CN108415067A - A kind of earthquake wave measuring system based on microstructured optical fibers distribution sound wave sensing - Google Patents

Abstract

The invention relates to a seismic wave measurement system based on microstructure optical fiber distributed acoustic wave sensing. Fiber optic cables, signal control and processing modules. The signal light generated by the narrow-linewidth laser is injected into the microstructure sensing optical cable by the circulator, and the external seismic wave signal is measured through the optical cable. The reflected light signal carrying the seismic wave information and the reference light form a heterodyne signal and send it to the balanced photodetector. The electrical signal is finally demodulated by the signal control processing module to realize the effective measurement of the seismic wave signal. The invention has the advantages of high measurement sensitivity and high spatial resolution. Compared with traditional electrical detectors, the invention has simple structure, small size and weight, easy installation, and is free from electromagnetic interference. The invention can be used in the fields of measuring seismic wave signals propagated underground, resource detection, geological exploration and the like.

Description

A Seismic Wave Measurement System Based on Microstructure Optical Fiber Distributed Acoustic Wave Sensing

technical field

The invention relates to a microstructure optical fiber distributed acoustic wave sensing system, in particular to a seismic wave measurement system based on microstructure optical fiber distributed acoustic wave sensing.

Background technique

Optical fiber sensing technology can be used to measure parameters such as downhole pressure, temperature, sound waves, and vibrations to obtain information such as geological conditions, fluid characteristics, and resource distribution. In the fields of petroleum exploration and geological testing, optical fiber sensing well logging and seismic wave measurement is a very important technology. At present, optical fiber distributed acoustic wave sensing is widely used in vertical seismic profile (VSP) detection, real-time fracturing monitoring, oil and gas pipeline leakage monitoring, and ground seismic wave propagation measurement and other fields. Therefore, the high-sensitivity and high-precision optical fiber distributed acoustic wave sensing technology is of great significance to resource exploration and earthquake detection.

The main means of current seismic exploration is to use electrical geophones to measure, which is currently an important seismic sensing component for seismic exploration. The technical level and performance quality are directly related to the fidelity effect of seismic data detection. Compared with other detection methods, the electrical geophone has higher measurement accuracy and sensitivity, but it is susceptible to electromagnetic interference due to its internal structure based on electrical domain measurement and mechatronics. At the same time, since seismic exploration is generally in a complex field environment, it is inevitable that the geophone will have a high failure rate. In addition, each geophone is distributed in different positions to detect the transmission of seismic waves at different positions for a period of time, so that each geophone needs to be connected to the terminal with a cable, because the geophone can only detect the seismic wave at a single point When measuring long-distance ground arrays, a large number of detector strings are required, and the huge number of detectors makes the installation process complicated and cumbersome.

Contents of the invention

The present invention proposes a seismic wave detection system based on micro-structured optical fiber distributed acoustic wave sensing. The geophone has technical problems such as large installation scale, susceptibility to electromagnetic interference, and susceptibility to environmental damage.

A seismic wave measurement system based on microstructure optical fiber distributed acoustic wave sensing, characterized in that it includes a narrow linewidth laser (1), a first optical coupler (2) and a second optical coupler (8), an optical circulator (5), acousto-optic modulator (3), erbium-doped optical fiber amplifier (4), balanced photodetector (7), microstructure sensing optical cable (6) and control processing module (9); wherein:

The output of the narrow linewidth laser (1) is connected to the first coupler (2) to provide a narrow linewidth laser signal; the laser linewidth is as narrow as possible, below 1MHz; the narrower the linewidth, the smaller the noise floor of the detection signal ;

The first coupler (2) includes a light source input end, a local oscillator light output end, and a signal light output end; the light source input end is used to receive narrow linewidth laser light, and the local oscillator light output end is connected to the second coupler (8 ) is connected to the local oscillator light input end to provide reference light; the signal light output end is connected to the input end of the acousto-optic modulator (3) to provide signal light to it;

The acousto-optic modulator (3) includes three ports of input, output and drive, which are used to modulate the laser into pulses and generate a certain frequency shift; the frequency shift causes the signal light to generate a frequency difference relative to the local oscillator light, forming an external Difference detection is used to demodulate the phase change caused by the seismic wave to realize the measurement of the seismic wave; the frequency shift is determined by the radio frequency signal applied to the acousto-optic modulator; The work also needs a driving signal; the pulse is a rectangular pulse, and the pulse period is related to the driving signal of the acousto-optic modulator. The greater the duty cycle of the driving control terminal signal, the greater the energy of the optical pulse;

