A method and system for pulse-modulated optical fiber distributed acoustic sensing seismic monitoring

Abstract

The application discloses a kind of pulse modulation optical fiber distributed sound sensing earthquake monitoring method and system, comprising: by optical fiber to each seismic measuring point place transmission linear frequency modulation pulse light wave;Obtain the backscattering power waveform of Rayleigh light wave returned by optical fiber;The backscattering power waveform of Rayleigh light wave returned by optical fiber and the reference backscattering power waveform of Rayleigh light wave returned by optical fiber when there is no earthquake caused environmental vibration are compared, the time offset of waveform at each measuring point at different measuring times is determined;According to the time offset of waveform at each measuring point at different measuring times, the optical fiber stress level at each measuring point at different measuring times is determined;From the optical fiber stress level at each measuring point at different measuring times, extract the measurement data related to earthquake. Improve the precision of earthquake monitoring.

Description

Technical Field

[0001] This invention relates to the field of earthquake monitoring technology, and in particular to a pulse-modulated fiber optic distributed sound sensing earthquake monitoring method and system. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Fiber-optic distributed acoustic sensing (DAS) technology monitors environmental vibrations along the sensing fiber by relying on Rayleigh scattering generated as light waves propagate forward. When a coherent pulse of light propagates along the sensing fiber, the slight non-uniformity of the fiber causes the generation of scattered light waves, much like randomly distributed imperfect mirrors within the fiber. Any disturbance caused by environmental vibrations along the fiber will alter the reflection characteristics of these mirrors, affecting the fiber's properties. Therefore, the DAS system can invert and calculate the duration and spatial location of environmental vibrations from the received backscattered Rayleigh signals.

[0004] However, due to the nonlinear characteristics of Rayleigh scattering intensity caused by stress in sensing optical fibers, directly measuring the Rayleigh scattering signal intensity along the fiber cannot provide a quantitative indicator of the environmental vibration amplitude. Any stress or temperature disturbance affecting the optical fiber can cause a linearly proportional phase change in backscattered Rayleigh scattering. Therefore, the environmental vibration amplitude can be quantitatively analyzed by measuring the phase of Rayleigh scattering.

[0005] In existing DAS systems based on phase demodulation, there are many methods for acquiring Rayleigh scattering phase, the most direct being heterodyne or homodyne coherent detection. However, coherent detection methods in DAS systems based on phase demodulation suffer from several problems. The most serious is that interference of backscattered Rayleigh waves causes signal fading, leading to unreliable phase measurements. This is particularly problematic in long-distance vibration monitoring, where the intensity of backscattered Rayleigh waves weakens at greater distances, resulting in a significant decrease in the signal-to-noise ratio and larger measurement errors. Furthermore, noise signals can cause instability in the phase unwinding process, and the laser itself contains phase noise, further deteriorating the accuracy of environmental vibration measurements by the DAS system.

[0006] Furthermore, the use of fiber optic DAS systems for seismic measurement presents new challenges. The optical instruments, electronic equipment, and vibration sources along the fiber optic cable route—whether man-made or environmental—can all act as noise sources interfering with seismic wave monitoring. The precise location of fiber optic cable laying is difficult to control, making it impossible to optimize fiber optic deployment to ensure that long-distance fibers only sense earthquake-related vibration signals. This exacerbates the interference of various noise sources on seismic wave monitoring. Summary of the Invention

[0007] To address the aforementioned problems, this invention proposes a pulse-modulated fiber optic distributed acoustic sensing earthquake monitoring method and system. This system eliminates the impact of signal power attenuation on earthquake detection sensitivity, maintains a consistently high signal-to-noise ratio at all locations along the long-distance fiber optic cable, allows for observation of earthquake events from multiple perspectives including frequency, wavenumber, and time domains, and more effectively filters out noise interference, thereby improving earthquake monitoring accuracy.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] Firstly, a pulse-modulated fiber-optic distributed acoustic sensing seismic monitoring method is proposed, including:

[0010] Linear frequency modulated pulse light waves are transmitted to each seismic measurement point via optical fiber;

[0011] Obtain the power waveform of the backscattered Rayleigh light wave transmitted through the optical fiber;

[0012] The power waveform of the backscattered Rayleigh light wave transmitted back through the optical fiber is compared with the reference backscattered Rayleigh light wave power waveform transmitted back through the optical fiber when there is no environmental vibration caused by earthquake, and the time offset of the waveform at each measurement point at different measurement times is determined.

[0013] The fiber stress level at each measurement point at different measurement times is determined based on the time offset of the waveform at each measurement point at different measurement times.

[0014] Earthquake-related measurement data were extracted from the fiber stress levels at each measurement point at different measurement times.

[0015] Secondly, a pulse-modulated fiber-optic distributed acoustic sensing seismic monitoring system is proposed, comprising:

[0016] A linear frequency modulated pulse light wave generation module is used to generate linear frequency modulated pulse light waves;

[0017] Optical fiber is used to transmit linear frequency modulated pulsed light waves to various seismic measurement points and to transmit back the backscattered Rayleigh light waves generated by the seismic monitoring unit.

