CN112162312A - Optical fiber multi-channel seismic system for detecting stratum shear wave velocity structure in ultra-shallow sea area - Google Patents
Optical fiber multi-channel seismic system for detecting stratum shear wave velocity structure in ultra-shallow sea area Download PDFInfo
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- G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
- G01V1/16—Receiving elements for seismic signals; Arrangements or adaptations of receiving elements
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- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
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- G01D5/35358—Sensor working in reflection using backscattering to detect the measured quantity
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- G01H9/004—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means using fibre optic sensors
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Abstract
The optical fiber multi-channel seismic system for detecting the structure of the transverse wave velocity of the stratum in the ultra-shallow sea area comprises an air explosion seismic source for exciting strong energy in the beach shallow sea land area and the sea bottom, a distributed acoustic sensing optical fiber array for sensing the strain and stress change of the sea bottom stratum, and a demodulation and recording unit for demodulating the external strain stress field information sensed by the distributed acoustic sensing optical fiber array into seismic wave signals and realizing continuous acquisition, recording and storage, wherein the distributed acoustic sensing optical fiber array comprises four parts, namely an optical fiber starting end, an optical fiber connecting section, an optical fiber array and an optical fiber tail end; linear, Z-shaped or O-shaped profiles may be used. The sensing distance of the invention reaches ten kilometers, the track spacing is dynamically adjustable, and the number of the collected seismic tracks reaches ten thousand, so that the invention is easily suitable for high-density space sampling of submarine transverse wave signals in an extremely shallow water area; meanwhile, the method has the advantages of multi-channel observation of 'streamer earthquake' and long-time continuous recording of 'ocean bottom seismograph', and can observe transverse wave velocity information which cannot be obtained by the traditional streamer earthquake.
Description
Technical Field
The invention relates to an optical fiber multi-channel seismic system for detecting a stratum shear wave velocity structure in an ultra-shallow sea area, and belongs to the field of marine geophysical and marine geology.
Background
The ultra-shallow sea area (the water depth is less than 5 meters) is a suture zone connecting the earth land and the sea, the connection with the human activity range is the most compact, almost every human offshore large-scale engineering construction such as a submarine tunnel, a cross-sea bridge, an offshore airport, a sea amusement place and the like needs to know the submarine shallow geological structure of the construction area and evaluate the submarine stability condition, and the transverse wave speed structure is an important parameter for evaluating the stratum bearing capacity and the crack development degree. The most common ocean survey equipment in the world today is a towline multi-channel seismic system, which is difficult to realize stratum detection operation in an extremely shallow sea area due to the limitation of the navigation water depth of a carried mother ship and the influence of obstacles such as shallow reefs, platforms, ships and the like, and has the problems of large track spacing, sparse space sampling, capability of only measuring and reflecting longitudinal waves at the sea surface and the like, and the problem that a submarine stratum shear wave velocity structure cannot be detected. At present, the earthquake observation technology capable of obtaining the submarine transverse wave information in China mainly comprises a submarine seismograph, submarine nodes, submarine cables, a submarine observation network and the like, but the technology needs to mount a series of sensors with different functions, the spatial sampling density is in direct proportion to the number of observation stations and the number of the sensors, the sampling interval is sparse, the price is high, seismic data with small channel interval and high density cannot be obtained, and the technology is only suitable for large-scale earth structure research or ocean oil and gas resource detection. Therefore, the detection of the small-scale ultra-shallow sea area shear wave velocity structure is still a weak link in the field of marine geology-geophysics at present. Therefore, the novel seismic exploration technology suitable for the extremely shallow sea area is researched and developed, and the method has important practical significance for solving the problems of basic engineering construction, marine ranching, seabed settlement, island reef seabed stability evaluation and the like of the extremely shallow sea area (such as ports, coastal zones, lagoons and other sea areas).
Disclosure of Invention
The invention aims to provide an optical fiber multi-channel seismic system for detecting a stratum transverse wave velocity structure in an ultra-shallow sea area, which overcomes the technical blank that the traditional towed multi-channel seismic system cannot carry out detection operation in the ultra-shallow sea area due to the limitation of the navigation water depth of a scientific investigation mother ship, provides a horizontal single-component surface wave and transverse wave seismic section for submarine engineering geological exploration of large-scale foundation construction in the ultra-shallow sea areas such as ports, coastal zones and lagoons, and lays the foundation for indoor post-processing and transverse wave velocity structure seismic imaging.
The optical fiber multi-channel seismic system for detecting the structure of the transverse wave velocity of the stratum in the ultra-shallow sea area is characterized by comprising an air explosion seismic source for exciting strong energy in the beach shallow sea land area and the sea bottom, a distributed acoustic sensing optical fiber array 2 for sensing the strain and stress change of the sea bottom stratum, and a demodulation and recording unit for demodulating the external strain stress field information sensed by the distributed acoustic sensing optical fiber array into seismic wave signals and realizing continuous acquisition, recording and storage;
the distributed acoustic sensing optical fiber array comprises an optical fiber starting end, an optical fiber connecting section, an optical fiber array and an optical fiber tail end;
the optical fiber starting end is used for being connected with the optical modulator in the demodulation recording unit, not only transmits optical pulses emitted by the laser, but also feeds back optical fiber strain scattering signals to the optical modulator, and the optical fiber starting end adopts an armored optical cable which is softer and thinner than an optical fiber connecting section so as to facilitate the connection between the optical fiber and the optical modulator;
the optical fiber connecting section is positioned between the optical fiber array on the seabed and the optical fiber starting end and plays a role in carrying out top-down connection and signal communication;
the optical fiber array is a main area for observing an artificial seismic wave field, the optical fiber in the optical fiber array comprises optical fibers with the diameter of 125-140 mu m at the innermost layer, a stainless steel sleeve, a Kevlar fiber layer, a lead wire winding layer and a PVC sleeve at the outermost layer are sequentially wrapped outside the optical fibers, a plurality of coupling bulges are arranged on the outer surface of the PVC sleeve, and the diameter of the optical fiber is 2mm in total; the PVC sleeve plays a role in protecting the appearance of the optical cable and preventing the optical cable from being corroded, and the outer surface of the PVC sleeve is embedded with a plurality of coupling bulges for enhancing the roughness of the surface of the optical cable and the biting force of seabed sediments and improving the coupling effect of the optical cable and the seabed; the middle lead wire winding layer is used for enhancing the weight of the optical cable, the Kevlar fiber layer enables the optical cable to have tensile capacity, and the stainless steel sleeve pipe of the inner layer enables the optical cable to have shear resistance, so that the optical fiber is prevented from being broken and damaged; the innermost layer is a single-mode non-dispersion sensing optical fiber with one core or two cores and is used for transmitting and feeding back optical signals and quantitatively sensing external stress and stress parameter change information;
the tail end of the optical fiber is a breakpoint of light propagation, and has strong reflection energy, which seriously interferes the demodulation of effective light scattering signals, so that the tail end of the optical fiber is wound with the optical fiber with about one meter of length and large curvature as flexible attenuation to reduce the influence of the strong reflection interference of the tail end.
