Measuring system, data processing device, measuring device, data processing method, and program

Abstract

A data processing device 1 includes: a data acquiring unit 183 that acquires a plurality of items of measurement data associated with an internal time clocked by a measuring device 4 that is installed on the seabed, the measurement data being generated by the measuring device 4 on the basis of seismic waves detected by the measuring device 4 in response to the emission, toward the seabed from a ship traveling on the sea, of vibration waves from a vibration source during a measuring period; a time difference identifying unit 184 that identifies (1) a first time difference, which is the difference between an absolute start time at the time the measuring period starts, and the internal time associated with the measurement data at the time the measuring period starts, and (2) a second time difference, which is the difference between an absolute end time at the time the measuring period ends, and the internal time associated with the measurement data at the time the measuring period ends; and a correcting unit 185 that corrects the internal time associated with the plurality of items of measurement data on the basis of at least the first time difference and the second time difference.

Description

Measurement system, data processing device, measurement device, data processing method and program

[0001] The present invention relates to a measurement system, a data processing device, a measurement device, a data processing method, and a program.

[0002] A system for measuring vibrations on the seafloor using seismic exploration equipment installed on the seafloor is known. Patent Document 1 discloses a technology for preventing the measurement results from being affected by clock drift in the measurement device by connecting the seismic exploration equipment to a Precision Time Protocol (PTP) network via a cable, thereby enabling the seismic exploration equipment to recognize the accurate time.

[0003] Special Publication No. 2021-501871

[0004] When installing a large number of measuring devices on the seabed and performing measurements using each of the measuring devices, there is a problem in that the cost of wiring the cables becomes high when connecting the large number of measuring devices to the PTP network with cables.

[0005] The present invention has been made in view of these points, and has as its object to reduce the influence of clock drift on the measurement results of a measurement device installed on the seabed.

[0006] A measurement system according to a first aspect of the present invention includes a measurement device installed on the seabed and a data processing device that analyzes measurement data of seismic waves detected by the measurement device in response to seismic waves emitted from a seismic source toward the seabed from a ship navigating the ocean during a measurement period. The measurement device includes an oscillator used for timekeeping and a measurement data creation unit that creates a plurality of pieces of measurement data associated with internal times kept based on the oscillator. Either the measurement device or the data processing device includes a time difference determination unit that determines (1) a first time difference, which is the difference between an absolute start time at the start of the measurement period and the internal time associated with the measurement data at the start of the measurement period, and (2) a second time difference, which is the difference between an absolute end time at the end of the measurement period and the internal time associated with the measurement data at the end of the measurement period, and a correction unit that corrects the internal times associated with the plurality of measurement data based on at least the first time difference and the second time difference.

[0007] The correction unit may determine the amount of change in the difference between the absolute time and the internal time per unit time based on the time difference between the start absolute time and the end absolute time and the difference between the first time difference and the second time difference, and correct the internal time associated with the multiple measurement data based on the determined amount of change.

[0008] The correction unit may determine the difference between the absolute time at the internal time and the internal time by adding to the first time difference a value obtained by multiplying the elapsed time from the start absolute time to the internal time by the amount of change, and correct the internal time based on the determined difference.

[0009] The data processing device may further include a signal transmitting unit that transmits a first acoustic signal including an absolute time, and the measurement device may further include an acoustic signal receiving unit that receives the first acoustic signal, and the time difference determining unit may determine at least one of the first time difference and the second time difference based on the difference between the absolute time indicated by the first acoustic signal and the internal time at the time the measurement device received the first acoustic signal.

[0010] The time difference determination unit may determine at least one of the first time difference and the second time difference based on the difference between the absolute time indicated by the first acoustic signal plus the time required for the first acoustic signal to reach the measuring device, and the internal time at the time the measuring device receives the first acoustic signal.

[0011] The measurement system may further include an optical communication device that emits a first optical signal indicating absolute time to the measurement device underwater, and the measurement device may further include an optical signal receiving unit that receives the first optical signal emitted by the optical communication device, and the time difference determining unit may determine at least one of the first time difference and the second time difference based on the difference between the emission time at which the optical communication device emits the first optical signal and the internal time at the time the measurement device receives the first optical signal.

[0012] The measurement system may further include an optical communication device that emits a first optical signal indicating absolute time to the measurement device underwater, the data processing device may further include a signal transmitting unit that transmits a first acoustic signal including control data for controlling the measurement device, the measurement device may further include an optical signal receiving unit that receives the first optical signal emitted by the optical communication device and an acoustic signal receiving unit that receives the first acoustic signal, and the time difference determining unit may determine the first time difference based on the difference between the absolute time at which the signal transmitting unit transmitted the first acoustic signal and the internal time at the time the measurement device received the first acoustic signal, and may determine the second time difference based on the difference between the emission time at which the optical communication device emitted the first optical signal and the internal time at the time the measurement device received the first optical signal.

[0013] The correction unit may determine the frequency deviation of the oscillator of the measurement device at the end of the measurement period based on the first time difference and the second time difference, and notify the measurement device of the determined frequency deviation, and the measurement device may further have a calibration unit that calibrates the frequency of the oscillator based on the frequency deviation notified by the data processing device.

[0014] The measurement system may include a plurality of measuring devices installed at different locations, and the data processing device may further include a signal transmitting unit that transmits a first acoustic signal including control data for controlling the measuring devices, and a signal receiving unit that receives a second acoustic signal emitted by the measuring devices that have received the first acoustic signal, wherein the signal transmitting unit may transmit the first acoustic signal including activation data for activating the measuring devices to each of the plurality of measuring devices, and may transmit the first acoustic signal including time data to a measuring device from which the signal receiving unit has received the second acoustic signal including response data to the activation data.

[0015] The data processing device is installed on the ship and has a memory unit that stores the positions of each of the multiple measuring devices in association with identification information of the measuring devices, and a position information acquisition unit that acquires position information indicating the position of the ship, and the signal transmission unit may transmit the first acoustic signal including the time data to the measuring devices that are within a predetermined range of the position indicated by the position information by referring to the positions of each of the multiple measuring devices stored in the memory unit.

[0016] The signal transmitting unit may transmit the first acoustic signal including recording start data indicating an instruction to start recording measurement data to the measuring device when the signal receiving unit receives a response to the first acoustic signal including the time data.

[0017] The measurement system may further include a control device that notifies the seismic source that it is in a state where it can generate the seismic waves after the signal transmitting unit transmits the first acoustic signal including the recording start data to the plurality of measurement devices.

[0018] A data processing device of a second aspect of the present invention includes a data acquisition unit that acquires a plurality of measurement data indicating seismic waves detected by a measurement device installed on the seabed in response to seismic waves emitted from an epicenter toward the seabed from a ship sailing on the sea during a measurement period, the measurement data being generated by the measurement device and associated with an internal time kept by the measurement device; a time difference determination unit that determines (1) a first time difference, which is the difference between the absolute start time at which the measurement period started and the internal time associated with the measurement data at the time the measurement period started, and (2) a second time difference, which is the difference between the absolute end time at which the measurement period ended and the internal time associated with the measurement data at the time the measurement period ended; and a correction unit that corrects the internal time associated with the plurality of measurement data based on at least the first time difference and the second time difference.

[0019] A third aspect of the measurement device of the present invention is a measurement device that measures seismic waves on the seabed that are generated in response to seismic waves emitted from an epicenter toward the seabed from a ship sailing on the sea during a measurement period, and includes: a data creation unit that creates multiple measurement data associated with internal times measured by the measurement device; a time difference determination unit that determines (1) a first time difference, which is the difference between the absolute start time at which the measurement period starts and the internal time associated with the measurement data at the time the measurement period starts, and (2) a second time difference, which is the difference between the absolute end time at which the measurement period ends and the internal time associated with the measurement data at the time the measurement period ends; and a correction unit that corrects the internal times associated with the multiple measurement data based on at least the first time difference and the second time difference.

[0020] A data processing method of a fourth aspect of the present invention includes the steps of: acquiring, executed by a computer, a plurality of measurement data indicating seismic waves detected by a measurement device installed on the seabed in response to seismic waves being emitted from an epicenter from a ship sailing on the sea toward the seabed during a measurement period, the plurality of measurement data being generated by the measurement device and associated with an internal time kept by the measurement device; (1) identifying a first time difference which is the difference between the absolute start time at which the measurement period started and the internal time associated with the measurement data at the time the measurement period started, and (2) a second time difference which is the difference between the absolute end time at which the measurement period ended and the internal time associated with the measurement data at the time the measurement period ended; and correcting the internal time associated with the plurality of measurement data based on at least the first time difference and the second time difference.

[0021] A fifth aspect of the present invention is a program for causing a computer to execute the following steps: acquiring a plurality of measurement data indicating seismic waves detected by a measurement device installed on the seabed in response to seismic waves emitted from an epicenter from a ship sailing on the sea toward the seabed during a measurement period, the plurality of measurement data being generated by the measurement device and associated with an internal time kept by the measurement device; identifying (1) a first time difference which is the difference between the absolute start time at which the measurement period started and the internal time associated with the measurement data at the time the measurement period started, and (2) a second time difference which is the difference between the absolute end time at which the measurement period ended and the internal time associated with the measurement data at the time the measurement period ended; and correcting the internal time associated with the plurality of measurement data based on at least the first time difference and the second time difference.