[0018] Multiple seismic monitoring units are deployed at multiple seismic measurement points, which can generate backscattered Rayleigh light waves when propagating linear frequency modulated pulse light waves;

[0019] The processor is used to acquire the backscattered Rayleigh power waveform transmitted through the optical fiber; compare the backscattered Rayleigh power waveform transmitted through the optical fiber with a reference backscattered Rayleigh power waveform transmitted through the optical fiber when there is no environmental vibration caused by an earthquake, and determine the time offset of the waveform at each measurement point at different measurement times; determine the fiber stress level at each measurement point at different measurement times based on the time offset of the waveform at each measurement point at different measurement times; and extract earthquake-related measurement data from the fiber stress level at each measurement point at different measurement times.

[0020] Thirdly, an electronic device is proposed, including a memory and a processor, as well as computer instructions stored in the memory and running on the processor, wherein the computer instructions, when executed by the processor, complete the steps described in a pulse-modulated fiber-optic distributed sound sensing seismic monitoring method.

[0021] Fourthly, a computer-readable storage medium is proposed for storing computer instructions, which, when executed by a processor, complete the steps described in a pulse-modulated fiber-optic distributed sound sensing seismic monitoring method.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] 1. This invention, by sending linear frequency modulated pulsed light waves to each seismic measurement point, introduces a mapping from light wavelength to time into the backscattered Rayleigh light wave waveform. This allows the fiber stress level at each measurement point at different measurement times to be determined by the time offset of the waveform. This fundamentally avoids the signal fading problem that exists in DAS systems based on phase demodulation when measuring earthquakes, so that the seismic detection sensitivity is no longer affected by the fading of the detection signal power. The signal-to-noise ratio of seismic monitoring at various locations along long-distance optical fibers can maintain the same high level, and the system complexity and hardware cost are lower.

[0024] 2. After obtaining the fiber stress levels at each measurement point at each time, this invention can perform statistical analysis on all fiber stress levels. It can observe earthquake events from multiple perspectives, such as the time domain, spatial domain, and wavenumber domain. By filtering the fiber stress levels at each measurement point, noise interference can be effectively removed, thereby improving the accuracy of earthquake monitoring.

[0025] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0026] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments of this application and their descriptions are used to explain this application and do not constitute an undue limitation of this application.

[0027] Figure 1 This is a structural diagram of the apparatus used in the method disclosed in Example 1.

[0028] Wherein: 1. Circulator first port, 2. Circulator second port, 3. Circulator third port, 4. Coupler first port, 5. Coupler second port, 6. Coupler third port. Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0030] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0031] Example 1

[0032] Fiber optic distributed sensing technology is based on scattering processes such as Brillouin scattering, Raman scattering, and Rayleigh scattering that occur when light waves propagate through optical fibers. Each scattering process has unique distributed sensing characteristics. Traditionally, fiber optic distributed sensing technology was mainly used to monitor quasi-static physical quantities, or slowly varying signals, such as ambient temperature. In the last decade, fiber optic distributed sensing technology has begun to be applied to monitor dynamic strain and environmental vibration physical quantities.

[0033] In existing fiber-optic distributed sensing technologies, both optical time-domain reflectometry (OTDR) and optical frequency-domain reflectometry (OFDR) based on Rayleigh scattering can be used for distributed acoustic sensing (DAS) to monitor environmental vibrations within a kHz range along the fiber optic cable. However, OFDR-based DAS systems are more suitable for short-distance, high-spatial-resolution vibration monitoring over distances of several hundred meters. Therefore, OTDR-based DAS systems are more commonly used in distributed fiber-optic seismic monitoring.

[0034] Fiber-optic distributed acoustic sensing technology is a vibration sensing technology that can convert long-distance optical fibers laid underground or on the seabed into single-component seismic detector arrays for earthquake monitoring. By connecting a DAS host to one end of a fiber optic cable that is tens of kilometers long, time-series geomorphic deformation measurement data at various measurement points along the fiber can be acquired, with the spatial interval between each measurement point being only a few meters.

[0035] Fiber optic DAS technology significantly enhances current earthquake monitoring capabilities, making it possible to deploy dense arrays of seismic detectors in previously impractical areas, such as urban or seabed regions. Fiber optic cables can be laid on rugged land or seabed, are less affected by environmental temperature variations, and DAS is a far from in-situ sensing technology; the main unit is located at one end of the fiber, requiring no power supply along the fiber optic cable. Furthermore, fiber optic DAS technology can be applied to existing commercially available fiber optic cables, avoiding the high costs of laying separate long-distance fiber optic cables for earthquake monitoring. A typical spatial sampling density for fiber optic DAS technology is one monitoring point (vibration detector) every 10 meters, with typical sampling frequencies reaching 100–1000 Hz, meaning vibration measurements at all monitoring points along the fiber optic cable are completed every 1–10 ms. Therefore, fiber optic DAS technology has great potential to supplement or even replace current measurement methods that rely on arrays of multiple seismic detectors.

