A direct and coherent hybrid survey system and method for seismic exploration

Abstract

This invention discloses a hybrid direct and coherent seismic exploration system and method. It employs multi-frequency time-delay dual-pulse direct detection to acquire the detection signal, and then uses moving average differential detection to determine the vibration location. Heterodyne coherent detection acquires multi-frequency optical pulse electrical signals, which are then bandpass filtered. The resulting signals of multiple different frequencies are then subjected to I / Q quadrature demodulation. Phase reconstruction is performed based on the phase curves at the determined vibration location to obtain the final vibration phase curve. By using a direct detection structure to initially determine the vibration location, and a coherent detection structure to directly reconstruct the vibration curve based on the obtained location, the detection efficiency is improved. This also enhances the signal-to-noise ratio and sensing distance of the distributed fiber optic vibration sensing system.

Description

Technical Field

[0001] This invention belongs to the field of seismic exploration technology, specifically a direct and coherent hybrid detection system and method for seismic exploration. Background Technology

[0002] Seismic exploration plays a crucial role in oil and gas and mineral resource exploration. The principle is that seismic detectors receive reflected or refracted waves generated in the subsurface medium by active sources (explosives, hydraulic sources, air guns, etc.) or passive sources (earthquakes, etc.). These waves are then interpreted through data inversion to obtain information about the subsurface structure, thus completing energy exploration. However, with my country's ever-increasing energy demand, energy extraction in complex and harsh environments is paramount. Therefore, higher demands are placed on seismic exploration instruments. Traditional seismic detectors cannot simultaneously meet requirements for high sensitivity, low cost, large capacity, and long detection range.

[0003] Distributed fiber optic vibration sensing technology is an emerging sensing technology in the sensor field. Its advantages of being passive, low-cost, resistant to electromagnetic interference, resistant to high temperature and high pressure, and capable of long-term monitoring have led to its widespread attention in perimeter security, power line monitoring, transportation, and other fields. Among them, the Φ-OTDR system receives backscattered Rayleigh light generated in the sensing fiber, demodulates it to obtain amplitude and phase information, and completes the accurate location and vibration information reconstruction of external vibration events.

[0004] Traditional Φ-OTDR system structures include direct detection structures and coherent detection structures. Direct detection structures have advantages such as simple structure, low system cost, low susceptibility to laser noise interference, and high efficiency, but they cannot accurately and directly obtain vibration signals and are not suitable for use in complex environments. Coherent detection structures can perform precise quantitative measurement of vibration signals and have a high signal-to-noise ratio, but problems such as coherent fading noise affect the signal-to-noise ratio and sensing distance, and they also have high requirements for the performance of optical components.

[0005] Therefore, the structural problems of the Φ-OTDR system limit its use in complex seismic exploration environments. Solving problems such as optimizing the system structure or addressing coherent fading noise will promote the application of the Φ-OTDR system in the field of seismic exploration. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a direct and coherent hybrid detection system and method for seismic exploration, which solves the problems of the Φ-OTDR system structure and the problem of coherent fading noise in the detection process.

[0007] This invention is implemented as follows:

[0008] A direct and coherent hybrid detection system for seismic exploration, comprising: a laser, a first coupler, a frequency modulation and frequency shifting unit, an erbium-doped fiber amplifier, a second coupler, a first optical switch, a second optical switch, a delay fiber, a third coupler, a fourth coupler, a circulator, a sensing fiber, a control unit, a photoelectric balance detector, a high-speed acquisition card, and a host computer.

