Seismic wave measurement method, program, and seismic wave measurement system

Abstract

To enable accurate correction of the internal time. [Solution] The control device 1 includes a time identification unit 177 that identifies the seabed reception time, which is the absolute time when the measuring device received the first signal, based on the sea surface transmission time, which is the absolute time when a communication device located at a depth based on wave information indicating the state of waves on the sea surface detects seismic waves in response to vibration waves being emitted from the epicenter 2, and the sea surface reception time, which is the absolute time when the measuring device received the second signal, which is the response to the first signal from the measuring device, and a correction unit 179 that corrects the internal time of the measuring device associated with the measurement data based on the time difference, which is the difference between the internal time of the measuring device when the measuring device received the first signal and the identified seabed reception time.

Description

Technical Field

[0001] The present invention relates to a seismic wave measurement method, program, and seismic wave measurement system for measuring seismic waves.

Background Art

[0002] A technique for correcting the error between the time of a clock in a seafloor seismometer and the absolute time is known (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] A measuring device installed on the seabed that detects seismic waves in response to the generation of vibration waves from a seismic source measures the time when the seismic waves are detected by an oscillator built therein. However, since the frequency of the oscillator changes with the passage of time due to aging characteristics, a difference (time drift) occurs between the internal time, which is the time measured by the oscillator built in the measuring device, and the absolute time. As a result, the analysis accuracy of the subsurface geological structure based on seismic waves decreases.

[0005] In order to suppress the decrease in analysis accuracy, there is a method of specifying the absolute time when the measuring device receives the signal transmitted by the communication device using the time required for the signal transmitted by the communication device connected to the ship to the measuring device to travel back and forth, and correcting the internal time. However, when the communication device moves due to the influence of waves on the sea surface, a difference occurs between the time required for the signal to travel in the forward path and the time required for the signal to travel in the return path. As a result, the absolute time when the measuring device receives the signal transmitted by the communication device cannot be accurately specified, and as a result, there is a problem that the internal time cannot be accurately corrected.

[0006] Therefore, the present invention has been made in view of these points, and aims to enable accurate correction of the internal time. [Means for solving the problem]

[0007] A seismic wave measurement method according to a first aspect of the present invention includes a time identification step in which a seabed-side reception time is the absolute time when a measuring device installed on the seabed receives a first signal, based on a sea surface-side transmission time, which is the absolute time when a communication device located at a depth based on wave information indicating the state of waves on the sea surface transmits a first signal to a measuring device, which creates measurement data of the seismic wave associated with an internal time when the seismic wave was detected in response to the emission of vibration waves from the epicenter; and a correction step in which the internal time of the measuring device associated with the measurement data is corrected based on a time difference, which is the difference between the internal time of the measuring device when the measuring device received the first signal and the identified seabed-side reception time.

[0008] In the time determination step, the time midway between the sea surface transmission time and the sea surface reception time may be determined as the seabed reception time.

[0009] In the time determination step, after determining the seabed reception time, the determined seabed reception time may be corrected based on the horizontal movement distance, which is the distance the communication device moved horizontally between the sea surface transmission time and the sea surface reception time.

[0010] The seismic wave measurement method may further include a calculation step of calculating a second distance, which is the distance from the position of the communication device to the position of the measuring device after movement, based on a first distance, which is the distance from the position of the communication device to the position of the measuring device before movement, and the horizontal movement distance. In the time determination step, the determined seabed reception time may be corrected based on the ratio of the first distance and the second distance.

[0011] The seismic wave measurement method may further include a depth determination step in which the depth of the communication device from the sea surface is determined based on the wave information.

[0012] The seismic wave measurement method may further include a wave information acquisition step of acquiring the wave height at the sea surface as wave information, and in the depth determination step, the depth from the sea surface associated with the wave height indicated by the wave information acquired in the wave information acquisition step may be determined by referring to depth management data which associates the wave height at the sea surface with the depth from the sea surface of the communication device.

[0013] In the wave information acquisition step, the wave height on the sea surface may be acquired based on the magnitude of the ship's oscillation detected by a vibration detection sensor on the ship navigating the sea, which is connected to the communication device.

[0014] In the depth determination step, the depth of the earthquake source from the sea surface may be determined based on the wave information.

[0015] A program in a second aspect of the present invention causes the processor of the information processing device to function as a time identification unit that identifies the seabed reception time, which is the absolute time when the measuring device received the first signal, based on the sea surface transmission time, which is the absolute time when the measuring device detected the seismic wave in response to the emission of vibration waves from the source, and the sea surface reception time, which is the absolute time when the measuring device received the second signal, which is a response to the first signal, from the measuring device; and a correction unit that corrects the internal time of the measuring device associated with the measurement data based on the time difference, which is the difference between the internal time of the measuring device when the measuring device received the first signal and the identified seabed reception time.

[0016] A third aspect of the present invention provides a seismic wave measurement system comprising: a communication device that transmits a signal to a measuring device installed on the seabed that creates measurement data of seismic waves associated with an internal time at which seismic waves were detected in response to seismic waves being emitted from a source; and a control device installed on a ship navigating the sea that controls the depth of the communication device, wherein the control device has a depth control unit that controls the depth of the communication device so that the communication device is located at a depth based on wave information indicating the state of waves on the sea surface, and either the control device or the communication device has a time identification unit that identifies a seabed reception time, which is the absolute time at which the measuring device received a first signal, based on a sea surface transmission time, which is the absolute time at which the communication device located at the depth based on the wave information transmitted a first signal to the measuring device, and a sea surface reception time, which is the absolute time at which the measuring device received a second signal, which is a response to the first signal from the measuring device; and a correction unit that corrects the internal time of the measuring device associated with the measurement data based on a time difference, which is the difference between the internal time of the measuring device when the measuring device received the first signal and the identified seabed reception time. [Effects of the Invention]

[0017] The present invention has the effect of enabling accurate correction of the internal time. [Brief explanation of the drawing]

[0018] [Figure 1] This is a diagram illustrating the seismic wave measurement system S. [Figure 2] This figure shows an example of the path of an acoustic signal traveling back and forth between the communication device 5 and the measuring device 4. [Figure 3] This flowchart shows an overview of the processing flow according to this embodiment. [Figure 4] This is a diagram illustrating the procedure for activating multiple measuring devices 4. [Figure 5] This figure shows an example of a management table that indicates the status of each measuring device 4. [Figure 6] This is a diagram showing the configuration of the control device 1. [Figure 7] It is a diagram showing an example of depth management data. [Figure 8] It is a diagram showing an example of time management data. [Figure 9] It is a flowchart showing the flow of processing related to correction of the underwater reception time. [Figure 10] It is a schematic diagram for explaining the correction of the underwater reception time. [Figure 11] It is a diagram showing the configuration of the communication device 5. [Figure 12] It is a diagram showing the configuration of the measuring device 4. [Embodiment for Carrying out the Invention]

[0019] [Outline of the Measurement System S] FIG. 1 is a diagram showing the outline of the measurement system S. The measurement system S is an ocean physical exploration system for analyzing the submarine geological structure. In the measurement system S, a seismic wave is generated from a seismic source 2 such as an air gun or a sparker, and the control device 1 analyzes the submarine geological structure using the results of the seismic wave measured by a number of measuring devices 4 installed on the seabed.