[0036] Fiber-optic distributed acoustic sensing (DAS) technology monitors environmental vibrations along the sensing fiber by relying on Rayleigh scattering generated as light waves propagate forward. When a coherent pulse of light propagates along the sensing fiber, the slight non-uniformity of the fiber causes the generation of scattered light waves, much like randomly distributed imperfect mirrors within the fiber. Any disturbance caused by environmental vibrations along the fiber will alter the reflection characteristics of these mirrors, affecting the fiber's properties. Therefore, the DAS system can invert and calculate the duration and spatial location of environmental vibrations from the received backscattered Rayleigh signals.

[0037] However, due to the nonlinear characteristics of Rayleigh scattering intensity caused by stress in sensing optical fibers, directly measuring the Rayleigh scattering signal intensity along the fiber cannot provide a quantitative indicator of the environmental vibration amplitude. Any stress or temperature disturbance affecting the optical fiber can cause a linearly proportional phase change in backscattered Rayleigh scattering. Therefore, the environmental vibration amplitude can be quantitatively analyzed by measuring the phase of Rayleigh scattering.

[0038] In existing DAS systems based on phase demodulation, there are many methods for acquiring Rayleigh scattering phase, the most direct being heterodyne or homodyne coherent detection. However, coherent detection methods in DAS systems based on phase demodulation suffer from several problems. The most serious is that interference of backscattered Rayleigh waves causes signal fading, leading to unreliable phase measurements, especially in long-distance vibration monitoring. The weakening intensity of backscattered Rayleigh waves at greater distances results in a significant decrease in the signal-to-noise ratio, leading to larger measurement errors. Furthermore, noise signals cause instability in the phase unwinding process, and the laser itself contains phase noise, further deteriorating the accuracy of environmental vibration measurements in DAS systems. To reduce the impact of laser phase noise in backscattered Rayleigh phase measurements, DAS systems based on phase demodulation typically employ narrow-linewidth, high-coherence lasers, which significantly increases the system's emergency costs. Additionally, backscattered light is randomly polarized, requiring the use of expensive polarization diversity detectors or Mach-Zehnder interferometers with 3×3 couplers in DAS systems.

[0039] Furthermore, the use of fiber optic DAS systems for seismic measurement presents new challenges. The optical instruments, electronic equipment, and vibration sources along the fiber optic cable route—whether man-made or environmental—can all act as noise sources interfering with seismic wave monitoring. The precise location of fiber optic cable laying is difficult to control, making it impossible to optimize fiber optic deployment to ensure that long-distance fibers only sense earthquake-related vibration signals. This exacerbates the interference of various noise sources on seismic wave monitoring.

[0040] Therefore, in this embodiment, a pulse-modulated fiber optic distributed sound sensing seismic monitoring method is disclosed, comprising:

[0041] S1: Transmit linear frequency modulated pulse light waves to each seismic measurement point via optical fiber; and acquire the power waveform of the backscattered Rayleigh light waves transmitted back through the optical fiber.

[0042] like Figure 1 As shown, this embodiment generates linear frequency modulated pulse light waves through a linear frequency modulated pulse light wave generation module; the linear frequency modulated pulse light waves generated by the linear frequency modulated pulse light wave generation module are propagated to each seismic measurement point through an optical fiber, and the backscattered Rayleigh light waves at each seismic measurement point are also transmitted back to the processor through an optical fiber.

[0043] Seismic monitoring units are set up at each seismic measurement point. Multiple seismic monitoring units are connected to optical fibers. The optical fibers propagate linear frequency modulated pulse light waves to each seismic monitoring unit. While receiving the linear frequency modulated pulse light waves, each seismic monitoring unit also generates backscattered Rayleigh light waves.

[0044] The backscattered Rayleigh light wave transmitted back through the processor's optical fiber is then used to form the power waveform P of the backscattered Rayleigh light wave transmitted back through the optical fiber. s (t).

[0045] The linear frequency modulated pulsed light wave generation module includes a signal generator, a laser, a semiconductor optical power amplifier, a Raman laser, and a coupler.

[0046] The signal generator is used to generate ramp modulation signals and pulse signals; the ramp modulation signals can drive the laser to emit linearly modulated light waves; the linearly modulated light waves enter the semiconductor optical power amplifier, and the pulse signals can drive the semiconductor optical power amplifier to convert the linearly modulated light waves into frequency-modulated pulse light waves; the Raman laser is used to output Raman effect light waves; both the frequency-modulated pulse light waves and the Raman effect light waves can enter the coupler, which is used to couple the frequency-modulated pulse light waves and the Raman effect light waves to obtain a pulse light wave that combines Raman light waves, which is a linearly frequency-modulated pulse light wave.

