OBN seabed node clock drift correction method

Abstract

The invention discloses an OBN seabed node clock drift correction method. The method is characterized by comprising the following steps: S1, obtaining a theoretical sampling point number M; s2, acquiring an actual sampling point number N; s3, acquiring a sampling interval L = P * N / M, wherein P represents a theoretical sampling interval; and S4, resampling the time-voltage sequence of the node by using the sampling interval L to obtain corrected data. According to the invention, the constant temperature crystal oscillator is used as an internal high-stability clock source, secondary time synchronization is carried out through an external GNSS high-precision time service module, and high precision and reliability of OBN data acquisition time can be realized in combination with a data resampling algorithm.

Description

Technical Field

[0001] The invention belongs to the field of exploration, and in particular relates to an OBN seabed node clock drift correction method. Background Art

[0002] Countries around the world have shifted the focus of oil and gas resource development to the ocean. Conventional offshore seismic exploration usually uses offshore tow cables TS (Tower Streamer), submarine cables OBC (Ocean Bottom Cable) and submarine nodes OBN (Ocean Bottom Node) as the main means. The emergence of ocean bottom node seismic exploration technology not only overcomes the influence of seawater, but also greatly improves the accuracy of exploration data, representing the development direction of marine exploration technology. The ocean bottom node is a multi-component seismometer located on the seabed that can independently collect and record seismic signals. Each OBN node works relatively independently, and the failure of a single acquisition node will not propagate to other nodes, greatly improving the overall reliability of the system. For TS and OBC, the cables and electrical connectors between internal acquisition stations have always been one of the main sources of failure of underwater seismic exploration acquisition equipment. The OBN instrument eliminates the connecting cables between stations, and the electrical connectors are only used on the water surface. Underwater, they are completely isolated from the environment by waterproof and pressure-resistant plugs. This completely solves the reliability problems that may be caused by the connection, simplifies the structure, and expands the environmental tolerance of OBN (for example, Magseis's Z700 can work at a water depth of 700 meters, and Z3000 can reach a water depth of 3000 meters). As for the problem of missing seismic channels caused by the failure of a single acquisition node, it can be solved by high-density acquisition. The seabed node is an independent "detector". It does not require cables to transmit data, gets rid of the constraints of cables, and can be flexibly deployed on the seabed. Due to its own physical characteristics, the positioning is more accurate and the quality of the collected data is higher. Currently, the more commonly used seabed nodes include Magseis's Z series and GeoSpace's OBX series.

[0003] At present, OBN instruments in the world generally use atomic clocks or constant temperature crystal oscillators as high-stability system operation clocks. The advantages of atomic clocks are high clock stability and low power consumption. At present, the clock drift of high-level rubidium clocks is about 7us / day and the power consumption is about 150mW; however, its disadvantages are high price (unit price ≥ $4000) and single supply, which is easily affected by international trade policies.

[0004] Oven-controlled crystal oscillators (OCXOs) use thermostatic bath technology to maintain the temperature stability of the crystal oscillator. The crystal is placed in a thermostatic bath and the temperature in the bath is adjusted to the zero temperature coefficient point of the crystal oscillator to keep the bath basically at a constant temperature, minimizing the change in the oscillator output frequency caused by ambient temperature changes, ensuring stable output frequency. However, oven-controlled crystal oscillators are susceptible to temperature and aging, and frequency offsets may occur, causing clock drift. Summary of the invention

[0005] In view of the problems existing in the background technology, the present invention selects a constant temperature crystal oscillator as the system operating clock for the sake of cost-effectiveness and supply stability. An external GNSS high-precision clock module is connected to the OBN before and after it enters the water, and a clock drift correction algorithm is combined to solve the problem of reduced frequency accuracy of the constant temperature crystal oscillator in the absence of a reference clock for a long time.