[0020] The measurement system S includes a control device 1, a seismic source 2, an optical communication device 3, a plurality of measuring devices 4, and a communication device 5. The control device 1, the seismic source 2, and the optical communication device 3 are mounted on a ship 100 that can move in the ocean. The control device 1 may be mounted on a USV (unmanned high-functional observation device) that can communicate with the ship 100. The USV is a survey boat that sails on the sea and conducts ocean surveys. The plurality of measuring devices 4 are installed on the seabed at intervals of a predetermined distance or more. The communication device 5 is a device that can communicate with the ship 100 or the USV via a cable C. The communication device 5 exists in the sea.

[0021] The control device 1 is, for example, a computer, which acquires measurement data indicating the vibration state of the seabed observed by multiple measuring devices 4 at the timing when seismic waves are emitted, and analyzes the acquired measurement data. In other words, the control device 1 analyzes the measurement data of seismic waves detected by the measuring devices 4 in response to seismic waves emitted from the epicenter 2 towards the seabed from a ship navigating the sea during the measurement period. As shown in Figure 1(a), the control device 1 controls multiple measuring devices 4 by sending and receiving acoustic signals via a communication device 5, and also receives measurement data generated by multiple measuring devices 4. The control device 1 also acquires information indicating the absolute time from, for example, a PTP network or GPS (Global Positioning System).

[0022] The control device 1 controls the depth of the communication device 5 from the sea surface. The control device 1 controls the depth of the communication device 5 from the sea surface by, for example, transmitting a control signal to the communication device 5 via cable C to control the movement of the communication device 5.

[0023] The seismic source 2 generates vibration waves during the measurement period. The seismic source 2 generates vibration waves based on the control of, for example, the control of the control device 1, but it may also generate vibration waves based on the control of a different control device (for example, a computer installed on a ship different from ship 100). Alternatively, a ship different from ship 100 may be equipped with the seismic source 2.

[0024] The optical communication device 3 acquires measurement data from at least one measuring device 4 by communicating optically with it based on the control of the control device 1. The optical communication device 3 emits a first optical signal to the measuring device 4 underwater, and receives a second optical signal transmitted by the measuring device 4 that received the first optical signal. As shown in Figure 1(b), the optical communication device 3 is connected to the control device 1 by cable C, and after diving to a position where it can communicate optically with the measuring device 4 based on the control of the control device 1, it communicates optically with the measuring device 4. The optical communication device 3 moves sequentially to the vicinity of multiple measuring devices 4 and acquires measurement data from multiple measuring devices 4 sequentially. Note that the measurement system S may have multiple optical communication devices 3, and multiple optical communication devices 3 may acquire measurement data from multiple measuring devices 4.

[0025] The measuring device 4 generates seismic wave measurement data associated with the internal time at which it detected seismic waves in response to the emission of seismic waves from the earthquake source 2. The measurement data indicates the amount of vibration of the measuring device 4 caused by the seismic waves. The measurement data indicates the magnitude of vibration detected by the sensors of the measuring device 4, and includes, for example, measured values ​​generated by sampling the signal output by the sensors every 1 millisecond. The measurement data is associated with the internal time measured by the oscillator inside the measuring device 4. The measuring device 4 transmits the measurement data to the optical communication device 3 using an optical signal.

[0026] The communication device 5 is, for example, an ROV (remotely operated vehicle). The communication device 5 is located within a predetermined distance from the ship 100 or USV and is situated at a depth of several meters to tens of meters below the surface. The communication device 5 can communicate with the control device 1 via cable C. The communication device 5 has thrusters as a means of moving itself and changes its depth by moving based on control signals received from the control device 1.

[0027] Incidentally, the internal time of the measuring device 4 is the time measured by the oscillator built into the measuring device 4, and therefore differs from the absolute time. Furthermore, due to the aging characteristics of the oscillator in the measuring device 4, the oscillator frequency changes over time. As a result, a difference arises between the internal time associated with the measurement data by the measuring device 4 and the absolute time. Even if a chip-scale atomic oscillator (CSAC), which has relatively good aging characteristics, is used as the oscillator, a frequency offset will occur over time. If there is a difference between the absolute time and the internal time, it becomes impossible to determine with high accuracy the relationship between the timing when the earthquake source 2 emitted vibration waves and the timing of the seismic waves indicated by the measurement data, which leads to a problem in that the accuracy of the analysis of the subseafloor geological structure based on seismic waves decreases.

[0028] To suppress the decrease in analysis accuracy, one method is to use the time required for the signal transmitted by the communication device 5 connected to the ship 100 to travel back and forth to the measuring device 4 to determine the absolute time when the measuring device 4 received the signal transmitted by the communication device 5, and then correct the internal time associated with the measurement data. However, the communication device 5 may move due to the effects of waves on the sea surface.

[0029] Figure 2 shows an example of the path of an acoustic signal traveling back and forth between the communication device 5 and the measuring device 4. Conventionally, as shown in Figure 2(a), the communication device 5 was attached to the tip of a structure (e.g., pole P) firmly fixed to the ship 100. Therefore, the communication device 5, located in the sea, may move as the ship 100 moves due to rocking. Specifically, between the absolute time when the communication device 5 transmits the first acoustic signal (first signal) to the measuring device 4 and the absolute time when the communication device 5 receives the second acoustic signal (second signal), which is the response to the first acoustic signal from the measuring device 4, the communication device 5 may move due to the rocking of the ship 100. In this case, as shown in Figure 2(a), the distance from the communication device 5 to the measuring device 4 before the movement and the distance from the communication device 5 to the measuring device 4 after the movement will be different, resulting in a difference between the first time required for the first acoustic signal to travel the outward path and the second time required for the second acoustic signal to travel the return path.

[0030] If there is no difference between the first and second time periods, for example, the midpoint between the transmission time of the first acoustic signal and the reception time of the second acoustic signal can be identified as the absolute time when the measuring device 4 received the signal transmitted by the communication device 5. However, if there is a difference between the first and second time periods, it becomes impossible to accurately identify the absolute time when the measuring device 4 received the signal transmitted by the communication device 5, resulting in the problem that accurate correction of the internal time associated with the measurement data becomes impossible. Furthermore, even if the communication device 5 is connected to the ship 100 with a loose cable C or the like to make it less susceptible to the rocking of the ship 100, if the depth of the communication device 5 is shallow, the communication device 5 will move due to the influence of waves on the sea surface, causing a similar problem.

[0031] Therefore, the measurement method according to this embodiment is characterized in that the communication device 5 is positioned at a depth that is not affected by or is less affected by waves on the sea surface. As a result, even if the ship 100 moves due to rocking, the communication device 5 is less likely to move due to the influence of waves on the sea surface, so as shown in Figure 2(b), the difference between the distance from the communication device 5 to the measurement device 4 before movement and the distance from the communication device 5 to the measurement device 4 after movement becomes less likely. As a result, the difference between the first time required for the first acoustic signal to travel along the outward path and the second time required for the second acoustic signal to travel along the return path becomes less likely, and the accuracy of determining the absolute time when the measurement device 4 receives the signal transmitted by the communication device 5 can be improved. As a result, the accuracy of correcting the internal time associated with the measurement data can be improved. The outline of the processing flow according to this embodiment will be explained below with reference to Figure 3.

[0032] [Overview of the processing flow] Figure 3 is a flowchart illustrating the overview of the processing flow according to this embodiment. The control device 1 acquires, for example, the wave height at the sea surface measured by a microwave wave height meter installed on the ship 100 as wave information (S1). The control device 1 may also determine the wave height at the sea surface based on the magnitude of the oscillation of the ship 100 detected by a vibration detection sensor installed on the ship 100, and acquire the determined wave height as wave information.