[0047] Preferably, the ramp modulation signal and the pulse signal have the same repetition frequency, both being f. r The duration of the high-level pulse signal, i.e., the time width of the pulsed light wave, is τ. p In long-distance earthquake monitoring applications, f r Typically below 1kHz, τ p Typically, it lasts for several hundred ns. When the pulse signal is high, the ramp modulation signal outputs a voltage signal with a fixed slope; when the pulse signal is low, the ramp modulation signal outputs a constant voltage signal, which is usually the lowest voltage value of the ramp modulation signal.

[0048] The laser should possess high coherence, with a linewidth typically within 1 MHz; an external cavity laser (ECL) can be selected. The continuous light wave output from the laser enters a semiconductor optical power amplifier. Driven by a ramp modulation signal, the frequency of the output continuous light wave exhibits linear modulation characteristics. When the ramp modulation signal outputs a constant voltage signal, the frequency of the output continuous light wave remains constant; when the ramp modulation signal outputs a voltage signal with a fixed slope, the frequency of the output continuous light wave changes linearly. Let the frequency modulation bandwidth be B. Correspondingly, the linear modulation slope of the laser output light wave can be expressed as:

[0049] Semiconductor optical power amplifiers are used to suppress in-band coherent noise. The light wave entering the semiconductor optical power amplifier from the laser is a continuous light wave. Driven by a pulse signal, the output light wave of the semiconductor optical power amplifier is converted into a pulsed light wave. When the pulse signal is high, the semiconductor optical power amplifier outputs a linearly modulated light wave; when the pulse signal is low, the semiconductor optical power amplifier does not output a light wave.

[0050] In addition, the linear frequency modulated pulse light wave generation module also includes an isolator, a transmitter erbium-doped fiber amplifier, a transmitter bandpass filter, and a circulator. The frequency modulated pulse light wave output by the semiconductor optical power amplifier can enter the transmitter erbium-doped fiber amplifier for power amplification through the isolator. The power-amplified signal output by the transmitter erbium-doped fiber amplifier enters the transmitter bandpass filter for filtering. The filtered signal can enter the coupler through the circulator and couple with the Raman effect light wave in the coupler.

[0051] Preferably, the isolator is used to isolate the front-end fiber optic devices from the back-end fiber optic devices, preventing reflected and scattered light waves generated by the back-end devices from adversely affecting the front-end devices. The pulsed light wave output by the isolator enters the erbium-doped fiber amplifier at the transmitting end.

[0052] The erbium-doped fiber amplifier at the transmitter is used to amplify the power of the pulsed light wave, improve the signal-to-noise ratio of the system, and the output pulsed light wave enters the bandpass filter at the transmitter.

[0053] The transmitter bandpass filter is used to reduce spontaneous emission noise introduced by the erbium-doped fiber amplifier at the transmitter, thereby improving the system signal-to-noise ratio. The output pulsed light wave enters the circulator through the first port 1. The circulator outputs the pulsed light wave through the second port 2.

[0054] The pulsed light wave output from port 2 of the circulator enters the coupler through port 4 of the coupler.

[0055] The Raman laser outputs Raman effect light waves, which enter the coupler through the second port 5 of the coupler to realize distributed power amplification of Raman scattering. This can compensate for the power attenuation of scattered light waves caused by loss of sensing fiber and improve the signal-to-noise ratio of the system.

[0056] After the coupler couples the two light waves, it obtains a pulsed light wave that combines the Raman light wave. This pulsed light wave that combines the Raman light wave is a linear frequency modulated pulsed light wave. The linear frequency modulated pulsed light wave enters the sensing fiber through the third port 6 of the coupler.

[0057] The sensing fiber can be a long-distance communication fiber already laid underground or on the seabed, typically spanning tens of kilometers. The length of the sensing fiber is denoted as L. s To address the issue that each seismic monitoring unit at various seismic measurement points along the optical fiber only has single-component seismic wave observation capabilities, the sensing optical fiber can be laid in a polygonal layout such as quadrilateral, pentagonal, or hexagonal. A linear frequency-modulated pulsed light wave incorporating Raman light propagates along the forward direction of the sensing optical fiber, while simultaneously, backscattered Rayleigh light waves are generated at each seismic monitoring unit at each measurement point.

[0058] The backscattered Rayleigh light wave re-enters the third port 6 of the coupler, and then enters the second port 2 of the circulator. The circulator outputs the received backscattered Rayleigh light wave from the third port 3 of the circulator, which then enters the erbium-doped fiber amplifier and the bandpass filter at the receiving end in sequence.

[0059] The erbium-doped fiber amplifier at the receiver is used to amplify the power of the backscattered Rayleigh light wave and improve the system signal-to-noise ratio.

[0060] The receiver bandpass filter is used to reduce the spontaneous emission noise introduced by the erbium-doped fiber amplifier at the receiver, improve the system signal-to-noise ratio, and allow the output pulsed light wave to enter the photodetector.

[0061] The photodetector converts the received backscattered Rayleigh light waves into analog electrical signals, which are then sent to the analog-to-digital converter. The operating bandwidth of the photodetector is at least twice the frequency modulation bandwidth of the light wave, i.e., the operating bandwidth of the photodetector is ≥ B.

