A marine controlled source electromagnetic detection timing and time synchronization system and its working process

Abstract

The present invention discloses a marine controlled source electromagnetic detection timing and time synchronization system and its workflow, comprising: a GNSS module, an NTP server module, a photoelectric conversion module, a switch, a PTP master clock module, a PTP slave clock module, an electromagnetic field acquisition system, an MCU, and a network data acquisition card; the NTP server module is connected to the output end of the GNSS module and synchronization is achieved through the rising edge of the PPS; the PTP master clock module is used for synchronization between the PTP master clock module and the PTP slave clock module; the PTP slave clock module synchronizes the MCU; the electromagnetic field acquisition system is connected to the TTL and serial port output ports of the PTP master clock module and synchronizes with the PTP master clock module. The present invention can perform regular sampling of the time drift of the underwater timing system, obtain its cumulative drift at different times, establish a nonlinear drift model, and use it for time calibration of the acquisition point.

Description

Technical Field

[0001] The present invention relates to the technical field of marine geophysical exploration, and in particular to a marine controlled-source electromagnetic detection timing and time synchronization system and its workflow. Background Art

[0002] The exploration and development of marine oil and gas resources has become a crucial component of global energy strategies. Among marine geophysical exploration methods, controlled-source electromagnetic (CSE) technology uses artificially induced alternating electromagnetic signals to probe the electrical structure of the seafloor medium. It has a remarkable ability to identify high-resistance anomalies beneath the seafloor. This technology plays an irreplaceable role in the discovery and assessment of submarine oil, gas, and natural gas hydrate resources.

[0003] The main components of a marine controlled-source electromagnetic detection system consist of a transmitting system and a receiving system. The transmitting system generates alternating current signals from a dipole current source towed by a mother ship, while the receiving system consists of multiple signal acquisition units that record electromagnetic field data. The time synchronization requirements of a marine controlled-source electromagnetic detection system are reflected in three aspects:

[0004] Time synchronization between the transmitting system and the receiving system: During the preprocessing of marine controlled source electromagnetic data, the output current data recorded by the transmitting system and the electromagnetic field data recorded by the receiving system need to be normalized to eliminate the influence of energy differences at different frequencies. This requires that the data acquisition modules of the transmitting system and the receiving system record data based on the same time reference (UTC time, Coordinated Universal Time).

[0005] Time synchronization between multiple receiving units in the receiving system: Due to the requirements of marine electromagnetic data processing and inversion, the data acquisition systems of different receiving devices need to record data based on the same time reference (UTC time) for multi-station joint processing and interpretation.

[0006] Time synchronization of different network devices in the launch system: The launch system contains various network devices, such as sensor servers, NAS (Network Attached Storage) servers, optoelectronic cabin servers, network data acquisition cards, deck clients, and network monitoring equipment. These devices perform functions such as logging, storing data, and recording sensor data. Therefore, each node in the network communication topology must be synchronized to the same time base (UTC).

[0007] There are three issues that need to be addressed in the clock synchronization of marine controlled source electromagnetic detection equipment:

[0008] (1) Network delay in long-distance transmission

[0009] Because the launch system is towed underwater for extended periods, the time calibration signal must be transmitted to each underwater network node via communication relay equipment such as optoelectronic conversion modules, optoelectronic composite cables, and switches. This signal transmission is significantly affected by network latency, making it difficult to provide an accurate time reference for underwater network devices. If an NTP (Network Time Protocol) server were used as the time reference for each underwater launch system device, each device could periodically access the NTP server to correct its local time. However, the NTP protocol's requirement for round-trip delay symmetry and its software timestamp mechanism can introduce millisecond-level errors. For example, due to the limitations of the operating system's clock granularity, software timestamps typically exhibit errors ranging from 1 to 15 ms. The time accuracy provided by the NTP server is insufficient to provide an accurate time reference for devices such as the underwater launch system's main control module and data acquisition module, which require precise clocks.

[0010] (2) Nonlinear drift of underwater timing system

[0011] To avoid the potential for time distortion at high-frequency sampling points caused by frequent clock synchronization, the launch system of a marine controlled-source electromagnetic survey (MSEM) system synchronizes its clock with a satellite time signal at the start of operation, then relies on an internal timing system (OCXO, oven-controlled crystal oscillator) to maintain its RTC (real-time clock). Similarly, due to the attenuation characteristics of electromagnetic waves in seawater, each acquisition unit in the acquisition system, after synchronizing with the satellite signal on deck, also relies on its own timing system to maintain its real-time clock when collecting electromagnetic field data on the seabed. Drift correction for the timing system's OCXO is typically performed by linearly interpolating the total drift during operation. However, factors such as crystal oscillator aging, voltage fluctuations, mechanical vibration, and significant external temperature fluctuations generally have a nonlinear effect on clock drift. Conventional linear correction methods cannot meet the required time synchronization accuracy.