The erbium-doped fiber amplifier (4) includes two ports of optical input and optical output, for amplifying input optical signals;

The first port of the optical circulator (5) is connected to the output end of the erbium-doped fiber amplifier (4), the second port is connected to the sensing optical cable (6), and the third port is connected to the input end of the second coupler (8); The optical circulator (8) is used to transmit the light from the first port to the second port for output, and transmit the light entering from the second port, that is, the light reflected by the microstructure points in the optical fiber, to the third port, and pass through the The port is sent to the second coupler (8); the second coupler (8) is used to combine the reference light and the signal light into one path and send them to the input end of the balanced photodetector (7);

The microstructure sensing optical cable (6) is used as a sensing unit for sensing the propagation of seismic waves, and detects seismic waves by demodulating the reflected optical signal in the optical cable; it contains several 5- The number of 20dB microstructure points can reach hundreds, and the area between every two microstructure points is a detection unit, and its reflected light carries the detected seismic wave signal; too strong reflection will make the optical insertion loss of forward transmission Too large, that is, a large part of the light is reflected back to the terminal, which is not conducive to the increase of the sensing distance;

The balanced photodetector (7) includes two input ports and a differential output port; the detector output is the difference between the two inputs, that is, the difference between the converted currents of the two detectors inside it; the second coupler ( 8) The output light enters the input end of the balanced photodetector (7); the two ports of the input end of the photodetector are respectively connected with the two output ports of the second coupler; the output terminal of the balanced photodetector (7) is connected to the control The input terminal of the processing module (9); the balanced photodetector (7) is used to convert the optical signal after the interference of the light reflected by the optical cable with the local oscillator light, that is, the beat frequency signal formed by the second coupler, into an electrical signal, Send into control processing module (9);

The control processing module (9) includes an input end, a drive port and a PC connection data port; the input end of the control processing module (9) is connected to the output end of the balanced photodetector (7), and the drive end is connected to the acousto-optic modulator (3) It is used to drive the acousto-optic modulator to work normally; the PC is connected to the data port for an external computer to realize real-time result display and data storage; the control processing module (9) is used to demodulate the electrical signal output by the detector (7), and demodulate and recover The external seismic wave signal detected by the optical cable generates a driving signal to control the operation of the acousto-optic modulator. The main functions of the control processing module (9) are summarized as generation, transformation, multiplication, filtering and amplitude peak finding of the reference signal. The purpose is to demodulate the optical phase change caused by the external seismic wave signal and obtain the phase change amount.

Further, the microstructure optical fiber (11) is encapsulated in the wear-resistant sheath outer cable (13), and the elastic medium (12) is filled between the optical fiber and the outer cable sheath, so as to protect the microstructure optical fiber and enhance the resistance to external seismic waves. Sensitivity of detection; the microstructure optical fiber is written on the common single-mode optical fiber with several scattering-enhanced microstructure points (10) at the same interval through the ultraviolet writing technology, which is used to reflect the optical signal in the optical fiber.

Further, the light splitting ratio of the first optical coupler is 1:99 (reference light: signal light), and the light splitting ratio of the second optical coupler is 1:1.

Further, the elastic medium has better transmission force between the optical fiber and the external environment, preferably Kevlar fiber. The elastic medium should have good transmission characteristics, that is, it can better transmit the external force to the internal optical fiber; at the same time, it has the characteristics of high strength, corrosion resistance, and tensile strength; Kevlar fiber has high strength, high temperature resistance, etc. , corrosion resistance and other characteristics, can ensure the normal operation of the optical cable in harsh environments, and is suitable as the filling medium for the outer layer of the optical fiber;

Further, each scattering-enhanced microstructure point written on the optical fiber of the microstructure sensing optical cable (6) has full-band reflection characteristics, and the reflectivity is 5-20dB higher than Rayleigh scattering;

Further, the control processing module includes an embedded processing module (15), a D/A module (16), an A/D module (18), a bandpass filter (17); the A/D module (18) Be used for gathering signal and be converted into digital signal, send into band-pass filter (17); Described band-pass filter (17) is used for realizing intermediate frequency band-pass, obtains heterodyne intermediate frequency signal, filters out noise, and its output connects Described embedded processing module (15); Described embedded processing module (15) is used for demodulating the signal that gathers, and produces drive signal, sends into acousto-optic modulator (7) after D/A conversion, To drive the acousto-optic modulator to work normally. The seismic wave propagates along the direction of fiber optic laying, and reaches different fiber segments at different times. After demodulation, the seismic wave waveform of each segment of fiber at the corresponding time can be obtained;