[0033] Next, the control device 1 determines the depth of the communication device 5 from the sea surface based on the acquired wave information (S2). Depth is, for example, the distance from the sea surface to the object in the vertical direction, which is perpendicular to the sea surface. For example, the control device 1 determines a larger value for depth the greater the wave height indicated by the acquired wave information. The control device 1 transmits a control signal to the communication device 5 to move it to the determined depth. Upon receiving the control signal, the communication device 5 moves to the determined depth indicated by the control signal (S3). In this way, the communication device 5 can move to a depth that is less affected by waves at the sea surface.

[0034] The control device 1 may transmit a control signal to the communication device 5 to move it to a predetermined depth (a depth at which wave information is generally not expected to have an effect). If the distance traveled by the communication device 5 after it has submerged to the predetermined depth due to the influence of waves is less than a predetermined threshold, the communication device 5 will remain at that predetermined depth. On the other hand, if the distance traveled by the communication device 5 after it has submerged to the predetermined depth due to the influence of waves is greater than or equal to a predetermined threshold, the communication device 5 will submerge until it is at a depth at which the distance traveled due to the influence of waves is less than the predetermined threshold.

[0035] The communication device 5, having moved to a depth less affected by waves, transmits a first acoustic signal to the measuring device 4 installed on the seabed (S4). The absolute time when the communication device 5 transmits the first acoustic signal is called the surface transmission time. Upon receiving the first acoustic signal, the measuring device 4 transmits a second acoustic signal, which is a response to the first acoustic signal, to the communication device 5. The communication device 5 receives the second acoustic signal transmitted from the measuring device 4 (S5). The absolute time when the communication device 5 receives the second acoustic signal is called the surface reception time.

[0036] The control device 1 identifies the seabed reception time, which is the absolute time when the measuring device 4 receives the first acoustic signal transmitted from the communication device 5 (S6). The control device 1 identifies the seabed reception time as, for example, the time midway between the surface transmission time and the surface reception time.

[0037] The control device 1 identifies the internal time of the measuring device 4 when it receives the first acoustic signal (S7). Next, the control device 1 identifies the time difference between the seabed reception time identified in S6 and the internal time identified in S7 (S8). Then, based on the identified time difference, the control device 1 corrects the internal time to be corrected, which is the internal time to be corrected associated with the measurement data (S9).

[0038] In this way, by moving the communication device 5 to a depth where it is not affected by waves at the sea surface, and then transmitting and receiving acoustic signals with the measuring device 4 installed on the seabed, the difference between the time it takes for the acoustic signal to travel along the outward path and the time it takes for it to travel along the return path becomes less likely. As a result, the accuracy of determining the absolute time at which the measuring device 4 receives the signal transmitted by the communication device 5 can be improved. Consequently, the accuracy of correcting the internal time to be corrected can be improved.

[0039] [Startup procedure for measuring device 4] Measurements for analyzing the subseafloor geological structure are carried out periodically. For example, measurements are carried out annually over a period of several days to several weeks. Multiple measuring devices 4 are installed on the seabed when the measurement period arrives, and retrieved from the seabed when the measurement is completed. However, installing multiple measuring devices 4 each time the measurement period arrives in this way requires a great deal of time for installation work, resulting in poor measurement efficiency. Therefore, the measurement system S in this embodiment may be configured to measure seismic waves using multiple measuring devices 4 that have been pre-installed on the seabed over multiple measurement periods spanning several years.

[0040] Since the measuring device 4 is battery-powered, if it operates while installed on the seabed for a long period of time, the battery will be depleted quickly. Therefore, in the measuring system S, multiple measuring devices 4 may be configured to start up when the measurement period begins, and to stop their measurement operations when the measurement period ends. When the measuring device 4 stops its measurement operations, it maintains the function of receiving acoustic signals from the control device 1, but enters a sleep state in which the oscillator of the measuring device 4 is stopped to reduce power consumption. The measuring device 4 has, for example, a measurement state in which it performs measurements, a standby state in which the oscillator is operating but no measurements are being performed, and a sleep state in which the oscillator is stopped and no measurements are being performed.

[0041] Figures 4 and 5 illustrate the procedure for activating multiple measuring devices 4. Figure 4 schematically shows multiple measuring devices 4 viewed from above. The circles (〇) in Figure 4 represent measuring devices 4 installed on the seabed. The numbers below the circles are identification information (ID) to identify each measuring device 4.

[0042] While the ship 100 is moving, the control device 1 transmits an acoustic signal containing control information to the measuring device 4 within the range to which the acoustic signal can reach (for example, the area within the dashed frame in Figure 4), thereby preparing the measuring device 4 to begin measurement. Specifically, the control device 1 prepares the measuring device 4 to begin measurement by transmitting a start command, a synchronization command, and a recording start command. In Figure 4, the dashed arrows represent the start command, and the solid arrows represent the synchronization command. The control device 1 may also transmit parameters necessary for measurement (for example, sampling interval or preamplifier gain) to the measuring device 4.

[0043] The startup command includes a string corresponding to an instruction to transition the measuring device 4 from sleep state to a measurable state, and the ID of the measuring device 4. The synchronization command includes a string corresponding to an instruction to request the internal time of the measuring device 4, and the ID of the measuring device 4. The control device 1 may also send a synchronization command that includes the absolute time recognized by the control device 1. In the following description, the process by which the control device 1 determines the relationship between the absolute time and the internal time of the measuring device 4 based on the internal time received from the measuring device 4 by sending a synchronization command to the measuring device 4 is referred to as "synchronization". The recording start command includes a string corresponding to an instruction to start recording measurement data, and the ID of the measuring device 4.

[0044] In Figure 4, measuring devices 4 without any letters inside the circle (for example, measuring device 4 with ID 0606) are in a stopped state. Measuring devices 4 with a dashed circle containing "W" indicate that they have received a start command and are in the process of starting up. Measuring devices 4 with a solid circle containing "W" indicate that they have finished starting up but have not yet completed synchronization. Measuring devices 4 with a dashed circle containing "S" indicate that they have received a synchronization command and are in the process of synchronizing. Measuring devices 4 with a solid circle containing "S" indicate that synchronization has been completed.

[0045] Figure 5 shows an example of a management table that indicates the status of each measuring device 4. In the management table shown in Figure 5, the ID of the measuring device 4 is associated with information indicating whether or not startup is complete, information indicating whether or not synchronization is complete, the previous action (i.e., the action performed immediately before), and the time at which that action was performed. As can be seen from the status of the multiple measuring devices 4 within the dashed area in Figure 4 and the time in the management table in Figure 5, the control device 1 sends startup commands and synchronization commands to different measuring devices 4 in a time-division multiplexer manner.

[0046] Specifically, the control device 1 sends a synchronization command to the already-activated second measuring device 4 between the time it sends a start command to the first measuring device 4 and the time it takes for the first measuring device 4 to complete its startup. By sequentially starting up and synchronizing multiple measuring devices 4 in this way, the control device 1 can make multiple measuring devices 4 ready for measurement in a shorter time than if it were to wait for a measuring device 4 to start up after sending a start command to it. Furthermore, it can make the measuring devices 4 ready for measurement in a shorter time and with greater reliability compared to a person starting each measuring device 4 individually.

[0047] Furthermore, the control device 1 activates multiple measuring devices 4 while moving in a constant direction. As an example, as shown in Figure 4, the control device 1 sends an activation command to multiple measuring devices 4 located in front of the ship 100, and after the ship 100 has moved to a position ahead of the activated multiple measuring devices 4, it sends a synchronization command to the multiple measuring devices 4.