[0062] An analog-to-digital converter (ADC) converts analog electrical signals into digital signals, which are then sent to a processor. The operating bandwidth of an ADC must be at least twice the bandwidth of optical frequency modulation, i.e., the operating bandwidth of an ADC is ≥2B.

[0063] S2: Compare the backscattered Rayleigh light wave power waveform transmitted back through the optical fiber with the reference backscattered Rayleigh light wave power waveform transmitted through the optical fiber when there is no environmental vibration caused by an earthquake, and determine the time offset of the waveform at each measurement point at different measurement times.

[0064] In a single probe pulse light wave, changing the instantaneous frequency of the light wave generates a linearly frequency-modulated (LFM) pulse light wave. When the LFM pulse light wave propagates along the optical fiber, it introduces a wavelength-to-time mapping into the backscattered Rayleigh light wave waveform. If there is environmental vibration near the optical fiber, the wavelength of the scattered light wave generated at the vibration location will shift proportionally to the intensity of the environmental vibration. When there is no environmental vibration, the waveform of the backscattered Rayleigh light wave power over time caused by the LFM pulse light wave propagating along the optical fiber is denoted as P0(t), where t represents time. When there is environmental vibration at a certain location in the optical fiber, the backscattered Rayleigh light wave generated at that location experiences a wavelength shift, corresponding to a time shift proportional to the intensity of the environmental vibration in the waveform of the backscattered Rayleigh light wave power over time. This waveform is denoted as P1(t). Compared to P0(t), at the sensing fiber location where there is no environmental vibration, the waveforms of P1(t) and P0(t) are theoretically completely identical; at the fiber location where there is environmental vibration, the waveforms of P1(t) and P0(t) have a time offset, and the amount of time offset has a linear relationship with the intensity of environmental vibration.

[0065] In this embodiment, the backscattered Rayleigh light wave transmitted back through the optical fiber by the processor is time-series data. In order to realize the time delay calculation of the power waveform of the backscattered Rayleigh light wave at each measurement point at different measurement times, this embodiment also transmits linear frequency modulated pulse light waves to each earthquake measurement point through optical fiber when there is no environmental vibration caused by earthquake.

[0066] It also acquires multiple backscattered Rayleigh light waves transmitted back through the optical fiber when there is no environmental vibration caused by an earthquake.

[0067] When there is no environmental vibration caused by an earthquake, the average of multiple backscattered Rayleigh waves transmitted back through the optical fiber is taken to obtain the reference backscattered Rayleigh wave power waveform when there is no environmental vibration caused by an earthquake, denoted as P. r (t).

[0068] The processor acquires the power waveform P of the backscattered Rayleigh light wave transmitted through the optical fiber. s After (t), the power waveform P of the backscattered Rayleigh wave transmitted back through the optical fiber is analyzed using the window shifting method. s (t) and the reference backscattered Rayleigh light wave power waveform P r (t) Divide the time axis into windows, compare the two waveforms in each time window, and determine the time offset of the waveform at each measurement point at each measurement time. Specifically:

[0069] Calculate P sequentially along the time axis using a fixed time window as the unit. r (t) and P s (t) The time offset of the waveform in each time window. The time window is usually set to be the same size as the duration of a single probe pulse, i.e., equal to the time width of the frequency-modulated pulse, and is denoted as τ. p .

[0070] By using a time window of the same length as the duration of a single detection pulse, it is possible to obtain the backscattered Rayleigh light waves at all seismic measurement points by sending a single pulse into the optical fiber. The time for sending the pulse is the measurement time; that is, the waveform within a single time window can contain the backscattered Rayleigh light wave power waveforms at all measurement points.

[0071] By comparing two waveforms at different seismic measurement points within the same time window, the time offset of the waveforms at different measurement points at the same measurement time is determined.

[0072] The time offset of the waveform at each measurement point obtained in different time windows is the waveform offset at each measurement point at different measurement times.

[0073] In this embodiment, the number of seismic measurement points is determined based on the ratio of optical fiber length to the spatial resolution of seismic monitoring, wherein the spatial resolution of seismic monitoring is determined based on the time width of the frequency-modulated pulse light wave.

[0074] The number of seismic monitoring units deployed on the optical fiber is the ratio of the fiber length to the spatial resolution of the seismic monitoring, which is determined based on the time width of the frequency-modulated pulse light wave.

[0075] When the time width of the modulated pulse light wave is τ p At that time, the spatial resolution ΔL for earthquake monitoring is: Among them, c n ≈2×10 8 m / s is the propagation speed of light in an optical fiber. Therefore, in a pulse-modulated fiber-optic distributed acoustic sensing seismic monitoring device, the sensing fiber is equivalent to an array of single-component seismic monitoring units, and the number of seismic monitoring units in the array is N. u for At each measurement point, a monitoring unit measures the average stress change over a fiber length of ΔL in real time, thereby achieving seismic monitoring. The measurement frequency f of each monitoring unit... u The upper limit is determined by the pulse signal repetition frequency f. r It is determined that the reciprocal of the measurement frequency of each unit, that is, the measurement period of each unit, must be greater than the round-trip transmission time of the pulse light wave in the optical fiber.