[0022] The present invention has the effect of reducing the influence of clock drift on the measurement results of a measurement device installed on the seabed.

[0023] 1 is a diagram illustrating an overview of a measurement system S. FIG. 1 is a diagram illustrating an overview of a measurement system S. FIG. 2 is a diagram illustrating a procedure for starting up a plurality of measurement devices 4. FIG. 3 is a diagram illustrating an example of a management table showing the state of each measurement device 4. FIG. 4 is a diagram illustrating a configuration of a data processing device 1. FIG. 5 is a diagram illustrating a configuration of a measurement device 4. FIG. 6 is a diagram illustrating the aging characteristics of an oscillator. FIG. 7 is a diagram illustrating the aging characteristics of an oscillator. FIG. 8 is a diagram illustrating the aging characteristics of an oscillator. FIG. 9 is a diagram illustrating the difference between calculating a second time difference using an acoustic signal at the end of measurement and calculating a second time difference using an optical signal. FIG. 10 is a diagram illustrating the difference between calculating a second time difference using an acoustic signal at the end of measurement and calculating a second time difference using an optical signal. FIG. 11 is a diagram illustrating the configuration of a measurement device 4 capable of calibrating frequency. FIG. 12 is a diagram illustrating the calibration of the frequency of an oscillator 41 by a calibration unit 483. FIG. 13 is a diagram illustrating the calibration of the frequency of an oscillator 41 by a calibration unit 483. FIG. 14 is a flowchart illustrating the flow of processing in a data processing device 1. FIG. 15 is a flowchart illustrating the flow of measurement data correction processing (S25). FIG. 16 is a diagram illustrating the configuration of a data processing device 1A according to a first modified example. Fig. 1 is a diagram showing the configuration of a measurement device 4A according to a first modified example. Fig. 2 is a flowchart showing the flow of processing in a data processing device 1A according to the first modified example. Fig. 3 is a flowchart showing the flow of processing in a measurement device 4A according to the first modified example. Fig. 4 is a diagram showing the configuration of a data processing device 1B according to a second modified example. Fig. 5 is a diagram showing the configuration of a measurement device 4B according to the second modified example.

[0024] [Outline of Measurement System S] Figure 1 is a diagram showing an outline of the measurement system S. The measurement system S is a marine geophysical exploration system for analyzing the subseafloor geological structure. In the measurement system S, seismic waves are generated from a seismic source 2 such as an air gun or sparker, and a data processing device 1 analyzes the subseafloor geological structure using the results of measurements of the seismic waves by multiple measuring devices 4 installed on the seafloor.

[0025] The measurement system S includes a data processing device 1, a seismic source 2, an optical communication device 3, and a plurality of measurement devices 4. The data processing device 1, the seismic source 2, and the optical communication device 3 are mounted on a ship 100 capable of moving on the ocean. The plurality of measurement devices 4 are installed on the seabed at intervals of at least a predetermined distance.

[0026] The data processing device 1 is, for example, a computer, which acquires measurement data indicating the seismic state of the seabed observed by multiple measuring devices 4 at the timing when seismic waves are emitted, and analyzes the acquired measurement data. That is, the data processing device 1 analyzes measurement data of seismic waves detected by measuring devices 4 installed on the seabed in response to seismic waves emitted from a seismic source 2 toward the seabed from a ship sailing at sea during a measurement period. As shown in FIG. 1A , the data processing device 1 controls the multiple measuring devices 4 by transmitting and receiving acoustic signals, and receives measurement data generated by the multiple measuring devices 4. The data processing device 1 also acquires information indicating absolute time from, for example, a PTP network or a GPS (Global Positioning System).

[0027] The seismic source 2 generates seismic waves during the measurement period. The seismic source 2 generates seismic waves, for example, under the control of the data processing device 1, but may also generate seismic waves under the control of a control device different from the data processing device 1 (for example, a computer installed on a ship different from the ship 100).

[0028] The optical communication device 3 acquires measurement data from at least one measurement device 4 by optically communicating with it based on the control of the data processing device 1. The optical communication device 3 emits a first optical signal to the measurement device 4 underwater and receives a second optical signal transmitted by the measurement device 4 that has received the first optical signal. The optical communication device 3 is connected to the data processing device 1 by a cable C, and, based on the control of the data processing device 1, dives to a position where optical communication with the measurement device 4 can be performed, and then performs optical communication with the measurement device 4. The optical communication device 3 sequentially moves close to multiple measurement devices 4 and sequentially acquires measurement data from the multiple measurement devices 4. Note that the measurement system S may have multiple optical communication devices 3, and the multiple optical communication devices 3 may acquire measurement data from the multiple measurement devices 4.

[0029] The measuring device 4 generates measurement data indicating the amount of vibration of the measuring device 4. The measurement data indicates the amount of vibration of the measuring device 4 caused by seismic waves. The measurement data is data indicating the magnitude of vibration detected by a sensor possessed by the measuring device 4, and is, for example, data including a measurement value generated by sampling a signal output by the sensor every millisecond. The measurement data is associated with an internal time kept by an oscillator inside the measuring device 4. The measuring device 4 transmits the measurement data to the optical communication device 3 by an optical signal.

[0030] The internal time of the measurement device 4 is measured by an oscillator built into the measurement device 4 and is therefore different from absolute time. Furthermore, due to the aging characteristics of the oscillator of the measurement device 4, the oscillator frequency changes over time. As a result, a difference occurs between the internal time associated with the measurement data by the measurement device 4 and absolute time. Even if a chip-scale atomic clock (CSAC), which has relatively good aging characteristics, is used as the oscillator, a frequency offset occurs over time. If there is a difference between absolute time and internal time, it is impossible to accurately determine the relationship between the timing at which the seismic source 2 emitted seismic waves and the timing of the seismic waves indicated by the measurement data, which results in a problem of reduced accuracy in analyzing the subseafloor geological structure based on seismic waves.

[0031] Therefore, the data processing device 1 determines the time difference between the absolute time and the internal time of the measurement device 4 using an acoustic signal or an optical signal at the start and end of measurement, and corrects the internal time associated with the measurement data based on the determined result. If the oscillator of the measurement device 4 has a frequency offset that changes approximately constant per unit time regardless of the passage of time, the time difference between the absolute time and the internal time at each measurement point within the measurement period also changes linearly. In other words, the time difference per unit time is approximately constant. The data processing device 1 utilizes this characteristic to calculate the difference between the internal time and the absolute time corresponding to each measurement value by multiplying the elapsed time from the start of measurement by the change in the time difference per unit time. The data processing device 1 corrects the internal time associated with the measurement data based on the calculated time difference.

[0032] By operating the data processing device 1 in this manner, the data processing device 1 does not need to continuously use acoustic or optical signals while the measurement device 4 is measuring to determine the time difference between absolute time and the internal time of the measurement device 4, but only needs to determine the time difference between the start and end of the measurement. Therefore, even if multiple measurement devices 4 are not connected to a PTP network, the data processing device 1 can analyze measurement data with high accuracy without significantly reducing measurement efficiency.

[0033] [Startup Procedure for Measuring Device 4] Measurements for analyzing the subseafloor geological structure are carried out periodically. For example, measurements are carried out for several days to several weeks once a year. If multiple measuring devices 4 are installed each time a measurement period arrives, the installation work requires a significant amount of time, resulting in poor measurement efficiency. Therefore, the measurement system S in this embodiment is configured to have multiple measuring devices 4 installed in advance on the seabed measure seismic waves over multiple measurement periods spanning multiple years.

[0034] Since the measurement devices 4 are battery-powered, the batteries will be depleted in a short time if they are installed on the seabed and operated for a long period of time. Therefore, the measurement system S is configured so that multiple measurement devices 4 are activated at the start of a measurement period and stop measuring operation when the measurement period ends. The measurement devices 4 that have stopped measuring operation enter a sleep state in which their oscillators are stopped to reduce power consumption while maintaining the function of receiving acoustic signals from the data processing device 1. The measurement devices 4 have, for example, a measurement state in which measurements are performed, a standby state in which the oscillator is operating but no measurement is being performed, and a sleep state in which the oscillator is stopped and no measurement is being performed.

[0035] 2 and 3 are diagrams for explaining the procedure for starting up multiple measurement devices 4. Fig. 2 schematically shows the multiple measurement devices 4 as viewed from above. The circles (◯) shown in Fig. 2 indicate measurement devices 4 installed on the seabed. The numbers below the circles are identification information (ID) for identifying the measurement devices 4.

[0036] While the ship 100 is moving, the data processing device 1 transmits an acoustic signal containing control information to the measuring device 4 within the range reached by the acoustic signal (for example, the range within the dashed frame in FIG. 2 ), thereby enabling the measuring device 4 to start a measurement operation. Specifically, the data processing device 1 transmits a start command, a synchronization command, and a recording start command, thereby enabling the measuring device 4 to start a measurement operation. In FIG. 2 , the dashed arrow represents the start command, and the solid arrow represents the synchronization command. The data processing device 1 may transmit parameters required for measurement (for example, a sampling interval or a preamplifier gain) to the measuring device 4.