The optical fiber multi-channel seismic system for detecting the stratum shear wave velocity structure in the ultra-shallow sea area is characterized in that in the optical fiber array, different optical fiber lengths are dynamically set within the range of 0.8-10 m to be used as a seismic channel for observing signals.
The optical fiber multi-channel seismic system for detecting the stratum shear wave velocity structure in the ultra-shallow sea area is characterized in that the optical fiber array adopts linear distribution, Z-shaped distribution or O-shaped distribution.
The optical fiber multi-channel seismic system for detecting the stratum shear wave velocity structure in the ultra-shallow sea area is characterized in that the gas explosion seismic source comprises an excitation seismic source device, a gas source storage tank and a gas injection system;
the seismic source excitation device structure comprises a seismic source main body, a control firing component and an excitation end; the seismic source main body is a gun body cast by steel, and a reaction cavity for storing reaction gas and a gas injection valve are arranged in the gun body; the detonation control component comprises a GPS time service clock crystal oscillator module, a thermal detonation wire, a high-voltage ignition circuit, a timing circuit, an embedded MCU and a battery cabin; the excitation end is composed of an arched diaphragm; the gas source storage tanks comprise an oxygen cylinder storage tank and a hydrogen/methane cylinder storage tank;
the gas injection system controls an industrial standard oxygen cylinder and a hydrogen/methane cylinder to respectively pass through a pressure reducing valve, a one-way valve and a stop valve gas injection pipeline in a time-sharing manner, and gas is injected into the excitation seismic source device through a gas injection console, so that the output of seismic waves of artificial sources with different energies and frequency spectrums is realized.
The optical fiber multi-channel seismic system for detecting the stratum shear wave velocity structure in the ultra-shallow sea area is characterized in that the demodulation and recording unit comprises a light emitting module, an interferometer module, a photoelectric conversion module, an electrical module and a data acquisition and processing module;
the optical transmission module comprises an exciter, an acousto-optic modulator, an optical isolator, an optical amplifier, a fiber grating and a circulator; the optical transmitting module is used for generating a coding pulse with narrow line width and high peak power and eliminating the influence of nonlinear effect and ASE noise;
the interferometer module comprises a Faraday rotator mirror, piezoelectric ceramics and an optical coupler, generates phase-modulated interference signals by loading PGC carrier signals and solves the problems of polarization fading and environmental interference;
the photoelectric conversion module converts weak coherent Rayleigh scattering optical signals into electrical signals through an optical detector;
the electrical module generates synchronous pulse code signals and PGC carrier signals based on the carrier generator, wherein the pulse code signals are used for modulating the optical modulator, and the PGC carrier signals are used for modulating the interferometer;
the data acquisition processing module comprises a data acquisition card and a signal processing module, is used for acquiring data at a high speed, and finishes the real-time extraction of phase signals through data recombination, PGC demodulation and a noise stabilization algorithm.
The optical fiber multi-channel seismic system for detecting the structure of the transverse wave velocity of the stratum in the ultra-shallow sea area is characterized in that the demodulation recording unit further comprises a human-computer interaction and display module, and after the data acquisition processing module finishes the real-time extraction of the phase signals, the data acquisition processing module displays the phase signals through the human-computer interaction and display module.
The system with the planar O-shaped spread optical fiber array is applied to observation of the transverse wave splitting phenomenon in shale gas exploration.
The method for detecting the stratum shear wave velocity structure in the ultra-shallow sea area by using the optical fiber multi-channel seismic system is characterized by comprising the following steps of:
0) selecting the layout of the optical fiber array, wherein the layout comprises linear spread, Z-shaped plane spread and O-shaped spread:
1) laying submarine optical fibers:
arranging the optical fiber array of the distributed acoustic sensing optical fiber array on the seabed through a boat or a ship according to a preset design route, wherein the arrangement of the optical fiber array is not more than 1.25 km;
2) gas explosion seismic source excitation:
the gas explosion seismic source is shot on land or is excited on the shallow sea bottom; before the gas explosion seismic source starts to inject gas, satellite signal receiving time synchronization is carried out for not less than 10 minutes, the number of satellites is not less than 3, and the time synchronization is carried out until a crystal oscillator module clock is locked; then the gas injection system injects gas to the excitation seismic source device; after gas injection is finished, frequency division is carried out on a crystal oscillator signal into 1Hz pulses, the 1Hz pulses are input to a counter and are compared with the preset detonation time, once the set detonation time is reached, a detonation control signal is immediately output to trigger a high-voltage ignition circuit, the high-voltage ignition circuit immediately outputs pulse high voltage to ignite mixed gas for a thermal explosion igniter, and ignition excitation of a gas explosion seismic source is realized; after excitation, the interior of the cavity of the seismic source excitation device instantly reaches high temperature and high pressure, and water vapor and carbon dioxide generated by reaction instantly expand to break through the arch-shaped diaphragm at the excitation end and release huge energy in submarine sediments or land strata to form seismic waves of an artificial source;
4) the demodulation recording system works:
firstly, automatically searching satellite signals by using a GPS time service clock crystal oscillator module of the equipment, completing clock correction and clock locking, and carrying out satellite signal time service for 10 minutes, wherein the number of satellites is not less than 3;
a second step of generating synchronous pulse code signals and PGC carrier signals by using an electrical module, wherein the pulse code signals are transmitted to a Laser of an optical emission module to modulate excited light pulses, the optical emission module generates light code pulses with narrow line width and high peak power through the Laser (Laser), An Optical Modulator (AOM), an optical Isolator (ISO) optical amplifier (EDFA), an optical fiber grating (FBG), an optical interconnection device and the like, and emits the light pulses to a sensing optical fiber array at equal time intervals, and at the moment, the backscattered light information carrying seismic wave information is transmitted to an interferometer module through a Circulator (Circulator); the electrical module generates a synchronous PGC Carrier signal (Carrier) through a load Generator (plus Generator) and transmits the signal to the interferometer module; converting the carrier signal into a phase modulation interference signal through piezoelectric ceramics (PZT) in the interferometer module;
when seismic waves excited by the gas explosion seismic source carry information such as reflection, refraction and surface waves of a seabed stratum to be transmitted to the sensing optical fiber array, the sensing optical fibers and the seabed are integrated to generate stress and strain changes, the changes directly cause the sensing optical fibers to generate tensile deformation in the horizontal direction, and then emitted light is subjected to backscattering in the transmission process inside the sensing optical fibers; the stronger the seismic wave amplitude is, the more severe the tensile deformation of the sensing optical fiber is, and the larger the light backscattering amplitude is;
synchronously decoupling the back scattering information transmitted by the sensing optical fiber and interference signals modulated by PGC phase carriers through an Optical Coupler (OC) to obtain coherent Rayleigh scattering optical signals, and further converting weak coherent Rayleigh scattering optical signals into electrical signals by using a Photoelectric Detector (PD) of a photoelectric conversion module;
fourthly, transmitting the electrochemical signals to a data acquisition and processing module, realizing data recombination, PGC demodulation and noise stabilization algorithm on the electrochemical signals by using a Signal processing module (Signal Processor Unit) to complete real-time extraction of phase signals, and realizing equal-time-interval high-speed sampling of seismic signals by using a data acquisition card (DAQ); the data acquisition part configures parameters of the data acquisition card by calling a hardware drive of the data acquisition card and transmits the configuration information to a PGC demodulation algorithm; the data processing part takes a Phase Generation Carrier (PGC) algorithm as a core; the data result processing part is used for displaying and storing the wavelength signals obtained by the PGC algorithm; and finally, processing, displaying and storing the acquired original data in real time.