Further, the purpose of demodulation in the control processing module is to extract the optical phase change amount caused by the seismic wave, that is, to find the phase difference between when the seismic wave is transmitted and when there is no seismic wave, including the following steps:

The light reflected back from the collected microstructure points and the beat frequency signal (19) formed by the local oscillator light are band-pass filtered (20) to filter out noise, and the heterodyne signal is obtained for phase extraction processing (24): respectively and the reference The signal 1 (23) is multiplied by the reference signal 2 (25), and then subjected to low-pass filtering respectively, and the two signals thus obtained are subjected to a division operation to obtain the tangent value of the phase quantity, and the phase value is obtained after the arctangent operation, Wherein, the reference signal 1 and the reference signal 2 are sinusoidal signals with a phase difference of π/2, and their frequency is equal to the frequency shift generated by the acousto-optic modulator;

At the same time, the automatic peak-seeking (21) process is performed on the signal amplitude to realize the positioning of the microstructure point reflection peak of each section of optical fiber, and the corresponding phase change is extracted through the above process, and then the phase difference between two adjacent microstructure points is calculated (22) , get the phase change on the optical cable connecting two points, and get the seismic wave information on this section of optical fiber according to the change, so as to realize the demodulation of the seismic wave signal.

Further, each section of microstructure fiber optic cable distributed in different positions can be regarded as a seismic wave detection unit to realize the detection of seismic wave signals, and its equivalent position is regarded as the middle of the optical fiber between two microstructure points, which can detect the position within a period of time The seismic wave propagation information at the location, and then the time and position distribution map of the seismic transmission waveform is obtained. This is equivalent to using an optical cable as a seismic wave detection unit. The optical cable between every two microstructure points is a detection unit. All seismic wave detection units are connected in series on one cable to detect seismic wave transmission at different locations. The optical cable is both a transmission signal. The medium is a section of detection units distributed in different positions.

Further, the balanced detector (9) has two built-in photodetectors with completely similar characteristics, and the direct current component can be removed after the subtraction of the two beams of light, which can eliminate receiver noise in optical coherent detection, and improve the system signal-to-noise ratio and sensitivity , which is beneficial to the detection of weak signals; different from ordinary photodetectors, it has two built-in channels, unlike single-tube detectors that only contain one probe.

Further, the seismic wave measurement system further includes a PC (14), used for displaying seismic wave signals and storing data. Combined with PC(14) software development, the collected data is processed by the corresponding demodulation algorithm, and the restored seismic wave measurement results can be displayed on the PC(14), and the PC can also control the embedded processing module accordingly , In this way, the PC is equivalent to the upper computer and is mainly used for software programming development and storage.

Compared with the prior art, the present invention mainly has the following advantages:

1. High measurement sensitivity and high spatial resolution. Each section of optical cable between two reflection points is used as a detection unit in the corresponding interval, which can detect the seismic wave transmission information at this position within a certain period of time;

2. Compared with the traditional electric detector, its structure is simple, small in size and easy to install, and it is not subject to electromagnetic interference.

The invention can measure seismic wave signals propagated underground, and can be applied to fields such as resource detection and geological exploration.

Description of drawings

Figure 1 is a block diagram of the system device;

Figure 2 is a detailed illustration of the optical cable;

Fig. 3 is the internal structure of the control and processing module;

Fig. 4 is algorithm processing flowchart;

Fig. 5 is the time position distribution map of seismic wave.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not constitute a conflict with each other.

The present invention will be further described in detail with reference to the drawings and embodiments. The invention provides a microstructure optical fiber distributed acoustic wave measurement system that can be used in seismic wave detection technology. The system consists of the following components: narrow linewidth lasers, 1:99 and 1:1 optical couplers, acousto-optic modulators, erbium-doped fiber amplifiers, optical circulators, microstructure sensing cables, balanced photodetection modules, and control and processing Embedded data processing module, DA module, AD module and filtering module in the module.

Among them, the microstructured optical cable is composed of an internal microstructured optical fiber and an outer package. Micro-structured optical fiber refers to the use of ultraviolet writing technology on ordinary single-mode optical fiber to continuously write several Rayleigh scattering enhancement points with extremely low reflectivity and the same at the same pitch. Outer encapsulation refers to encapsulating a layer of protective leather cable outside the micro-structured optical fiber. The elastic medium is filled between the optical fiber and the leather cable to play the role of force transmission. Here, Kevlar fiber is selected as an example.