[0037] The startup command includes a character string corresponding to an instruction to transition the measurement device 4 from a sleep state to a state where it is capable of measurement, and the ID of the measurement device 4. The synchronization command includes a character string corresponding to an instruction to request the internal time of the measurement device 4, and the ID of the measurement device 4. The data processing device 1 may send a synchronization command that includes the absolute time recognized by the data processing device 1. In the following description, the process by which the data processing device 1 determines the relationship between the absolute time and the internal time of the measurement device 4 based on the internal time received from the measurement device 4 by sending a synchronization command to the measurement device 4 is referred to as "synchronization." The recording start command includes a character string corresponding to an instruction to start recording measurement data, and the ID of the measurement device 4.

[0038] A measuring device 4 with no letter written in a circle shown in FIG. 2 (for example, a measuring device 4 with an ID of 0606) is in a stopped state. A measuring device 4 with a "W" written in a dashed circle indicates that it has received a startup command and is currently executing startup processing. A measuring device 4 with a "W" written in a solid circle indicates that it has completed startup but has not yet completed synchronization. A measuring device 4 with an "S" written in a dashed circle indicates that it has received a synchronization command and is currently executing synchronization processing. A measuring device 4 with an "S" written in a solid circle indicates that it has completed synchronization.

[0039] Fig. 3 is a diagram showing an example of a management table showing the status of each measurement device 4. In the management table shown in Fig. 3, the ID of the measurement device 4, information indicating whether startup has been completed, information indicating whether synchronization has been completed, the previous action (i.e., the action executed immediately before), and the time at which the action was executed are associated with each other. As can be seen from the states of the multiple measurement devices 4 within the dashed line area in Fig. 2 and the times in the management table in Fig. 3, the data processing device 1 transmits startup commands and synchronization commands to different measurement devices 4 in a time-division manner.

[0040] Specifically, the data processing device 1 transmits a synchronization command to the already-activated second measuring device 4 during the period from when the data processing device 1 transmits a startup command to the first measuring device 4 until startup of the first measuring device 4 is completed. By having the data processing device 1 sequentially start up and then synchronize the multiple measuring devices 4 in this manner, the multiple measuring devices 4 can be brought into a state where they can measure in a shorter time than if the data processing device 1 transmits a startup command to each measuring device 4 and then waits until the measuring device 4 starts up. Furthermore, the measuring devices 4 can be brought into a state where they can measure in a shorter time and more reliably than if a person starts up each measuring device 4.

[0041] Furthermore, while traveling in a fixed direction, the data processing device 1 activates the multiple measuring devices 4. As an example, as shown in Fig. 2, the data processing device 1 transmits activation commands to the multiple measuring devices 4 located in the front of the ship 100, and transmits synchronization commands to the multiple measuring devices 4 after the ship 100 has moved to a position ahead of the activated multiple measuring devices 4.

[0042] In this way, the data processing device 1 transmits commands to a plurality of measuring devices 4 that are located at the front of the vessel 100 on which the data processing device 1 is mounted and that are capable of receiving acoustic signals, and a plurality of measuring devices 4 that are located at the rear of the vessel 100 and that are also capable of receiving acoustic signals. By operating in this manner, the data processing device 1 can bring the plurality of measuring devices 4 into a state where they can measure in a shorter time than when commands are transmitted only to measuring devices 4 that are located at either the front or rear of the vessel 100.

[0043] 2 and 3, the data processing device 1 may transmit a recording start command to the measurement devices 4 that have completed synchronization after receiving a response to the synchronization command from the measurement devices 4 and completing synchronization. The recording start command may be a command that includes an instruction to start recording immediately, or may be a command that indicates the time to start recording. The data processing device 1 may transmit recording start commands successively to multiple measurement devices 4 after synchronization of all measurement devices 4 has been completed and before the seismic source generates seismic waves.

[0044] Installing multiple measuring devices 4 on the seabed every time a measurement is performed would require significant installation time and expense. Meanwhile, leaving the measuring devices 4 in operation for extended periods of time would result in battery depletion. In the measurement system S, the data processing device 1 sequentially starts and synchronizes multiple measuring devices 4 before a measurement period, and then puts the measuring devices 4 into a sleep state when the measurement period ends to reduce battery consumption. This configuration of the measurement system S allows multiple measuring devices 4 to efficiently measure seismic waves over extended periods of time.

[0045] 4 is a diagram showing the configuration of the data processing device 1. The data processing device 1 includes a position information acquisition unit 11, an acoustic signal transmission unit 12, an acoustic signal reception unit 13, a data transmission / reception unit 14, an absolute time acquisition unit 15, an external communication unit 16, a storage unit 17, and a control unit 18. The control unit 18 includes a hypocenter control unit 181, a command creation unit 182, a data acquisition unit 183, a time difference identification unit 184, and a correction unit 185. Note that some of the functional units of the control unit 18 may be provided in a device other than the data processing device 1.

[0046] The position information acquisition unit 11 acquires position information indicating the position of the data processing device 1, i.e., the position of the ship 100 on which the data processing device 1 is mounted. The position information acquisition unit 11 acquires, for example, radio waves received from a GPS satellite as position information, and identifies the latitude and longitude based on the acquired position information. The position information acquisition unit 11 notifies the command creation unit 182 of the identified latitude and longitude.

[0047] The acoustic signal transmitting unit 12 is an acoustic communication unit that transmits a first acoustic signal to the measurement device 4. For example, the acoustic signal transmitting unit 12 transmits a first acoustic signal including control data (e.g., various commands) input from the command creating unit 182 based on the control of the command creating unit 182. The acoustic signal receiving unit 13 transmits a first acoustic signal including a command to a measurement 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 acquiring unit 11, by referring to the respective positions of the multiple measurement devices 4 stored in the storage unit 17. The predetermined range is a range within which the measurement device 4 can receive the first acoustic signal transmitted by the acoustic signal transmitting unit 12.

[0048] As an example, the acoustic signal transmitter 12 transmits, to each of the plurality of measuring devices 4, a first acoustic signal including a startup command that is startup data for starting up the measuring device 4. The acoustic signal transmitter 12 transmits a first acoustic signal including a synchronization command indicating absolute time (i.e., a synchronization command including time data) to each measuring device 4 whose acoustic signal receiver 13 has received a second acoustic signal including response data to the startup command.

[0049] Furthermore, the acoustic signal transmitter 12 transmits a first acoustic signal including a recording start command, which is recording start data indicating an instruction to start recording measurement data, to a measuring device 4 whose acoustic signal receiver 13 has received a response to the first acoustic signal including the synchronization command. That is, the acoustic signal transmitter 12 transmits a first acoustic signal including recording start data to a measuring device 4 that has transmitted response data to the synchronization command.

[0050] The acoustic signal transmitting unit 12 may transmit a recording start command including the ID of one measuring device 4, or may transmit a recording start command including the IDs of multiple measuring devices 4 that have completed synchronization. The acoustic signal transmitting unit 12 may transmit a recording start command including information indicating that the command is intended for all measuring devices 4. By transmitting such a recording start command from the acoustic signal transmitting unit 12, multiple measuring devices 4 can start recording seismic waves by transmitting the recording start command once, thereby improving measurement efficiency.

[0051] The acoustic signal receiving unit 13 is an acoustic communication unit that receives a second acoustic signal emitted by the measuring device 4 that has received the first acoustic signal. The acoustic signal receiving unit 13 receives, for example, the second acoustic signal that indicates the internal time of the measuring device 4. The acoustic signal receiving unit 13 identifies the internal time based on the time data included in the received acoustic signal, and notifies the data acquiring unit 183 of the identified internal time.

[0052] The data transmitter / receiver 14 is a communication interface for transmitting and receiving data to and from the optical communication device 3. For example, the data transmitter / receiver 14 transmits data input from the data acquisition unit 183 to the optical communication device 3, the data including an instruction to acquire the internal time from the measurement device 4, and receives time data indicating the internal time acquired by the optical communication device 3 from the measurement device 4.

[0053] The data transmitter / receiver 14 may notify the optical communication device 3 of the absolute time acquired by the absolute time acquisition unit 15, and may receive time data in which the internal time is associated with the absolute time at the time when the optical communication device 3 acquired the internal time from the measurement device 4. The data transmitter / receiver 14 receives data indicating the internal time acquired by the optical communication device 3 from the measurement device 4, for example, at a time within a predetermined range from the time when the last measurement is made within the measurement period. The data transmitter / receiver 14 notifies the data acquisition unit 183 of the acquired time data.

[0054] The absolute time acquisition unit 15 acquires the absolute time from, for example, a GPS satellite. The absolute time acquisition unit 15 notifies the acquired absolute time to the time difference determination unit 184. The absolute time acquisition unit 15 may also notify the data transmission / reception unit 14 of the absolute time.