[0012] (3) Alignment of sampling start time

[0013] Refer to the highest frequency f of the effective harmonics in the current waveform output by the transmitting system max , set a sampling frequency f in the data acquisition system s If the sampling time is not synchronized with the current emission time, the exact time of current output cannot be determined, which may introduce a period less than (1 / f s ) seconds of alignment error, which will have an adverse effect on subsequent data processing. Summary of the Invention

[0014] The purpose of the present invention is to provide a marine controlled source electromagnetic detection timing and time synchronization system and its workflow to solve the above problems.

[0015] The present invention solves the technical problem by adopting the following technical solutions:

[0016] A marine controlled source electromagnetic detection timing and time synchronization system, comprising: a GNSS module, an NTP server module, a photoelectric conversion module, a switch, a PTP master clock module, a PTP slave clock module, an electromagnetic field acquisition system, an MCU and a network data acquisition card;

[0017] The NTP server module is connected to the output end of the GNSS module, parses the $GPRMC statement input from the serial port, and achieves synchronization through the rising edge of PPS; the NTP server module distributes the message information to each NTP client in the form of a network data packet through a photoelectric conversion module and a switch; the PTP master clock module has a network output port and a PPS and TOD output port, and its network output port is connected to the PTP slave clock module for synchronization between the PTP master clock module and the PTP slave clock module; the PPS output end of the PTP slave clock module is connected to the MCU, and the TOD output port is connected to the MCU serial port input pin to synchronize the MCU; the electromagnetic field acquisition system is connected to the TTL and serial port output ports of the PTP master clock module, and is synchronized with the PTP master clock module in time.

[0018] Furthermore, it also includes OCXO. The MCU is connected to the OCXO, which is used to provide accurate frequency signals to maintain the timing system of underwater equipment; the electromagnetic field acquisition system is equipped with an OCXO inside, and the electromagnetic field acquisition system relies on the OCXO for timing.

[0019] Furthermore, the NTP client includes a deck NTP client and an underwater NTP client, which, combined with the timestamp and time delay information in the network data packet, correct their respective local times to meet the needs of generating log files, recording sensor data, network monitoring, and software running tasks.

[0020] Furthermore, the PTP master clock module uses the rising edge of the PPS output by the GNSS module as a synchronization reference, parses the TOD information contained in its $GPRMC statement, and synchronizes with the PTP slave clock module in the form of network data packets through the optoelectronic conversion module and the switch.

[0021] Further, the method to improve the time resolution of the MCU real-time clock is as follows:

[0022] Set the asynchronous prescaler value in the RTC prescaler to a smaller value and the synchronous prescaler value to a larger value; configure the MCU's external interrupt pin, and after receiving the time synchronization instruction from the host computer, enable the external interrupt. In the interrupt callback function, set the initial value of the RTC's sub-second register according to the value of the synchronous prescaler, parse the TOD data input from the serial port, obtain the second pulse time t0, and write the time (t0+1s) into the internal RTC module on the next PPS rising edge; after completing the time synchronization, turn off the MCU's external interrupt, and the RTC uses the pulse signal provided by OCXO1 as the reference for time synchronization; in subsequent work processes, the time register value of the MCU real-time clock will no longer be modified according to the PTP slave clock module information.

[0023] A working process of a marine controlled source electromagnetic detection timing and time synchronization system includes the following steps:

[0024] Step 1: The GNSS module starts up and searches for navigation satellite signals through the antenna. After locking onto the satellite signal, it outputs an accurate second pulse signal and a fixed-format time message, namely, the time of day (TOD). The second pulse signal and TOD information are transmitted to the NTP server module and the PTP master clock module respectively. The receiving end parses them according to the set time message format.

[0025] Step 2: The NTP server module outputs time information that is synchronized with the GNSS module time signal and meets the accuracy requirements through the network transmission protocol. The deck-side NTP client and the underwater NTP client regularly access the NTP server according to the set cycle to realize the local system time correction of each device.

[0026] Step 3: The PTP master clock module receives and processes the PPS signal and TOD signal input by the GNSS module, and sends them to the PTP slave clock module through the network;

[0027] Step 4: After synchronizing with the PTP master clock module, the PTP slave clock module outputs the corrected PPS signal and TOD information to the MCU and network data acquisition card. The rising edge of the PPS signal output by the PTP slave clock module is used as the external trigger signal. The MCU obtains the time in the TOD message at this time as t0. When the MCU's external trigger pin receives the next PPS rising edge, the time (t0+1s) is written into the time register of the real-time clock module inside the MCU, realizing the synchronization of the MCU's internal real-time clock with the GNSS signal, completing the time synchronization task for the MCU. After synchronization is completed, the MCU's internal real-time clock module relies on its own OCXO for time synchronization.

[0028] Step 5: Use the PPS and TOD signals output by the PTP master clock module to synchronize the timing of each electromagnetic field data acquisition unit in the receiving system in the same way. Each acquisition unit relies on its internal OCXO for timing when collecting electromagnetic field data on the seabed.