[0006] Technical solution:

[0007] A method for correcting clock drift of an OBN seafloor node comprises the following steps:

[0008] S1, obtain the theoretical number of sampling points M;

[0009] S2, obtain the actual number of sampling points N;

[0010] S3, obtaining a sampling interval L = P*N / M, where P represents a theoretical sampling interval, which is determined when designing acquisition parameters and is a known quantity;

[0011] S4. Resample the time-voltage series of the node using a sampling interval L, and obtain the resampled data as the corrected data.

[0012] Preferably, the theoretical number of sampling points M is obtained by the following steps:

[0013] Before and after OBN enters the water, it is connected to an external GNSS high-precision clock module for satellite timing. The secondary timestamps are recorded and the difference between the two is taken to obtain the real time. The real time is divided by the signal sampling interval to obtain the theoretical number of sampling points M.

[0014] Preferably, the OBN is connected to a GNSS high-precision clock module via an external timing module.

[0015] Preferably, the timestamps of the constant temperature crystal oscillator corresponding to the GNSS moments before and after the OBN enters the water are recorded, the difference between the two is taken to obtain the theoretical time, and the theoretical time is divided by the signal sampling interval to obtain the actual number of sampling points N.

[0016] Preferably, the specifications of the constant temperature crystal oscillator are: frequency stability: within ±15ppb; thermal drift: within ±0.5ppb.

[0017] Beneficial Effects of the Invention

[0018] The present invention uses a constant temperature crystal oscillator as the internal clock source because of its low price (much lower than the atomic clock) and stable supply channels. At the same time, it can significantly reduce the manufacturing cost of the submarine OBN. Using a constant temperature crystal oscillator as the internal high-stability clock source, and performing secondary time synchronization through an external GNSS high-precision timing module, combined with a data resampling algorithm, can achieve high accuracy and reliability of OBN data acquisition time. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 OBN system architecture diagram

[0020] Figure 2 Schematic diagram of the process of collecting GNSS timing for OBN underwater

[0021] Figure 3 Schematic diagram of theoretical sampling points in the embodiment

[0022] Figure 4 Schematic diagram of actual sampling points in the embodiment DETAILED DESCRIPTION

[0023] The present invention will be further described below in conjunction with embodiments, but the protection scope of the present invention is not limited thereto:

[0024] like Figure 1 As shown, the circuit system architecture of the OBN instrument in the present invention includes a master control system, a high-precision acquisition and digitization unit, an SD memory card, an external timing module (external GNSS high-precision clock module), a low-temperature drift time reference (high-accuracy constant-temperature crystal oscillator, in this embodiment, model: EWOS8HP-5V0), a three-component detector, a hydrophone, and an internal three-axis acceleration sensor, a temperature and humidity sensor, and an internal battery pack.

[0025] As the master control processor, the low-power ARM microprocessor is responsible for system workflow control and data recording and storage. The system has a built-in low-power attitude measurement unit consisting of a three-axis geomagnetic sensor, an inclination sensor, and a three-axis acceleration sensor, which provides high-precision vector field information for the attitude solution of the acquisition station. The main control system and the peripheral functional modules / test modules adopt a star topology structure to provide communication interfaces and power interfaces for detector signals, hydrophone signals, and other sensor channels that may be added in the future. At the same time, the OBN instrument circuit integration design is equipped with a high-speed data reading channel to realize the return of the acquisition station status and the high-speed reading back of the collected data.

[0026] The auxiliary sensors inside the master control system are all digital interfaces, and the collected data can be directly read through the microprocessor digital interface. The three-component detector and hydrophone need to cooperate with high-precision data acquisition modules and serial communication modules to realize data acquisition, transmission, and storage. The data of various sensors are written into the SD memory card by the master control system in the specified format. The watchdog circuit and high-precision clock inside the master control system provide guarantee for the reliable operation of the master control system.

[0027] The synchronous data acquisition process of the three-component detector requires an external GNSS high-precision clock module to obtain accurate time. When the high-precision acquisition module inside the instrument completes the clock synchronization with the GNSS clock module, all the collected data will have corresponding time. When the instrument loses the GNSS signal after entering the water, the low-temperature drift clock reference (high-precision constant-temperature crystal oscillator) inside the system generates the acquisition time.