[0048] In this manner, the control device 1 transmits commands to multiple measuring devices 4 located in front of the vessel 100 on which the control device 1 is installed and capable of receiving acoustic signals, and to multiple measuring devices 4 located behind the vessel 100 and capable of receiving acoustic signals. By operating in this manner, the control device 1 can bring multiple measuring devices 4 into a measurement-ready state in a shorter time compared to when commands are transmitted only to measuring devices 4 located in either the front or rear of the vessel 100.

[0049] Although not shown in Figures 4 and 5, the control device 1 may send a recording start command to the synchronized measuring device 4 after receiving a response to the synchronization command from the measuring device 4 and completing the synchronization. The recording start command may be a command that includes an instruction to start recording immediately, or it may be a command that indicates the time to start recording. The control device 1 may send recording start commands to multiple measuring devices 4 in succession after the synchronization of all measuring devices 4 is complete and before the earthquake source generates vibration waves.

[0050] Installing numerous measuring devices 4 on the seabed each time a measurement is performed would require considerable time and expense for installation. On the other hand, keeping the measuring devices 4 in operation for extended periods would lead to battery depletion. In the measurement system S, before the measurement period begins, the control device 1 sequentially starts and synchronizes multiple measuring devices 4, and when the measurement period ends, the measuring devices 4 are put into sleep mode to suppress battery consumption. With the measurement system S configured in this way, it is possible to efficiently measure seismic waves with numerous measuring devices 4 over a long period of time.

[0051] [Configuration of Control Device 1] Figure 6 shows the configuration of the control device 1. The control device 1 includes a vibration detection sensor 11, a position information acquisition unit 12, a data transmission / reception unit 13, an absolute time acquisition unit 14, an external communication unit 15, a storage unit 16, and a control unit 17. The control unit 17 includes a wave information acquisition unit 171, a depth determination unit 172, a depth control unit 173, an earthquake source control unit 174, a command creation unit 175, a data acquisition unit 176, a time identification unit 177, a time difference identification unit 178, a correction unit 179, and a calculation unit 180. Note that some of the functions of the control unit 17 may be provided in devices other than the control device 1.

[0052] The vibration detection sensor 11 is a sensor that detects the rocking motion of the ship 100 and is installed on the ship 100.

[0053] The position information acquisition unit 12 acquires position information indicating the position of the control device 1, that is, the position of the ship 100 on which the control device 1 is installed. The position information acquisition unit 12 acquires position information, for example, from radio waves received from GPS satellites, and determines the latitude and longitude based on the acquired position information. The position information acquisition unit 12 notifies the command creation unit 175 of the determined latitude and longitude.

[0054] The data transmission / reception unit 13 is a communication interface for sending and receiving data with the optical communication device 3 or the communication device 5. For example, the data transmission / reception unit 13 transmits data to the optical communication device 3, which includes an instruction to acquire the internal time from the measuring device 4, input from the data acquisition unit 176, and the optical communication device 3 receives time data indicating the internal time acquired from the measuring device 4.

[0055] The data transmission / reception unit 13 may notify the optical communication device 3 of the absolute time acquired by the absolute time acquisition unit 14, and the optical communication device 3 may receive time data in which the absolute time at the time the internal time was acquired from the measuring device 4 is associated with the internal time. The data transmission / reception unit 13 then notifies the data acquisition unit 176 of the acquired time data.

[0056] The data transmission / reception unit 13 may transmit a control signal, input from the depth control unit 173, to the communication device 5 via cable C, which is used to move the communication device 5 to the determined depth. The data transmission / reception unit 13 may also transmit data, input from the data acquisition unit 176, which includes an instruction to transmit a first acoustic signal to the measuring device 4, via cable C to the communication device 5. The data transmission / reception unit 13 may also receive data from the communication device 5 via cable C, which indicates that the communication device 5 has received a second acoustic signal.

[0057] The absolute time acquisition unit 14 acquires the absolute time, for example, from GPS satellites. The absolute time acquisition unit 14 notifies the time identification unit 177 of the acquired absolute time. The absolute time acquisition unit 14 may also notify the data transmission / reception unit 13 of the absolute time.

[0058] The external communication unit 15 transmits the measurement results, including the measurement data after the internal time has been corrected, which are input from the correction unit 179. The external communication unit 15 may transmit the measurement results to an external computer that performs processing to analyze the measurement results and identify the subseafloor geological structure, or it may transmit them to other processing units of the control unit 17.

[0059] The storage unit 16 has storage media such as ROM (Read Only Memory), RAM (Random Access Memory), and SSD (Solid State Drive). The storage unit 16 stores programs executed by the control unit 17. The storage unit 16 also stores various data for causing the multiple measuring devices 4 to perform measurements. For example, the storage unit 16 stores the location of each of the multiple measuring devices 4 in association with the identification information of each measuring device 4. Specifically, the storage unit 16 stores the latitude and longitude of the multiple measuring devices 4 in association with their IDs.

[0060] Furthermore, the storage unit 16 stores a management table as shown in Figure 5. In addition, the storage unit 16 stores multiple measurement data acquired from the multiple measuring devices 4, associated with the IDs of the multiple measuring devices 4. The storage unit 16 stores multiple measurement data associated with the internal time of the measuring device 4 at the time the measurement data was generated. Subsequently, when the time corrected from the internal time by the correction unit 179 is associated with the measurement data, the storage unit 16 stores the measurement data associated with the corrected time.

[0061] Furthermore, the memory unit 16 stores depth management data. Figure 7 shows an example of depth management data. In the depth management data, wave height and depth are associated. Wave height is the height of the waves at the sea surface. Depth is the depth of the communication device 5 from the sea surface. The greater the wave height, the greater the effect of the waves on the communication device 5. Therefore, in order to reduce the effect of waves on the communication device 5, a larger value is set for the depth in the depth management data as the wave height increases. This makes it possible to position the communication device 5 in a location that is less affected by waves at the sea surface.

[0062] Furthermore, the memory unit 16 stores time management data. Figure 8 shows an example of time management data. In the time management data, the acoustic signal ID, date, sea surface transmission time, seabed reception time, sea surface reception time, internal time at reception, and time difference are associated. The acoustic signal ID is an ID used to identify the first acoustic signal transmitted by the communication device 5 to the measuring device 4.

[0063] The surface-side transmission time is the absolute time when the communication device 5 transmitted the first acoustic signal to the measuring device 4, and is the absolute time when the data acquisition unit 176 input data including the instruction to transmit the first acoustic signal to the data transmission / reception unit 13, for example as described later. The seabed-side reception time is the absolute time when the measuring device 4 received the first acoustic signal, and is the absolute time identified by the time identification unit 177, for example as described later. The surface-side reception time is the absolute time when the communication device 5 received the second acoustic signal, which is a response to the first acoustic signal, from the measuring device 4, and is the absolute time when the data acquisition unit 176 received data from the data transmission / reception unit 13 indicating that the second acoustic signal had been received, for example as described later.

[0064] The internal time at reception is the internal time of the measuring device 4 when it receives the first acoustic signal, and is the internal time when the measuring device 4 received the first acoustic signal, as indicated by the second acoustic signal, as described later. The time difference is the difference between the internal time at reception and the time received on the seabed. As mentioned above, the frequency of the oscillator in the measuring device 4 changes over time, so as shown in Figure 8, the time difference between the internal time at reception and the time received on the seabed increases over time.

[0065] The control unit 17 includes, for example, a CPU (Central Processing Unit). By executing a program stored in the memory unit 16, the control unit 17 functions as a wave information acquisition unit 171, a depth determination unit 172, a depth control unit 173, a seismic source control unit 174, a command creation unit 175, a data acquisition unit 176, a time identification unit 177, a time difference identification unit 178, a correction unit 179, and a calculation unit 180.