[0055] The external communication unit 16 transmits the measurement results including the measurement data after correcting the internal time, which are input from the correction unit 185. The external communication unit 16 may transmit the measurement results to an external computer that analyzes the measurement results and executes a process to identify the sub-seafloor geological structure, or may transmit the measurement results to another processing unit included in the control unit 18.

[0056] The storage unit 17 has storage media such as a read-only memory (ROM), a random access memory (RAM), and a solid-state drive (SSD). The storage unit 17 stores programs executed by the control unit 18. The storage unit 17 also stores various data for causing the multiple measuring devices 4 to perform measurements. The storage unit 17 stores, for example, the locations of the multiple measuring devices 4 and the identification information of the measuring devices 4 in association with each other. Specifically, the storage unit 17 stores the latitude and longitude of the multiple measuring devices 4 in association with the IDs of the measuring devices 4.

[0057] 3. Furthermore, the storage unit 17 stores a management table such as that shown in FIG. 3. Furthermore, the storage unit 17 stores a plurality of pieces of measurement data acquired from the measurement devices 4, in association with the IDs of the plurality of measurement devices 4. The storage unit 17 stores the plurality of pieces of measurement data in association with the internal time of the measurement device 4 at the time the measurement data was generated. After that, when the correction unit 185 associates the time corrected from the internal time with the measurement data, the storage unit 17 stores the measurement data in association with the corrected time.

[0058] The control unit 18 includes, for example, a CPU (Central Processing Unit). The control unit 18 executes the programs stored in the storage unit 17 to function as a hypocenter control unit 181, a command creation unit 182, a data acquisition unit 183, a time difference identification unit 184, and a correction unit 185.

[0059] The epicenter control unit 181 transmits an instruction to generate seismic waves to the epicenter 2. The epicenter control unit 181 causes the epicenter 2 to generate seismic waves, for example, after the acoustic signal transmission unit 12 transmits a first acoustic signal including a recording start command to the plurality of measuring devices 4. The epicenter control unit 181 transmits an instruction to generate seismic waves to the epicenter 2, for example, after receiving a notification from the data acquisition unit 183 that all of the measuring devices 4 are ready to perform measurement. The epicenter control unit 181 may cause the epicenter 2 to generate seismic waves at a predetermined date and time, or may cause the epicenter 2 to generate seismic waves in response to an instruction from an external device. The data processing device 1 may not have the epicenter control unit 181, and an external control device may function as the epicenter control unit 181.

[0060] The command creation unit 182 creates commands to be transmitted by the acoustic signal transmission unit 12 to the measuring devices 4. The command creation unit 182 creates, for example, a startup command, a synchronization command, and a recording start command, and inputs the created commands to the acoustic signal transmission unit 12. When creating a command, the command creation unit 182 selects measuring devices 4 within a predetermined range from the latitude and longitude input from the position information acquisition unit 11 by referencing the latitude and longitude of the locations where multiple measuring devices 4 are installed, which are stored in the storage unit 17. The command creation unit 182 creates a command including the ID of the selected measuring device 4.

[0061] 2 , the command creation unit 182 creates a startup command for a measuring device 4 that is in a sleeping state among multiple measuring devices 4 within a predetermined range. In response to receiving a notification from the data acquisition unit 183 that a measuring device 4 corresponding to the created startup command has started up, the command creation unit 182 creates a synchronization command for that measuring device 4. In response to receiving a notification from the data acquisition unit 183 that a measuring device 4 corresponding to the created synchronization command has completed synchronization, the command creation unit 182 creates a recording start command for that measuring device 4.

[0062] When the command creation unit 182 inputs the created command to the acoustic signal transmission unit 12, it updates the "last action" in the management table stored in the storage unit 17. When the command creation unit 182 inputs a start-up command to the acoustic signal transmission unit 12, it sets the "last action" corresponding to the ID of the measuring device 4 included in the start-up command to "starting up." When the command creation unit 182 inputs a synchronization command to the acoustic signal transmission unit 12, it sets the "last action" corresponding to the ID of the measuring device 4 included in the synchronization command to "synchronizing."

[0063] The data acquisition unit 183 acquires various data transmitted from the measuring device 4. The data acquisition unit 183 acquires response data in response to a command transmitted by the acoustic signal transmission unit 12 via the acoustic signal reception unit 13. The data acquisition unit 183 notifies the command creation unit 182 that the response data has been acquired.

[0064] When the data acquisition unit 183 acquires response data, it updates the content of the "last action" in the management table stored in the storage unit 17. For example, when the data acquisition unit 183 acquires response data indicating startup, it updates the "last action" corresponding to the ID of the measuring device 4 included in the response data to "startup completed." When the data acquisition unit 183 acquires response data including the internal time of the measuring device 4 transmitted by the measuring device 4 in response to receiving a synchronization command, it updates the "last action" corresponding to the ID of the measuring device 4 included in the response data to "synchronization completed." The data acquisition unit 183 associates the absolute time when the synchronization command was transmitted and the internal time indicated in the response data with the ID of the measuring device 4 and stores them in the storage unit 17.

[0065] When the data acquisition unit 183 acquires response data indicating that the measuring device 4 has started recording, it updates the "last action" corresponding to the ID of the measuring device 4 included in the response data to "start recording." When the data acquisition unit 183 receives response commands to the recording start command from all measuring devices 4, that is, when the "last action" of all measuring devices 4 has become "start recording," it notifies the epicenter control unit 181 that measurement can start.

[0066] The data acquiring unit 183 may further acquire an emission time, which is the absolute time when the optical communication device 3 emits the first optical signal, and an 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 measurement device 4 in response to receiving the first optical signal. The data acquiring unit 183 associates the emission time and the internal time with the ID of the measurement device 4, stores them in the storage unit 17, and notifies the time difference identifying unit 184.

[0067] Furthermore, the data acquiring unit 183 acquires measurement data from the measurement device 4 via the data transmitting / receiving unit 14. The data acquiring unit 183 acquires multiple pieces of measurement data each indicating a measurement value corresponding to a different time. For example, after the measurement period has ended, the data acquiring unit 183 acquires multiple pieces of measurement data that the optical communication device 3 has collected from the measurement device 4 via optical communication from the data transmitting / receiving unit 14. The data acquiring unit 183 associates the acquired measurement data with the ID of the measurement device 4 and stores the data in the storage unit 17, thereby enabling the time difference identifying unit 184 to refer to the measurement data.

[0068] The time difference determination unit 184 determines the time difference between the absolute time and the internal time of the measurement device 4 associated with the measurement data by referring to the measurement data stored in the storage unit 17. Specifically, the time difference determination unit 184 determines a first time difference, which is the difference between the absolute start time when the measurement period starts and the internal time of the measurement device 4 associated with the measurement data at the time the measurement period starts, by referring to the absolute time and internal time stored in the storage unit 17 in association with the ID of the measurement device 4. The time difference determination unit 184 also determines a second time difference, which is the difference between the absolute end time when the measurement period ends and the internal time associated with the measurement data at the time the measurement period ends.

[0069] The time difference determining unit 184 determines at least one of the first time difference and the second time difference based on the difference between the absolute time at which the acoustic signal transmitting unit 12 transmitted the first acoustic signal and the internal time indicated by the second acoustic signal received by the acoustic signal receiving unit 13. The time difference determining unit 184 determines the first time difference based on, for example, the difference between the absolute time transmitted by the command creating unit 182 in the synchronization command and the internal time of the measurement device 4 included in the response data to the synchronization command. Furthermore, the time difference determining unit 184 determines the second time difference based on the difference between the absolute time included in the measurement data acquisition request command transmitted by the optical communication device 3 to the measurement device 4 after the measurement period has ended and the internal time of the measurement device 4 included in the response data to the measurement data acquisition request command.

[0070] There is a time required for the acoustic signals to propagate between when the acoustic signal transmitter 12 transmits the first acoustic signal and when the acoustic signal receiver 13 receives the second acoustic signal. Therefore, the absolute time at which the measurement device 4 receives the first acoustic signal differs from the absolute time at which the acoustic signal transmitter 12 transmits the first acoustic signal. Therefore, the time difference determining unit 184 may determine at least one of the first time difference and the second time difference based on the difference between the internal time indicated by the second acoustic signal received by the acoustic signal receiver 13 and the time obtained by adding the time required for the first acoustic signal to the absolute time at which the acoustic signal transmitter 12 transmits the first acoustic signal.

[0071] The time difference determination unit 184 may determine at least one of the first time difference and the second time difference based on the difference between the internal time indicated by the second acoustic signal and the time obtained by adding the time required for the measurement device 4 to determine the internal time after receiving the first acoustic signal to the absolute time at which the acoustic signal transmission unit 12 transmitted the first acoustic signal. In this way, the time difference determination unit 184 uses the propagation time of the first acoustic signal and the processing time in the measurement device 4, thereby improving the accuracy of determining the difference between absolute time and internal time.