[0028] The core functional modules of the OBN instrument are the three-component geophone and hydrophone digital acquisition modules. The acquisition unit consists of a high-resolution, low-noise analog-to-digital conversion channel and surrounding analog signal conditioning and input protection circuits. The analog-to-digital conversion adopts the internationally advanced 32-bit analog-to-digital conversion circuit solution. By optimizing and improving the high-precision digital acquisition technology, the high-precision and large dynamic range acquisition of the analog signal output by the front-end sensor is achieved.

[0029] The master control processor of the OBN instrument can automatically detect the current working status. When abnormal monitoring instrument data, communication failure, storage space abnormality, etc. occur, it can independently analyze the cause of the failure and perform control and emergency treatment. Considering the particularity of the underwater measurement environment, an offline self-test method is used before the instrument is deployed. The test items cover the key parameters of the detector, key indicators of the acquisition system, clock zero bias and temperature drift status, etc. The periodic self-test method is used to achieve online self-test and continuous recording of working conditions with minimal energy loss after the instrument is deployed and all functions are started normally.

[0030] Combination Figure 2 , the OBN seafloor node clock drift correction method is as follows:

[0031] OBN has a built-in high-accuracy constant-temperature crystal oscillator (±15ppb) as an internal high-accuracy clock source;

[0032] Before and after the OBN enters the water, it is connected to an external GNSS high-precision clock module for satellite timing. The secondary timestamp is recorded, and the difference between the two is taken to obtain the real time, which is divided by the signal sampling interval to obtain the theoretical number of sampling points.

[0033] Record the timestamps of the constant temperature crystal oscillator corresponding to the GNSS time before and after the OBN enters the water, take the difference between the two to get the theoretical time, and divide it by the sampling interval to get the actual number of sampling points;

[0034] The actual number of sampling points divided by the theoretical number of sampling points multiplied by the theoretical sampling interval is used as the sampling interval. The actual number of sampling points is resampled by interpolation to obtain the real number of sampling points, thereby realizing the correction of OBN clock drift.

[0035] In a specific example, the theoretical number of sampling points between two synchronization times is M (see Figure 3 ), the actual number of sampling points of OBN nodes is N (see Figure 4 ), p is the theoretical sampling interval. The node time-voltage series is resampled with p*N / M as the sampling interval, so that the number of points in each sequence after resampling is M, and the corrected data is obtained. That is, the correction of OBN time drift is completed.

[0036] The specific embodiments described herein are merely examples of the spirit of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in similar ways, but they will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A method for correcting clock drift of an OBN submarine node, characterized in that It includes the following steps: S1, obtain the theoretical number of sampling points M; S2, obtain the actual number of sampling points N; S3, obtaining a sampling interval L=P*N / M, where P represents a theoretical sampling interval; S4. Resample the time-voltage series of the node using the sampling interval L to obtain corrected data.

2. The method according to claim 1, characterized in that The theoretical number of sampling points M is obtained by the following steps: Before and after OBN enters the water, it is connected to an external GNSS high-precision clock module for satellite timing. The secondary timestamps are recorded and the difference between the two is taken to obtain the real time. The real time is divided by the theoretical sampling interval P to obtain the theoretical number of sampling points M.

3. The method according to claim 2, characterized in that OBN is connected to the GNSS high-precision clock module through an external timing module.

4. The method according to claim 1, characterized in that The number of sampling points N is obtained by the following steps: The timestamps of the constant temperature crystal oscillator corresponding to the GNSS moments before and after the OBN enters the water are recorded, and the difference between the two is taken to obtain the theoretical time. The theoretical time is divided by the signal sampling interval to obtain the actual number of sampling points N.

5. The method according to claim 4, characterized in that The specifications of the constant temperature crystal oscillator are: frequency stability: within ±15ppb; thermal drift: within ±0.5ppb.

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