the other wave-receiving boats are not affected and exploration can continue. The control unit 30 automatically reconfigures the formation excluding the failed wave-receiving boat and continues to acquire the most optimal exploration data possible with the remaining wave-receiving boats. This high fault tolerance is particularly advantageous in long-term unmanned operation.

[0115] Although embodiments of the present invention have been described above, the present invention is not limited to these embodiments. Modifications and applications that can be easily conceived by those skilled in the art within the scope of the technical idea described in the claims are also included in the present invention. For example, although the above embodiments mainly described a wave-receiving boat that navigates on the surface of the water, it is also possible to use a wave-receiving boat that navigates underwater, and it is also possible to operate a mixture of surface boats and underwater boats. Furthermore, the target of exploration is not limited to seabed geological structures, but can also be applied to the exploration of floating objects in water, schools of fish, underwater structures, etc. [Explanation of symbols]

[0116] 1 Exploration system, 10 Transmitting boat, 10a, 10b Transmitting boat, 11 Transmitting unit, 20a~20n Wave receiving boat, 21, 21a~21m Wave receiver, 22 GNSS receiver, 23 High-precision clock, 24 Data recording device, 25 Communication device, 26 Propulsion device, 30 Control unit, 40 Processing unit, 50 Cable, 60 Short holding member, 70 Short multi-channel array, 71 Short streamer cable, L, L1, L2 Survey line, d Distance between boats, Tr Wave receiving period, C1 Inter-boat communication layer, C2 Exploration data transfer layer, C3 Wide-area communication layer, S31~S35 Operation cycle steps

Claims

1. A search system based on reflected wave signals for searching for objects in a body of water, A vibrating boat having an oscillator that emits exploration waves consisting of elastic waves, A plurality of wave-receiving boats, each having a wave receiver that receives reflected waves corresponding to the exploration waves emitted by the oscillation unit of the aforementioned wave-receiving boat, A control unit controls the plurality of wave-receiving boats in a survey formation while they are physically disconnected, and controls them to a low-noise state during the wave-receiving period after the wave-receiving boat emits the survey wave, including at least one of the following states: reducing the propulsion output of the plurality of wave-receiving boats, stopping propulsion, transitioning to a drifting state, or performing only minimal attitude control to maintain the distance between the boats. A processing unit that acquires reflected wave signal data received by the receiver from each of the plurality of wave-receiving boats, along with the position information and time information of the wave-receiving boat, synchronizes the time axis of the reflected wave signal data based on the time information, and integrates the reflected wave signal data based on the position information to generate exploration data, A search system based on reflected wave signals, equipped with the necessary features.

2. The aforementioned survey formation is a linear formation in which the multiple wave-receiving boats are arranged in a roughly linear fashion along the survey line. A search system based on reflected wave signals as described in claim 1.

3. The aforementioned survey formation is a planar formation that includes at least one of the following: a grid, a staggered pattern, a fan shape, or an arc shape. A search system based on reflected wave signals as described in claim 1.

4. The control unit moves the multiple wave-receiving boats to a transition formation different from the survey formation when turning during exploration along the survey line based on the survey formation or when switching survey lines. A search system based on reflected wave signals according to claim 2 or 3.

5. The wave receiver is fixed to the hull of the wave-receiving boat, suspended from the wave-receiving boat, or held near the wave-receiving boat by a short holding member. A search system based on reflected wave signals as described in claim 1.

6. At least one of the wave-receiving boats supports a short multi-channel array in which multiple wave receivers are arranged along a short streamer cable. A search system based on reflected wave signals as described in claim 1.

7. The control unit changes one of the following depending on the target depth of the object, the orientation of the target structure of the object, the required resolution, sea conditions or obstacle distribution, and the oscillation or reception conditions: the distance between the wave-receiving boats, the shape of the survey formation, or the selection of wave-receiving boats. A search system based on reflected wave signals as described in claim 1.

8. The processing unit performs interpolation or regularization processing to correct the irregularities in the positions of the plurality of wave-receiving boats. A search system based on reflected wave signals as described in claim 1.

9. The control unit re-selects the roles of the oscillating boat and the wave-receiving boat according to external information or the state of either the oscillating boat or the wave-receiving boat. A search system based on reflected wave signals as described in claim 1.