The narrow linewidth laser output a narrow linewidth laser signal of a certain wavelength, which is divided into two beams by a 1:99 optical coupler, and 1% of the light is output to the other 1:1 coupler input end as a local oscillator reference light, 99 % of the light is transmitted to the next device, the acousto-optic modulator, as the probe signal light.

The acousto-optic modulator is driven by the embedded processing module in the control and processing module through the DA module to work normally. Its function is to modulate the optical signal into pulsed light and make the optical signal generate a certain frequency shift. The frequency shift here is 200MHz for example.

The pulsed light is then amplified by the erbium-doped fiber amplifier and input to the 1 port of the circulator, and the light from the 1 port of the circulator is sent to the 2 port of the circulator, that is, transmitted to the sensing optical cable connected to the 2 port.

There are a series of scattering-enhanced microstructure points in the sensing optical cable. The reflectivity here is 10dB higher than Rayleigh scattering, which can reflect the forward transmitted light. The external seismic wave acts on the sensing optical cable, and the internal elastic medium transmits the force Acting on the internal microstructure fiber, causing a change in the phase of the reflected light. The reflected light enters the 2 port of the circulator, and the light at the 2 port of the circulator is sent to the 3 port of the circulator, and the optical signal to be tested at the 3 port is sent to the other input end of the 1:1 coupler, and the light through the coupler and the local oscillator Coupled into the same optical path to form a heterodyne detection, the two beams combined into a beat frequency signal.

The optical signal output by the coupler is received by the balanced photoelectric detection module and converted into an electrical signal, filtered by the band-pass filter, collected by the AD module, and sent to the embedded data processing module. After the algorithm processing process as shown in Figure 4, each beat frequency signal collected can be demodulated to obtain its phase change amount, and the connection between two points can be obtained through the differential operation of the phase change amount between two adjacent microstructure points. The phase change amount produced by the optical fiber, and the phase change of the optical fiber section caused by the seismic wave transmission to the optical fiber section in the corresponding interval, so that the seismic wave signal at the moment in the interval can be known, and by analogy, the entire optical cable distribution interval can be known. Seismic wave information at each moment.

Every optical cable between two adjacent microstructure points is a seismic wave detection unit, which not only transmits light but also serves as a sensing unit, and a single optical cable is divided into seismic wave detection units at different positions. Buried it underground, when seismic waves are transmitted underground, information at different times at each location can be obtained. Figure 5 shows the transmission information of seismic waves at different locations and at different times.

It is easy for those skilled in the art to understand that the above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, All should be included within the protection scope of the present invention.

Claims (10)

1. a kind of earthquake wave measuring system based on microstructured optical fibers distribution sound wave sensing, which is characterized in that including narrow linewidth Laser (1), the first photo-coupler (2) and the second photo-coupler (8), optical circulator (5), acousto-optic modulator (3), erbium-doped fiber Amplifier (4), balance photodetector (7), micro-structure sensing optic cable (6) and control process module (9)；Wherein：

Narrow linewidth laser (1) output is connected with the first coupler (2), for providing narrow-linewidth laser signal；

First coupler (2) includes light source input terminal, local oscillator light output end and signal light output end；The light source input terminal For receiving narrow-linewidth laser, local oscillator light output end is connected with the second coupler (8) local oscillator light input end, provides it reference Light；Signal light output end is connected with acousto-optic modulator (3) input terminal, provides it signal light；

The acousto-optic modulator (3) includes three input, output and driving ports, for Laser Modulation at pulse and to be generated one Fixed frequency displacement；

The EDFA Erbium-Doped Fiber Amplifier (4) includes light input and two ports of light output, for amplifying input optical signal；

The first port of the optical circulator (5) is connected with EDFA Erbium-Doped Fiber Amplifier (4) output end, and second port is passed with micro-structure Sensing optical cable (6) is connected, and third port connects the second coupler (8) input terminal；The optical circulator (8) is used for the light of first port It is delivered to second port output, the light entered from second port is delivered to third port, the second coupling is sent to through the port Device (8)；Second coupler (8) is used to reference light and signal light being combined into all the way, is sent into balance photodetector (7) and inputs End；

The micro-structure sensing optic cable (6) is used as sensing unit, the propagation for perceiving seismic wave；

The balance photodetector (7) includes two input ports and a difference output port, the second coupler (8) output Light enter balance photodetector (7) input terminal；It balances photodetector (7) and exports the defeated of termination control process module (9) Enter end；The light that balance photodetector (7) is used to optical cable being reflected back and the optical signal after the local oscillator interference of light, are converted into telecommunications Number, it is sent into control process module (9)；

The control process module (9) includes input terminal, port is driven to connect data port with PC；Control process module (9) is defeated Enter the output end of end connection balance photodetector (7), driving end connection acousto-optic modulator (3) is for driving acousto-optic modulator just Often work；PC connection data port is used for external computer；Realize that real-time results are shown and data store；Control process module (9) it is used to demodulate the electric signal of detector (7) output, demodulation restores the extraneous seismic signal that optical cable is detected, and generates drive Dynamic signal is to control the work of acousto-optic modulator.