[0072] The time difference determination unit 184 may determine both the first time difference and the second time difference based on the difference between the absolute time when the acoustic signal transmitting unit 12 transmitted the first acoustic signal and the internal time included in the second acoustic signal received by the acoustic signal receiving unit 13, or may determine at least one of the first time difference and the second time difference based on the difference between the light emission time, which is the absolute time when the optical communication device 3 transmitted the first optical signal, and the internal time included in the second optical signal received by the optical communication device 3 from the measurement device 4.

[0073] The time difference determination unit 184 may determine at least one of the first time difference and the second time difference based on the difference between the absolute time when the optical communication device 3 transmitted the first optical signal plus the time required for the first optical signal to reach the measurement device 4, and the internal time indicated by the second optical signal received by the optical communication device 3. The absolute time when the optical communication device 3 transmitted the first optical signal may be the time when the data transmitter / receiver 14 instructed the optical communication device 3 to transmit the first optical signal.

[0074] Light has a faster propagation speed than sound and its propagation is more stable than sound. Furthermore, the optical communication device 3 emits the first optical signal at a position closer to the measurement device 4 than the data processing device 1. Therefore, the propagation time required for the first optical signal emitted by the optical communication device 3 to reach the measurement device 4 is shorter than the propagation time required for the first acoustic signal transmitted by the acoustic signal transmitter 12 to reach the measurement device 4, and therefore the variance in propagation time is small. As a result, the accuracy of determining the time difference is improved by using an optical signal to determine the time difference.

[0075] However, if the optical communication device 3 moves close to the measurement device 4 both at the start and end of measurement, it will require time for the optical communication device 3 to move close to the measurement device 4. Therefore, the time difference determining unit 184 determines the first time difference using the acoustic signal at the start of measurement, and determines the second time difference using the optical signal at the end of measurement when the optical communication device 3 moves close to the measurement device 4 to collect the measurement data. By operating the time difference determining unit 184 in this manner, the time difference can be determined with high accuracy based on the optical signal without increasing the time it takes for the optical communication device 3 to move close to the measurement device 4 to acquire the internal time, thereby achieving both measurement efficiency and measurement accuracy.

[0076] The correction unit 185 reads out the plurality of measurement data stored in the storage unit 17, and corrects the internal times associated with the plurality of measurement data based on at least the first time difference and the second time difference. The correction unit 185 stores the plurality of measurement data with the corrected internal times in the storage unit 17.

[0077] Specifically, the correction unit 185 first determines the amount of change per unit time in the difference between the absolute time and the internal time based on the time difference between the absolute start time (the absolute time when measurement starts) and the absolute end time (the absolute time when measurement ends), and the difference between the first time difference and the second time difference. Next, the correction unit 185 corrects the internal times associated with the multiple measurement data based on the determined amount of change.

[0078] The correction unit 185 specifies the difference between the internal time and the absolute time corresponding to the measurement data to be corrected by adding the value obtained by multiplying the elapsed time from the absolute start time to the internal time by the amount of change per unit time to the first time difference. The correction unit 185 corrects the internal time of the measurement data to be corrected based on the specified difference. If the amount of change per unit time in the frequency offset of the oscillator of the measurement device 4 is approximately constant regardless of the passage of time, the time difference determination unit 184 can thus efficiently correct the internal time corresponding to the measurement data using the first time difference at the start of measurement and the second time difference at the end of measurement.

[0079] Note that the data acquisition unit 183 may also send a synchronization command during the measurement period to acquire the internal time of the measurement device 4, and the correction unit 185 may correct the internal time of the measurement data further based on the difference between the absolute time during the measurement period and the internal time of the measurement device 4. The correction unit 185 may also correct the internal time of the measurement data based on the time difference between the internal time and the absolute time identified based on synchronization commands issued at a number of points corresponding to the linearity of the aging characteristics of the oscillator of the measurement device 4. By operating in this manner, the correction unit 185 can appropriately correct the internal time corresponding to the measurement data in accordance with the aging characteristics of the oscillator of the measurement device 4.

[0080] 5 is a diagram showing the configuration of the measuring device 4. The measuring device 4 has 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 has a data creating unit 481 and a data communication unit 482.

[0081] The oscillator 41 generates an oscillation signal used to measure the internal time in the measurement device 4. As described above, the oscillator 41 is, for example, a chip-scale atomic oscillator, but may be another type of oscillator.

[0082] The sensor 42 generates a detection signal whose level changes in response to vibration of the measuring device 4. The sensor 42 inputs the detection signal to the data creation unit 481.

[0083] The acoustic signal receiving unit 43 receives a first acoustic signal transmitted from the data processing device 1. The acoustic signal receiving unit 43 inputs data such as the command and absolute time contained 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 indicating the internal time at which the first acoustic signal was received to the data processing device 1.

[0084] 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 a command 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 the data communication unit 482, for example, to the optical communication device 3.

[0085] The storage unit 47 has storage media such as a ROM, a RAM, an SSD, etc. The storage unit 47 stores programs executed by the control unit 48. The storage unit 47 also stores measurement data created by the data creation unit 481.

[0086] The control unit 48 includes, for example, a CPU. The control unit 48 executes the programs stored in the storage unit 47 to function as a data creation unit 481 and a data communication unit 482.

[0087] The data creation unit 481 functions as a measurement data creation unit that creates multiple pieces of measurement data associated with the internal time kept based on the oscillator 41. The data creation unit 481 creates multiple pieces of measurement data indicating the sampled signal levels (i.e., measurement values) by, for example, sampling the detection signal input from the sensor 42 at predetermined time intervals (e.g., 1 millisecond intervals). The data creation unit 481 associates the multiple pieces of measurement data with the internal time and stores them in the storage unit 47. Note that the data creation unit 481 may keep the internal time by counting the oscillation signal input from the oscillator 41, or may determine the internal time based on data indicating the internal time input from the oscillator 41.

[0088] The data communication unit 482 transmits, via the acoustic signal transmission unit 44, response data to a command included in the first acoustic signal received from the data processing device 1 via the acoustic signal reception unit 43. The data communication unit 482 also transmits, via the optical signal transmission unit 46, response data to a command included in the optical signal received from the optical communication device 3 via the optical signal reception unit 45. When the data communication unit 482 receives a synchronization command, it acquires 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 acquired internal time.

[0089] Furthermore, the data communication unit 482 transmits the plurality of pieces of 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 the plurality of pieces of measurement data stored in the storage unit 47 in association with the internal time.

[0090] [Oscillator Aging Characteristics] Figures 6 and 7 are diagrams for explaining the aging characteristics of an oscillator. Figure 6 shows the characteristics of an oscillator with worse aging characteristics than oscillator 41, and Figure 7 shows the aging characteristics of oscillator 41. Figures 6A and 7A show the characteristics in the initial state, and Figures 6B and 7B show the characteristics after a long period of time (several years) has passed.

[0091] The horizontal axis in Figures 6 and 7 represents the number of days elapsed since the start of measurement. The thin solid line in Figures 6 and 7 represents the amount of clock drift (left vertical axis). The thick solid line represents the state in which the amount of clock drift changes linearly over time, and the correction unit 185 corrects the internal time corresponding to multiple measurement data items, assuming that the amount of clock drift changes as shown by the thick solid line. The dashed line represents the residual error (right vertical axis) after the correction unit 185 corrects the internal time. The residual error is greatest near the center of the measurement period, and in Figure 6, a maximum residual error of approximately 0.44 milliseconds occurs.

[0092] 7 , the residual error is approximately 0.009 milliseconds, and it can be seen that the internal time corresponding to the measurement data is corrected with high accuracy by the correction process of the correction unit 185. In this way, when a chip-scale atomic oscillator with good aging characteristics is used as the oscillator 41, the process of correcting the internal time corresponding to the measurement data during the measurement period based on the first time difference at the start of measurement and the second time difference at the end of measurement is particularly effective.

[0093] 8A and 8B are diagrams illustrating the difference between calculating the second time difference using an acoustic signal at the end of measurement and calculating the second time difference using an optical signal. The solid line indicates the amount of clock drift, and the dashed lines indicate the maximum and minimum values ​​of the error due to correction. FIG. 8A shows a case where the time difference is determined based on the acoustic signal at the start and end of measurement. FIG. 8B shows a case where the first time difference is determined based on the acoustic signal at the start of measurement and the second time difference is determined based on the optical signal at the end of measurement.

[0094] Here, it is assumed that there is an error of ±0.15 milliseconds in determining the time difference based on the acoustic signal and an error of ±0.01 milliseconds in determining the time difference based on the optical signal. The maximum residual error in the example shown in Fig. 8A is 0.254 milliseconds, while the maximum residual error in the example shown in Fig. 8B is 0.179 milliseconds. Thus, it can be confirmed that the residual error can be reduced by having the time difference determination unit 184 determine the second time difference based on the optical signal at the end of measurement.

[0095] [Calibration of Oscillator 41] The processing of the correction unit 185 according to this embodiment can reduce the effect of the frequency offset of the oscillator 41. However, when comparing measurement data acquired during different measurement periods, it is desirable for the frequency offset to be small. Therefore, the measurement system S may be configured to calibrate the frequency of the oscillator 41 when the measurement period ends.