Further comprising the step 5) of long-term continuous observation:
the demodulation recording unit and the gas detonation source are internally embedded with a satellite signal receiver clock high-precision synchronization module, and a quartz crystal oscillator (quartz clock) is used for preventing the system time drift of the equipment; therefore, stress and strain field change information sensed by the sensing optical fiber array on the seabed for a long time is fed back to the demodulation recording unit according to the Universal Time (UTC) sequence, and the demodulation recording unit continuously repeats the step 4) to obtain continuous seismic wave field information of months to years; the seismic waves can be gas explosion seismic sources from artificial sources or natural earthquakes from the nature; the surface wave and transverse wave information of the seismic wave field are utilized to obtain the transverse wave velocity structure of the underground space of the detection area.
In the step 1), during the arrangement of the submarine optical fibers, an armor or lead weight is additionally arranged outside the optical fiber cable to protect the optical fiber cable and increase the weight of the optical fiber cable, so that the coupling performance with the seabed is enhanced, and the optical fiber cable is placed for several days after the arrangement, and the coupling effect of the optical fiber cable and seabed sediments is further increased by utilizing the natural acting force of ocean tides, waves and sediment sedimentation.
The invention has the beneficial effects that:
a set of seismic detection system of a structure for acquiring submarine stratum shear wave velocity at high density in an extremely shallow sea area is provided. The earthquake focus is excited by adopting a gas reaction type earthquake focus which can adapt to dual environments of land and seabed, and is green, environment-friendly, small in size and convenient to transport and use; the sensor adopts a specially-made seabed coupling reinforced optical fiber cable, and utilizes Rayleigh scattering phase sensitive optical time domain reflection technology to ensure that the sensing distance of an optical fiber array reaches ten kilometers, the track spacing is dynamically adjustable (the minimum reaches 0.8 meter/track), and the number of collected seismic tracks reaches ten thousand, so that the sensor is easily suitable for high-density space sampling of submarine transverse wave signals in an extremely shallow water area; compared with land optical cables, the seabed coupling reinforced optical fiber cable has a rougher surface, a coupling bulge and larger self weight, and is thinner than a seabed communication optical cable, so that an optical fiber sensing array has a better coupling effect with seabed sediments, the observed seabed earthquake transverse wave information has better quality, and different azimuth angles and two-dimensional earthquake wave observation can be realized by designing different sensing optical fiber array shapes; a high-precision synchronization module of a satellite signal receiver clock is embedded in a demodulation recording unit and an air detonation source of the device on the basis of the prior art, and a quartz crystal oscillator (quartz clock) is used for preventing the system time drift of the device. The method has the advantages of multi-channel observation of 'streamer earthquake' and long-time continuous recording of 'ocean bottom seismograph', is a novel ocean bottom multi-channel seismic exploration technology for long-time continuous recording, the obtained data volume is increased by tens of times compared with that of an exploration type multi-channel earthquake or ocean bottom seismograph, transverse wave velocity information which cannot be obtained by the traditional streamer earthquake can be observed, the method is a novel technical means which has potential for carrying out ocean bottom engineering geological exploration in ports, coastal zones, lagoons and other extremely shallow sea areas, and has industrial and practical application prospects.
Drawings
FIG. 1 is a schematic view of an optical fiber multi-channel seismic system of the present invention.
FIG. 2 is an overall architecture of the fiber optic multi-track seismic system of the present invention.
Fig. 3 is a schematic diagram of a gas explosion seismic source structure.
FIG. 4 is a schematic diagram of a seismic source device.
Fig. 5 is a cross-sectional view of an excitation source apparatus.
Fig. 6 is a schematic structural diagram of a detonation control component.
FIG. 7 is a cross-sectional view of a fiber optic cable for a distributed acoustic sensing fiber array.
Fig. 8 shows several forms of the distributed acoustic sensing fiber array, in which fig. 8A is a linear spread belonging to two-dimensional seismic exploration, fig. 8B is a Z-type spread belonging to three-dimensional multi-channel seismic observation, and fig. 8C is an O-type spread for seismic wave omni-directional observation.
Fig. 9 shows a distributed acoustic sensing fiber array structure (taking a Z-shape as an example).
Fig. 10 is a schematic diagram of the internal structure of the demodulation recording unit.
In the figure, Laser-exciter; AOM-acousto-optic modulator; ISO-optical isolators; an EDFA-amplifier; FBG-grating; circular-Circulator; FRM-faraday rotator mirror; PZT-piezoelectric ceramics; an OC-optical coupler; PD-photodetector; a Pluse Generator-carrier Generator; Carrier-Carrier; DAQ-data acquisition card; signal Processor Unit-Signal processing module.
Fig. 11 is a schematic structural diagram of a data acquisition and processing module.
FIG. 12 human-computer interaction and display Module interface
FIG. 13 shows an original fiber optic seismic record observed in a lagoon area using the system of the present invention; in the figure, S: shear-waves Shear waves; SW: Surface-Wave Surface waves; ch.1-4, the optical fiber sensing array penetrates through the artificial water channel region; seg.1-4: fiber array segment number.
FIG. 14 is a one-dimensional shear wave velocity structure obtained based on fiber optic multi-channel seismic data inversion
Wherein, 1-gas explosion seismic source, 2-distributed acoustic sensing optical fiber array, 3-demodulation recording unit, 4-excitation seismic source device, 5-gas source storage tank, 6-gas injection system, 7-seismic source main body, 8-control ignition component, 9-excitation end, 10-reaction cavity, 11-gas injection valve, 12-GPS time service clock crystal oscillator module, 13-thermal explosion wire, 14-high-pressure ignition circuit, 15-timing circuit, 16-embedded MCU, 17-battery cabin, 18-oxygen cylinder storage tank, 19-hydrogen (methane) gas cylinder storage tank, 20-gas injection pipeline, 21-gas injection console, 22-remote controller, 23-coupling bulge, 24-PVC sleeve pipe, 25-lead wire winding layer, 26-Kevlar fiber, 27-stainless steel sleeve, 28-optical fiber, 29-linear spread, 30-Z-type spread, 31-O-type spread, 32-optical fiber initial end, 33-optical fiber connecting section, 34-sensing optical fiber array, 35-seismic channel, 36-optical fiber tail end, 37-emission module, 38-interferometer module, 39-photoelectric conversion module, 40-electrical module, 41-data acquisition and processing module and 42-PC man-machine interaction and display module.
Detailed Description
The optical fiber multi-channel seismic system for detecting the stratum shear wave velocity structure in the ultra-shallow sea area comprises an air explosion seismic source 1, a distributed acoustic sensing optical fiber array 2 and a demodulation recording unit 3.
The distributed acoustic sensing optical fiber array 2 comprises an optical fiber starting end 32, an optical fiber connecting section 33, an optical fiber array 34 and an optical fiber tail end 36.