10. A group of exploration vessels based on reflected wave signals, It includes an oscillator boat having an oscillator unit that emits exploration waves consisting of elastic waves, and a plurality of wave-receiving boats, Each of the aforementioned wave-receiving boats is configured to independently support at least one wave receiver and to output waveform data of the reflected wave signal received by the wave receiver, along with the position information and time information of the wave-receiving boat. The plurality of wave-receiving boats are configured to form a survey formation in a physically unconnected state, and are configured to be in a low-noise state during the wave-receiving period after the oscillation of the survey wave by the oscillating boat, including at least one of the following states: reduction of propulsion output of the plurality of wave-receiving boats, cessation of propulsion, transition to a drifting state, or minimal attitude control to maintain the distance between boats. A group of exploration vessels that use reflected wave signals.

11. The plurality of wave-receiving boats are further configured to maintain the survey formation and enter a low-noise state during the wave-receiving period after the wave-receiving boat emits a survey wave, and to form a transition formation different from the survey formation when the plurality of wave-receiving boats turn or when the survey line is switched. A group of exploration vessels based on reflected wave signals as described in claim 10.

12. A control device used for exploration based on reflected wave signals, A formation control unit controls multiple wave-receiving boats into a survey formation while they are physically disconnected, monitors the position of each wave-receiving boat, and transmits navigation commands to each wave-receiving boat so that the target survey formation is maintained. During the wave reception period following the oscillation of the exploration wave by the wave-receiving boat, which emits an exploration wave consisting of elastic waves, a low-noise control unit controls the output of the propulsion engine of the wave-receiving boat in synchronization with the oscillation timing of the exploration wave by the wave-receiving boat, A control device equipped with the following features.

13. An input unit that acquires waveform data of the reflected wave signal of the exploration wave, received by receivers independently supported on each of the plurality of wave-receiving boats, along with position information and time information of the plurality of wave-receiving boats, A processing unit that synchronizes the time axis of the waveform data based on the time information and integrates the waveform data based on the position information to generate exploration data, The control device according to claim 12, further comprising:

14. A search method based on reflected wave signals for searching for objects in a body of water, A step of preparing an oscillator boat having an oscillator unit that emits exploration waves consisting of elastic waves, A process of controlling a plurality of wave-receiving boats, each having a receiver that receives reflected waves corresponding to the exploration waves emitted by the oscillation unit of the aforementioned wave-receiving boat, into a survey formation while physically disconnected, The process of emitting a probe wave from the aforementioned oscillator, A step of controlling the plurality of wave-receiving boats to a low-noise state during the wave-receiving period after the oscillation of the exploration wave by the oscillating boat, which includes at least one of the following states: reducing the propulsion output of the plurality of wave-receiving boats, stopping propulsion, transitioning to a drifting state, or performing only minimal attitude control to maintain the distance between the boats; The process of receiving the reflected waves with the wave receivers of the plurality of wave-receiving boats, The process of acquiring reflected wave signal data received by the receiver from each of the plurality of wave-receiving boats, along with the position information and time information of the wave-receiving boat, synchronizing the time axis of the reflected wave signal data based on the time information, and integrating the reflected wave signal data based on the position information to generate exploration data, A search method based on reflected wave signals, including those mentioned above.

15. Computers, In a search method based on reflected wave signals for searching for objects in a body of water, Using a plurality of wave-receiving boats, each having an oscillator that emits a probe wave consisting of elastic waves and a receiver that receives a reflected wave corresponding to the probe wave emitted by the oscillator of the oscillator, The process of controlling the aforementioned multiple wave-receiving boats into a survey formation while they are physically disconnected, The process of emitting a probe wave from the aforementioned oscillator, A step of controlling the plurality of wave-receiving boats to a low-noise state during the wave-receiving period after the oscillation of the exploration wave by the oscillating boat, which includes at least one of the following states: reducing the propulsion output of the plurality of wave-receiving boats, stopping propulsion, transitioning to a drifting state, or performing only minimal attitude control to maintain the distance between the boats; The process of receiving the reflected waves with the wave receivers of the plurality of wave-receiving boats, The process of acquiring reflected wave signal data received by the receiver from each of the plurality of wave-receiving boats, along with the position information and time information of the wave-receiving boat, synchronizing the time axis of the reflected wave signal data based on the time information, and integrating the reflected wave signal data based on the position information to generate exploration data, A program that executes the command.

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