[0096] 9 is a diagram showing the configuration of a measurement device 4 capable of frequency calibration. The measurement device 4 shown in FIG. 9 differs from the measurement device 4 shown in FIG. 5 in that it further includes a calibration unit 483, but is otherwise the same.

[0097] To calibrate the frequency of the oscillator 41, the correction unit 185 determines the frequency deviation of the oscillator of the measurement device 4 at the end of the measurement period based on the first time difference and the second time difference, and notifies the measurement device 4 of the determined frequency deviation. The correction unit 185 calculates the amount of internal time deviation per unit time, for example, by dividing the difference between the first time difference and the second time difference by the measurement period. Because the amount of internal time deviation per unit time is proportional to the magnitude of the frequency deviation, the correction unit 185 can calculate the frequency deviation based on the amount of internal time deviation per unit time.

[0098] The calibration unit 483 calibrates the frequency of the oscillator 41 based on the frequency deviation notified from the data processing device 1. Specifically, the calibration unit 483 changes the voltage of the control signal used to control the oscillation frequency of the oscillator 41 in accordance with the frequency deviation. When the frequency deviation is, for example, +1.0×10 －１０ If －１０The calibration unit 483 may change the control voltage so as to lower the oscillation frequency of the oscillator 41 by a frequency corresponding to the threshold value when the frequency deviation at the end of the measurement period is equal to or greater than the threshold value.

[0099] In order to improve the accuracy of the calibration, the calibration unit 483 may calibrate the frequency of the oscillator 41 using the average value of the time difference between the absolute time transmitted by the optical communication device 3 via an optical signal and the internal time at the time the measuring device 4 receives the optical signal, obtained multiple times over a certain period of time (e.g., 10 minutes).

[0100] FIG. 10 is a diagram for explaining the calibration of the frequency of the oscillator 41 by the calibration unit 483. In FIG. 10, it is assumed that there is a 40-day measurement period per year. FIG. 10 shows that the frequency offset increases while the oscillator 41 is operating, and that the frequency offset does not change while the oscillator 41 is stopped. In FIG. 10, the error bars extending above and below the black dots indicating the frequency offset indicate the error range (magnitude of instability) caused by power cycles.

[0101] 10A shows the change in frequency deviation when the calibration unit 483 does not calibrate the frequency of the oscillator 41. FIG. －１０ 10B shows a change in frequency deviation when the calibration unit 483 calibrates the frequency of the oscillator 41 when the frequency deviation is equal to or greater than the reference frequency. As shown in FIG. 10B, the data acquisition unit 183 calibrates the frequency of the oscillator 41, so that the frequency deviation can be kept within a certain range.

[0102] [Processing Flow in Data Processing Device 1] Figures 11 to 13 are flowcharts showing the processing flow in the data processing device 1. Figures 11 and 12 show the processing flow from the start to the end of measurement in one measurement period. Figure 13 shows the flow of correction processing for the internal time corresponding to the measurement data.

[0103] 11 starts from the point when all the measurement devices 4 are in a sleep state. As an example, the command creation unit 182 monitors whether the timing to start the measurement devices 4, i.e., whether the measurement period has arrived (S11). When the command creation unit 182 determines that the measurement period has arrived (YES in S11), it selects a measurement device 4 that is within a predetermined range from the position of the ship 100 and is in a sleep state (S12). The command creation unit 182 transmits a start-up command to the selected measurement device 4 (S13).

[0104] In parallel with the operations of S12 and S13, or after the operation of S13, the command creation unit 182 selects a measurement device 4 that is within a predetermined range from the position of the ship 100 and for which synchronization has not been completed (S14). The command creation unit 182 transmits a synchronization command to the selected measurement device 4 (S15).

[0105] When the data acquiring unit 183 receives the internal time of the measurement device 4 from the measurement device 4 that received the synchronization command, the time difference identifying unit 184 identifies a first time difference based on the relationship between the internal time and the absolute time at which the command creating unit 182 transmitted the synchronization command. The time difference identifying unit 184 associates the identified first time difference with the ID of the measurement device 4 and stores them in the storage unit 17 (S16). The time difference identifying unit 184 may not identify the first time difference at this point, and the data acquiring unit 183 may associate the absolute time at which the synchronization command was transmitted with the internal time received from the measurement device 4 and store them in the storage unit 17.

[0106] The command creation unit 182 sends a recording start command to the measuring devices 4 that have received the response data to the synchronization command (S17). The command creation unit 182 refers to the management table stored in the storage unit 17 and determines whether synchronization of all measuring devices 4 has been completed and preparations for recording measurement data are complete (S18). If preparations for all measuring devices 4 have not been completed (NO in S18), the command creation unit 182 repeats the processes from S12 to S17. If preparations for all measuring devices 4 have been completed (YES in S18), the command creation unit 182 notifies the epicenter control unit 181 that preparations for measurement are complete, and the epicenter control unit 181 causes the epicenter 2 to generate seismic waves (S19).

[0107] The epicenter control unit 181 causes the epicenter 2 to generate seismic waves until the final scheduled measurement is completed (NO in S20). When the final scheduled measurement is completed (YES in S20), the epicenter control unit 181 notifies the command creation unit 182 that the measurement has been completed.

[0108] 12 , the command creation unit 182 transmits a recording end command to the measuring device 4 via the data transmission / reception unit 14 (S21). Upon receiving the recording end command, the measuring device 4 stops recording the measurement data and transitions to a standby state. The data acquisition unit 183 acquires multiple pieces of measurement data via the optical communication device 3 (S22). The data acquisition unit 183 stores the acquired multiple pieces of measurement data in the storage unit 17, associating them with the ID of the measuring device 4.

[0109] Furthermore, the data acquiring unit 183 acquires, via the data transmitting / receiving unit 14, the absolute time at the time when the optical communication device 3 requested the internal time from the measurement device 4 in response to the end of measurement, and the internal time acquired by the optical communication device 3 from the measurement device 4 (S23). The time difference identifying unit 184 identifies a second time difference based on the absolute time at the time when the measurement ended acquired by the data acquiring unit 183 and the internal time of the measurement device 4 at that time (S24). The time difference identifying unit 184 associates the identified second time difference with the ID of the measurement device 4 and stores it in the storage unit 17.

[0110] The correction unit 185 then analyzes the plurality of measurement data and corrects the internal times associated with the plurality of measurement data based on the first time difference and the second time difference stored in the storage unit 17 in association with the ID of the common measurement device 4 (S25). The correction unit 185 outputs the measurement results including the plurality of measurement data after correction (S26).

[0111] 13 is a flowchart showing the flow of the measurement data correction process (S25). First, the correction unit 185 selects a measurement device 4 for which measurement data is to be corrected (S31). The method by which the correction unit 185 selects the measurement device 4 is arbitrary, and the correction unit 185 may select the measurement device 4 in order of its ID, for example.

[0112] Next, the correction unit 185 identifies the absolute start time stored in the storage unit 17 in association with the ID of the selected measurement device 4 (S32). The correction unit 185 also identifies the internal start time corresponding to the absolute start time (S33). The correction unit 185 calculates a first time difference based on the absolute start time and the internal start time (S34). As described above, the correction unit 185 may calculate the first time difference based further on the time required for the first acoustic signal to reach the measurement device 4 after the position information acquisition unit 11 transmits the first acoustic signal including the synchronization command.

[0113] The correction unit 185 identifies the end absolute time and the end internal time stored in the storage unit 17 in association with the ID of the selected measurement device 4 (S35, S36). The correction unit 185 calculates a second time difference based on the end absolute time and the end internal time (S37). The correction unit 185 may calculate the second time difference based further on the time required for the first optical signal to arrive at the measurement device 4 after the position information acquisition unit 11 transmits the first optical signal to request the internal time.

[0114] Next, the correction unit 185 calculates the amount of change ΔT per unit time in the difference between the absolute time and the internal time based on the first time difference and the second time difference (S38). Specifically, the correction unit 185 calculates the amount of change ΔT by dividing the difference between the first time difference and the second time difference by the elapsed time from the start absolute time to the end absolute time.

[0115] Next, the correction unit 185 calculates a correction value corresponding to each internal time by multiplying the elapsed time from the internal measurement start time to the internal time at which each piece of measurement data was acquired by a unit amount ΔT (S39). The correction unit 185 calculates a corrected internal time by adding the calculated correction value to the internal time at which each piece of measurement data was acquired. The correction unit 185 corrects the time corresponding to the measurement data by updating the internal time of each piece of measurement data corresponding to the selected measurement device 4 to the corrected internal time (S40). The correction unit 185 may correct the internal time corresponding to all of the measurement data, or may correct the internal time corresponding to a portion of the measurement data necessary for analyzing seismic waves. The correction unit 185 associates the measurement data with the corrected time and stores them in the memory unit 17.

[0116] If the correction of the measurement data of all the measurement devices 4 has not been completed (NO in S41), the correction unit 185 repeats the processes from S31 to S40. If the correction of the measurement data of all the measurement devices 4 has been completed (YES in S41), the correction unit 185 ends the correction process.