The optical fiber starting end 32 is used for being connected with the optical modulator in the demodulation recording unit 3, not only transmits the optical pulse emitted by the laser, but also feeds back the optical fiber strain scattering signal to the optical modulator, and the optical fiber starting end 32 adopts an armless optical cable (the diameter is 0.05 mm) which is more flexible and thinner than the optical fiber connecting section 33, so that the connection between the optical fiber and the optical modulator is facilitated.
The optical fiber connection section 33 connects the optical fiber array 34 located at the bottom of the sea with the optical fiber start end 32, and performs a top-down connection function, the specific extension length is determined according to the actual situation, but the optical fiber cable is poorly coupled with the bottom of the sea, and is not usually used for signal demodulation purposes, and is only used for transmitting and transmitting optical information.
The optical fiber array 34 is a main area for observing an artificial seismic wave field, can be arranged in an array with different rules according to different target targets of seismic detection, and can be used for carrying out three-dimensional seismic observation and two-dimensional linear observation; the optical cable in the optical fiber array 34 comprises an optical fiber 28 with the innermost layer and the diameter of 125-; the PVC sleeve 24 can protect the appearance of the optical cable and prevent the optical cable from being corroded, and the outer surface of the PVC sleeve is embedded with a plurality of coupling bulges 23 with the height of about 0.2mm, so that the surface roughness of the optical cable and the biting force of seabed sediments are enhanced, and the coupling effect of the optical cable and the seabed is improved; the middle lead wire winding layer 25 is used for enhancing the weight of the optical cable, the Kevlar fiber layer 26 enables the optical cable to have tensile capacity (larger than 100 kg), and the stainless steel sleeve 27 on the inner layer enables the optical cable to have shear resistance and prevents the optical fiber from being broken and damaged; the innermost layer is a single-mode non-dispersion sensing optical fiber 28 with one core or two cores, and is used for transmitting and feeding back optical signals and quantitatively sensing external stress and stress parameter change information; the optical fiber array 34 can be usually arranged for ten kilometers, armor or lead weight can be continuously added outside the optical fiber cable for protection and the weight of the body can be improved during construction, the coupling property with the sea bottom is enhanced, different optical fiber lengths can be dynamically set within the range of 0.8-10 meters to be used as a seismic channel 35 for observing signals, the number of seismic channels acquired by one-time gas explosion seismic source excitation can reach ten thousand, the high-density spatial sampling is achieved, and the detection advantage of high transverse resolution is achieved; the tail end 36 of the optical fiber is a breakpoint of light propagation, has strong reflection energy, and seriously interferes the demodulation of effective light scattering signals, so tail end processing must be carried out; the scheme of the invention is to wind the optical fiber with a length of about one meter and a large curvature at the tail end 36 of the optical fiber to serve as flexible attenuation to reduce the influence of strong reflection interference at the tail end.
The optical fiber array 34 may adopt several distribution modes, such as linear distribution 29, Z-shaped distribution 30, and O-shaped distribution 31; the linear spread 29 is a two-dimensional optical fiber multi-channel seismic observation, and two-dimensional space slice data spread along the optical fiber is obtained, so that the method belongs to two-dimensional seismic exploration and is suitable for a coarse-scale seismic detection method; the planar Z-shaped spread 30 is a three-dimensional optical fiber multi-channel seismic observation, a three-dimensional data body in the transverse-longitudinal-depth direction of the optical fiber spread is obtained, the three-dimensional transverse wave structure imaging of the submarine stratum can be realized, and the method is a fine-scale seismic detection method; the plane O-shaped spreading 31 is used for observing all azimuth angles of seismic waves, a multi-wave multi-component seismic data body is obtained, and the transverse wave splitting phenomenon in shale gas exploration can be well recorded.
The specific structure of the gas explosion seismic source 1 is as follows: comprises an excitation seismic source device 4, an air source storage tank 5 and an air injection system 6.
The structure of the seismic source excitation device 4 comprises a seismic source main body 7, a detonation control component 8 and an excitation end 9; the seismic source main body 7 is a gun body cast by steel, and a reaction cavity 10 for storing reaction gas and a gas injection valve 11 are designed in the gun body; the volume of the cavity can be set according to output parameters such as excitation capacity, gas injection pressure and theoretical excitation energy required to be achieved; the detonation control component 8 comprises a GPS time service clock crystal oscillator module 12, a thermal fuse 13, a high-voltage ignition circuit 14, a timing circuit 15, an embedded MCU 16 and a battery cabin 17; the excitation end 9 is composed of an arched diaphragm 14, the compression resistance of the area is weak, and high-temperature and high-pressure gas breaks through the diaphragm to release energy during reaction; the gas source storage tank 5 comprises an oxygen cylinder storage tank 18 and a hydrogen/methane gas cylinder storage tank 19, and has the function of storing oxygen cylinders, hydrogen or methane gas cylinders of 40L in industrial standard in a separated manner so as to ensure the safety of the oxygen cylinders, the hydrogen or the methane gas cylinders in long-term use; the gas injection system 6 controls an industrial standard oxygen cylinder and a hydrogen/methane cylinder to respectively pass through a pressure reducing valve, a one-way valve and a stop valve gas injection pipeline 20 in a time-sharing manner, gas is injected into the excitation seismic source device 4 through a gas injection console 21, gas injection pressure and component proportion are precisely controlled by a remote controller 22, and artificial source seismic wave output with different energy and frequency spectrums is realized by injecting fuel ratios with different proportions and pressures.
The demodulation recording unit 3 comprises a light emitting module 37, an interferometer module 38, a photoelectric conversion module 39, an electrical module 40, a data acquisition and processing module 41, and a human-computer interaction and display module 42.
The light emitting module 37 comprises an exciter, an acousto-optic modulator, an optical isolator, an optical amplifier, a fiber grating and a circulator; the optical transmitting module 37 is used for generating a narrow-linewidth high-peak-power coded pulse and eliminating the influence of nonlinear effect and ASE noise; the interferometer module 38 comprises a faraday rotator mirror, piezoelectric ceramics and an optical coupler, and generates a phase-modulated interference signal by loading a PGC carrier signal, so as to solve the problems of polarization fading and environmental interference; the photoelectric conversion module 39 converts the weak coherent rayleigh scattered light signal into an electrical signal through the optical detector; the electrical module 40 generates a synchronized pulse code signal and a PGC carrier signal based on the carrier generator, wherein the pulse code signal is used to modulate the optical modulator and the PGC carrier signal is used to modulate the interferometer 38; the data acquisition processing module 41 comprises a data acquisition card and a signal processing module, and is used for acquiring data at a high speed and completing real-time extraction of phase signals through data recombination, PGC demodulation and a noise stabilization algorithm; after the data acquisition and processing module 41 finishes the real-time extraction of the phase signals, the phase signals are displayed through the human-computer interaction and display module 42. The human-computer interaction and display module 42 includes a hardware driver layer, an operating system layer, an API function layer and a user application layer, and the user interface is used as the user application layer for interactive operation, so as to implement real-time processing, display and storage of the acquired original data. The designed main interface comprises an operation control button part, a channel setting part, a filter parameter setting part, an error warning part, a file storage part, a trigger timing part and a display main interface.