[0066] The wave information acquisition unit 171 acquires wave information indicating the wave conditions on the sea surface. Wave conditions include, for example, wave height or period, or tidal currents. For example, the wave information acquisition unit 171 acquires the wave height on the sea surface measured by a microwave wave height meter installed on the vessel 100 as wave information. The wave information acquisition unit 171 may also determine the wave height on the sea surface based on the magnitude of the oscillation of the vessel 100 detected by a vibration detection sensor 11 installed on the vessel 100, and acquire the determined wave height as wave information. The wave information acquisition unit 171 may also refer to wave height management data (not shown) which associates the magnitude of the oscillation of the vessel 100 with the wave height on the sea surface, and acquire the wave height associated with the magnitude of the oscillation of the vessel 100 detected by the vibration detection sensor 11 as wave information. The wave information acquisition unit 171 inputs the acquired wave information to the depth determination unit 172.

[0067] The depth determination unit 172 determines the depth of the communication device 5 from the sea surface based on wave information. The depth determination unit 172 refers to depth management data (Figure 7) which associates wave height at the sea surface with the depth of the communication device 5 from the sea surface, for example, and determines the depth from the sea surface associated with the wave height indicated by the wave information acquisition unit 171. The depth determination unit 172 may also refer to the depth management data and determine the depth associated with the wave height that is closest to the wave height indicated by the wave information input from the wave information acquisition unit 171, among wave heights equal to or greater than the wave height indicated by the wave information input from the wave information acquisition unit 171. The depth determination unit 172 may determine a deeper depth the longer the wave period indicated by the wave information. The depth determination unit 172 inputs the depth information indicating the determined depth of the communication device 5 to the depth control unit 173.

[0068] Incidentally, in order to improve the accuracy of the analysis of the subseafloor geological structure, as mentioned above, it is important to determine with high precision the relationship between the timing at which the epicenter 2 emitted seismic waves and the timing of the seismic waves indicated by the measurement data. In order to perform this determination precisely, it is preferable to keep the depth of the epicenter 2 constant during the measurement period for the same measurement device 4, and therefore it is preferable to make the epicenter 2 less susceptible to movement due to the influence of waves at the sea surface.

[0069] Therefore, the depth determination unit 172 may determine the depth of the epicenter 2 from the sea surface based on wave information. The depth determination unit 172 may also determine the depth from the sea surface associated with the wave height indicated by the wave information acquired by the wave information acquisition unit 171 by referring to depth management data (not shown) for the epicenter 2, which associates wave height with the depth of the epicenter 2 from the sea surface. In this way, by determining a depth at which the epicenter 2 is less likely to move due to the influence of waves at the sea surface, the distance between the measuring device 4 and the epicenter 2 can be kept constant during the measurement period. As a result, the accuracy of the analysis of the subseafloor geological structure is improved. The depth determination unit 172 inputs depth information indicating the determined depth of the epicenter 2 to the depth control unit 173.

[0070] The depth control unit 173 controls the depth of the communication device 5 so that it is located at a depth based on wave information indicating the wave conditions at the sea surface. For example, the depth control unit 173 inputs a control signal to the data transmission / reception unit 13 to move the communication device 5 to the depth indicated by the depth information input from the depth determination unit 172.

[0071] The depth control unit 173 may control the depth of the earthquake source 2 so that it is located at a depth based on wave information indicating the wave conditions at the sea surface. The depth control unit 173 may also input a control signal to the earthquake source control unit 174 to move the earthquake source 2 to the depth indicated by the depth information input from the depth determination unit 172.

[0072] The seismic source control unit 174 transmits an instruction to the seismic source 2 to generate vibration waves. The seismic source control unit 174 generates vibration waves at the seismic source 2 after, for example, the acoustic signal transmission unit 52 of the communication device 5 transmits a first acoustic signal including a recording start command to the multiple measuring devices 4. The seismic source control unit 174 transmits an instruction to the seismic source 2 to generate vibration waves after, for example, receiving notification from the data acquisition unit 176 that all measuring devices 4 are ready to measure. The seismic source control unit 174 may generate vibration waves at the seismic source 2 at a predetermined date and time, or it may generate vibration waves at the seismic source 2 in response to an instruction received from an external device. The control device 1 does not have a seismic source control unit 174, and an external control device may function as the seismic source control unit 174.

[0073] The seismic source control unit 174 may control the seismic source 2 to move to the determined depth in response to a control signal input from the depth control unit 173 for moving the seismic source 2 to the determined depth. The seismic source control unit 174 may also control the thrusters of the seismic source 2 to generate thrust to move the seismic source 2 to the determined depth.

[0074] The command creation unit 175 creates commands for the acoustic signal transmission unit 52 of the communication device 5 to transmit to the measuring device 4. The command creation unit 175 creates, for example, a startup command, a synchronization command, and a recording start command, and inputs the created commands to the data acquisition unit 176. When creating a command, the command creation unit 175 selects a measuring device 4 within a predetermined range from the latitude and longitude input from the location information acquisition unit 12 by referring to the latitude and longitude of the locations where multiple measuring devices 4 are installed, which are stored in the storage unit 16. The command creation unit 175 creates a command that includes the ID of the selected measuring device 4.

[0075] As explained with reference to Figure 4, the command creation unit 175 creates a start command for a sleep-state measuring device 4 among a plurality of measuring devices 4 within a predetermined range. When the command creation unit 175 receives notification from the data acquisition unit 176 that the measuring device 4 corresponding to the created start command has been started, it creates a synchronization command for that measuring device 4. When the command creation unit 175 receives notification from the data acquisition unit 176 that the measuring device 4 corresponding to the created synchronization command has completed synchronization, it creates a recording start command for that measuring device 4.

[0076] When the command creation unit 175 inputs the created command to the data acquisition unit 176, it updates the "previous action" in the management table stored in the storage unit 16. When the command creation unit 175 inputs a startup command to the data acquisition unit 176, it sets the "previous action" corresponding to the ID of the measuring device 4 included in the startup command to "starting up". When the command creation unit 175 inputs a synchronization command to the data acquisition unit 176, it sets the "previous action" corresponding to the ID of the measuring device 4 included in the synchronization command to "synchronizing".

[0077] The data acquisition unit 176 acquires various data transmitted from the measuring device 4. The data acquisition unit 176 acquires response data to commands transmitted by the acoustic signal transmitting unit 52 of the communication device 5 via the acoustic signal receiving unit 53 of the communication device 5. The data acquisition unit 176 notifies the command creation unit 175 that it has acquired the response data.

[0078] When the data acquisition unit 176 acquires response data, it updates the content of the "previous action" in the management table stored in the storage unit 16. For example, when the data acquisition unit 176 acquires response data indicating that the device has been started, it updates the "previous action" corresponding to the ID of the measuring device 4 included in the response data to "startup complete". When the data acquisition unit 176 acquires response data that includes the internal time of the measuring device 4 transmitted by the measuring device 4 in response to receiving a synchronization command, it updates the "previous action" corresponding to the ID of the measuring device 4 included in the response data to "synchronization complete". The data acquisition unit 176 stores the absolute time when the synchronization command was sent and the internal time indicated by the response data in the storage unit 16, associating them with the ID of the measuring device 4.

[0079] When the data acquisition unit 176 acquires response data indicating that the measuring device 4 has started recording, it updates the "previous action" corresponding to the ID of the measuring device 4 included in the response data to "start recording". When the data acquisition unit 176 receives response commands to the start recording command from all measuring devices 4, that is, when the "previous action" of all measuring devices 4 becomes "start recording", it notifies the seismic source control unit 174 that measurement can be started.