[0117] [Effects of Data Processing Device 1] As described above, the data processing device 1 includes a time difference determination unit 184 that determines a first time difference, which is the difference between the internal time and the absolute start time at the start of the measurement period, and a second time difference, which is the difference between the internal time and the absolute end time at the end of the measurement period, and a correction unit 185 that corrects the internal time associated with multiple pieces of measurement data based on the first time difference and the second time difference. By correcting the internal time of the measurement data in this manner, the data processing device 1 can determine the relationship between the seismic waves and the seismic waves detected by the measurement device 4 with high accuracy, with almost no effect on the time required for measurement, even if the measurement device 4 is not connected to the PTP network and cannot recognize the absolute time.

[0118] [First Modification] In the measurement system S described above, the data processing device 1 identifies the first time difference and the second time difference, but the measurement device 4 may identify the first time difference and the second time difference and correct the internal time associated with the measurement data. In this case, the measurement device 4 creates measurement data associated with the corrected internal time (i.e., time that is approximately equal to absolute time), and the data processing device 1 can acquire the measurement data associated with the corrected internal time.

[0119] Fig. 14 is a diagram showing the configuration of a data processing device 1A according to this modified example. Fig. 15 is a diagram showing the configuration of a measurement device 4A according to this modified example. Fig. 16 is a flowchart showing the processing flow in the data processing device 1A according to this modified example. Fig. 17 is a flowchart showing the processing flow in the measurement device 4A according to this modified example.

[0120] First, with reference to Fig. 14, differences in the configuration of the data processing device 1A from the data processing device 1 will be described. The data processing device 1A differs from the data processing device 1 in that it does not have the time difference identification unit 184 and the correction unit 185 that are included in the data processing device 1 shown in Fig. 4. The functions of the units shown in Fig. 14 are equivalent to the functions of the units with the same reference numerals in the data processing device 1 shown in Fig. 4.

[0121] The data acquisition unit 183 acquires measurement data whose internal time has been corrected in the measurement device 4 via the data transmission / reception unit 14. The time associated with the measurement data acquired by the data acquisition unit 183 is almost equal to absolute time, and can therefore be used for analysis as is. Therefore, the data acquisition unit 183 transmits the measurement results, including the acquired measurement data, to the outside via the external communication unit 16.

[0122] Next, differences in the configuration of the measurement device 4A from the measurement device 4 will be described with reference to Fig. 15. The measurement device 4A differs from the measurement device 4 in that it includes a time difference specifying unit 484 and a correction unit 485 in addition to the configuration of the measurement device 4 shown in Fig. 5. The measurement device 4A may further include the calibration unit 483 shown in Fig. 9.

[0123] When the data communication unit 482 receives the synchronization command via the acoustic signal receiving unit 43, the data communication unit 482 notifies the time difference determination unit 484 of the absolute time included in the synchronization command. The time difference determination unit 484 determines the time difference between the internal time at that time and the notified absolute time as the first time difference at the start of measurement. The time difference determination unit 484 stores the determined first time difference in the storage unit 47.

[0124] Furthermore, when the data communication unit 482 acquires the absolute time at which the measurement ended via the acoustic signal receiving unit 43 or the optical signal receiving unit 45, the data communication unit 482 notifies the acquired absolute time to the time difference determining unit 484. The time difference determining unit 484 determines the time difference between the internal time at that time and the notified absolute time as a second time difference at the end of the measurement. The time difference determining unit 484 stores the determined second time difference in the storage unit 47.

[0125] The time difference determining unit 484 can execute processing equivalent to that of the time difference determining unit 184. For example, the time difference determining unit 484 calculates a first time difference based on the difference between the absolute time and the internal time and the propagation time required for the first acoustic signal transmitted from the data processing device 1A to reach the measurement device 4A. The time difference determining unit 484 also calculates a second time difference based on the difference between the absolute time and the internal time and the propagation time required for the first optical signal transmitted from the optical communication device 3 to reach the measurement device 4A.

[0126] The correction unit 485 corrects the internal time at which the sensor 42 outputs the measurement data, based on the first time difference and second time difference identified by the time difference identification unit 484. Similar to the correction unit 185, the correction unit 485 identifies the amount of change per unit time in the difference between the absolute time and the internal time, based on the time difference between the absolute start time, which is the absolute time at the start of measurement, and the absolute end time, which is the absolute time at the end of measurement, and the difference between the first time difference and the second time difference. Next, the correction unit 485 corrects the internal time associated with the multiple measurement data created by the data creation unit 481, based on the identified amount of change. The correction unit 485 notifies the data communication unit 482 of the corrected internal time, associating it with the measurement data.

[0127] The data communication unit 482 transmits the plurality of measurement data associated with the corrected internal time notified by the correction unit 485 to the optical communication device 3 via the optical signal transmission unit 46 .

[0128] The flowchart of the data processing device 1A shown in Fig. 16 shows the flow of processing from the start of measurement in one measurement period to the acquisition of measurement data by the data processing device 1A. The processing from S11 to S20 in the flowchart shown in Fig. 16 differs from the flowchart shown in Fig. 11 in that it does not include the processing of identifying the first time difference (S16), but is otherwise the same.

[0129] The flowchart of the measurement device 4A shown in Figure 17 begins when the data processing device 1 transmits a first acoustic signal including a synchronization command. When the acoustic signal receiving unit 43 receives the first acoustic signal including the synchronization command (S41), the time difference determining unit 484 determines the absolute time included in the synchronization command as the start absolute time (S42). The time difference determining unit 484 also determines the internal time at this time as the start internal time (S43). The time difference determining unit 484 calculates the difference between the start absolute time and the start internal time as the first time difference (S44).

[0130] The measurement device 4A then measures seismic waves (S45), and the data creation unit 481 creates measurement data associated with the internal time output by the oscillator 41. When the final measurement is complete, the optical signal receiving unit 45 receives a first optical signal including absolute time from the optical communication device 3 (S46). The time difference determination unit 484 determines the absolute time included in the first optical signal as the end absolute time (S47). The time difference determination unit 484 also determines the internal time at this point as the end internal time (S48). The time difference determination unit 484 calculates the difference between the end absolute time and the end internal time as the second time difference (S49).

[0131] Next, the correction unit 485 calculates the amount of change ΔT per unit time in the difference between the absolute time and the internal time based on the first time difference and the second time difference (S50). Specifically, the correction unit 485 calculates the amount of change ΔT by dividing the difference between the first time difference and the second time difference by the elapsed time from the start absolute time to the end absolute time.

[0132] Next, the correction unit 485 calculates a correction value corresponding to each internal time by multiplying the elapsed time from the internal measurement start time to the internal time at which each piece of measurement data was acquired by a unit amount ΔT (S51). The correction unit 485 calculates a corrected internal time by adding the calculated correction value to the internal time at which each piece of measurement data was acquired. The correction unit 485 corrects the time corresponding to the measurement data by updating the internal time of each piece of measurement data to the corrected internal time (S52). The correction unit 485 may correct the internal time corresponding to all of the measurement data, or may correct the internal time corresponding to a portion of the measurement data necessary for analyzing seismic waves.

[0133] The correction unit 485 inputs the measurement data associated with the corrected time to the data communication unit 482. The data communication unit 482 transmits the measurement data with the corrected time to the optical communication device 3 via the optical signal transmission unit 46 (S53).

[0134] [Second Modification] Fig. 18 is a diagram showing the configuration of a data processing device 1B according to a second modification. Fig. 19 is a diagram showing the configuration of a measurement device 4B according to the second modification. The second modification differs from the data processing device 1A according to the first modification in that the data processing device 1B has a correction unit 185. Also, while the measurement device 4A has a time difference specifying unit 484 and a correction unit 485 in the first modification, the second modification differs in that the measurement device 4B has a time difference specifying unit 484 but does not have a correction unit 485.

[0135] The time difference determining unit 484 of the measurement device 4B operates in conjunction with the time difference determining unit 484 of the measurement device 4A. That is, the time difference determining unit 484 calculates a first time difference based on the difference between the absolute time and the internal time and the propagation time required for the first acoustic signal transmitted from the data processing device 1A to reach the measurement device 4A. The time difference determining unit 484 also calculates a second time difference based on the difference between the absolute time and the internal time and the propagation time required for the first optical signal transmitted from the optical communication device 3 to reach the measurement device 4A. The time difference determining unit 484 inputs the determined first time difference and second time difference to the data communication unit 482.

[0136] Furthermore, the data creation unit 481 of the measurement device 4B associates the multiple pieces of measurement data output by the sensor 42 with the internal time and inputs the associated data to the data communication unit 482. The data communication unit 482 associates the multiple pieces of measurement data with the internal time and transmits them to the data processing device 1B via the optical signal transmission unit 46, and also transmits the first time difference and second time difference identified by the time difference identification unit 484 to the data processing device 1B.

[0137] In the data processing device 1B, when the data acquisition unit 183 acquires multiple pieces of measurement data and the first and second time differences, the correction unit 185 corrects the internal times associated with each of the multiple pieces of measurement data, similar to the data processing device 1. In this way, it is optional whether the time difference identification unit and the correction unit are provided in the data processing device or the measurement device.

