A device for quickly calibrating acoustic pressure sensitivity of distributed hydrophone cable based on comparison method

Abstract

The present application belongs to the field of optical fiber hydrophone design, and specifically provides a device for quickly calibrating acoustic pressure sensitivity of distributed hydrophone optical cable based on comparison method. The device places the hydrophone optical cable to be measured in a cascade closed cavity, and calibrates it by using the closed cavity comparison method. Since the length of the sensing unit of the distributed optical fiber hydrophone optical cable can reach several meters, using a single large cavity for calibration will lead to the decrease of the stability and uniformity of the sound field, so the present application uses a cascade of multiple small-size closed cavities, which can ensure that the acoustic pressure around the sensing unit of the hydrophone optical cable is equal everywhere, and improve the calibration accuracy. The calibration method uses the closed cavity comparison method, and by measuring the acoustic pressure around the standard hydrophone, the acoustic pressure acting on the optical cable can be quickly obtained, and this method can arbitrarily select the calibration points, making the calibration more flexible. At the same time, with the assistance of the roller, the motor module is used to control the movement of the optical cable, and the continuous calibration of the tens of kilometers of hydrophone optical cable can be quickly completed.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic hydrophones, specifically relating to a rapid calibration device for the acoustic pressure sensitivity of a distributed hydrophone cable based on a comparison method. Background Technology

[0002] Due to their advantages such as ultra-large array networking, ultra-large aperture, high sensitivity, and high packaging reliability, distributed fiber optic hydrophones are widely used in deep-sea exploration, seismic monitoring, oil and gas exploration, and marine hydrological monitoring. Their principle is as follows: a narrow-linewidth laser is emitted into an optical fiber. The backscattered signal from the fiber can be received at the laser incident end. The backscattering characteristics of the fiber are affected by the sound waves surrounding the fiber. Therefore, by detecting changes in the phase information of the backscattered light signal, the sound waves around the fiber can be detected. In practical applications, fiber optic hydrophones require calibration to achieve accurate measurement of underwater acoustic parameters; therefore, sensitivity calibration is crucial. Considering the technical characteristics of distributed fiber optic hydrophones and the form of hydrophone cables, core requirements such as the calibration of long-length sensing units and rapid large-array calibration of hydrophone cables need to be addressed.

[0003] The national standard "Low-Frequency Calibration Method for Acoustic Hydrophones" (GB / T 4300-2017) specifies various calibration methods for traditional piezoelectric hydrophones in the frequency range of 0.01Hz to 3.15kHz, such as the coupled-cavity reciprocity method, piezoelectric compensation method, vibrating liquid column method, and closed-cavity comparison method. However, these methods cannot be used for continuous calibration of next-generation distributed hydrophone optical cables. Furthermore, because the calibration size is limited by the wavelength of the sound wave, the calibration length is short at higher frequencies, making it impossible to rapidly calibrate large-size, long-length sensing units.

[0004] Although a rapid sensitivity calibration device and method for an optical fiber hydrophone array is disclosed in patent CN110006516A, this method cannot accurately calibrate the sound pressure sensitivity of a fixed portion of the optical cable and is not suitable for continuous calibration of distributed optical fiber hydrophones.

[0005] In summary, there is currently no method for rapidly, continuously, and over long-length calibrating the acoustic pressure sensitivity of distributed hydrophones. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a rapid calibration device for the acoustic pressure sensitivity of distributed hydrophone optical cables based on the comparison method, which can calibrate the sensing units of distributed hydrophone optical cables quickly and over long lengths.

[0007] To achieve the above technical objectives, the solution proposed by this invention is:

[0008] A rapid calibration device for the acoustic pressure sensitivity of a distributed hydrophone based on a comparison method specifically includes: a hydrophone to be tested, a liquid tank, clamping devices, multiple sealed cavities, and a motor module; the number of sealed cavities is not less than four, all of which are cascaded and have a total length of not less than 1m, with a clamping device on each side fixed in the liquid tank; both sides of the liquid tank have circular holes and are fitted with elastic flanges; the hydrophone to be tested passes sequentially through the circular hole on one side of the liquid tank, the first clamping device, the cascaded sealed cavities, the second clamping device, and the circular hole on the other side of the liquid tank, and is connected to the motor module; the motor module drives the optical cable to move and determine the calibration position; both the sealed cavities and the liquid tank are filled with liquid;

[0009] The sealed cavity includes a transmitting transducer, a standard hydrophone, a rigid material, and a locking device. The inner wall of the sealed cavity is made of the rigid material, and there is a circular hole on each side of the cavity. The locking device, consisting of two rigid clips, is installed on the circular holes and can retract and lock the hydrophone fiber optic cable under test to prevent sound leakage. The transmitting transducer is installed at the top of the cavity to generate the sound field required for the test. The standard hydrophone is inserted into the cavity from the top to monitor the sound pressure inside the cavity.

