Three-component detector for seismic wave directional detection and optimization method

Abstract

The invention discloses a three-component geophone for directional detection of seismic waves and an optimization method, and the geophone achieves the three-dimensional collection of the seismic waves through orthogonally arranging three steel elastic sheets on a metal frame type fixing frame, connecting the free end of each elastic sheet with a mass block and packaging a fiber bragg grating. The optimization method comprises the steps of material and structure optimization, parameter calculation and simulation verification, performance test and the like, the stability is improved by adopting a glass solder packaging process, and the consistency of triaxial performance is ensured through theoretical calculation and simulation analysis, so that the detector has good sensitivity response and anti-interference capability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of seismic exploration, and particularly relates to a three-component geophone for directional detection of seismic waves and an optimization method. BACKGROUND

[0002] In recent years, seismic exploration technology is becoming more and more mature, which promotes the rapid development of seismic geophones. In the past, well seismic exploration technology mainly depends on electric geophones, which have problems such as being susceptible to electromagnetic interference and not resistant to high temperature in practical application.

[0003] Compared with electric geophones, a fiber Bragg grating (FBG) geophone detects the changes of grating period and effective refractive index caused by external vibration, which leads to the shift of Bragg wavelength, so as to realize the measurement of vibration signals. The output signal of the FBG geophone is consistent with the acceleration signal characteristics of conventional seismic geophones, and can be directly used for seismic wave analysis. Moreover, the FBG geophone has advantages such as strong anti-electromagnetic interference ability, high sensitivity, corrosion resistance, high temperature resistance, safety and reliability, and multiplexing, and has gradually become a mainstream geophone in existing seismic geophones.

[0004] Current FBG seismic detection sensors can be divided into single-component and multi-component according to the information collection dimension. The single-component geophone can only obtain vibration signals of a single axis, which has obvious limitations in complex geological exploration: on the one hand, it is difficult to analyze the polarization characteristics of seismic wave field (such as shear wave splitting), and on the other hand, it cannot meet the accuracy requirements of three-dimensional velocity modeling. In comparison, the three-component FBG geophone can realize vector reconstruction of wave field and improve the detection sensitivity of low-frequency signals (<1Hz) by synchronously collecting x / y / z three-axis vibration data, which has key significance for deep resource exploration and fine structure interpretation. Therefore, developing a high-performance three-component FBG geophone has become an important research direction of seismic sensing technology. SUMMARY

[0005] The application provides a three-component geophone for directional detection of seismic waves and an optimization method. The three-component geophone is based on a conventional single-component seismic geophone, and three cantilever beams are orthogonally arranged on the base to realize three-dimensional collection of seismic wave information. The structure parameters are optimized through theoretical calculation and experiment, so that the three-component geophone can not only well complete three-component collection of seismic waves, but also ensure good sensitivity response.

[0006] The technical scheme adopted by the application is as follows:

[0007] A three-component geophone for directional detection of seismic waves, comprising a metal frame type fixing frame 1, three steel elastic sheets 2 respectively embedded on the inner wall of the frame type fixing frame 1, wherein two are vertically fixed and the other is horizontally fixed, three mass blocks 3 respectively fixedly connected to the free ends of the steel elastic sheets 2, three groups of fiber Bragg gratings FBG5, FBG6 and FBG7 respectively encapsulated on the steel elastic sheets 2, and an optical fiber 4 in series communication with the three groups of fiber Bragg gratings.

[0008] Further, the steel elastic sheet 2 is in triangular structure.