[0009] The laser output is connected to the input of the first coupler, and the output of the first coupler is connected to the input of the frequency modulation and frequency shifting unit and the first optical switch, respectively. The output of the frequency modulation and frequency shifting unit is connected to the input of the erbium-doped fiber amplifier, and the output of the erbium-doped fiber amplifier is connected to the input of the second coupler. The output of the second coupler is connected to the input of the third coupler and the input of the second optical switch, respectively. The output of the second optical switch is connected to the input of the delay fiber, and the output of the delay fiber is connected to the input of the third coupler. The output of the third coupler is connected to the first port of the circulator, the second port of the circulator is connected to the sensing fiber, and the third port of the circulator is connected to the input of the fourth coupler. The output of the second optical switch is connected to the input of the fourth coupler. The output of the fourth coupler is connected to the input of the photoelectric balance detector, the output of the photoelectric balance detector is connected to the input of the high-speed acquisition card, and the output of the high-speed acquisition card is connected to the input of the host computer. The output of the control unit is connected to the control terminal of the first optical switch, the control terminal of the frequency modulation and frequency shifting unit, the control terminal of the second optical switch, and the control terminal of the high-speed acquisition card, respectively.

[0010] Furthermore, the control unit controls the frequency modulation and frequency shifting unit to modulate continuous light into multi-frequency pulse light by controlling the first optical switch to be in the off state and the second optical switch to be in the on state, thereby realizing direct detection of multi-frequency time-delay dual pulses; the control unit controls the frequency modulation and frequency shifting unit to modulate continuous light into multi-frequency pulse light by controlling the first optical switch to be in the on state and the second optical switch to be in the off state, thereby realizing heterodyne coherent detection.

[0011] Furthermore, during the multi-frequency time-delay dual-pulse direct detection, the laser emits continuous light. After passing through the first coupler, all the continuous light enters the frequency modulation and frequency shifting unit controlled by the control unit. The continuous light is modulated into optical pulses composed of multiple frequencies. After being amplified by an erbium-doped fiber amplifier, the optical pulse signal is split into two optical pulses through the second coupler. One optical pulse directly enters the third coupler, and the other optical pulse enters the third coupler through a time-delay fiber. The two optical pulses form a multi-frequency time-delay optical pulse pair in the third coupler. The multi-frequency optical pulse pair enters the sensing fiber through a circulator and interferes with the backscattered Rayleigh light generated in the sensing fiber. The interference light is converted into an electrical signal by a photoelectric balance detector through the fourth coupler. Finally, a high-speed acquisition card is used to acquire the signal and upload it to the host computer for signal processing.

[0012] Furthermore, during heterodyne coherent detection, the laser emits continuous light. After passing through the first coupler, part of the continuous light enters the frequency modulation and shifting unit controlled by the control unit, while the other part of the continuous light enters the fourth coupler as the local oscillator. The continuous light entering the frequency modulation and shifting unit is modulated into multi-frequency optical pulses. After being amplified by an erbium-doped fiber amplifier, the optical pulse signal enters the sensing fiber through the second and third couplers. The backscattered Rayleigh light generated in the sensing fiber couples with the local oscillator at the fourth coupler. Next, it is converted into an electrical signal by a photoelectric balance detector. Finally, a high-speed acquisition card is used to acquire the signal and upload it to the host computer for signal processing.

[0013] Furthermore, the host computer directly performs moving average differential analysis on the electrical signal of the multi-frequency optical pulse directly detected by the multi-frequency time-delay dual pulse to determine the vibration position;

[0014] After the multi-frequency optical pulse electrical signal of heterodyne coherent detection is bandpass filtered, the generated signals of different frequencies are subjected to I / Q quadrature demodulation. The phase is then restored according to the phase curve at the determined vibration position to obtain the final vibration phase curve.

[0015] A hybrid direct and coherent detection method for seismic exploration, comprising:

[0016] Multi-frequency time-delay dual-pulse direct detection is used to acquire the detection signal, and moving average differential is used to determine the vibration location;

[0017] Heterodyne coherent detection is performed to acquire multi-frequency optical pulse electrical signals. After bandpass filtering, the generated signals of different frequencies are subjected to I / Q quadrature demodulation. Based on the phase curves at the determined vibration locations, phase reconstruction is performed to obtain the final vibration phase curve.