[0076] In long-distance earthquake monitoring applications, f r Typically below 1kHz, the corresponding unit measurement period will not be less than 1ms; the length L of the optical fiber s Typically tens of kilometers, the propagation speed of light in optical fiber is c. n ≈2×10 8 m / s, corresponding to the round-trip propagation time of the pulsed light wave in the optical fiber. It lasts for approximately several hundred μs.

[0077] Since the frequency range of seismic waves is typically 0.01–10 Hz, and considering the data storage capacity requirements, the measurement frequency f of each unit is... u It does not need to be equal to the pulse signal repetition frequency f r Theoretically, as long as f u A frequency of ≥20Hz is sufficient to meet the needs of earthquake monitoring.

[0078] S3: Determine the fiber stress level at each measurement point at different measurement times based on the time offset of the waveform at each measurement point at different measurement times.

[0079] In practice, the fiber stress level at each measurement point is determined based on the time offset of the waveform and the linear relationship between the time offset and the intensity of environmental vibration.

[0080] Among them, the linear relationship between the time offset and the intensity of environmental vibration is a known variable.

[0081] S4: Extract earthquake-related measurement data from the fiber optic stress levels at each measurement point at different measurement times, specifically including:

[0082] S41: Construct a two-dimensional matrix containing time and spatial dimensions by using the fiber stress levels at all measurement points over all times.

[0083] After the processor performs time delay calculations on the acquired backscattered Rayleigh light waves, it obtains a set of two-dimensional matrix measurement data D that reflects the fiber stress level in each unit of the seismic monitoring unit array. s The size of the two-dimensional matrix is ​​N. t ×N u , where N t The time dimension represents the number of measurement points along the time axis for each seismic monitoring unit, and the number of measurement times represents the number of measurement periods. If the data acquisition time is T, then N... t =T×f u N u The two-dimensional matrix represents the spatial dimension, specifically the number of seismic monitoring units within the optical fiber, and thus the number of seismic measurement points. The data in the two-dimensional matrix is ​​time-domain and spatial-domain measurement data. Along the time dimension, the measurement data reflects the fiber stress level at a specific seismic measurement point at different measurement times, allowing us to obtain the amplitude curve of the fiber stress variation over time at that seismic measurement point. Along the spatial dimension, the measurement data reflects the fiber stress level at different seismic measurement points at the same moment, allowing us to obtain the amplitude curve of the stress along the fiber's path along distance at the same moment.

[0084] S42: Extract earthquake-related measurement data from the two-dimensional matrix data.

[0085] Most environmental noise exhibits spatial locality, generating vibration noise only in specific regions of the sensing optical fiber. Therefore, in two-dimensional matrix measurement data, the stress changes in the optical fiber caused by environmental noise show a high correlation along the time axis, but are more distinct along the spatial axis. In contrast, seismic waves, once they arrive, typically affect the entire optical fiber, but their duration is relatively short. Thus, the stress changes in the optical fiber caused by seismic waves show a high correlation along the spatial axis, but are more significant along the time axis. Therefore, based on this characteristic, seismic signals and environmental noise signals can be distinguished from the data itself in the two-dimensional matrix. However, due to the pervasiveness of environmental noise, two-dimensional matrix measurement data always contains a significant amount of noise, resulting in severe overall background noise that still affects the accuracy of seismic wave monitoring.

[0086] This embodiment performs a two-dimensional Fourier transform on the data in the two-dimensional matrix to obtain frequency domain-wavenumber domain measurement data; and uses a two-dimensional linear bandpass-lowpass filter to extract earthquake-related measurement data from the frequency domain-wavenumber domain measurement data.

[0087] Preferably, the frequency domain bandpass range is 0.01–10 Hz, and the wavenumber domain low-pass range is 0–10 Hz. -2 A two-dimensional linear bandpass-lowpass filter of m-1 is used to extract earthquake-related measurement data from frequency domain-wavenumber domain measurement data.

[0088] In practical implementation, the processor will measure the two-dimensional matrix data D s A two-dimensional Fourier transform is performed to obtain the frequency domain-wavenumber domain measurement data FD. s This calculation process is denoted as: FT 2D (D s The frequency domain-wavenumber domain measurement data remains a two-dimensional matrix with a matrix size of N. f ×N k N f Greater than or equal to N t And it is the closest to N t The power of 2, N k Greater than or equal to N u And it is the closest to N u The power of 2. Along the frequency dimension, the frequency domain-wavenumber domain measurement data reflects the spectral curve of fiber stress at a certain location of a seismic monitoring unit; along the wavenumber dimension, the frequency domain-wavenumber domain measurement data reflects the amplitude curve of stress along the sensing fiber in the wavenumber domain at the same measurement time.