[0010] The continuous calibration method for optical cables is as follows:

[0011] (1) Determine the initial calibration position of the hydrophone cable to be tested and set the calibration frequency;

[0012] (2) The same sound wave signal is generated in each sealed cavity;

[0013] (3) By detecting the sound pressure acting on the standard hydrophone, the sound pressure around the hydrophone cable under test is obtained, and the sensing phase difference of the hydrophone cable under test is obtained by a distributed fiber optic sensor, and the sound pressure sensitivity is calculated.

[0014] (4) The motor module pulls the optical cable to a suitable position, pauses briefly for calibration, and then continues to operate after completion;

[0015] Furthermore, the clamping device consists of two rollers and a fixing device, which are used to assist the movement of the optical cable and provide support.

[0016] Furthermore, the motor module is a power unit that controls the mechanical claw through a servo motor assembly to grasp the optical cable and drive the optical cable to move with the assistance of the clamping device.

[0017] Furthermore, the device is calibrated to have a sound frequency range of 0.01 Hz to 2 kHz.

[0018] Furthermore, the size of the sealed cavity needs to be much smaller than the wavelength of the sound wave to ensure a uniform distribution of the sound field within the cavity.

[0019] Furthermore, the calibration method includes:

[0020] (1) Connect all instruments and preheat them, set the calibration frequency, and control the motor module to adjust the hydrophone optical cable to be tested to the beginning.

[0021] (2) Drive the transmitting transducer to generate sound waves in each sealed cavity;

[0022] (3) After detection by the lock-in amplifier, the voltage of the standard hydrophone in each cavity is obtained, and the input of the corresponding signal source is adjusted according to the voltage value of the standard hydrophone in the first sealed cavity, so that the detection voltage of the standard hydrophone in the subsequent sealed cavities is consistent with that of the first cavity, so that the sound pressure in each sealed cavity is the same.

[0023] (4) The sound pressure detected by the standard hydrophone in each sealed cavity is calculated. Since the sound field is uniformly distributed in the cavity, the sound pressure is equal to the sound pressure acting on the hydrophone optical cable under test. The calculation formula is as follows:

[0024]

[0025] Where P is the intracavity sound pressure, U c M is the detection voltage of a standard hydrophone. S The sound pressure sensitivity of a standard hydrophone is expressed in V / Pa.

[0026] (5) Read the sensing phase difference of the distributed optical fiber sensor at this time. The calibration position of the optical cable is recorded, and the sound pressure sensitivity of that section of the optical cable is calculated using the following formula:

[0027]

[0028] Expressed in sound pressure level, M ref If the value is 1 rad / uPa, then the above formula becomes:

[0029]

[0030] in, M(i) represents the sensing phase difference of the optical cable in the calibration area as read by the distributed optical fiber sensor, and M(i) represents the sound pressure sensitivity level of the optical cable segment.

[0031] (6) Stop the signal source output, release the cascaded sealed cavity locking device, control the hydrophone cable under test to move to a suitable position through the motor module, lock it and restart the output of the transmitting transducer;

[0032] (7) Repeat steps (5) to (6) until all the hydrophones under test are calibrated. Record the data, turn off all instruments, and plot the calibration curve using a computer.

[0033] Compared with the prior art, the advantages of this invention are:

[0034] This invention addresses the current lack of a method for continuous, rapid, and long-length calibration of distributed fiber optic hydrophones in the field of fiber optic hydrophones by proposing a rapid calibration device for the sound pressure sensitivity of distributed hydrophone cables based on a comparison method. The device places the hydrophone cable under test in cascaded sealed cavities and calibrates it using a sealed cavity comparison method. Since the sensing units of distributed fiber optic hydrophone cables can reach several meters in length, using a single large cavity during calibration would lead to a decrease in the stability and uniformity of the sound field. Therefore, this invention uses multiple small-sized sealed cavities cascaded together to ensure that the sound pressure around the sensing units of the hydrophone cable is equal everywhere, improving calibration accuracy. The calibration method uses a sealed cavity comparison method. By measuring the sound pressure around a standard hydrophone, the sound pressure acting on the optical cable can be quickly obtained. This method allows for arbitrary selection of calibration points, making calibration more flexible. Simultaneously, with the assistance of rollers, a motor module controls the movement of the optical cable, enabling rapid, segment-by-segment continuous calibration of tens of kilometers of hydrophone cable. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the structure of the rapid sound pressure sensitivity calibration device for distributed hydrophone optical cables based on the comparison method of the present invention.