[0029] Step 6: Before the launch system starts working, the PPS signal output from the clock module is used as the IGBT drive signal output by the MCU and the synchronization signal for the network data acquisition card to start collecting data. The rising edge of the second pulse will be used as an external trigger signal to start the IGBT drive signal output and the data acquisition device to achieve synchronization of the two tasks. The second pulse corresponds to the full second time t in the TOD signal. start This is the moment when the transmitting system outputs current and starts data acquisition. This moment is the reference moment for data synchronization between the transmitting system and the receiving system.

[0030] Step 7: During the system operation, the time is recalibrated every time a period of time passes. During the i-th time calibration, the MCU is restarted to connect the external interrupt of the PTP slave clock module pulse pin and read the time register of the RTC inside the MCU at the time t mi , read the TOD data at the rising edge of the PPS of the PTP slave clock and record it as t gi , drift Represents the accumulated clock drift of the OCXO at the current moment; read the time t of the real-time clock module inside the electromagnetic field acquisition system in the same way r And the TOD output time t of the PTP master clock module at this moment g , and thus the cumulative drift of the electromagnetic field acquisition system when working underwater is obtained r -t g During the operation, the same receiving device is deployed and recovered multiple times. When the number of recovery times is N, a total of N sets of time-accumulated drift data of the timing system of the device under the working conditions are obtained, which serve as the basic data for establishing the clock drift model of the receiving system.

[0031] Step 8: Perform nonlinear fitting on multiple sets of accumulated drift data obtained during the measurement to obtain the nonlinear clock drift model of the OCXO under this working condition. This model regards the drift error of the OCXO as a discrete time-varying system error; using the differential slope calculation method, ΔT n is the actual drift of the nth sampling point, T(n) is the cumulative drift of the nth sampling point obtained according to the clock drift model, thus we can get: ΔT n= T(n) - T(n-1); Substituting the sampling points into the above formula, the actual drift of each point can be obtained, thereby achieving accurate synchronization of the transmitting system current output, transmitting system data acquisition, receiving system data acquisition and UTC time.

[0032] Beneficial effects of the present invention:

[0033] (1) In response to the different clock accuracy requirements of different modules, different clock synchronization strategies are adopted to solve the network delay problem during long-distance transmission and provide an accurate time reference for the underwater system;

[0034] (2) The modular design of the NTP server module, PTP master clock module, and PTP slave clock module can adjust the synchronization frequency in real time based on the network bandwidth, thus achieving controllable synchronization.

[0035] (3) Make full use of the synchronized second pulse signal and use it as the trigger signal for timing, current output, and data acquisition. In subsequent data processing, the whole second corresponding to the second pulse is used as the starting time to improve the synchronization of the transmission system and the receiving system.

[0036] (4) Through multiple time synchronizations during the operation, the accumulated drift of the MCU timing system and the receiving device timing system at different times is obtained, and a nonlinear clock drift model of the OCXO is established, which can more accurately correct the clock drift of each sampling point. The synchronization of the transmitting system, the receiving system and the GNSS clock signal is achieved, reducing the impact of clock drift on subsequent data processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of the system structure of the present invention;

[0038] Figure 2 This is a flow chart of the present invention for performing clock synchronization on the system MCU and time synchronization between the current output and data acquisition of the transmitting system;

[0039] Figure 3 A schematic diagram of the process of establishing an OCXO nonlinear drift model and performing error correction in the present invention;

[0040] Figure 4 This is a schematic diagram showing how the accumulated clock drift of an OCXO changes over time according to Example 2 of the present invention. DETAILED DESCRIPTION

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0042] Addressing the time accuracy requirements of various modules in a marine controlled-source electromagnetic detection system, this invention provides a marine controlled-source electromagnetic detection timing and time synchronization system. This system utilizes multiple strategies, including GNSS (Global Navigation Satellite System) signals, PTP (Precision Time Protocol), and NTP, to synchronize the time of devices at each node. High-precision time synchronization between surface and underwater systems inevitably consumes a certain amount of network bandwidth. The modular design of the NTP server module, PTP master clock module, and PTP slave clock module in the time synchronization system allows for the frequency of time synchronization to be adjusted based on network communication conditions, thus better utilizing network bandwidth resources. This time synchronization system regularly samples the time drift of the underwater timing system (OCXO), obtains the accumulated drift at different times, and establishes a nonlinear drift model for time calibration at the acquisition point. The pulse-per-second signal generated by the PTP slave clock module in this time synchronization system is used to synchronize the current output and data acquisition modules, reducing sampling errors.