The method for detecting the stratum shear wave velocity structure in the ultra-shallow sea area by using the optical fiber multi-channel seismic system is characterized by comprising the following steps of:
(0) selecting a layout of the fiber array 34;
the layout of fiber array 34 includes a linear spread 29, a Z-shaped planar spread 30, and an O-shaped spread 31:
the linear spread is two-dimensional optical fiber multi-channel seismic observation, and two-dimensional space slice data distributed along the optical fiber spread is obtained, so that the method is suitable for a coarse-scale seismic detection method;
the planar Z-shaped spread is three-dimensional optical fiber multi-channel seismic observation, a three-dimensional data body in the transverse-longitudinal-depth direction of the optical fiber spread is obtained, the three-dimensional transverse wave structure imaging of the seabed stratum can be realized, and the method is a fine-scale seismic detection method;
the plane O-shaped spread is used for observing the seismic waves in all azimuth angles, a multi-wave multi-component seismic data body is obtained, and the transverse wave splitting phenomenon in shale gas exploration can be well recorded. Different spreading modes can meet the requirements of detection targets and scientific research personnel.
(1) Laying submarine optical fibers;
because the optical cable is light, the optical fiber array can be arranged on the seabed by a small boat or a ship according to a preset design route, but the distributed optical fiber arrangement should not exceed 1.25km in principle. If the arrangement length is exceeded, the emission power of the optical fiber needs to be increased, and the signal to noise ratio is low; if the bottom flow of the detection area is larger, lead sheath winding or weight addition can be continuously carried out on the outside during the optical fiber sensing array so as to improve the weight of the body and enhance the coupling degree with seabed sediments; after the optical fiber is laid, the optical fiber can be placed for several days, and the coupling effect of the optical fiber and seabed sediments is further increased by utilizing natural acting forces of ocean tides, waves, sediment settlement and the like.
(2) Exciting a gas explosion seismic source; the gas explosion seismic source can be shot on land and can also be excited in shallow sea bottom, and the excitation station position design with different offset distances and azimuth angles can be carried out according to different detection targets so as to meet different scientific and industrial detection requirements;
before the gas explosion seismic source starts to inject gas, satellite signal receiving time synchronization is carried out for not less than 10 minutes, and the number of satellites is not less than 3 until the clock of the crystal oscillator module is locked; then the gas injection system injects gas to the excitation seismic source device through a pressure reducing valve, a one-way valve and a stop valve pipeline respectively in a time-sharing way through an industrial standard oxygen cylinder and a hydrogen (methane) cylinder, the gas injection pressure and the component proportion are precisely controlled by a remote controller, and the proportion is controlled to be 1: 2, complete reaction, the internal pressure of the gas-injected gun body can reach 3Mp/7.5Mp/9.0Mp (specifically set according to output parameters such as required theoretical excitation energy); after gas injection is finished, the detonation control circuit is controlled to divide frequency of a crystal oscillator signal into 1Hz pulses, the 1Hz pulses are input to a counter and are compared with the preset detonation time, once the set detonation time is reached, a detonation control signal is immediately output to trigger a high-voltage ignition circuit, and the high-voltage ignition circuit immediately outputs pulse high voltage to a thermal detonation igniter to detonate mixed gas so as to realize ignition excitation of a gas detonation focus; after excitation, the interior of the cavity of the seismic source excitation device instantly reaches high temperature and high pressure, and water vapor and carbon dioxide generated by reaction instantly expand to break through the arch-shaped diaphragm at the excitation end and release huge energy in submarine sediments or land strata to form seismic waves of the artificial source.
(4) The demodulation recording system works;
firstly, automatically searching satellite signals by using a GPS time service clock crystal oscillator module of equipment, completing clock correction and clock locking, and requiring that the number of satellite signals is not less than 3 in 10 minutes of satellite signal time synchronization in principle, wherein the specific time length is determined according to the actual on-site satellite signal intensity;
and a second step of generating synchronous pulse coding signals and PGC carrier signals by using an electrical module, wherein the pulse coding signals are transmitted to a Laser of the light emitting module to modulate excited light pulses, the light emitting module generates light coding pulses with narrow line width and high peak power through the Laser (Laser), An Optical Modulator (AOM), an optical Isolator (ISO) optical amplifier (EDFA), an optical fiber grating (FBG), an optical interconnection device and the like, and emits the light pulses to a sensing optical fiber array and the like at equal time intervals, and at the moment, the backscattered light information carrying seismic wave information is transmitted to the interferometer module through a Circulator (Circulator). The electrical module generates a synchronous PGC Carrier signal (Carrier) by a Carrier Generator (plus Generator) and sends it to the interferometer module. The carrier signal is converted into a phase modulation interference signal by piezoelectric ceramics (PZT) in the interferometer module.
When seismic waves excited by the gas explosion seismic source 1 carry information such as reflection, refraction and surface waves of a seabed stratum to be transmitted to the sensing optical fiber array, the sensing optical fibers and the seabed are integrated to generate stress and strain changes, the changes directly cause the sensing optical fibers to generate tensile deformation in the horizontal direction, and then emitted light is subjected to backscattering in the transmission process inside the sensing optical fibers. The stronger the seismic wave amplitude, the more severe the tensile deformation of the sensing fiber, and the larger the light backscatter amplitude.
And thirdly, synchronously decoupling the back scattering information transmitted by the sensing optical fiber and interference signals modulated by PGC phase carriers through an Optical Coupler (OC) to obtain coherent Rayleigh scattering optical signals, and further converting weak coherent Rayleigh scattering optical signals into electrical signals by using a Photoelectric Detector (PD) of a photoelectric conversion module. The photodetector is required to have high sensitivity, wide frequency and high linearity.
And fourthly, transmitting the electrochemical signals to a data acquisition and processing module, realizing data recombination, PGC demodulation and noise stabilization algorithm on the electrochemical signals by using a Signal processing module (Signal Processor Unit) to finish the real-time extraction of phase signals, and realizing the equal-time-interval high-speed sampling of the seismic signals by using a data acquisition card (DAQ). The data acquisition part configures parameters of the data acquisition card mainly by calling a hardware drive of the data acquisition card and transmits the configuration information to a PGC demodulation algorithm; the data processing part takes a Phase Generation Carrier (PGC) algorithm as a core, and sets parameters of a filter and a display function related to the PGC algorithm through a user interaction interface; the data result processing part is used for displaying and storing the wavelength signals obtained by the PGC algorithm and providing a software interface for the data processing software according to the condition; and finally, the acquired original data is processed, displayed and stored in real time through a human-computer interaction and display module (PC). The man-machine interaction and display main interface designed at the present stage comprises an operation control button part, a channel setting part, a filter parameter setting part, an error warning part, a file storage part, a trigger timing part and a display main interface.