[0018] Furthermore, when using multi-frequency time-delay dual-pulse direct detection, the laser emits continuous light. After passing through the first coupler, all the continuous light enters the frequency modulation and frequency shifting unit controlled by the control unit. The continuous light is modulated into optical pulses composed of multiple frequencies. After being amplified by an erbium-doped fiber amplifier, the optical pulse signal is split into two optical pulses through the second coupler. One optical pulse directly enters the third coupler, and the other optical pulse enters the third coupler through a time-delay fiber. The two optical pulses form a multi-frequency time-delay optical pulse pair in the third coupler. The multi-frequency optical pulse pair enters the sensing fiber through a circulator and interferes with the backscattered Rayleigh light generated in the sensing fiber. The interference light is converted into an electrical signal by a photoelectric balance detector after passing through the fourth coupler.

[0019] Furthermore, during heterodyne coherent detection, the laser emits continuous light. After passing through the first coupler, part of the continuous light enters the frequency modulation and shifting unit controlled by the control unit, while the other part of the continuous light enters the fourth coupler as the local oscillator. The continuous light entering the frequency modulation and shifting unit is modulated into multi-frequency optical pulses. After being amplified by the erbium-doped fiber amplifier, the optical pulse signal enters the sensing fiber through the second and third couplers. The backscattered Rayleigh light generated in the sensing fiber couples with the local oscillator at the fourth coupler, and is then converted into an electrical signal by the photoelectric balance detector.

[0020] Compared with the prior art, the beneficial effects of this invention are as follows:

[0021] (1) The vibration location is initially determined by direct detection of the structure, and the vibration curve is directly reconstructed based on the obtained vibration location by coherent detection structure, which improves the detection efficiency. This improves the signal-to-noise ratio and sensing distance of the distributed fiber optic vibration sensing system; (2) Detection is completed by generating multi-frequency pulse light, which more effectively suppresses coherent fading noise and improves the detection performance of the distributed fiber optic vibration sensing system. Attached Figure Description

[0022] Figure 1 This is a structural diagram of a distributed fiber optic vibration sensing system for seismic exploration, which is a direct and coherent hybrid (joint) (novel) detection structure according to an embodiment of the present invention.

[0023] Figure 2 This is a structural diagram of a frequency modulation and frequency shifting unit according to an embodiment of the present invention;

[0024] Figure 3 This is a flowchart of a detection process according to an embodiment of the present invention;

[0025] Figure 4 This is a correlation coefficient diagram of an embodiment of the present invention;

[0026] Figure 5 This is a flowchart of a signal processing method according to an embodiment of the present invention;

[0027] Figure 6 This is a flowchart of I / Q quadrature demodulation according to an embodiment of the present invention. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0029] This invention discloses a direct and coherent hybrid detection system for seismic exploration, comprising: a laser, a first coupler, a frequency modulation and shifting unit, an erbium-doped fiber amplifier, a second coupler, a first optical switch, a second optical switch, a delay fiber, a third coupler, a fourth coupler, a circulator, a sensing fiber, a control unit, a photoelectric balanced detector, a high-speed acquisition card, and a host computer. See also... Figure 1 :

[0030] The laser output is connected to the input of the first coupler, and the output of the first coupler is connected to the input of the frequency modulation and shifting unit and the first optical switch, respectively. The output of the frequency modulation and shifting unit is connected to the input of the erbium-doped fiber amplifier, and the output of the erbium-doped fiber amplifier is connected to the input of the second coupler. The output of the second coupler is connected to the input of the third coupler and the input of the second optical switch, respectively. The output of the second optical switch is connected to the input of the delay fiber, and the output of the delay fiber is connected to the input of the third coupler. The output of the third coupler is connected to the first port of the circulator, the second port of the circulator is connected to the sensing fiber, and the third port of the circulator is connected to the input of the fourth coupler. The output of the second optical switch is connected to the input of the fourth coupler. The output of the fourth coupler is connected to the input of the photoelectric balance detector, the output of the photoelectric balance detector is connected to the input of the high-speed acquisition card, and the output of the high-speed acquisition card is connected to the input of the host computer. The output of the control unit is connected to the control terminal of the first optical switch, the control terminal of the frequency modulation and shifting unit, the control terminal of the second optical switch, and the control terminal of the high-speed acquisition card, respectively.