[0043] refer to Figure 1 The present invention provides a marine controlled source electromagnetic detection timing and time synchronization system, which can optimize the time synchronization method of each module, improve the output current frequency accuracy and data sampling accuracy, and correct the data by establishing a crystal oscillator nonlinear drift model to reduce the error of subsequent data processing. The system includes: a GNSS module, an NTP server module, a photoelectric conversion module, a switch, a PTP master clock module, a PTP slave clock module, an electromagnetic field acquisition system, an MCU and a network data acquisition card;

[0044] The NTP server module is connected to the output end of the GNSS module, parses the $GPRMC statement input from the serial port, and achieves synchronization through the rising edge of PPS; the NTP server module distributes the message information to each NTP client in the form of a network data packet through a photoelectric conversion module and a switch; the PTP master clock module has a network output port and a PPS and TOD output port, and its network output port is connected to the PTP slave clock module for synchronization between the PTP master clock module and the PTP slave clock module; the PPS output end of the PTP slave clock module is connected to the MCU, and the TOD output port is connected to the MCU serial port input pin to synchronize the MCU; the electromagnetic field acquisition system is connected to the TTL and serial port output ports of the PTP master clock module, and is synchronized with the PTP master clock module in time.

[0045] The present invention also includes an OCXO, the MCU is connected to the OCXO, the OCXO is used to provide accurate frequency signals to maintain the timing system of the underwater equipment; the electromagnetic field acquisition system is internally provided with the OCXO, and the electromagnetic field acquisition system relies on the OCXO for timing.

[0046] The NTP client includes the deck NTP client and the underwater NTP client. It combines the timestamp, time delay and other information in the network data packet to correct their respective local times to meet the needs of tasks such as generating log files, recording sensor data, network monitoring, and software operation.

[0047] The GNSS module receives and processes GNSS signals from satellites, generates pulses per second (PPS), and time of day (TOD), providing a precise time reference for the system.

[0048] NTP server module provides a common time reference signal for network devices in the system to ensure the consistency of device clocks;

[0049] The PTP master clock module is connected to the output of the GNSS module, corrects its own time through data analysis, sends the time information to the slave clock through the network, and generates PPS and TOD signals synchronized with the GNSS signal.

[0050] The PTP slave clock module is connected to the output of the PTP master clock module through the network to correct its own time. It can output the corrected PPS signal and TOD signal to provide accurate clock signals for the underwater MCU;

[0051] OCXO: Oven-controlled crystal oscillator, provides accurate frequency signals to maintain the timing system of underwater equipment;

[0052] Photoelectric conversion module: Through the conversion between electrical signals and optical signals, it realizes network communication between above-water and underwater equipment, and improves the transmission distance and transmission speed of network communication;

[0053] Switch: used for network data forwarding in LAN;

[0054] Electromagnetic field acquisition system: This is the "receiving system" mentioned above. After being time-synchronized with the GNSS signal, it receives and records the electromagnetic field signal on the seabed and relies on the internal oven-controlled crystal oscillator (OCXO) for timing.

[0055] Deck-side NTP client: Network devices on the deck with local system clocks, including deck host computers, various sensor devices, network-attached storage devices, etc.

[0056] Underwater NTP client: underwater network equipment with local system clock, including underwater sensor data acquisition computers, underwater control computers, etc.

[0057] Network data acquisition card: used to record the output current and voltage data of the transmission system;

[0058] MCU: It is used to receive host computer instructions, output IGBT drive signals, collect and save data, transmit data back, etc. It is the main control unit of the underwater system.

[0059] To further optimize the technical solution, the PTP master clock module uses the rising edge of the PPS output by the GNSS module as the synchronization reference, parses the TOD information contained in its $GPRMC statement, and synchronizes with the PTP slave clock module in the form of network data packets through the photoelectric conversion module and the switch. The synchronization process is as follows: Figure 2 The hardware timestamp Follow_up in PTP protocol messages directly records the arrival and departure times of data packets at the physical layer, eliminating latency interference from the operating system and software protocol stack. The synchronization message Sync and the timestamp message Follow_up are delivered and parsed in a two-step process, compensating for transmission delays. After synchronization through the above process, the PTP slave clock module can output PPS and TOD information in $GPRMC format, synchronized with the PTP master clock module, via TTL level and serial port.

[0060] The following are some methods to further optimize the technical solution and improve the time resolution of the MCU real-time clock:

[0061] Set the asynchronous prescaler value in the RTC prescaler to a smaller value and the synchronous prescaler value to a larger value; configure the MCU's external interrupt pin, and after receiving the time synchronization instruction from the host computer, enable the external interrupt. In the interrupt callback function, set the initial value of the RTC's sub-second register according to the value of the synchronous prescaler, parse the TOD data input from the serial port, obtain the second pulse time t0, and write the time (t0+1s) into the internal RTC module on the next PPS rising edge; after completing the time synchronization, turn off the MCU's external interrupt, and the RTC uses the pulse signal provided by OCXO1 as the reference for time synchronization; in subsequent work processes, the time register value of the MCU real-time clock will no longer be modified according to the PTP slave clock module information.