[0036] Figure 2 This is a schematic diagram of the structure of a sealed cavity;

[0037] Figure 3 This is a structural diagram of the locking device;

[0038] Figure 4 This is a connection control diagram for each unit in the device;

[0039] Figure 5 This is a flowchart of the sound pressure sensitivity calibration method based on the device of the present invention;

[0040] Reference numerals in the attached figures: 1 is the hydrophone cable under test, 2 is the liquid tank, 3 is the clamping device, 4 is the sealed cavity, 5 is the motor module; 401 is the transmitting transducer, 402 is the standard hydrophone, 403 is the rigid material, 404 is the locking device; 10 is the computer, 20 is the signal source, 30 is the power amplifier, 40 is the lock-in amplifier, 50 is the electronic switch, and 60 is the distributed fiber optic sensor. Detailed Implementation

[0041] The present invention will now be described in further detail with reference to the accompanying drawings and specific examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0042] Traditional piezoelectric hydrophones have a sound pressure sensitivity M P It can be represented as:

[0043]

[0044] Where U is the open-circuit voltage at the output of the hydrophone in the sound field, P is the actual sound pressure level acting on the hydrophone, and M... P The unit is (V / Pa).

[0045] To calibrate the sound pressure sensitivity of piezoelectric hydrophones, a closed-cavity comparison method was proposed. The specific procedure involves establishing a sound field within a rigid, sealed cavity filled with liquid using a transmitting transducer. The hydrophone under test and a standard hydrophone (as a reference) are placed in an area with the same sound pressure. By comparing the measured data, the sound pressure sensitivity of the hydrophone under test is calculated using the following formula:

[0046]

[0047] Among them, M X M represents the sound pressure sensitivity of the piezoelectric hydrophone under test. S For the sound pressure sensitivity of a standard hydrophone, U X U is the output voltage of the hydrophone under test. S This is the output voltage of a standard hydrophone;

[0048] However, this traditional closed-cavity comparison method can only calibrate point-type hydrophones, and cannot continuously calibrate distributed fiber optic hydrophone cables, nor can it calibrate the sensing units of a single distributed fiber optic hydrophone cable that is several meters long. This invention, based on the traditional closed-cavity comparison method, innovatively achieves meter-level calibration of a single sensing unit through the cascading of multiple closed cavities. Because each closed cavity is small, the stability and uniformity of the sound field within the cavity are ensured, making the sound pressure equal everywhere within the cavity. By only monitoring the sound pressure of a standard hydrophone, the sound pressure around the cable under test can be quickly obtained, and the sensitivity can then be calculated. Combined with a transmission device, continuous calibration of distributed fiber optic hydrophones with a total sensing length of tens of kilometers can be quickly completed.

[0049] Specific embodiments of the present invention are as follows:

[0050] like Figure 1As shown, the present invention provides a rapid calibration device for the acoustic pressure sensitivity of a distributed hydrophone based on the comparison method, specifically including: the hydrophone to be tested (1), a liquid tank (2), a clamping device (3), a sealed cavity (4), and a motor module (5); wherein, the sealed cavity and supporting devices need to be cascaded in multiple sets, and four sets are used as an example in this embodiment. Four chambers are arranged horizontally and fixed by threaded locks, located in a liquid tank; both the sealed chamber and the liquid tank are filled with degassed water; there are round holes on both sides of the liquid tank, and elastic flanges are installed for locking and sealing; there is a clamping device on each side of the cascaded sealed chamber; the hydrophone optical cable under test passes sequentially through the round hole on one side of the liquid tank, the first clamping device, the cascaded sealed chamber, the second clamping device, and the round hole on the other side of the liquid tank, and is connected to the motor module; the motor module, as the power unit, is controlled by a servo motor assembly to grip the hydrophone optical cable with a mechanical claw, thereby driving the movement of the optical cable; the clamping device consists of two rollers and a fixing device, which assists the movement of the optical cable and provides support after the hydrophone optical cable passes through the middle;

[0051] The structure of the sealed cavity is as follows Figure 2 As shown, it specifically includes a transmitting transducer (401), a standard hydrophone (402), a rigid material (403), and a locking device (404); the inner wall of the sealed cavity is the rigid material, and there is a round hole on each side of the cavity, with a locking device on the round hole, and its structure is as follows. Figure 3 As shown, it consists of two retractable rigid clips. After the hydrophone cable to be tested passes through, it is locked to prevent sound leakage. A transmitting transducer is installed at the top of the cavity to generate the sound field required for the test inside the cavity. A standard hydrophone is inserted into the cavity from the top to monitor the sound pressure inside the cavity.