(5) Long-term continuous observation;
the demodulation recording unit 3 and the gas explosion seismic source 1 are internally embedded with a high-precision satellite signal receiver clock synchronization module on the basis of the prior art, and a quartz crystal oscillator (quartz clock) is used for preventing the system time drift of the equipment. Therefore, stress and strain field change information sensed by the sensing optical fiber array on the seabed for a long time is fed back to the demodulation recording unit according to the Universal Time (UTC) sequence, and the demodulation recording unit repeats the step (4) continuously, so that continuous seismic wave field information of months to years can be obtained. These seismic waves can be both gas-exploded seismic sources from artificial sources and natural earthquakes from nature. The surface wave and transverse wave information of the seismic wave field are utilized to obtain the transverse wave velocity structure of the underground space of the detection area.
Examples
The optical fiber multi-channel seismic system for detecting the shear wave velocity structure of the stratum in the ultra-shallow sea area is described below with reference to the accompanying drawings, the system is composed of an air explosion seismic source 1, a distributed acoustic sensing optical fiber array 2 and a demodulation recording unit 3, the system observation mode is shown in figure 1, and the overall system architecture is shown in figure 2.
The gas explosion seismic source 1 is mainly used for exciting artificial source seismic waves with strong energy in the beach shallow sea land area and the sea bottom, and comprises surface waves and body waves; the distributed acoustic sensing optical fiber array 2 is mainly used for sensing strain and stress changes of a submarine stratum at a high density and a long distance, namely, the strain and stress dynamic changes caused in the horizontal direction of the optical fiber after the artificial seismic waves excited by an air explosion seismic source are reflected and refracted (body waves) and interface propagation (surface waves) in the process of propagating to the deep part of the earth; the demodulation recording unit 3 has the main functions of demodulating strain and stress information carrying stratum structure information sensed by the distributed acoustic sensing optical fiber array 2 into a seismic wave field based on a Rayleigh scattering phase sensitive optical time domain reflection technology and a phase generation carrier demodulation algorithm, and recording, processing, storing and displaying demodulated data in a channel time-sharing mode.
The gas explosion seismic source 1 comprises an excitation seismic source device, a gas source storage tank and a gas injection system (figures 3 and 4). The seismic source excitation device structure comprises a seismic source main body, a detonation control component and an excitation end. The seismic source main body is a gun body cast by steel, and a reaction cavity for storing reaction gas and a gas injection valve (shown in figure 5) are designed in the gun body; the volume of the cavity can be set according to output parameters such as excitation capacity, gas injection pressure and theoretical excitation energy required to be achieved; the detonation control component comprises a GPS time service clock crystal oscillator module, a high-voltage ignition circuit, a thermal explosion wire, a battery cabin and an embedded MCU (figure 6); the excitation end is mainly composed of an arched membrane, the compression resistance of the area is weak, and high-temperature and high-pressure gas breaks through the membrane to release energy in reaction (figures 3, 4 and 5); the gas source storage box mainly has the function of storing an oxygen cylinder and a hydrogen (methane) cylinder which are 40L in industrial standard in a separated mode (figure 3) so as to ensure the safety of the oxygen cylinder and the hydrogen (methane) cylinder in long-term use; the gas injection system controls an industrial standard oxygen cylinder and a hydrogen (methane) cylinder to respectively pass through a pressure reducing valve, a one-way valve and a stop valve pipeline in a time-sharing manner, gas is injected into the excitation seismic source device through a gas injection console, the gas injection pressure and the component proportion are precisely controlled by a remote controller, and the output of seismic waves of the artificial source with different energy and frequency spectrums is realized by injecting fuel ratios with different proportions and pressures.
The distributed acoustic sensing optical fiber array 2 is composed of a special optical cable with the diameter of 2mm (figure 7), and the structure of the distributed acoustic sensing optical fiber array comprises six parts, namely a coupling bulge, a PVC sleeve, a lead wire winding layer, Kevlar fibers, a stainless steel sleeve and optical fibers. The PVC sleeve on the outermost layer of the optical cable can protect the appearance of the optical cable and prevent the optical cable from being corroded, 11 coupling bulges are embedded on the outer surface of the PVC sleeve, the height of the coupling bulges is about 0.2mm, the PVC sleeve is used for enhancing the surface roughness of the optical cable and the biting force of seabed sediments, and the coupling effect of the optical cable and the seabed is improved; the middle lead wire winding layer is used for enhancing the weight of the optical cable, the Kevlar fiber enables the optical cable to have the tensile capacity (more than 100 kg), and the stainless steel sleeve pipe on the inner layer enables the optical cable to have the shear resistance and prevents the optical fiber from being broken and damaged; the innermost layer is a single-mode non-dispersion sensing optical fiber with one core or two cores, and the single-mode non-dispersion sensing optical fiber is mainly used for transmitting and feeding back optical signals and quantitatively sensing external stress and stress parameter change information. The distributed acoustic sensing optical fiber array can be subdivided into four parts, namely an optical fiber starting end, an optical fiber connecting section, a sensing optical fiber array and an optical fiber tail end (figure 9). The optical fiber starting end is mainly used for being connected with a laser and an optical modulator of the demodulation recording unit, not only transmitting optical pulses emitted by the laser, but also feeding back optical fiber strain scattering signals to the optical modulator; the optical fiber connecting section has the main functions of connecting the optical fiber sensing array and the demodulation recording unit in the target area, and has the starting and stopping connection functions, the specific extension length is determined according to the actual situation, but the optical fiber cable is poor in coupling with the seabed, is not usually used for the signal demodulation purpose, and is only used for transmitting and transmitting optical information; the optical fiber sensing array is the main area for observing artificial seismic wave field, and can be arranged in different regular arrays according to different targets of seismic detection, and can be designed into linear spread, planar Z-shaped or O-shaped spread (figure 8). The linear spread is two-dimensional optical fiber multi-channel seismic observation, and two-dimensional space slice data distributed along the optical fiber is obtained to perform two-dimensional seismic exploration; the plane Z-shaped spread is three-dimensional optical fiber multi-channel seismic observation, a three-dimensional data body in the transverse-longitudinal-depth direction of the optical fiber spread is obtained, and the imaging of the three-dimensional transverse wave structure of the seabed stratum can be realized; the plane O-shaped spread is used for observing all azimuth angles of seismic waves, a multi-wave multi-component seismic data body is obtained, and the transverse wave splitting phenomenon in shale gas exploration can be well recorded. The optical fiber sensing array can be generally arranged for ten kilometers, different optical fiber lengths (0.8-10 meters) can be dynamically set to serve as a seismic channel to observe signals, the number of seismic channels acquired by one-time gas explosion seismic source excitation can reach ten thousand, the optical fiber sensing array belongs to high-density space sampling, and the optical fiber sensing array has the detection advantage of high transverse resolution. The tail end of the optical fiber is a breakpoint of light propagation, has strong reflection energy and seriously interferes the demodulation of effective light scattering signals, so tail end processing must be carried out. The invention winds the optical fiber with the length of about one meter and the large curvature at the tail end of the optical fiber to serve as flexible attenuation to reduce the influence of strong reflection interference at the tail end.