[0009] An optimization method of a three-component geophone for directional detection of seismic waves, comprising the following steps:

[0010] S1, material and structure optimization:

[0011] The fixing frame 1 and the steel elastic sheet 2 are integrally processed by using 316L stainless steel, so as to ensure mechanical strength and environmental adaptability;

[0012] The fiber Bragg gratings are encapsulated along the central axis of the steel elastic sheet 2 and are encapsulated by using glass solder to improve high temperature and high pressure stability;

[0013] The structure parameters of the geophone are simulated and optimized by using Matlab software, and the steel elastic sheets 2 are orthogonally arranged on X / Y / Z three axes;

[0014] S2, parameter optimization:

[0015] The orthogonal arrangement parameters of the steel elastic sheets 2 are determined by theoretical calculation and COMSOL modal analysis simulation;

[0016] Based on the vibration model formula, the length / thickness of the steel elastic sheet and the mass of the mass block 3 are optimized;

[0017] The three-axis natural frequency of the geophone is ensured to be lower than 50Hz and consistent;

[0018] S3, simulation verification:

[0019] Modal analysis is carried out by using COMSOL software, the consistency of the three-axis natural frequency is verified, and the first-order, second-order and third-order vibration mode diagrams are obtained;

[0020] S4, performance test:

[0021] The natural frequency is determined by amplitude-frequency response test, 0.1g acceleration reference is adopted, vibration frequency starts from 0.6Hz, measurement step is 5Hz, and when approaching the natural frequency, the step becomes 1Hz;

[0022] The anti-interference ability is verified by transverse interference test, 20Hz working frequency band is selected, and the interference degree is required to be less than 5%;

[0023] The acceleration response characteristics are verified by a sensitivity test, the frequency is fixed at 30 Hz, and the acceleration amplitude is adjusted from 0.1 g to 1 g at a step of 0.1 g.

[0024] Further, in step S1, the fiber Bragg grating is packaged with glass solder, and the packaging position is located at the center line of the surface of the steel elastic sheet 2. The packaging mode avoids high-temperature creep.

[0025] Further, in step S2, the simulation includes Matlab software simulation and COMSOL modal analysis, and the parameter optimization is based on a vibration model formula, including an equivalent mass calculation formula, an inherent frequency calculation formula, a strain displacement relationship formula and a wavelength drift formula.

[0026] Further, in step S4, the amplitude-frequency response test uses an air floating vibrator, and the wavelength range of the FBG dynamic demodulator is 1510-1590 nm.

[0027] The transverse interference test requires that the interference degree is less than 5%, the sensitivity test records the wavelength drift, and the data is processed by a computer.

[0028] The beneficial effects of the present application are:

[0029] 1. The frame-shaped fixing frame and the triangular steel elastic sheet are orthogonally arranged in an integrated manner, the X / Y / Z three-axis sensing unit is integrated in a single compact structure, and the volume limitation of the traditional split detector is broken; FBG5 / FBG6 / FBG7 three groups of gratings are accurately packaged along the center line of the elastic sheet, forming a specific receiving array for P wave, SVH wave and SVP wave. The glass solder packaging process improves the grating strain transmission efficiency by 35%, and eliminates the baseline drift problem caused by the aging of the adhesive.

[0030] 2. The optimal structure parameters of the detector are determined by theoretical calculation and simulation, so that the structure not only can well complete the three-component acquisition of seismic waves, but also can ensure that the detector has good accuracy and sensitivity response. Compared with the traditional single-component detector, the three-component FBG seismic detector has higher detection accuracy and wider application scenarios. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0032] Figure 1 is a structure diagram of the three-component seismic detector of the present application;

[0033] Figure 2 is a detector vibration model;

[0034] Figure 3 is a detector amplitude-frequency response test curve in x, y and z directions;

[0035] Figure 4 is a detector operating frequency range in x, y and z directions;

[0036] Figure 5 is a detector transverse anti-interference test curve in x, y and z directions;

[0037] Figure 6 is a detector sensitivity test curve in x, y and z directions.

[0038] In the figure, 1. fixed frame, 2. steel spring, 3. mass block, 4. optical fiber, 5. FBG, 6. FBG, 7. FBG. DETAILED DESCRIPTION

[0039] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0040] A three-component detector for directional detection of seismic waves, as shown in Figure 1 , comprises a frame-shaped fixed frame 1 of metal material, three steel springs 2 in a triangular shape are respectively embedded on the inner wall of the frame-shaped fixed frame 1, and the other end of the steel spring 2 is respectively fixedly connected with a cuboid mass block 3. The fixed frame 1 and the steel spring 2 are integrally processed and manufactured by stainless steel.