[0052] The connection control of each unit of the device is as follows: Figure 4 As shown, there are four sets of signal source (20), power amplifier (30), transmitting transducer (401) and standard hydrophone (402); computer (10) connects to and controls signal source (20), which is amplified by power amplifier (30) and drives transmitting transducer (401) to generate sound pressure in sealed cavity; electronic switch (50) switches the signal of standard hydrophone (402), which is transmitted to computer via lock-in amplifier (40); the hydrophone optical cable (1) under test is detected by distributed optical fiber sensor (60) to detect the sensing phase difference; at the same time, computer controls motor module (5) to realize the movement of optical cable;

[0053] To ensure a uniform sound field distribution within the sealed cavity and equal sound pressure around the hydrophone, the size of the sealed cavity must be much smaller than the wavelength of the sound wave. In this embodiment, each sealed cavity has an inner diameter of 70 mm and a length of approximately 250 mm. Four sealed cavities are cascaded, enabling the calibration of a hydrophone sensing unit up to 1 m in length at a time.

[0054] Based on the above-mentioned distributed hydrophone optical cable sound pressure sensitivity calibration device, the following is a detailed description of the sound pressure sensitivity calibration method in this embodiment;

[0055] Unlike piezoelectric hydrophones, the acoustic pressure sensitivity of a phase-sensitive distributed fiber optic hydrophone is defined as the change in the sensing phase of the fiber optic hydrophone caused by the sound field. The ratio of the actual sound pressure P acting on the fiber optic hydrophone to the actual sound pressure P, expressed in radians per Pa (rad / Pa), is:

[0056]

[0057] The sensitivity Mp of the fiber optic hydrophone is usually compared with the reference sound pressure sensitivity Mref, and then the logarithm base 10 is multiplied by 20 to obtain the sound pressure sensitivity level M of the hydrophone, i.e.:

[0058]

[0059] Where M is in dB, M ref The value is usually 1 rad / uPa.

[0060] To calibrate the acoustic pressure sensitivity of a distributed fiber optic hydrophone, the calibration method of this invention is as follows: Figure 5 As shown, the specific steps include:

[0061] (1) Connect all instruments and preheat them, set the calibration frequency, and control the motor module to adjust the hydrophone optical cable to be tested to the beginning.

[0062] (2) The control signal source generates a signal, which is then amplified by a power amplifier to drive the transmitting transducer, so that sound waves are generated in all four sealed cavities.

[0063] (3) Switch the electronic switch, and after detection by the lock-in amplifier, obtain the voltage of the standard hydrophone in each cavity. Based on the voltage value of the standard hydrophone in the first sealed cavity, adjust the input of the corresponding signal source so that the detection voltage of the standard hydrophone in the last three sealed cavities is consistent with that in the first cavity, so that the sound pressure in the four sealed cavities is the same.

[0064] (4) The sound pressure detected by the standard hydrophone in each sealed cavity is calculated. Since the sound field is uniformly distributed in the cavity, the sound pressure is equal to the sound pressure acting on the hydrophone optical cable under test. The calculation formula is as follows:

[0065]

[0066] Where P is the intracavity sound pressure, U c M is the detection voltage of a standard hydrophone. S The sound pressure sensitivity of a standard hydrophone is expressed in V / Pa.

[0067] (5) Read the sensing phase difference of the distributed optical fiber sensor at this time. The calibration position of the optical cable is recorded, and the sound pressure sensitivity of that section of the optical cable is calculated using the following formula:

[0068]

[0069] Expressed in sound pressure level, M ref If the value is 1 rad / uPa, then the above formula becomes:

[0070]

[0071] in, M(i) represents the sensing phase difference of the optical cable in the calibration area as read by the distributed optical fiber sensor, and M(i) represents the sound pressure sensitivity level of the optical cable segment.

[0072] (6) Stop the signal source output, release the cascaded sealed cavity locking device, control the hydrophone cable under test to move to a suitable position through the motor module, lock it and restart the signal source output;

[0073] (7) Repeat steps (5) to (6) until all the hydrophones under test are calibrated. Record the data, turn off all instruments, and plot the calibration curve using a computer.

[0074] In addition, the entire calibration process is computer-controlled, which can automatically adjust parameters and strictly control calibration time, further improving calibration speed.