[0080] Furthermore, the data acquisition unit 176 may acquire the emission time, which is the absolute time when the optical communication device 3 emitted the first optical signal, and the internal time included in the second optical signal received by the optical communication device 3. The second optical signal is an optical signal transmitted by the measuring device 4 in response to the receipt of the first optical signal. The data acquisition unit 176 stores the emission time and the internal time in the storage unit 16, associating them with the ID of the measuring device 4, and notifies the time identification unit 177.

[0081] Furthermore, the data acquisition unit 176 acquires measurement data from the measuring device 4 via the communication device 5 and the data transmission / reception unit 13. The data acquisition unit 176 acquires multiple measurement data, each representing a measurement value corresponding to a different time. For example, after the measurement period has ended, the data acquisition unit 176 acquires multiple measurement data recovered from the measuring device 4 by the optical communication device 3 via optical communication from the data transmission / reception unit 13. The data acquisition unit 176 stores the acquired measurement data in the storage unit 16, associating it with the ID of the measuring device 4, so that the time identification unit 177 can refer to the measurement data.

[0082] The data acquisition unit 176 may input data to the data transmission / reception unit 13 that includes an instruction to transmit a first acoustic signal, in response to receiving a command input from the command creation unit 175. The data acquisition unit 176 may store the time of this input in the storage unit 16, associating it with the ID of the first acoustic signal as the sea surface side transmission time in the time management data.

[0083] Furthermore, the data acquisition unit 176 may receive data from the data transmission / reception unit 13 indicating that the second acoustic signal has been received. The data acquisition unit 176 may store the time of this reception in the storage unit 16, associating it with the ID of the first acoustic signal as the sea surface reception time in the time management data. Alternatively, the data acquisition unit 176 may store the internal time indicated by the second acoustic signal, when the measuring device 4 received the first acoustic signal, in the storage unit 16, as the reception internal time in the time management data, associating it with the ID of the first acoustic signal.

[0084] The time identification unit 177 identifies the seabed reception time, which is the absolute time when the measuring device 4 received the first acoustic signal, based on the sea surface transmission time, which is the absolute time when the communication device 5 located at a depth based on wave information transmitted the first acoustic signal to the measuring device 4, and the sea surface reception time, which is the absolute time when the measuring device 4 received the second acoustic signal, which is the response to the first acoustic signal. The time identification unit 177 may, for example, refer to time management data stored in the storage unit 16 to identify the seabed reception time based on the sea surface transmission time and the sea surface reception time, and store the identified seabed reception time in the storage unit 16 in association with the ID of the first acoustic signal.

[0085] If the measuring device 4 transmits the second acoustic signal immediately after receiving the first acoustic signal, the time determination unit 177 determines, for example, the midpoint between the sea surface transmission time and the sea surface reception time as the seabed reception time. In this way, the seabed reception time can be determined with high accuracy using a simple method.

[0086] On the other hand, there are cases where the measuring device 4 does not transmit the second acoustic signal immediately after receiving the first acoustic signal (when the response time required from the time the measuring device 4 receives the first acoustic signal until it transmits the second acoustic signal is greater than or equal to a threshold). In this case, the time determination unit 177 determines the time obtained by subtracting half the response time from the midpoint between the sea surface transmission time and the sea surface reception time as the seabed reception time. In this way, even when the measuring device 4 does not transmit the second acoustic signal immediately after receiving the first acoustic signal, the seabed reception time can be determined with high accuracy.

[0087] The time difference identification unit 178 identifies the internal time of reception, which is the internal time when the measuring device 4 received the first acoustic signal, and the seabed reception time, which is the absolute time when the measuring device 4 received the first acoustic signal. The time difference identification unit 178 identifies the time difference between the internal time of reception and the seabed reception time by referring to time management data stored in the storage unit 16, for example, and stores the identified time difference in association with the ID of the first acoustic signal in the storage unit 16.

[0088] The correction unit 179 corrects the internal time of the measuring device 4 associated with the measurement data based on the time difference identified by the time difference identification unit 178. For example, the correction unit 179 corrects the internal time to be corrected by adding the time difference in the time management data stored in the storage unit 16 to the internal time to be corrected, which is the internal time to be corrected associated with the measurement data stored in the storage unit 16.

[0089] As explained above, although the communication device 5 communicates acoustically with the measuring device 4 at a depth less affected by waves on the sea surface, for example, if the ship 100 moves horizontally for work, the communication device 5 connected to the ship 100 by cable C also moves horizontally. In this case, a difference occurs between the first time required for the first acoustic signal to travel along the outward path and the second time required for the second acoustic signal to travel along the return path, which may make it impossible to correct the internal time with high accuracy. Therefore, as explained below, the time determination unit 177 may, after determining the seabed reception time, correct the determined seabed reception time based on the horizontal movement distance, which is the distance the communication device 5 has moved horizontally between the sea surface transmission time and the sea surface reception time.

[0090] Figure 9 is a flowchart showing the processing flow related to the correction of the seabed reception time. Figure 10 is a schematic diagram illustrating the correction of the seabed reception time. First, the calculation unit 180 determines the distance from the sea surface to the measuring device 4 (Figure 9: S91, Figure 10: a1). The distance from the sea surface to the measuring device 4 is recorded, for example, when the measuring device 4 is installed on the seabed, and the storage unit 16 stores it in association with the ID of the measuring device 4.

[0091] Next, the calculation unit 180 determines the depth of the communication device 5 (Figure 9: S92, Figure 10: a2). The calculation unit 180 determines the depth of the communication device 5 based on the depth information of the communication device 5 input from the depth determination unit 172, for example.

[0092] Next, the calculation unit 180 calculates a first distance by subtracting the depth specified in S92 from the distance specified in S91 (Figure 9: S93, Figure 10: a). The first distance is the distance from the position of the communication device 5 before movement to the position of the measuring device 4. Note that the first distance may be the distance measured by the communication device 5, rather than the distance calculated by the calculation unit 180.

[0093] Next, the calculation unit 180 determines the horizontal movement distance of the communication device 5 (Figure 9: S94, Figure 10: b). The calculation unit 180 determines the horizontal movement distance of the communication device 5 based, for example, on the value measured by the acceleration sensor mounted on the ship 100.

[0094] Next, the calculation unit 180 calculates a second distance based on the first distance calculated in S93 and the horizontal movement distance identified in S94 (Figure 9: S95, Figure 10: c). The second distance is the distance from the position of the communication device 5 after movement to the position of the measuring device 4. The calculation unit 180 calculates, for example, "(first distance a) 2 +(horizontal movement distance b) 2 =(second distance c) 2 The second distance c is calculated using the following formula:

[0095] Then, the time determination unit 177 corrects the specified seabed reception time based on the ratio of the first distance determined in S93 and the second distance calculated in S95 (Figure 9: S96). Specifically, the time determination unit 177 calculates the round-trip time of the acoustic signal by subtracting the sea surface transmission time from the sea surface reception time. Next, the round-trip time of the calculated acoustic signal is multiplied by the value of "(first distance) / (sum of first distance and second distance)" to calculate the forward journey time required for the first acoustic signal to travel along the forward path. Then, the time determination unit 177 calculates the corrected seabed reception time by adding the calculated forward journey time to the sea surface transmission time.

[0096] In this way, the time determination unit 177 corrects the seabed reception time based on the horizontal movement distance of the communication device 5, thereby enabling the determination of a more accurate corrected seabed reception time. As a result, the internal time can be corrected with high accuracy even when the communication device 5 moves horizontally.