[0062] The present invention also provides a workflow of a marine controlled source electromagnetic detection timing and time synchronization system, comprising the following steps:

[0063] Step 1: The GNSS module starts up and searches for navigation satellite signals through the antenna. After locking onto the satellite signal, it can output a precise second pulse signal and a fixed-format time message, namely, the time of day (TOD) information. The second pulse signal and TOD information are transmitted to the NTP server module and the PTP master clock module respectively. The receiving end parses them according to the set time message format.

[0064] Step 2: The NTP server module outputs time information that is synchronized with the GNSS module time signal and meets the accuracy requirements through the network transmission protocol. The deck-side NTP client and the underwater NTP client regularly access the NTP server according to the set cycle to correct the local system time of each device. The main functions of each NTP client are generally sensor data storage, log file generation, data monitoring, data backup, etc. The frequency of these operations is generally low (usually in seconds). The time information obtained through the NTP server can meet the time accuracy requirements of these devices.

[0065] Step 3: The PTP master clock module receives and processes the PPS and TOD signals input by the GNSS module and transmits them to the PTP slave clock module over the network. Compared to the NTP protocol, the PTP module generates hardware timestamps at the network interface card (NIC) level, reducing timestamp inaccuracies caused by operating system scheduling and other process interference. Furthermore, the module measures and corrects the dwell time of data packets as they pass through switches or routers, including this information in PTP messages and transmitting it to downstream devices, enabling more accurate calculation of actual network latency.

[0066] Step 4: After synchronization with the PTP master clock module, the PTP slave clock module outputs the corrected PPS signal and TOD information to the MCU and network data acquisition card. These two modules are responsible for outputting the IGBT drive signal and recording the emission current and voltage parameters, respectively, and require a high-precision time reference signal. The clock accuracy provided by the PTP protocol can reach sub-microseconds, which can meet the functional requirements of this part. The rising edge of the PPS signal output by the PTP slave clock module is used as the external trigger signal, and the MCU obtains the time in the TOD message at this time as t0; when the external trigger pin of the MCU receives the next PPS rising edge, the time (t0+1s) is written into the time register of the real-time clock module inside the MCU, realizing the synchronization of the internal real-time clock of the MCU with the GNSS signal, and completing the time synchronization task of the MCU; after the synchronization is completed, the internal real-time clock module of the MCU relies on its own OCXO for time synchronization;

[0067] Step 5: Synchronously, the PPS signal and TOD signal output by the PTP master clock module are used to synchronize the timing of each electromagnetic field data acquisition unit in the receiving system in the same manner. Each acquisition unit relies on the internal OCXO for timing when collecting electromagnetic field data on the seabed.

[0068] Step 6: Before the launch system starts working, the PPS signal output from the clock module is used as the IGBT drive signal output by the MCU and the synchronization signal for the network data acquisition card to start collecting data. The rising edge of the second pulse will be used as an external trigger signal to start the IGBT drive signal output and the data acquisition device to achieve synchronization of the two tasks. The second pulse corresponds to the full second time t in the TOD signal. start This is the moment when the transmitting system outputs current and starts data acquisition. This moment is the reference moment for data synchronization between the transmitting system and the receiving system.

[0069] Step 7: During the system operation, the system will re-calibrate the time every once in a while. This step only records the time difference between the MCU's real-time clock and the GNSS time, that is, the accumulated drift of the timing system, and does not re-time the MCU. In the i-th time calibration, restart the MCU to connect the external interrupt of the PTP slave clock module pulse pin and read the time register of the MCU's internal RTC at time t mi , read the TOD data at the rising edge of the PPS of the PTP slave clock and record it as t gi , drift Represents the accumulated clock drift of the OCXO at the current moment; considering the drift characteristics and stabilization process of the OCXO, the frequency adjustment is more important in the early stage of the operation, so as to more finely characterize the nonlinear characteristics of the crystal oscillator in the early stage. For the receiving system, after it is recovered to the deck, the PPS and TOD signal output terminals of the PTP master clock module are reconnected. The time t of the real-time clock module inside the electromagnetic field acquisition system is read in the same way. r And the TOD output time t of the PTP master clock module at this moment g , from which we can get the cumulative drift t of the electromagnetic field acquisition system when working underwater r -t g During the operation, the same receiving device needs to be deployed and recovered multiple times. When the number of recovery times is N, a total of N sets of time-accumulated drift data of the timing system of the device under the working conditions can be obtained, which serves as the basic data for establishing the clock drift model of the receiving system.

[0070] Step 8: Since the drift characteristics of crystal oscillators of different devices are different, different clock drift models need to be established to correct them separately. A nonlinear fitting is performed on the multiple sets of accumulated drift data obtained by the OCXO during the measurement to obtain the nonlinear clock drift model of the OCXO under this working condition. This model can regard the drift error of the OCXO as a discrete time-varying system error; using the differential slope calculation method, ΔT n is the actual drift of the nth sampling point, T(n) is the cumulative drift of the nth sampling point obtained according to the clock drift model, thus we can get: ΔT n = T(n) - T(n-1); Substituting the sampling points into the above formula, the actual drift of each point can be obtained, thereby achieving accurate synchronization of the transmitting system current output, transmitting system data acquisition, receiving system data acquisition and UTC time.