[0041] As shown in Figure 1 , of which two steel springs 2 are fixed vertically, and the other steel spring 2 is fixed horizontally, and three groups of fiber Bragg gratings (FBG) are respectively encapsulated on the surface middle line positions of the steel springs 2, which are FBG5, FBG6 and FBG7 respectively, and the optical fiber 4 is arranged along the middle line of the steel spring 2 and the middle line of the fixed frame 1, and is connected with the three groups of gratings.

[0042] First, the detector is fixed on the vibration table, when the detector is subjected to vibration signals in different directions, the mass block 3 under the action of inertia forces the free end of the steel spring 2 to move, driving the FBG to strain, the FBG is stretched or compressed, and the center wavelength of the FBG will also change accordingly, until the vibration signal stops, the FBG returns to the initial state. At this time, only the amount of change of the FBG center wavelength needs to be measured, the vibration signal and vibration direction of the external environment can be obtained. Among them, FBG6 is the main measurement grating for determining the detection direction, which receives the reflected P wave in the detection direction, FBG5 receives the reflected SVH wave in the detection direction, and FBG7 receives the reflected SVP wave in the detection direction.

[0043] The optimization method of the three-component detector for seismic wave directional detection comprises the following steps:

[0044] S1, optimizing material selection and structure design;

[0045] The 316L stainless steel integrated processing fixing frame and the steel spring are adopted to ensure the mechanical strength and environmental adaptability. The mass block material is selected as high-density alloy, the FBG is packaged along the central axis of the steel spring with 502 glue to avoid high-temperature creep, and glass solder is used to improve the high-temperature and high-pressure stability.

[0046] The parameters of the detector are determined by Matlab software simulation, and the structure of the detector is optimized. The steel spring needs to be arranged orthogonally (X / Y / Z three-axis), the length and thickness parameters are optimized by Matlab simulation, the mass of the mass block is matched by theoretical calculation and simulation (COMSOL modal analysis), and the natural frequency is ensured to be below 50Hz.

[0047] S2, theoretical analysis and parameter optimization;

[0048] The natural frequency is analyzed and compared with the COMSOL simulation results for verification:

[0049] The vibration model of the detector is shown in Figure 2 When the detector is in a static state, the displacement of the mass block and the horizontal line is y0, and the effective length of the optical fiber is L at this time. When the external vibration signal is applied, the displacement of the mass block under the acceleration state is y1, and the effective length of the optical fiber is L1 at this time, and m is the mass of the mass block.

[0050] When the detector is subjected to vibration signals in the vertical y direction, the vibration acceleration can be obtained according to Newton's second law:

[0051]

[0052] Among them, M eff is the equivalent mass of the detector, K eff is the equivalent stiffness of the detector, and ΔL is the change of the FBG.

[0053] According to the mass distribution of the beam, the equivalent mass of the detector is:

[0054] M eff = m + 0.45m1 (2)

[0055] Where m1 is the total mass of the cantilever beam.

[0056] Since the three cantilever beams and the mass block of the detector are of the same size, the first, second, and third natural frequencies of the three-component FBG seismic detector are equal, and the calculation formula is:

[0057]

[0058] Optimize the sensitivity by adjusting the elastic coefficient of the steel spring and the mass of the mass block:

[0059] The surface axial strain of the FBG is i The relationship of the present transverse displacement is as follows:

[0060]

[0061] When i = 1, it represents the strain of the FBG when there is vibration acceleration; when i = 0, it represents the strain of the FBG in the static state. The FBG is fixed between the base above the cantilever beam and the mass block, and the relationship between the axial strain of the FBG fixed between the base and the mass block and the axial strain of the FBG on the surface of the cantilever beam is as follows: i

[0062]

[0063] Where t is the vertical distance between the FBG and the cantilever beam after being pasted.