The demodulation recording unit 3 comprises a light emitting module, an interferometer module, a photoelectric conversion module, an electrical module, a data acquisition and processing module, and a man-machine interaction and display module (fig. 10), wherein the light emitting module comprises an exciter, an acousto-optic modulator, an optical isolator, an optical amplifier, a fiber grating and optical devices required by optical interconnection. The optical transmitting module is mainly used for generating encoding pulses with narrow line width and high peak power and eliminating the influence of nonlinear effect and ASE noise; the interferometer module generates phase modulation interference signals by loading PGC carrier signals, and solves the problems of polarization fading and environmental interference; the photoelectric conversion module converts weak coherent Rayleigh scattering optical signals into electrical signals, and the photoelectric detector is required to have high sensitivity, wide frequency and high linearity; the electrical module is used for generating synchronous pulse code signals and PGC carrier signals, wherein the pulse code signals are used for modulating the optical modulator, and the PGC carrier signals are used for modulating the interferometer; the data acquisition and processing module is used for acquiring data at a high speed, finishing real-time extraction of phase signals through algorithms such as data recombination, PGC demodulation, noise stabilization and the like, and displaying the phase signals through the man-machine interaction and display module (figure 11); the man-machine interaction and display module comprises a hardware driving layer, an operating system layer, an API function layer and a user application program layer, and a user interface is used as the user application program layer for interaction operation to realize real-time processing, display and storage of the acquired original data. The designed main interface includes an operation control button part, a channel setting part, a filter parameter setting part, an error warning part, a file storage part, a trigger timer and a display main interface (fig. 12).
Examples of the applications
Fig. 13 is an original optical fiber seismic record obtained in the lagoon new village lagoon area of the hainan island by using the optical fiber multi-channel seismic system of the present invention, and the main observation parameters are set as follows: the length of the optical fiber is 1 kilometer, 1-100 meters and 900-100 meters are optical fiber connection sections, 100-300 meters (Seg.1), 300-500 meters (Seg.2), 500-700 meters (Seg.3) and 700-900 meters (Seg.4) are optical fiber sensing arrays which are arranged in a reciprocating mode, and an air explosion seismic source is excited 1 time on the land side. As can be seen from the graph of the seismic data variable density color block shown in FIG. 11, the transverse wave (S) and the Surface Wave (SW) with strong energy can be received, and the recording time of the surface wave exceeds 2 seconds. Based on the surface wave data obtained in fig. 13, a one-dimensional shear wave velocity structure of the test lake drainage region is obtained by using active source multi-channel surface wave inversion calculation, as shown in fig. 14.
Claims (10)
1. The optical fiber multi-channel seismic system for detecting the stratum shear wave velocity structure in the ultra-shallow sea area is characterized by comprising an air explosion seismic source (1) for exciting strong energy in the beach shallow sea land area and the sea bottom, a distributed acoustic sensing optical fiber array (2) for sensing the strain and stress change of the sea bottom stratum, and a demodulation and recording unit (3) for demodulating the external strain stress field information sensed by the distributed acoustic sensing optical fiber array (2) into seismic wave signals and realizing continuous acquisition, recording and storage;
the distributed acoustic sensing optical fiber array (2) comprises an optical fiber initial end (32), an optical fiber connecting section (33), an optical fiber array (34) and an optical fiber tail end (36);
the optical fiber starting end (32) is used for being connected with the optical modulator in the demodulation recording unit (3), not only transmits light pulses emitted by the laser, but also feeds back optical fiber strain scattering signals to the optical modulator, and the optical fiber starting end (32) adopts an armored optical cable which is softer and thinner than the optical fiber connecting section (33) so as to facilitate the connection between the optical fiber and the optical modulator;
the optical fiber array (34) and the optical fiber initial end (32) of the optical fiber connecting section (33) are positioned at the sea bottom, and have the functions of top-down connection and signal communication;
the optical fiber array (34) is a main area for observing an artificial seismic wave field, the optical fiber in the optical fiber array (34) comprises optical fibers (28) with the diameter of 125-140 mu m at the innermost layer, a stainless steel sleeve (27), a Kevlar fiber layer (26), a lead wire winding layer (25) and a PVC sleeve (24) at the outermost layer are sequentially arranged outside the optical fibers (28), and a plurality of coupling bulges (23) are arranged on the outer surface of the PVC sleeve (24); the PVC sleeve (24) protects the appearance of the optical cable and prevents the optical cable from being corroded, and the outer surface of the PVC sleeve is embedded with a plurality of coupling bulges (23) for enhancing the surface roughness of the optical cable and the biting force of seabed sediments and improving the coupling effect of the optical cable and the seabed; the middle lead wire winding layer (25) is used for enhancing the weight of the optical cable, the Kevlar fiber layer (26) enables the optical cable to have tensile capacity, and the stainless steel sleeve (27) of the inner layer enables the optical cable to have shear resistance and prevents the optical fiber from being broken and damaged; the innermost layer is a single-mode non-dispersion sensing optical fiber (28) with one core or two cores and is used for transmitting and feeding back optical signals and quantitatively sensing external stress and stress parameter change information;
the tail end (36) of the optical fiber is a breakpoint of light propagation, has strong reflection energy and seriously interferes the demodulation of effective light scattering signals, so that the tail end (36) of the optical fiber is wound by the optical fiber with a long and large curvature of about one meter to serve as flexible attenuation to reduce the influence of the strong reflection interference of the tail end.
2. The fiber optic multi-channel seismic system for ultra-shallow sea formation shear wave velocity structure detection as claimed in claim 1, wherein in said fiber optic array (34), different fiber lengths are dynamically set within the range of 0.8-10 meters as a seismic channel (35) to observe signals.
3. The fiber optic multi-channel seismic system for ultra-shallow sea formation shear wave velocity structure detection as claimed in claim 1 wherein said fiber optic array (34) employs, for example, a linear spread (29), a Z-spread (30) or an O-spread (31).
4. The optical fiber multi-channel seismic system for the detection of the structure of the shear wave velocity of the stratum in the ultra-shallow sea area according to claim 1, wherein the gas explosion seismic source 1 comprises an excitation seismic source device (4), a gas source storage tank (5) and a gas injection system (6);
the structure of the seismic source excitation device (4) comprises a seismic source main body (7), a detonation control component (8) and an excitation end (9); the seismic source main body (7) is a gun body cast by steel, and a reaction cavity (10) for storing reaction gas and a gas injection valve (11) are designed in the gun body; the detonation control component (8) comprises a GPS time service clock crystal oscillator module (12), a thermal explosion wire (13), a high-voltage ignition circuit (14), a timing circuit (15), an embedded MCU (16) and a battery cabin (17); the excitation end (9) is composed of an arched diaphragm (14); the gas source storage tank (5) comprises an oxygen cylinder storage tank (18) and a hydrogen/methane cylinder storage tank (19);
the gas injection system (6) controls an industrial standard oxygen cylinder and a hydrogen/methane cylinder to respectively pass through a pressure reducing valve, a one-way valve and a stop valve gas injection pipeline (20) in a time-sharing manner, and gas is injected into the excitation seismic source device (4) through a gas injection console (21), so that the output of seismic waves of artificial sources with different energies and frequency spectrums is realized.