[0031] like Figure 2 As shown, the frequency modulation and frequency shifting unit includes a phase modulator and an acousto-optic modulator, which then modulate the laser into multi-frequency pulsed light.

[0032] This system enables multi-frequency time-delayed dual-pulse direct detection and heterodyne coherent detection, with switching achieved through optical switches and a control unit. During multi-frequency time-delayed dual-pulse direct detection, the control unit keeps the first optical switch off and the second optical switch on, and controls the frequency modulation and shifting unit to modulate continuous light into multi-frequency pulse light. During heterodyne coherent detection, the control unit keeps the first optical switch on and the second optical switch off, and controls the frequency modulation and shifting unit to modulate continuous light into multi-frequency pulse light. After completing one multi-frequency time-delayed dual-pulse direct detection, the vibration location is initially determined. Finally, the control unit controls heterodyne coherent detection, and the vibration curve is reconstructed based on the determined vibration location. The flowchart is shown below. Figure 3 As shown.

[0033] The process of direct detection using multi-frequency time-delayed dual pulses to initially determine the vibration location is as follows: The laser emits continuous light, which, after passing through the first coupler, enters the frequency modulation and shifting unit controlled by the control unit. The continuous light is modulated into optical pulses composed of multiple frequencies. After being amplified by an erbium-doped fiber amplifier, the optical pulse signal is split into two optical pulses through the second coupler. One optical pulse directly enters the third coupler, while the other enters the third coupler through a time-delayed fiber. The two optical pulses form a multi-frequency time-delayed optical pulse pair in the third coupler. The multi-frequency optical pulse pair then enters the sensing fiber through a circulator and interferes with the backscattered Rayleigh light generated in the sensing fiber. The interference light is converted into an electrical signal by a photoelectric balance detector after passing through the fourth coupler. Finally, a high-speed acquisition card is used to acquire the signal and upload it to the host computer for signal processing.

[0034] The time delay of the multi-frequency optical pulse pair is In the formula L T denoted as the length of the delay fiber, n as the refractive index of the delay fiber, and c as the speed of light.

[0035] The heterodyne coherent detection process is as follows: The laser emits continuous light, which passes through the first coupler. Part of the continuous light enters the frequency modulation and frequency shifting unit controlled by the control unit, while the other part of the continuous light enters the fourth coupler as the local oscillator. The continuous light entering the frequency modulation and frequency shifting unit is modulated into multi-frequency optical pulses. After the optical pulse signal is amplified by the erbium-doped fiber amplifier, it enters the sensing fiber through the second and third couplers. The backscattered Rayleigh light generated in the sensing fiber is coupled with the local oscillator at the fourth coupler. Then, it is converted into an electrical signal by the photoelectric balance detector. Finally, the signal is acquired by a high-speed acquisition card and uploaded to the host computer for signal processing.

[0036] Among them, the backscattering intensities generated by the multi-frequency optical pulses are different, and the correlation coefficient between them can be expressed as:

[0037]

[0038] In the formula, Δf is the pulse frequency difference, and ω is the pulse width. From the above formula, we can see that when Δf > 0.808 / ω, the correlation coefficient approaches 0, as shown below. Figure 4 As shown, this indicates that optical pulses of different frequencies are uncorrelated. When the backscattering intensity of one optical pulse is low at a certain position due to coherent fading, the backscattering intensity of the other optical pulse is relatively high at the same position. This method suppresses coherent fading noise.

[0039] The signal processing by the host computer is as follows: the collected multi-frequency optical pulse electrical signal containing vibration information is directly subjected to moving average difference to determine the vibration location, and the peak of the obtained moving average difference curve is the vibration location.