[0138] The present invention has been described above using embodiments, but the technical scope of the present invention is not limited to the scope described in the above embodiments, and various modifications and changes are possible within the scope of the gist of the present invention. For example, all or part of the device can be configured by functionally or physically distributing or integrating in any unit. Furthermore, new embodiments resulting from any combination of multiple embodiments are also included in the embodiments of the present invention. The effects of the new embodiments resulting from the combination also have the effects of the original embodiments.

[0139] REFERENCE SIGNS LIST 1 Data processing device 2 Epicenter 3 Optical communication device 4 Measuring device 11 Position information acquisition unit 12 Acoustic signal transmission unit 13 Acoustic signal reception unit 14 Data transmission / reception unit 15 Absolute time acquisition unit 16 External communication unit 17 Memory unit 18 Control unit 41 Oscillator 42 Sensor 43 Acoustic signal reception unit 44 Acoustic signal transmission unit 45 Optical signal reception unit 46 Optical signal transmission unit 47 Memory unit 48 Control unit 100 Ship 181 Epicenter control unit 182 Command creation unit 183 Data acquisition unit 184 Time difference determination unit 185 Correction unit 481 Data creation unit 482 Data communication unit 483 Calibration unit 484 Time difference determination unit 485 Correction unit

Claims

1. A measurement system comprising: a measuring device installed on the seabed; and a data processing device that analyzes measurement data of seismic waves detected by the measuring device in response to seismic waves emitted from a seismic source toward the seabed from a ship sailing on the sea during a measurement period, wherein the measuring device has an oscillator used for timekeeping; and a measurement data creation unit that creates the multiple pieces of measurement data associated with an internal time kept based on the oscillator, wherein either the measuring device or the data processing device has: (1) a time difference determination unit that determines a first time difference, which is the difference between an absolute start time when the measurement period starts and the internal time associated with the measurement data at the time the measurement period starts, and (2) a second time difference, which is the difference between an absolute end time when the measurement period ends and the internal time associated with the measurement data at the time the measurement period ends; and a correction unit that corrects the internal time associated with the multiple pieces of measurement data based on at least the first time difference and the second time difference.

2. The measurement system of claim 1, wherein the correction unit determines the amount of change in the difference between the absolute time and the internal time per unit time based on the time difference between the start absolute time and the end absolute time and the difference between the first time difference and the second time difference, and corrects the internal time associated with the multiple measurement data based on the determined amount of change.

3. The measurement system described in claim 2, wherein the correction unit determines a difference between the absolute time in the internal time and the internal time by adding to the first time difference a value obtained by multiplying the elapsed time from the starting absolute time to the internal time by the amount of change, and corrects the internal time based on the determined difference.

4. The measurement system of claim 1, wherein the data processing device further has a signal transmitting unit that transmits a first acoustic signal including an absolute time, the measurement device further has an acoustic signal receiving unit that receives the first acoustic signal, and the time difference determining unit determines at least one of the first time difference and the second time difference based on the difference between the absolute time indicated by the first acoustic signal and the internal time at the time the measurement device receives the first acoustic signal.

5. The measurement system described in claim 4, wherein the time difference determination unit determines at least one of the first time difference and the second time difference based on the difference between a time obtained by adding the time required for the first acoustic signal to reach the measurement device to the absolute time indicated by the first acoustic signal and the internal time at the time the measurement device receives the first acoustic signal.

6. The measurement system according to any one of claims 1 to 5, further comprising an optical communication device that emits a first optical signal indicating absolute time to the measurement device underwater, the measurement device further comprising an optical signal receiving unit that receives the first optical signal emitted by the optical communication device, and the time difference determining unit determines at least one of the first time difference and the second time difference based on the difference between the emission time at which the optical communication device emits the first optical signal and the internal time at the time the measurement device receives the first optical signal.

7. The measurement system described in claim 1, wherein the measurement system further comprises an optical communication device that emits a first optical signal indicating absolute time to the measurement device underwater, the data processing device further comprises a signal transmitting unit that transmits a first acoustic signal including control data for controlling the measurement device, the measurement device further comprises an optical signal receiving unit that receives the first optical signal emitted by the optical communication device, and an acoustic signal receiving unit that receives the first acoustic signal, and the time difference determining unit determines the first time difference based on the difference between the absolute time when the signal transmitting unit transmits the first acoustic signal and the internal time at the time when the measurement device receives the first acoustic signal, and determines the second time difference based on the difference between the emission time when the optical communication device emits the first optical signal and the internal time at the time when the measurement device receives the first optical signal.

8. The measurement system of claim 1, wherein the correction unit determines a frequency deviation of the oscillator of the measurement device at the end of the measurement period based on the first time difference and the second time difference, and notifies the measurement device of the determined frequency deviation, and the measurement device further has a calibration unit that calibrates the frequency of the oscillator based on the frequency deviation notified by the data processing device.

9. The measurement system according to claim 1, wherein the measurement system has a plurality of measuring devices installed at different positions, and the data processing device further has a signal transmitting unit that transmits a first acoustic signal including control data for controlling the measuring devices, and a signal receiving unit that receives a second acoustic signal emitted by the measuring device upon receiving the first acoustic signal, and the signal transmitting unit transmits the first acoustic signal including start-up data for starting up the measuring device to each of the plurality of measuring devices, and transmits the first acoustic signal including time data to the measuring device from which the signal receiving unit has received the second acoustic signal including response data to the start-up data.

10. The measurement system described in claim 9, wherein the data processing device is installed on the ship and has a memory unit that stores the positions of each of the multiple measuring devices in association with identification information of the measuring devices, and a position information acquisition unit that acquires position information indicating the position of the ship, and the signal transmission unit transmits the first acoustic signal including the time data to the measuring device within a predetermined range of the position indicated by the position information by referring to the positions of each of the multiple measuring devices stored in the memory unit.

11. The measurement system described in claim 10, wherein the signal transmitting unit transmits the first acoustic signal including recording start data indicating an instruction to start recording measurement data to the measurement device from which the signal receiving unit has received a response to the first acoustic signal including the time data.

12. The measurement system according to claim 11, further comprising a control device that notifies the seismic source that it is in a state in which it can generate the vibration waves after the signal transmitting unit has transmitted the first acoustic signal including the recording start data to the plurality of measuring devices.

13. A data processing device having: a data acquisition unit that acquires a plurality of measurement data indicating seismic waves detected by a measuring device installed on the seabed in response to vibration waves emitted from a seismic source toward the seabed from a ship sailing on the sea during a measurement period, the plurality of measurement data being generated by the measuring device and associated with an internal time kept by the measuring device; a time difference determination unit that determines (1) a first time difference that is the difference between an absolute start time at which the measurement period starts and the internal time associated with the measurement data at the time the measurement period starts, and (2) a second time difference that is the difference between an absolute end time at which the measurement period ends and the internal time associated with the measurement data at the time the measurement period ends; and a correction unit that corrects the internal time associated with the plurality of measurement data based on at least the first time difference and the second time difference.

14. A measuring device that measures seismic waves on the seabed that are generated in response to seismic waves emitted from a seismic source toward the seabed from a ship sailing on the sea during a measurement period, the measuring device having: a data creation unit that creates multiple measurement data associated with internal times measured by the measuring device; a time difference determination unit that determines (1) a first time difference, which is the difference between an absolute start time at which the measurement period starts and the internal time associated with the measurement data at the time the measurement period starts, and (2) a second time difference, which is the difference between an absolute end time at which the measurement period ends and the internal time associated with the measurement data at the time the measurement period ends; and a correction unit that corrects the internal time associated with the multiple measurement data based on at least the first time difference and the second time difference.

15. A data processing method executed by a computer, comprising the steps of: acquiring a plurality of measurement data indicating seismic waves detected by a measuring device installed on the seabed in response to vibration waves emitted from a seismic source toward the seabed from a ship sailing on the sea during a measurement period, the plurality of measurement data being generated by the measuring device and associated with an internal time kept by the measuring device; (1) identifying a first time difference which is the difference between an absolute start time at which the measurement period starts and the internal time associated with the measurement data at the time the measurement period starts, and (2) a second time difference which is the difference between an absolute end time at which the measurement period ends and the internal time associated with the measurement data at the time the measurement period ends; and correcting the internal time associated with the plurality of measurement data based on at least the first time difference and the second time difference.

16. A program for causing a computer to execute the following steps: acquiring a plurality of measurement data indicating seismic waves detected by a measuring device installed on the seabed in response to vibration waves emitted from a seismic source toward the seabed from a ship sailing on the sea during a measurement period, the plurality of measurement data being generated by the measuring device and associated with an internal time kept by the measuring device; (1) identifying a first time difference which is the difference between an absolute start time at which the measurement period starts and the internal time associated with the measurement data at the time the measurement period starts, and (2) a second time difference which is the difference between an absolute end time at which the measurement period ends and the internal time associated with the measurement data at the time the measurement period ends; and correcting the internal time associated with the plurality of measurement data based on at least the first time difference and the second time difference.

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