[0089] The noise inherent in the optical instruments and electronic equipment of the system is mostly thermal noise, with frequencies typically concentrated in the frequency range below 0.1 Hz. However, it is almost ubiquitous in space and can cover the entire wavenumber range. Seismic waves, on the other hand, typically have a frequency range of 0.01–10 Hz, which is higher and wider than the frequency range of thermal noise. Furthermore, seismic waves typically propagate at speeds of several hundred m / s, exhibiting a distinct spatial distribution and corresponding to a lower wavenumber region. Other man-made mobile vibration sources along the fiber optic cable, such as automobiles, generally have significantly lower vibration propagation speeds than seismic waves, resulting in a wider wavenumber coverage. Therefore, in frequency-wavenumber domain measurement data, seismic waves exhibit more significant characteristics, allowing the system to achieve a better signal-to-noise ratio and thus improve the accuracy of earthquake monitoring.

[0090] In frequency-wavenumber domain measurement data, the range of the frequency dimension is -f. u ~f u The range of wavenumber dimension is For fiber optic stress change signals induced by seismic waves, the frequency range is typically 0.01–10 Hz, and the wavenumber range is typically below 10. -2 m-1. Therefore, a frequency domain bandpass range of 0.01–10 Hz and a wavenumber domain low-pass range of 0–10 Hz can be used. -2 A two-dimensional linear bandpass-lowpass filter of m⁻¹ is used to extract earthquake-related measurement data from frequency domain-wavenumber domain measurement data, further improving the system signal-to-noise ratio and earthquake monitoring accuracy. Using a bandpass filter with a frequency domain bandpass range of 0.01–10 Hz, compared to a frequency domain low-pass filter, it can more effectively filter out low-frequency noise components related to thermal noise.

[0091] This embodiment discloses a pulse-modulated fiber-optic distributed acoustic sensing earthquake monitoring method. After acquiring the backscattered Rayleigh light wave power waveform transmitted back through the fiber, the time offset of the waveform at different measurement points at different measurement times is calculated. Then, the fiber stress level at different measurement points at different measurement times is determined based on the time offset. A two-dimensional matrix containing time and spatial dimensions is constructed using the fiber stress level. A two-dimensional Fourier transform is performed on the data in the two-dimensional matrix to obtain frequency domain-wavenumber domain measurement data. Using a two-dimensional linear bandpass-lowpass filter, earthquake-related measurement data is extracted from the frequency domain-wavenumber domain measurement data. Earthquake events can be observed from multiple perspectives, including the frequency domain, wavenumber domain, and time domain. This method can more effectively filter out noise interference, improve the system signal-to-noise ratio and earthquake monitoring accuracy, and fundamentally avoid the signal fading problem that exists in phase demodulation-based DAS systems when measuring earthquakes. The earthquake detection sensitivity is no longer affected by the fading of the detection signal power. The earthquake monitoring signal-to-noise ratio at various locations along the long-distance sensing fiber can maintain the same high level, and the system complexity and hardware cost are lower.

[0092] Example 2

[0093] In this embodiment, a pulse-modulated fiber optic distributed sound sensing earthquake monitoring system is disclosed, comprising:

[0094] A linear frequency modulated pulse light wave generation module is used to generate linear frequency modulated pulse light waves;

[0095] Optical fiber is used to transmit linear frequency modulated pulsed light waves to various seismic measurement points and to transmit back the backscattered Rayleigh light waves generated by the seismic monitoring unit.

[0096] Multiple seismic monitoring units are deployed at multiple seismic measurement points, which can generate backscattered Rayleigh light waves when propagating linear frequency modulated pulse light waves;

[0097] The processor is used to acquire the backscattered Rayleigh power waveform transmitted through the optical fiber; compare the backscattered Rayleigh power waveform transmitted through the optical fiber with a reference backscattered Rayleigh power waveform transmitted through the optical fiber when there is no environmental vibration caused by an earthquake, and determine the time offset of the waveform at each measurement point at different measurement times; determine the fiber stress level at each measurement point at different measurement times based on the time offset of the waveform at each measurement point at different measurement times; and extract earthquake-related measurement data from the fiber stress level at each measurement point at different measurement times.

[0098] Furthermore, the linear frequency modulated pulsed light wave generation module includes a signal generator, a laser, a semiconductor optical power amplifier, a Raman laser, and a coupler;

[0099] The signal generator is used to generate ramp modulation signals and pulse signals; the ramp modulation signals can drive the laser to emit linearly modulated light waves; the linearly modulated light waves enter the semiconductor optical power amplifier, and the pulse signals can drive the semiconductor optical power amplifier to convert the linearly modulated light waves into frequency-modulated pulse light waves; the Raman laser is used to output Raman effect light waves; both the frequency-modulated pulse light waves and the Raman effect light waves can enter the coupler, which is used to couple the frequency-modulated pulse light waves and the Raman effect light waves to obtain a linearly frequency-modulated pulse light wave, which is a pulse light wave combined with the Raman light wave. This pulse light wave combined with the Raman light wave is a linearly frequency-modulated pulse light wave.