[0064] When the vibration frequency is much smaller than the natural frequency of the detector, the displacement of the free end of the cantilever beam is:

[0065]

[0066] According to formulas 2-6, we have

[0067]

[0068] Where Δε is the strain change of the optical fiber, and Δε = ε1- ε0. According to the principle of FBG, the wavelength shift of the FBG center is:

[0069] Δλ B = λ B (1-P e )Δε (8)

[0070] Where λ​B is the center wavelength of FBG, B = 1, 2, 3; P e is the effective photoelastic coefficient of the fiber. Since the three cantilever beams and the mass block of the three-component FBG seismometer are of the same size, the sensitivity calculation formulas of the first, second and third orders are as follows:

[0071]

[0072] S3, simulation verification is carried out to confirm the consistency of the three-axis natural frequency;

[0073] The COMSOL software is used to carry out modal analysis on the three-component FBG seismometer model, and the first, second and third order vibration mode diagrams of the seismometer are obtained. The steel elastic sheets reciprocate towards the x, y and z directions respectively, and the size and material parameters, fixed constraints and additional loads of the three steel elastic sheets are the same, so the simulation simulation obtains the equal first, second and third order natural frequencies, which proves that the simulation result is reliable.

[0074] S4, experimental test and performance verification;

[0075] The three-component FBG seismometer experimental system is composed of a computer, a FBG dynamic demodulator (micro optics, sm130), an air floating vibrator (ETZ-186-400), a signal generator (IM1208C) and a power amplifier (FDD-1). The seismometer is fixed on the air floating vibration platform, and the dynamic demodulator is composed of a self-emission (ASE) laser source and a demodulator, with a wavelength range of 1510-1590nm and a scanning frequency of 1kHz. The computer records the center wavelength of the seismometer.

[0076] Amplitude frequency response test:

[0077] In order to measure the natural frequency of the seismometer in three directions, the seismometer is first fixed on the vibration table according to the x, y and z directions respectively; then, the acceleration is fixed as 0.1g, the vibration frequency starts from 0.6Hz, the measurement step is 5Hz, when approaching the natural frequency, the measurement step becomes 1Hz, finally, the collected data is processed to obtain the amplitude frequency response test curve of the seismometer as shown in Figure 3 It can be seen that the first, second and third order natural frequencies of the seismometer are 56Hz, 59Hz and 63Hz respectively; in the range of 0.6-50Hz, the three directions of x, y and z have good frequency flat response, that is, the working frequency of the sensor is below 50Hz.

[0078] Lateral anti-interference test:

[0079] The transverse anti-interference capability of the detector is an important index for distinguishing the direction source of the vibration signal. In order to verify the transverse anti-interference capability of the detector, the detector is placed on a workbench according to the x, y and z working directions to perform experiments, and the working frequency ranges in the three directions are obtained as shown in Figure 4 . Since the working frequency band of the detector is only in the flat zone range, the present application selects the 20Hz working frequency band in the range, and the transverse anti-interference curves after applying 0.1g acceleration in the three directions are obtained as shown in Figure 5 . Tests show that in whichever direction the detector works, the transverse interference degree in the remaining two directions is less than 5%. Therefore, the transverse anti-interference capability of the detector is good, and the basic vibration direction can be identified.

[0080] Sensitivity verification:

[0081] In order to measure the sensitivity of the detector, the frequency is fixed at 30Hz during the experiment, the acceleration amplitude is adjusted from 0.1g to 1g with a step of 0.1g, and the wavelength drift corresponding to each acceleration is recorded. The present application performs three repeated experiments, and the sensitivity curves of the detector in the x, y and z directions are obtained as shown in Figure 6 . It can be seen that the sensitivities of the detector in the x, y and z directions are 218.22pm / g, 284.76pm / g and 249.67pm / g respectively, and the wavelength drift has a very good linear relationship with the acceleration, and the average linearity is as high as 99.8%. This shows that the repeatability performance of the detector is good and the response sensitivity is good.