[0097] [Configuration of communication device 5] Figure 11 shows the configuration of the communication device 5. The communication device 5 includes a data transmission / reception unit 51, an acoustic signal transmission unit 52, an acoustic signal reception unit 53, a propulsion unit 54, a storage unit 55, and a control unit 56. The control unit 56 includes a data communication unit 561.

[0098] The data transmission / reception unit 51 is a communication interface for sending and receiving data with the control device 1. For example, the data transmission / reception unit 51 receives a control signal from the control device 1 via cable C to move the communication device 5 to a determined depth, and inputs the received control signal to the data communication unit 561. The data transmission / reception unit 51 may also receive data from the control device 1 via cable C that includes an instruction to transmit a first acoustic signal to the measuring device 4, and input the received data to the data communication unit 561. The data transmission / reception unit 51 may also transmit data input from the data communication unit 561 indicating that the communication device 5 has received a second acoustic signal to the control device 1 via cable C.

[0099] The acoustic signal transmitting unit 52 is an acoustic communication unit that transmits a first acoustic signal to the measuring device 4. The acoustic signal transmitting unit 52 transmits a first acoustic signal to the measuring device 4 in response to receiving data input from the data communication unit 561, which includes an instruction to transmit a first acoustic signal to the measuring device 4. The acoustic signal transmitting unit 52 transmits a first acoustic signal that includes, for example, control data (e.g., various commands) input from the data communication unit 561. The acoustic signal transmitting unit 52 transmits a first acoustic signal including a command to a measuring device 4 that is within a predetermined range from the position of the ship 100 indicated by the position information acquired by the position information acquisition unit 12 of the control device 1, by referring to the position of each of the multiple measuring devices 4 stored in the storage unit 16 of the control device 1. The predetermined range is the range in which the measuring device 4 can receive the first acoustic signal transmitted by the acoustic signal transmitting unit 52.

[0100] As an example, the acoustic signal transmitting unit 52 transmits a first acoustic signal to each of the multiple measuring devices 4, which includes a startup command that is startup data for starting up the measuring device 4. The acoustic signal transmitting unit 52 transmits a second acoustic signal, which includes response data to the startup command, to the measuring device 4, which has been received by the acoustic signal receiving unit 53, and transmits a first acoustic signal that includes a synchronization command indicating the absolute time (i.e., a synchronization command including time data).

[0101] Furthermore, the acoustic signal transmitting unit 52 transmits a first acoustic signal, which is recording start data indicating the start of recording of measurement data, to the measuring device 4, whose response to the first acoustic signal, including the synchronization command, has been received by the acoustic signal receiving unit 53. In other words, the acoustic signal transmitting unit 52 transmits a first acoustic signal, which is recording start data, to the measuring device 4, which has transmitted the response data to the synchronization command.

[0102] The acoustic signal transmission unit 52 may send a recording start command that includes the ID of one measuring device 4, or it may send a recording start command that includes the IDs of multiple measuring devices 4 that have completed synchronization. The acoustic signal transmission unit 52 may also send a recording start command that includes information indicating that the command applies to all measuring devices 4. By sending such a recording start command, the acoustic signal transmission unit 52 can start recording vibration waves on multiple measuring devices 4 by sending the recording start command only once, thereby improving measurement efficiency.

[0103] The acoustic signal receiving unit 53 is an acoustic communication unit that receives a second acoustic signal emitted by the measuring device 4, which has received a first acoustic signal. The acoustic signal receiving unit 53 receives, for example, a second acoustic signal indicating the internal time of the measuring device 4. The acoustic signal receiving unit 53 inputs data indicating that the second acoustic signal has been received to the data communication unit 561. The acoustic signal receiving unit 53 identifies the internal time based on the time data contained in the received acoustic signal and inputs the identified internal time to the data communication unit 561.

[0104] The propulsion unit 54 is a component that generates a thrusting force to propel the communication device 5, and is, for example, a thruster. For example, in response to a control signal input from the data communication unit 561 for moving the communication device 5 to a determined depth, the propulsion unit 54 generates a thrusting force to move the communication device 5 to the determined depth indicated by the control signal. The communication device 5, moved by the thrusting force generated by the propulsion unit 54, will be positioned at the depth based on the wave information. The propulsion unit 54 may also generate a thrusting force to move the communication device 5 in the horizontal direction. This allows the communication device 5 to quickly return to its previous position, even if it moves horizontally due to, for example, the generation of ocean currents or the movement of the vessel 100.

[0105] The memory unit 55 has a storage medium such as ROM, RAM, and SSD. The memory unit 55 stores the program that the control unit 56 will execute.

[0106] The data communication unit 561 inputs the control signal received from the control device 1 via the data transmission / reception unit 51 to the propulsion unit 54. Furthermore, upon receiving data from the control device 1 via the data transmission / reception unit 51, including an instruction to transmit a first acoustic signal to the measuring device 4, the data communication unit 561 transmits the first acoustic signal to the measuring device 4 via the acoustic signal transmission unit 52. Additionally, upon receiving a second acoustic signal from the measuring device 4 via the acoustic signal receiving unit 53, the data communication unit 561 transmits data indicating the receipt of the second acoustic signal to the control device 1 via the data transmission / reception unit 51.

[0107] [Configuration of measuring device 4] Figure 12 shows the configuration of the measuring device 4. The measuring device 4 includes an oscillator 41, a sensor 42, an acoustic signal receiving unit 43, an acoustic signal transmitting unit 44, an optical signal receiving unit 45, an optical signal transmitting unit 46, a storage unit 47, and a control unit 48. The control unit 48 includes a data creation unit 481 and a data communication unit 482.

[0108] The oscillator 41 generates an oscillation signal used for timing the internal time in the measuring device 4. As described above, the oscillator 41 is, for example, a chip-scale atomic oscillator, but it may be of another type.

[0109] Sensor 42 generates a detection signal whose level changes in response to vibrations of the measuring device 4. Sensor 42 inputs the detection signal to the data creation unit 481.

[0110] The acoustic signal receiving unit 43 receives a first acoustic signal transmitted from the communication device 5. The acoustic signal receiving unit 43 inputs the command and absolute time data included in the received first acoustic signal to the data communication unit 482. In response to the acoustic signal receiving unit 43 receiving the first acoustic signal, the acoustic signal transmitting unit 44 transmits a second acoustic signal to the control device 1 indicating the internal time when the first acoustic signal was received.

[0111] The optical signal receiving unit 45 receives a first optical signal transmitted from the optical communication device 3. The optical signal receiving unit 45 inputs data such as commands and absolute time contained in the received first optical signal to the data communication unit 482. In response to the optical signal receiving unit 45 receiving the first optical signal, the optical signal transmitting unit 46 transmits a second optical signal indicating the internal time at which the first optical signal was received. The optical signal transmitting unit 46 transmits the second optical signal, including the internal time input from, for example, the data communication unit 482, to the optical communication device 3.

[0112] The storage unit 47 has a storage medium such as ROM, RAM, and SSD. The storage unit 47 stores the program executed by the control unit 48. The storage unit 47 also stores the measurement data created by the data creation unit 481. The storage unit 47 may also store the corrected time included in the acoustic signal transmitted to the measurement device 4 by the communication device 5 that received the second acoustic signal as time correction data.

[0113] The control unit 48 has, for example, a CPU. The control unit 48 functions as a data creation unit 481 and a data communication unit 482 by executing a program stored in the storage unit 47.