Claims

1. A rapid calibration device for the acoustic pressure sensitivity of a distributed hydrophone based on a comparison method, specifically comprising: The test hydrophone optical cable includes a liquid tank, clamping devices, multiple sealed cavities, and a motor module. There are at least four sealed cavities, all cascaded together, with a total length of at least 1 meter. Each cavity has a clamping device on one side, fixed within the liquid tank. Both sides of the liquid tank have circular holes with flexible flanges installed. The test hydrophone optical cable passes sequentially through a circular hole on one side of the liquid tank, the first clamping device, the cascaded sealed cavities, the second clamping device, and another circular hole on the other side of the liquid tank, connecting to the motor module. The motor module moves the optical cable to determine the calibration position. Both the sealed cavities and the liquid tank are filled with liquid. The sealed cavity includes a transmitting transducer, a standard hydrophone, a rigid material, and a locking device. The inner wall of the sealed cavity is made of the rigid material, and there is a circular hole on each side of the cavity. The locking device, consisting of two rigid clips, is installed on the circular holes and can retract and lock the hydrophone fiber optic cable under test to prevent sound leakage. The transmitting transducer is installed at the top of the cavity to generate the sound field required for the test. The standard hydrophone is inserted into the cavity from the top to monitor the sound pressure inside the cavity. The continuous calibration method for optical cables is as follows: (1) Determine the initial calibration position of the hydrophone cable to be tested and set the calibration frequency; (2) The same sound wave signal is generated in each sealed cavity; (3) The sound pressure around the hydrophone under test is obtained by detecting the sound pressure acting on the standard hydrophone, and the sensing phase difference of the hydrophone under test is obtained by using a distributed fiber optic sensor to calculate the sound pressure sensitivity. (4) The motor module pulls the optical cable to a suitable position, pauses briefly and calibrates, and then continues to operate.

2. The rapid calibration device for the acoustic pressure sensitivity of a distributed hydrophone optical cable based on the comparison method as described in claim 1, characterized in that, The clamping device consists of two rollers and a fixing device, which is used to assist the movement of the optical cable and provide support.

3. The rapid calibration device for the acoustic pressure sensitivity of a distributed hydrophone optical cable based on the comparison method as described in claim 1, characterized in that, The motor module is a power unit that controls the mechanical claw through a servo motor assembly to grasp the optical cable and drive the optical cable to move with the assistance of the clamping device.

4. The rapid calibration device for the acoustic pressure sensitivity of a distributed hydrophone optical cable based on the comparison method as described in claim 1, characterized in that, The device is calibrated to have a sound frequency range of 0.01 Hz to 2 kHz.

5. The rapid calibration device for the acoustic pressure sensitivity of a distributed hydrophone optical cable based on the comparison method as described in claim 1, characterized in that, The size of the sealed cavity is much smaller than the wavelength of the sound wave to ensure a uniform distribution of the sound field within the cavity.

6. The rapid calibration device for the acoustic pressure sensitivity of a distributed hydrophone optical cable based on the comparison method as described in claim 1, characterized in that, The calibration methods include: (1) Connect all instruments and preheat them, set the calibration frequency, and control the motor module to adjust the hydrophone optical cable to be tested to the beginning. (2) Drive the transmitting transducer to generate sound waves in each sealed cavity; (3) After detection by the lock-in amplifier, the voltage of the standard hydrophone in each cavity is obtained, and the input of the corresponding signal source is adjusted according to the voltage value of the standard hydrophone in the first sealed cavity, so that the detection voltage of the standard hydrophone in the subsequent sealed cavities is consistent with that of the first cavity, so that the sound pressure in each sealed cavity is the same. (4) The sound pressure detected by the standard hydrophone in each sealed cavity is calculated. Since the sound field is uniformly distributed in the cavity, the sound pressure is equal to the sound pressure acting on the hydrophone optical cable under test. The calculation formula is as follows: ； Where P is the intracavity sound pressure, U c M is the detection voltage of a standard hydrophone. S The sound pressure sensitivity of a standard hydrophone is expressed in V / Pa. (5) Read the sensing phase difference Δφ(i) of the distributed optical fiber sensor at this time, record the calibration position of the optical cable, and calculate the sound pressure sensitivity of this section of the optical cable. Its formula is: ； Using sound pressure level sensitivity, the above formula becomes: ； Where Δφ(i) is the sensing phase difference of the optical cable in the calibration area read by the distributed optical fiber sensor, and M(i) is the sound pressure sensitivity level of the optical cable segment. (6) Stop the signal source output, release the cascaded sealed cavity locking device, control the hydrophone cable under test to move to a suitable position through the motor module, lock it and restart the output of the transmitting transducer; (7) Repeat steps (5) to (6) until all the hydrophones under test are calibrated. Record the data, turn off all instruments, and plot the calibration curve using a computer.

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