[0071] The present invention addresses the differentiated clock accuracy requirements of different modules by adopting different clock synchronization strategies, addressing network delay issues during long-distance transmission and providing an accurate time reference for underwater systems. The modular design of the NTP server module, PTP master clock module, and PTP slave clock module allows real-time adjustment of the synchronization frequency based on network bandwidth, achieving controllable synchronization. The synchronized pulse-per-second signal is fully utilized as a trigger signal for timing, current output, and data acquisition. The full second corresponding to the pulse-per-second signal is used as the starting time in subsequent data processing, improving the synchronization of the transmitting and receiving systems. Multiple time synchronizations are performed during operation to obtain the cumulative drift of the MCU timing system and the receiving device timing system at different times. A nonlinear clock drift model for the OCXO is established, enabling more accurate correction of the clock drift at each sampling point. This achieves synchronization between the transmitting and receiving systems and the GNSS clock signal, minimizing the impact of clock drift on subsequent data processing.

[0072] In order to solve the synchronization between the surface system, underwater system, receiving system and UTC time during marine controlled source electromagnetic detection, this embodiment discloses a time synchronization system for marine electromagnetic detection equipment. Figure 1 This is the structural diagram of this system. Figure 1 As shown in the figure, it includes a GNSS module, an NTP server module, an optoelectronic conversion module, a network switch module, a PTP master and slave clock modules, an electromagnetic field acquisition system, an MCU, and a network data acquisition card. The GNSS module receives time signals from navigation satellites via an antenna and outputs synchronized TTL-level PPS and serial statements containing time-of-day (TOD) information in the $GPRMC format. The PPS and TOD signals are input to the NTP server module and the PTP master clock module, respectively.

[0073] The NTP server module is connected to the output of the GNSS module, parses the $GPRMC statements input from the serial port, and synchronizes the time using the rising edge of the PPS. Through optical-to-electrical conversion modules and switches, it distributes time information, delay information, and other information in the form of network packets to each NTP client. Each NTP client uses the timestamp, time delay, and other information in the network packets to adjust its local time to meet the needs of tasks such as generating log files, recording sensor data, network monitoring, and software execution.

[0074] The PTP master clock module has a network output port and a PPS and TOD output port. Its network output port is connected to the PTP slave clock module for synchronization between the PTP master and slave module clocks. The PTP master clock module uses the rising edge of the PPS output by the GNSS module as the synchronization reference, parses the TOD information contained in its $GPRMC statement, and synchronizes with the PTP slave clock module in the form of network data packets through the optoelectronic conversion module and the switch. The synchronization process is as follows: Figure 2 The hardware timestamp Follow_up in PTP protocol messages directly records the arrival and departure times of data packets at the physical layer, eliminating latency interference from the operating system and software protocol stack. The synchronization message Sync and the timestamp message Follow_up are delivered and parsed in a two-step process, compensating for transmission delays. After synchronization through the above process, the PTP slave clock module can output PPS and TOD information in $GPRMC format, synchronized with the PTP master clock module, via TTL level and serial port.

[0075] The underwater MCU model can be selected from STMicroelectronics STM32H7 series chips. The PTP slave clock module's PPS output is connected to the MCU, and the TOD output port is connected to the MCU's serial port input pin. The MCU time synchronization process is as follows: Figure 2 As shown in S1 to S4 in the figure. To maximize the time resolution of the MCU's real-time clock, the asynchronous prescaler value in the RTC's prescaler is set to a smaller value, while the synchronous prescaler value is set to a larger value. The MCU's external interrupt pin is configured. Upon receiving a time synchronization command from the host computer, the external interrupt is enabled. In the interrupt callback function, the initial value of the RTC's sub-second register is set based on the value of the synchronous prescaler. The TOD data input from the serial port is parsed to obtain the second pulse time t0. On the next rising edge of the PPS, the time (t0 + 1s) is written to the internal RTC module. After time synchronization is complete, the MCU's external interrupt is disabled, and the RTC uses the pulse signal provided by the OCXO as the reference for timing. In subsequent operations, the time register value of the MCU's real-time clock is no longer modified based on the PTP slave clock module information.

[0076] The electromagnetic field acquisition system is connected to the TTL and serial output ports of the PTP master clock module. Its time synchronization process is the same as the time synchronization process of the PTP slave clock module to the MCU. After completing the timing, the electromagnetic field acquisition system relies on the internal OCXO for timing and is deployed on the seabed to collect marine electromagnetic field data.