[0114] The data creation unit 481 functions as a measurement data creation unit that creates multiple measurement data associated with an internal time measured based on the oscillator 41. The data creation unit 481 creates multiple measurement data indicating sampled signal levels (i.e., measured values) by sampling the detection signal input from the sensor 42 at predetermined time intervals (e.g., 1-millisecond intervals). The data creation unit 481 stores the multiple measurement data in the storage unit 47, associating them with the internal time. The data creation unit 481 may measure the internal time by counting the oscillation signal input from the oscillator 41, or it may determine the internal time based on data indicating the internal time input from the oscillator 41.

[0115] The data communication unit 482 transmits response data for commands included in the first acoustic signal received from the communication device 5 via the acoustic signal receiving unit 43, via the acoustic signal transmitting unit 44. The data communication unit 482 also transmits response data for commands included in the optical signal received from the optical communication device 3 via the optical signal receiving unit 45, via the optical signal transmitting unit 46. When the data communication unit 482 receives a synchronization command, it obtains the internal time at the time the synchronization command was received from the oscillator 41 or the data creation unit 481, and transmits response data including the obtained internal time.

[0116] Furthermore, the data communication unit 482 transmits multiple measurement data created by the data creation unit 481 to the optical communication device 3 via the optical signal transmission unit 46. Specifically, the data communication unit 482 transmits multiple measurement data stored in the storage unit 47, associating them with the internal time.

[0117] [Effects of Measurement System S] As explained above, in the measurement system S, the communication device 5 moves to a depth where it is not affected by waves at the sea surface, and then transmits and receives acoustic signals with the measurement device 4 installed on the seabed. This reduces the difference between the time it takes for the acoustic signal to travel on its outward journey and the time it takes to travel on its return journey. As a result, the accuracy of determining the absolute time at which the measurement device 4 receives the signal transmitted by the communication device 5 can be improved. Consequently, the accuracy of correcting the internal time to be corrected can be improved.

[0118] [Differentiation] In the measurement system S described above, an example was described in which the control device 1 is mounted on either the ship 100 or the USV. However, it is also possible that a control device mounted on the USV performs some of the processing that the control device 1 performs, and the control device mounted on the ship 100 performs the remaining processing that the control device 1 performs. For example, the control device mounted on the USV may perform the processing performed by the data transmission / reception unit 13 and the processing performed by the data acquisition unit 176, and the control device mounted on the ship 100 may perform the remaining processing other than these.

[0119] Furthermore, in the measurement system S described above, an example was described in which the control device 1 transmits and receives data with the communication device 5 using cable C. However, the control device 1 may also transmit and receive data with the communication device 5 using acoustic signals instead of cable C. [Explanation of Symbols]

[0120] 1. Control device 2 earthquake epicenters 3. Optical communication device 4. Measuring device 5. Communication equipment 11. Vibration detection sensor 12 Location information acquisition section 13. Data transmission / reception unit 14 Absolute Time Acquisition Unit 15 External Communications Department 16 Memory section 17 Control Unit 41 Oscillator 42 sensors 43 Acoustic signal receiving section 44 Acoustic signal transmission section 45 Optical signal receiving section 46 Optical signal transmission unit 47 Memory section 48 Control Unit 51 Data transmission / reception unit 52 Acoustic signal transmission section 53 Acoustic signal receiving section 54 Promotion Department 55 Storage section 56 Control Unit 100 ships 171 Wave information acquisition department 172 Depth determination section 173 Depth Control Unit 174 Earthquake Source Control Unit 175 Command Creation Section 176 Data Acquisition Unit 177 Time identification part 178 Time difference identification part 179 Correction section 481 Data Creation Department 482 Data Communications Department 561 Data Communications Department

Claims

1. A time identification step in which a communication device located at a depth based on wave information indicating the state of waves on the sea surface transmits a first signal to a measuring device installed on the seabed, which creates measurement data of the seismic waves associated with an internal time when the seismic waves were detected in response to vibration waves being emitted from the epicenter, and the measuring device receives a second signal, which is a response to the first signal, from the measuring device, and the measuring device receives the first signal, which is the absolute time on the seabed, is determined based on this transmission time on the sea surface, and the transmission time on the sea surface, which is the absolute time on the sea surface, the measurement device receives the first signal. A correction step of correcting the internal time of the measuring device associated with the measurement data based on a time difference which is the difference between the internal time of the measuring device when the measuring device receives the first signal and the identified seabed reception time, A seismic wave measurement method having [specific characteristics].

2. In the aforementioned time determination step, the time midway between the sea surface transmission time and the sea surface reception time is determined as the seabed reception time. The seismic wave measurement method according to claim 1.

3. In the time determination step, after determining the seabed reception time, the determined seabed reception time is corrected based on the horizontal movement distance, which is the distance the communication device moved horizontally between the sea surface transmission time and the sea surface reception time. The seismic wave measurement method according to claim 1.

4. The method further includes a calculation step of calculating a second distance, which is the distance from the position of the communication device to the position of the measuring device after movement, based on a first distance, which is the distance from the position of the communication device to the position of the measuring device before movement, and the horizontal movement distance. In the time determination step, the determined seabed reception time is corrected based on the ratio of the first distance to the second distance. The seismic wave measurement method according to claim 3.

5. The system further includes a depth determination step of determining the depth of the communication device from the sea surface based on the wave information, The seismic wave measurement method according to claim 1.

6. The system further includes a wave information acquisition step that acquires the wave height on the sea surface as wave information, In the depth determination step, the depth from the sea surface is determined by referring to depth management data which associates the wave height at the sea surface with the depth from the sea surface of the communication device. The seismic wave measurement method according to claim 5.

7. In the wave information acquisition step, the wave height on the sea surface is acquired based on the magnitude of the ship's oscillation detected by a vibration detection sensor on the ship navigating the sea, which is connected to the communication device. The seismic wave measurement method according to claim 6.

8. In the depth determination step, the depth of the earthquake source from the sea surface is determined based on the wave information. The seismic wave measurement method according to claim 5.

9. The processor in the information processing device A communication device located at a depth based on wave information indicating the state of waves on the sea surface transmits a first signal to a measuring device installed on the seabed, which creates measurement data of the seismic waves associated with an internal time when the seismic waves were detected in response to vibration waves being emitted from the epicenter. A time identification unit identifies the seabed reception time, which is the absolute time when the measuring device received the first signal, based on the sea surface transmission time, which is the absolute time when the measuring device received the second signal, which is the response to the first signal from the measuring device. A correction unit corrects the internal time of the measuring device associated with the measurement data based on a time difference which is the difference between the internal time of the measuring device when the measuring device receives the first signal and the specified seabed reception time, A program designed to function as such.

10. The system comprises a communication device that transmits a signal to a measuring device installed on the seabed that creates measurement data of the seismic waves associated with an internal time when the seismic waves were detected in response to the emission of seismic waves from the epicenter, and a control device installed on a ship navigating the sea that controls the depth of the communication device, The control device includes a depth control unit that controls the depth of the communication device so that the communication device is located at a depth based on wave information indicating the wave conditions on the sea surface. Either the control device or the communication device, A time identification unit identifies the seabed reception time, which is the absolute time when the measuring device received the first signal, based on the sea surface transmission time, which is the absolute time when the communication device located at the depth based on the wave information transmitted the first signal to the measuring device, and the sea surface reception time, which is the absolute time when the measuring device received the second signal, which is the response to the first signal from the measuring device. A correction unit corrects the internal time of the measuring device associated with the measurement data based on a time difference which is the difference between the internal time of the measuring device when the measuring device receives the first signal and the specified seabed reception time, Having, Seismic wave measurement system.

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