2. according to the earthquake wave measuring system described in claim 1, which is characterized in that the microstructured optical fibers (11) are packaged in Within the outer cable (13) of rubber-insulated wire, elastic fluid (12) is filled between optical fiber and outer layer cable skin, to realize protection microstructured optical fibers and increasing By force to the sensitivity of extraneous seismic wave detection；The microstructured optical fibers be by uv-exposure technology, will be several with identical spacing A single-point Rayleigh scattering enhancing point (10) is scribed on general single mode fiber, for the optical signal in mirror based fiber optica.

3. according to the earthquake wave measuring system described in claim 1, which is characterized in that the first photo-coupler (2) reference The splitting ratio of light and signal light is 1：99, the splitting ratio of the second photo-coupler (8) reference light and signal light is 1：1.

4. according to the earthquake wave measuring system described in claim 2, which is characterized in that the elastic fluid is in optical fiber and the external world There is preferable conductance between environment, and need tension anticorrosive, preferably Kafra fiber.

5. according to the earthquake wave measuring system described in claim 2, which is characterized in that the micro-structure sensing optic cable (6) The each single-point Rayleigh scattering enhancing point inscribed on optical fiber has all band reflection characteristic, and reflectivity is low but higher than Rayleigh scattering 5-20dB。

6. according to the earthquake wave measuring system described in claim 1, which is characterized in that the control process module includes insertion Formula processing module (15), D/A modules (16), A/D modules (18), bandpass filter (17)；The A/D modules (18) are for acquiring Signal is simultaneously converted into digital signal, is sent into bandpass filter (17)；The bandpass filter (17) for realizing intermediate frequency band logical, Output connects the embedded processing module (15)；The embedded processing module (15) is for solving collected signal It adjusts, and generates drive signal, acousto-optic modulator (7) is sent into after D/A is converted, to drive acousto-optic modulator to work normally.

7. earthquake wave measuring system according to claim 6, which is characterized in that in the control process module demodulation include Following steps：

After beat signal (19) bandpass filtering (20) that the light that collected micro-structure point reflection is returned is formed with local oscillator light, make phase Position extraction process (24)：It is multiplied respectively with reference signal 1 (23) and reference signal 2 (25), it is low-pass filtered again respectively, thus Two signals arrived carry out division arithmetic, obtain the tangent value about phase mass, phase value is obtained after arctangent cp cp operation, In, reference signal 1 and reference signal 2 are the sinusoidal signals that phase difference is pi/2, and frequency is equal to frequency caused by acousto-optic modulator Move size；Automatic peak-seeking (21) is carried out to signal amplitude simultaneously, positions the micro-structure point reflection peak of every section of optical fiber, it is corresponding to extract it Phase change amount is obtained by phase difference (22) between calculus of differences 2 adjacent microstructures points of calculating on point-to-point transmission connection optical cable Phase change obtains the seismic wave information on this section of optical fiber according to variation, to realize the demodulation to seismic signal.

8. according to the earthquake wave measuring system described in claim 1, which is characterized in that seismic wave causes optical signal in optical cable Variation is distributed in every section of microstructured optical fibers optical cable of different location and is realized to seismic signal for a seismic wave detection unit Detection, equivalent position can be considered in the middle part of the optical fiber between 2 micro-structure points；Optical cable itself is detection unit, can detect one section Seimic wave propagation information in time at this position, and then obtain the time location distribution map of seismic acquisition waveform.

9. according to the earthquake wave measuring system described in claim 1, which is characterized in that two built in the balanced detector (9) The completely close photodetector of a characteristic, two-beam can remove DC component after subtracting each other, can eliminate in optical coherent detection and receive Machine noise, lifting system signal-to-noise ratio and sensitivity are conducive to the detection of small-signal.

10. earthquake wave measuring system according to claim 6, which is characterized in that further include (14) PC, for showing seismic wave Signal and storage data.