[0077] The synchronization between the transmitting system and the receiving system is as follows: Figure 2After receiving the start command from the host computer, the MCU restarts the external interrupt connected to the PTP slave clock PPS pin, enters the interrupt callback function at the next PPS rising edge, starts the drive signal and data acquisition module, and starts the second pulse time t s As the synchronization moment of the emission waveform output and data acquisition. In the subsequent data processing, in the time series recorded by the electromagnetic acquisition system, t s The moment is used as the starting point of waveform acquisition, which can perform current normalization more accurately.

[0078] In summary, this embodiment utilizes a comprehensive GNSS module, NTP server module, PTP master clock module, and PTP slave clock module to construct a time synchronization system for marine controlled-source electromagnetic exploration. This system addresses issues such as insufficient time synchronization accuracy in the underwater master control system and insufficient time synchronization between the transmitter system and the data acquisition system. Adjusting the time synchronization logic facilitates subsequent data processing.

[0079] Example 2

[0080] This embodiment discloses the establishment process and correction method of the OCXO nonlinear drift model of the transmitting system and receiving system of the marine controlled source electromagnetic time synchronization system based on Example 1. The model establishment process is as follows: Figure 3 shown.

[0081] During operation, the PTP slave clock module maintains constant synchronization with satellite time and can rely on its own high-precision crystal oscillator to maintain time even if the navigation satellite signal is temporarily lost. The underwater MCU, following instructions from the host computer, periodically calculates the accumulated clock drift relative to the standard time output by the PTP slave clock module. This process only saves the current drift and does not write to the MCU's time registers.

[0082] Based on the characteristics of the selected MCU, the RTC timestamp function of the STM32H7 series chip can be used to configure the timestamp event detection pin for rising edge detection. When the rising edge of the second pulse signal generated by the GNSS module is detected, the current time can be saved in the MCU's timestamp register. The interrupt callback function can parse the TOD information input from the serial port to calculate the accumulated drift of the OCXO at the current time.

[0083] After the electromagnetic field acquisition system completes data collection on the seabed and is recovered to the deck, it is connected to the PPS and TOD output ports of the PTP master clock module. According to the above method, its cumulative drift during the working time is calculated as the basic data for establishing the OCXO drift model of the receiving system.

[0084] Figure 4This is an example of the cumulative offset variation of the OCXO under the current working conditions over time. The data is fitted nonlinearly to obtain the OCXO clock drift model. Using the differential slope calculation method, ΔT n is the actual drift of the nth sampling point, T(n) is the cumulative drift of the nth point obtained according to the obtained clock drift model, and the actual drift of each data point during the sampling period can be calculated according to the formula ΔTn = T(n) - T(n-1).

[0085] For the electromagnetic field acquisition system, an OCXO clock drift model is established based on the accumulated drift data collected by each acquisition unit. The drift at the sampling points is corrected in the same manner to achieve precise synchronization between the transmitting and receiving systems. Furthermore, if the electromagnetic field acquisition system runs out of power while operating underwater, the fitted nonlinear model can be used to calculate the corresponding UTC time based on the end time of the electromagnetic field acquisition system's recording, thus resolving the issue of clock synchronization issues in this situation.

[0086] In summary, this embodiment demonstrates the process of constructing a nonlinear time drift model of the OCXO based on the change of the accumulated clock drift over time during the marine controlled source electromagnetic detection operation, and correcting the sampling time based on this model. This can achieve more accurate time synchronization between the transmitting system, the receiving system and the UTC time, and solve the problem of being unable to synchronize time after the receiving device is recovered due to power failure.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A marine controlled source electromagnetic detection timing and time synchronization system, characterized in that: include: GNSS module, NTP server module, photoelectric conversion module, switch, PTP master clock module, PTP slave clock module, electromagnetic field acquisition system, MCU and network data acquisition card; The NTP server module is connected to the output end of the GNSS module, parses the $GPRMC statement input from the serial port, and achieves synchronization through the rising edge of PPS; the NTP server module distributes the message information to each NTP client in the form of a network data packet through a photoelectric conversion module and a switch; the PTP master clock module has a network output port and a PPS and TOD output port, and its network output port is connected to the PTP slave clock module for synchronization between the PTP master clock module and the PTP slave clock module; the PPS output end of the PTP slave clock module is connected to the MCU, and the TOD output port is connected to the MCU serial port input pin to synchronize the MCU; the electromagnetic field acquisition system is connected to the TTL and serial port output ports of the PTP master clock module, and is synchronized with the PTP master clock module in time; It also includes OCXO. The MCU is connected to OCXO1, which is used to provide accurate frequency signals to maintain the timing system of underwater equipment; the electromagnetic field acquisition system is equipped with OCXO2, which relies on OCXO2 for timing.

2. The marine controlled source electromagnetic detection timing and time synchronization system according to claim 1, characterized in that: The NTP client includes the deck NTP client and the underwater NTP client. It combines the timestamp and time delay information in the network data packet to correct their respective local times to meet the needs of generating log files, recording sensor data, network monitoring, and software running tasks.