[0100] Example 3

[0101] In this embodiment, an electronic device is disclosed, including a memory and a processor, as well as computer instructions stored in the memory and running on the processor. When the processor executes the computer instructions, it completes the steps described in the pulse-modulated fiber optic distributed sound sensing seismic monitoring method disclosed in Embodiment 1.

[0102] Example 4

[0103] In this embodiment, a computer-readable storage medium is disclosed for storing computer instructions, which, when executed by a processor, complete the steps described in the pulse-modulated fiber-optic distributed sound sensing seismic monitoring method disclosed in Embodiment 1.

[0104] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A pulse-modulated fiber optic distributed acoustic sensing seismic monitoring method, characterized in that, include: Linear frequency modulated pulse light waves are transmitted to each seismic measurement point via optical fiber; Obtain the power waveform of the backscattered Rayleigh light wave transmitted through the optical fiber; The power waveform of the backscattered Rayleigh light wave transmitted back through the optical fiber is compared with the reference backscattered Rayleigh light wave power waveform transmitted back through the optical fiber when there is no environmental vibration caused by earthquake, and the time offset of the waveform at each measurement point at different measurement times is determined. The fiber stress level at each measurement point at different measurement times is determined based on the time offset of the waveform at each measurement point at different measurement times. Earthquake-related measurement data were extracted from the fiber optic stress levels at each measurement point at different measurement times. When there is no environmental vibration caused by an earthquake, the average of multiple backscattered Rayleigh light waves transmitted back through the optical fiber is taken to obtain the reference backscattered Rayleigh light wave power waveform when there is no environmental vibration caused by an earthquake. By using the shifting window method, the power waveform of the backscattered Rayleigh light wave transmitted by the optical fiber and the reference backscattered Rayleigh light wave power waveform transmitted by the optical fiber when there is no environmental vibration caused by earthquake are divided into windows on the time axis. The two waveforms in each time window are compared to determine the time offset of the waveform at each measurement point at each measurement time. A two-dimensional matrix containing both temporal and spatial dimensions is constructed using the fiber stress levels at all measurement points over all times; earthquake-related measurement data is then extracted from the two-dimensional matrix data. A two-dimensional Fourier transform is performed on the data in the two-dimensional matrix to obtain frequency domain-wavenumber domain measurement data; Earthquake-related measurement data are extracted from frequency domain-wavenumber domain measurement data using a two-dimensional linear bandpass-lowpass filter.

2. The pulse-modulated fiber optic distributed acoustic sensing seismic monitoring method as described in claim 1, characterized in that, In the absence of environmental vibrations caused by earthquakes, linear frequency modulated pulse light waves are transmitted to each earthquake measurement point via optical fiber; To obtain multiple backscattered Rayleigh light waves transmitted back by optical fiber when there is no environmental vibration caused by an earthquake.

3. The pulse-modulated fiber optic distributed acoustic sensing seismic monitoring method as described in claim 1, characterized in that, By comparing two waveforms within the same time window at different seismic measurement points, the time offset of the waveforms at different measurement points at the same measurement time can be determined.

4. The pulse-modulated fiber optic distributed acoustic sensing seismic monitoring method as described in claim 1, characterized in that, The number of seismic measurement points is determined by the ratio of optical fiber length to the spatial resolution of seismic monitoring, where the spatial resolution of seismic monitoring is determined by the time width of the frequency-modulated pulse light wave.

5. A pulse-modulated fiber optic distributed acoustic sensing seismic monitoring system, employing the pulse-modulated fiber optic distributed acoustic sensing seismic monitoring method as described in any one of claims 1-4, characterized in that, include: A linear frequency modulated pulse light wave generation module is used to generate linear frequency modulated pulse light waves; Optical fiber is used to transmit linear frequency modulated pulsed light waves to various seismic measurement points and to transmit back the backscattered Rayleigh light waves generated by the seismic monitoring unit. Multiple seismic monitoring units are deployed at multiple seismic measurement points, which can generate backscattered Rayleigh light waves when propagating linear frequency modulated pulse light waves; The processor is used to acquire the backscattered Rayleigh light wave power waveform transmitted through the optical fiber; compare the backscattered Rayleigh light wave power waveform transmitted through the optical fiber with the reference backscattered Rayleigh light wave power waveform transmitted through the optical fiber when there is no environmental vibration caused by earthquake, and determine the time offset of the waveform at each measurement point at different measurement times. Based on the time offset of the waveform at each measurement point at different measurement times, the fiber stress level at each measurement point at different measurement times is determined; earthquake-related measurement data are extracted from the fiber stress levels at each measurement point at different measurement times.

6. An electronic device, characterized in that, It includes a memory and a processor, as well as computer instructions stored in the memory and running on the processor, which, when executed by the processor, complete the steps of the pulse-modulated fiber-optic distributed sound sensing seismic monitoring method according to any one of claims 1-4.

7. A computer-readable storage medium, characterized in that, Used to store computer instructions, which, when executed by a processor, complete the steps of the pulse-modulated fiber optic distributed sound sensing earthquake monitoring method according to any one of claims 1-4.

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