[0082] The optimization method of the three-component detector for directional detection of seismic waves determines the optimization structure parameters of the detector through theoretical calculation and simulation, so that the structure can not only complete the three-component acquisition of seismic waves well, but also ensure that the detector has good sensitivity response. Experimental results show that the flat zone of the detector is 0.6-50Hz, the first-order, second-order and third-order natural frequencies are 56Hz, 59Hz and 63Hz respectively, the sensitivities in the x, y and z directions are 218.22pm / g, 284.76pm / g and 249.67pm / g respectively, and the transverse interference degree is less than 5%, which is close to the theoretical value. Compared with the traditional single-component detector, the detection accuracy of the three-component FBG seismic detector is higher, and the application scenarios are more extensive.

[0083] Each embodiment in the specification is described in a related manner, and the same and similar parts between each embodiment can be referred to each other. Each embodiment mainly explains the difference from other embodiments. Especially, for the system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and the related parts can be referred to the part of the method embodiment.

[0084] The above merely provides the preferred embodiments of the application, and not intended to limit the protection scope of the application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the application shall fall within the protection scope of the application.

Claims

1. A three-component geophone for directional detection of seismic waves, comprising a metal frame-type mount (1), characterized in that: Three steel springs (2) are respectively embedded on the inner wall of the frame-shaped fixing frame (1), two of which are vertically fixed, and the other is horizontally fixed, three mass blocks (3) are respectively fixedly connected to the free ends of the steel springs (2), three groups of fiber Bragg gratings (FBG5, FBG6, FBG7) are respectively packaged on the steel springs (2), and the optical fibers (4) are connected in series with the three groups of fiber Bragg gratings.

2. The geophone of claim 1, wherein: The steel spring (2) is a triangular structure.

3. A method for optimizing a three-component geophone for directional seismic wave detection, characterized by, The steps include: S1, material and structure optimization: The fixing frame (1) and the steel spring (2) are integrally processed by using 316L stainless steel to ensure mechanical strength and environmental adaptability; The fiber Bragg grating is packaged along the central axis of the steel spring (2), and glass solder is used for packaging to improve high temperature and high pressure stability; The structure parameters of the detector are simulated and optimized by Matlab software, and the steel springs (2) are orthogonally arranged in X / Y / Z three axes; S2, parameter optimization: The orthogonal arrangement parameters of the steel springs (2) are determined by theoretical calculation and COMSOL modal analysis simulation; Based on the vibration model formula, the length / thickness of the steel spring and the mass of the mass block (3) are optimized; Ensure that the natural frequency of the three-axis detector is lower than 50Hz and the consistency is matched; S3, simulation verification: Modal analysis is carried out by using COMSOL software to verify the consistency of the three-axis natural frequency, and the first-order, second-order and third-order vibration mode diagrams are obtained; S4, performance test: The natural frequency is determined by amplitude-frequency response test, 0.1g acceleration reference is used, vibration frequency starts from 0.6Hz, measurement step is 5Hz, and step becomes 1Hz when approaching natural frequency; The anti-interference ability is verified by transverse interference test, 20Hz working frequency band is selected, and the interference degree is required to be less than 5%; The acceleration response characteristics are verified by sensitivity test, the frequency is fixed at 30Hz, and the acceleration amplitude is adjusted from 0.1g to 1g with a step of 0.1g.

4. The optimization method of claim 3, wherein In step S1: The fiber Bragg grating is packaged with glass solder, and the packaging position is located on the center line of the surface of the steel spring (2), and the packaging method avoids high temperature creep.

5. The method of claim 3, wherein In step S2: The simulation includes Matlab software simulation and COMSOL modal analysis, parameter optimization is based on vibration model formula, including equivalent mass calculation formula, natural frequency calculation formula, strain displacement relationship formula and wavelength drift formula.

6. The method of claim 3, wherein In step S4: The amplitude-frequency response test uses air floating vibrator, and the wavelength range of FBG dynamic demodulator is 1510-1590nm; The transverse interference test requires that the interference degree is less than 5%, and the sensitivity test records the wavelength drift, and the data is processed by computer.

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