[0040] The acquired multi-frequency optical pulse electrical signals containing vibration information are bandpass filtered, and then I / Q quadrature demodulation is performed on the resulting signals of different frequencies. Next, phase reconstruction is performed based on the phase curves at the determined vibration locations (see...). Figure 6 As shown, phase restoration is the normal process of arctangent, phase spreading, and phase unwinding (which will not be elaborated here). This yields the final vibration phase curve. The vibration curve with the largest amplitude is then selected as the optimal result to suppress coherent fading noise, completing the distributed detection. The specific flowchart is shown below. Figure 5 As shown.

[0041] The present invention also provides a direct and coherent hybrid detection method for seismic exploration, which uses multi-frequency time-delayed dual-pulse direct detection to acquire detection signals and performs moving average differential to determine the vibration location;

[0042] Heterodyne coherent detection is performed to acquire multi-frequency optical pulse electrical signals. After bandpass filtering, the generated signals of different frequencies are subjected to I / Q quadrature demodulation. Based on the phase curves at the determined vibration locations, phase reconstruction is performed to obtain the final vibration phase curve.

[0043] It is worth mentioning that traditional distributed fiber optic vibration sensing systems suffer from problems such as coherent fading noise and the inability to accurately and directly obtain vibration signals. This system utilizes both direct detection and coherent detection structures, fully combining the advantages of both to improve the signal-to-noise ratio and sensing distance of the distributed fiber optic vibration sensing system. Furthermore, direct detection is achieved by generating multi-frequency time-delayed double pulses, which more effectively suppresses coherent fading noise and improves the detection performance of the distributed fiber optic vibration sensing system. Finally, the vibration location is initially determined through the direct detection structure, and the vibration curve is directly reconstructed based on the obtained vibration location using the coherent detection structure, improving detection efficiency and further advancing the application of the Φ-OTDR system in the field of seismic exploration.

[0044] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A hybrid direct and coherent detection system for seismic exploration, characterized in that, The system includes: a laser, a first coupler, a frequency modulation and frequency shifting unit, an erbium-doped fiber amplifier, a second coupler, a first optical switch, a second optical switch, a delay fiber, a third coupler, a fourth coupler, a circulator, a sensing fiber, a control unit, a photoelectric balance detector, a high-speed data acquisition card, and a host computer. The laser output is connected to the input of the first coupler, and the output of the first coupler is connected to the input of the frequency modulation and frequency shifting unit and the first optical switch, respectively. The output of the frequency modulation and frequency shifting unit is connected to the input of the erbium-doped fiber amplifier, and the output of the erbium-doped fiber amplifier is connected to the input of the second coupler. The output of the second coupler is connected to the input of the third coupler and the input of the second optical switch, respectively. The output of the second optical switch is connected to the input of the delay fiber, and the output of the delay fiber is connected to the input of the third coupler. The output of the third coupler is connected to the first port of the circulator, the second port of the circulator is connected to the sensing fiber, and the third port of the circulator is connected to the input of the fourth coupler. The output of the second optical switch is connected to the input of the fourth coupler. The output of the fourth coupler is connected to the input of the photoelectric balance detector, the output of the photoelectric balance detector is connected to the input of the high-speed acquisition card, and the output of the high-speed acquisition card is connected to the input of the host computer. The output of the control unit is connected to the control of the first optical switch, the control of the frequency modulation and frequency shifting unit, the control of the second optical switch, and the control of the high-speed acquisition card, respectively. The control unit controls the first optical switch to be in the off state and the second optical switch to be in the on state, and controls the frequency modulation and frequency shifting unit to modulate continuous light into multi-frequency pulse light, thereby realizing direct detection of multi-frequency time-delay dual pulses; the control unit controls the first optical switch to be in the on state and the second optical switch to be in the off state, and controls the frequency modulation and frequency shifting unit to modulate continuous light into multi-frequency pulse light, thereby realizing heterodyne coherent detection. During the multi-frequency time-delay dual-pulse direct detection, the laser emits continuous light. After passing through the first coupler, the continuous light enters the frequency modulation and frequency shifting unit controlled by the control unit. The continuous light is modulated into optical pulses composed of multiple frequencies. After being amplified by an erbium-doped fiber amplifier, the optical pulse signal is split into two optical pulses through the second coupler. One optical pulse directly enters the third coupler, and the other optical pulse enters the third coupler through a time-delay fiber. The two optical pulses form a multi-frequency time-delay optical pulse pair in the third coupler. The multi-frequency optical pulse pair enters the sensing fiber through a circulator and interferes with the backscattered Rayleigh light generated in the sensing fiber. The interference light is converted into an electrical signal by a photoelectric balance detector through the fourth coupler. Finally, a high-speed acquisition card is used to acquire the signal and upload it to the host computer for signal processing.