5. The fiber-optic multi-channel seismic system for detecting the structure of the shear wave velocity of the stratum in the ultra-shallow sea area according to claim 1, wherein the demodulation and recording unit (3) comprises a light emitting module (37), an interferometer module (38), a photoelectric conversion module (39), an electrical module (40) and a data acquisition and processing module (41);
the light emitting module (37) comprises an exciter, an acousto-optic modulator, an optical isolator, an optical amplifier, a fiber grating and a circulator; the optical transmitting module (37) is used for generating encoding pulses with narrow line width and high peak power and eliminating the influence of nonlinear effect and ASE noise;
the interferometer module (38) comprises a Faraday rotator mirror, piezoelectric ceramics and an optical coupler, and generates phase-modulated interference signals by loading PGC carrier signals, so that the problems of polarization fading and environmental interference are solved;
the photoelectric conversion module (39) converts weak coherent Rayleigh scattered light signals into electrical signals through a light detector;
the electrical module (40) generates a synchronized pulse code signal and a PGC carrier signal based on the carrier generator, wherein the pulse code signal is used for modulating the optical modulator and the PGC carrier signal is used for modulating the interferometer (38);
the data acquisition processing module (41) comprises a data acquisition card and a signal processing module, is used for acquiring data at a high speed, and finishes the real-time extraction of phase signals through data recombination, PGC demodulation and a noise stabilization algorithm.
6. The fiber-optic multi-channel seismic system for detecting the structure of the shear wave velocity of the stratum in the ultra-shallow sea area as claimed in claim 5, wherein the demodulation recording unit (3) further comprises a human-computer interaction and display module (42), and the human-computer interaction and display module (42) is used for displaying after the data acquisition processing module (41) finishes the real-time extraction of the phase signals.
7. Use of an optical fibre multi-track seismic system as claimed in claim 3, characterised in that the system with an optical fibre array (34) with a planar O-shaped spread (31) is used for shear wave splitting observation in shale gas exploration.
8. The method for detecting the structure of the shear wave velocity of the stratum in the ultra-shallow sea area by using the optical fiber multi-channel seismic system as claimed in claim 1, which is characterized by comprising the following steps:
0) selecting a layout of the fiber array (34) comprising a linear spread (29), a Z-shaped planar spread (30), and an O-shaped spread (31):
1) laying submarine optical fibers:
arranging an optical fiber array (34) of the distributed acoustic sensing optical fiber array (2) on the seabed through a boat or a ship according to a preset design route, wherein the arrangement of the optical fiber array (34) is not more than 1.25 km;
2) gas explosion seismic source excitation:
the gas explosion seismic source is shot on land or is excited on the shallow sea bottom; before the gas explosion seismic source starts to inject gas, satellite signal receiving time synchronization is carried out for not less than 10 minutes, the number of satellites is not less than 3, and the time synchronization is carried out until a crystal oscillator module clock is locked; then the gas injection system injects gas to the excitation seismic source device; after gas injection is finished, frequency division is carried out on a crystal oscillator signal into 1Hz pulses, the 1Hz pulses are input to a counter and are compared with the preset detonation time, once the set detonation time is reached, a detonation control signal is immediately output to trigger a high-voltage ignition circuit, the high-voltage ignition circuit immediately outputs pulse high voltage to ignite mixed gas for a thermal explosion igniter, and ignition excitation of a gas explosion seismic source is realized; after excitation, the interior of the cavity of the seismic source excitation device instantly reaches high temperature and high pressure, and water vapor and carbon dioxide generated by reaction instantly expand to break through the arch-shaped diaphragm at the excitation end and release huge energy in submarine sediments or land strata to form seismic waves of an artificial source;
4) the demodulation recording system works:
firstly, automatically searching satellite signals by using a GPS time service clock crystal oscillator module of the equipment, completing clock correction and clock locking, and carrying out satellite signal time service for 10 minutes, wherein the number of satellites is not less than 3;
a second step of generating synchronous pulse code signals and PGC carrier signals by using an electrical module, wherein the pulse code signals are transmitted to a Laser of an optical emission module to modulate excited light pulses, the optical emission module generates light code pulses with narrow line width and high peak power through the Laser (Laser), An Optical Modulator (AOM), an optical Isolator (ISO) optical amplifier (EDFA), an optical fiber grating (FBG), an optical interconnection device and the like, and emits the light pulses to a sensing optical fiber array at equal time intervals, and at the moment, the backscattered light information carrying seismic wave information is transmitted to an interferometer module through a Circulator (Circulator); the electrical module generates a synchronous PGC Carrier signal (Carrier) through a load Generator (plus Generator) and transmits the signal to the interferometer module; converting the carrier signal into a phase modulation interference signal through piezoelectric ceramics (PZT) in the interferometer module;
when seismic waves excited by the gas explosion seismic source (1) carry information such as seabed stratum reflection, refraction and surface waves to be transmitted to the sensing optical fiber array (34), the sensing optical fibers and the seabed are integrated to generate stress and strain changes, the changes directly cause the sensing optical fibers to generate tensile deformation in the horizontal direction, and then emitted light is subjected to backscattering in the transmission process inside the sensing optical fibers; the stronger the seismic wave amplitude is, the more severe the tensile deformation of the sensing optical fiber is, and the larger the light backscattering amplitude is;
synchronously decoupling the back scattering information transmitted by the sensing optical fiber and interference signals modulated by PGC phase carriers through an Optical Coupler (OC) to obtain coherent Rayleigh scattering optical signals, and further converting weak coherent Rayleigh scattering optical signals into electrical signals by using a Photoelectric Detector (PD) of a photoelectric conversion module;
fourthly, transmitting the electrochemical signals to a data acquisition and processing module, realizing data recombination, PGC demodulation and noise stabilization algorithm on the electrochemical signals by using a Signal processing module (Signal Processor Unit) to complete real-time extraction of phase signals, and realizing equal-time-interval high-speed sampling of seismic signals by using a data acquisition card (DAQ); the data acquisition part configures parameters of the data acquisition card by calling a hardware drive of the data acquisition card and transmits the configuration information to a PGC demodulation algorithm; the data processing part takes a Phase Generation Carrier (PGC) algorithm as a core; the data result processing part is used for displaying and storing the wavelength signals obtained by the PGC algorithm; and finally, processing, displaying and storing the acquired original data in real time.
9. The method of claim 8, further comprising step 5) long duration continuous observation:
the demodulation recording unit 3 and the gas explosion seismic source 1 are internally embedded with a satellite signal receiver clock high-precision synchronization module, and a quartz crystal oscillator is used for preventing the system time drift of the equipment; therefore, stress and strain field change information sensed by the sensing optical fiber array on the seabed for a long time is fed back to the demodulation recording unit according to the Universal Time (UTC) sequence, and the demodulation recording unit continuously repeats the step 4) to obtain continuous seismic wave field information of months to years; the seismic waves can be gas explosion seismic sources from artificial sources or natural earthquakes from the nature; the surface wave and transverse wave information of the seismic wave field are utilized to obtain the transverse wave velocity structure of the underground space of the detection area.
10. The method as claimed in claim 8, wherein in the step 1) of laying the submarine optical fibers, the outer part of the optical fiber cable is protected by an armor or lead weight to increase the weight of the body, so as to enhance the coupling with the seabed, and after laying, the optical fiber cable is placed for several days, so as to further increase the coupling effect between the optical fibers and the seabed sediments by using the natural acting force of ocean tides, waves and sediment settlement.
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