3. The marine controlled source electromagnetic detection timing and time synchronization system according to claim 2, characterized in that: The PTP master clock module uses the rising edge of the PPS output by the GNSS module as the synchronization reference, parses the TOD information contained in its $GPRMC statement, and synchronizes with the PTP slave clock module in the form of network data packets through the optoelectronic conversion module and the switch.

4. The marine controlled source electromagnetic detection timing and time synchronization system according to claim 3, characterized in that: The methods to improve the time resolution of the MCU real-time clock are as follows: Set the asynchronous prescaler value in the RTC's prescaler to a smaller value and the synchronous prescaler value to a larger value; configure the MCU's external interrupt pin, and after receiving the time synchronization instruction from the host computer, enable the external interrupt. In the interrupt callback function, set the initial value of the RTC's sub-second register according to the value of the synchronous prescaler, parse the TOD data input from the serial port, obtain the second pulse time t0, and write the time t0+1s into the internal RTC module at the next PPS rising edge; after completing the time synchronization, turn off the MCU's external interrupt, and the RTC uses the pulse signal provided by OCXO1 as the reference for time synchronization; in subsequent work processes, the time register value of the MCU real-time clock will no longer be modified according to the PTP slave clock module information.

5. A working process of a marine controlled source electromagnetic detection timing and time synchronization system, characterized in that: The steps include: Step 1: The GNSS module starts up and searches for navigation satellite signals through the antenna. After locking onto the satellite signal, it outputs an accurate second pulse signal and a fixed-format time message, namely, the time of day (TOD). The second pulse signal and TOD information are transmitted to the NTP server module and the PTP master clock module respectively. The receiving end parses them according to the set time message format. Step 2: The NTP server module outputs time information that is synchronized with the GNSS module time signal and meets the accuracy requirements through the network transmission protocol. The deck-side NTP client and the underwater NTP client regularly access the NTP server according to the set cycle to realize the local system time correction of each device. Step 3: The PTP master clock module receives and processes the PPS signal and TOD signal input by the GNSS module, and sends them to the PTP slave clock module through the network; Step 4: After synchronization with the PTP master clock module, the PTP slave clock module outputs the corrected PPS signal and TOD information to the MCU and network data acquisition card. The rising edge of the PPS signal output by the PTP slave clock module is used as the external trigger signal. The MCU obtains the time in the TOD message at this time as t0. When the MCU's external trigger pin receives the next PPS rising edge, the time (t0+1s) is written into the time register of the real-time clock module inside the MCU, realizing the synchronization of the MCU's internal real-time clock with the GNSS signal, completing the time synchronization task for the MCU. After synchronization is completed, the internal real-time clock module of the MCU relies on its own OCXO for time synchronization. Step 5: Use the PPS and TOD signals output by the PTP master clock module to synchronize the timing of each electromagnetic field data acquisition unit in the receiving system in the same way. Each acquisition unit relies on its internal OCXO for timing when collecting electromagnetic field data on the seabed. Step 6: Before the launch system starts working, the PPS signal output from the clock module is used as the IGBT drive signal output by the MCU and the synchronization signal for the network data acquisition card to start collecting data. The rising edge of the second pulse will be used as an external trigger signal to start the IGBT drive signal output and the data acquisition device to achieve synchronization of the two tasks. The second pulse corresponds to the full second time t in the TOD signal. start This is the moment when the transmitting system outputs current and starts data acquisition. This moment is the reference moment for data synchronization between the transmitting system and the receiving system. Step 7: During the system operation, the time is recalibrated every time a period of time passes. During the i-th time calibration, the MCU is restarted to connect the external interrupt of the PTP slave clock module pulse pin and read the time register of the RTC inside the MCU at the time t mi , read the TOD data at the rising edge of the PPS of the PTP slave clock and record it as t gi , drift T i =t mi -t gi Represents the accumulated clock drift of the OCXO at the current moment; read the time t of the real-time clock module inside the electromagnetic field acquisition system in the same way r And the TOD output time t of the PTP master clock module at this moment g , and thus the cumulative drift of the electromagnetic field acquisition system when working underwater is obtained r -t g During the operation, the same receiving device is deployed and recovered multiple times. When the number of recovery times is N, a total of N sets of time-accumulated drift data of the device's timing system when working underwater are obtained, which serve as the basic data for establishing the clock drift model of the receiving system. Step 8: Perform nonlinear fitting on multiple sets of accumulated drift data obtained during the measurement to obtain the nonlinear clock drift model of the OCXO under this working condition. This model regards the drift error of the OCXO as a discrete time-varying system error; using the differential slope calculation method, ΔT n is the actual drift of the nth sampling point, T(n) is the cumulative drift of the nth sampling point obtained according to the clock drift model, thus we can get: ΔT n =T(n)-T(n-1); Substituting the sampling points into the above formula, the actual drift of each point can be obtained, thereby achieving accurate synchronization of the transmitting system current output, transmitting system data acquisition, receiving system data acquisition and UTC time.

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