2. The direct and coherent hybrid detection system for seismic exploration according to claim 1, characterized in that, During heterodyne coherent detection, the laser emits continuous light. After passing through the first coupler, part of the continuous light enters the frequency modulation and shifting unit controlled by the control unit, while the other part of the continuous light enters the fourth coupler as the local oscillator. The continuous light entering the frequency modulation and shifting unit is modulated into multi-frequency optical pulses. After being amplified by an erbium-doped fiber amplifier, the optical pulse signal enters the sensing fiber through the second and third couplers. The backscattered Rayleigh light generated in the sensing fiber couples with the local oscillator at the fourth coupler. Then, it is converted into an electrical signal by a photoelectric balance detector. Finally, a high-speed acquisition card is used to acquire the signal and upload it to the host computer for signal processing.

3. The direct and coherent hybrid detection system for seismic exploration according to claim 1, characterized in that, The host computer directly performs moving average differential analysis on the electrical signal of the multi-frequency optical pulse directly detected by the multi-frequency time-delay dual pulse to determine the vibration position; After the multi-frequency optical pulse electrical signal of heterodyne coherent detection is bandpass filtered, the generated signals of different frequencies are subjected to I / Q quadrature demodulation. The phase is then restored according to the phase curve at the determined vibration position to obtain the final vibration phase curve.

4. A direct and coherent hybrid detection method for seismic exploration, employing the direct and coherent hybrid detection system for seismic exploration as described in claim 1, characterized in that, The method includes: Multi-frequency time-delay dual-pulse direct detection is used to acquire the detection signal, and moving average differential is used to determine the vibration location; Heterodyne coherent detection is performed to acquire multi-frequency optical pulse electrical signals. After bandpass filtering, I / Q quadrature demodulation is performed on the generated signals of different frequencies. Phase recovery is performed based on the phase curves at the determined vibration positions to obtain the final vibration phase curve. When using multi-frequency time-delayed dual-pulse direct detection, the laser emits continuous light. After passing through the first coupler, the continuous light enters the frequency modulation and frequency shifting unit controlled by the control unit. The continuous light is modulated into optical pulses composed of multiple frequencies. After being amplified by an erbium-doped fiber amplifier, the optical pulse signal is split into two optical pulses through the second coupler. One optical pulse directly enters the third coupler, and the other optical pulse enters the third coupler through a time-delayed fiber. The two optical pulses form a multi-frequency time-delayed optical pulse pair in the third coupler. The multi-frequency optical pulse pair enters the sensing fiber through a circulator and interferes with the backscattered Rayleigh light generated in the sensing fiber. The interference light is converted into an electrical signal by a photoelectric balance detector after passing through the fourth coupler.

5. The direct and coherent hybrid detection method for seismic exploration according to claim 4, characterized in that, During heterodyne coherent detection, the laser emits continuous light. After passing through the first coupler, part of the continuous light enters the frequency modulation and shifting unit controlled by the control unit, while the other part of the continuous light enters the fourth coupler as the local oscillator. The continuous light entering the frequency modulation and shifting unit is modulated into multi-frequency optical pulses. After being amplified by an erbium-doped fiber amplifier, the optical pulse signal enters the sensing fiber through the second and third couplers. The backscattered Rayleigh light generated in the sensing fiber couples with the local oscillator at the fourth coupler, and is then converted into an electrical signal by a photoelectric balance detector.

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