Earthquake detection system and method and application of earthquake detection system and method in earthquake surface survey

Abstract

The invention provides an earthquake detection system and method and application of the earthquake detection system and method in earthquake surface survey, and relates to the technical field of seismic exploration. The optical fiber sensor is arranged in the detection well and / or buried in the ground; the optical fiber data acquisition module is connected with one end of the optical fiber sensor and is used for providing a light source and acquiring detection data; the excitation device is arranged in the detection well and / or on the ground close to the wellhead of the detection well and is used for forming a seismic source; at least one problem existing in an earthquake surface survey technology in the prior art is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of seismic exploration, and more specifically, relates to a seismic detection system, method and its application in seismic surface investigation. Background Art

[0002] Seismic exploration is the most important and effective method for solving oil and gas exploration problems in geophysical exploration. Seismic exploration refers to a geophysical exploration method that uses the elastic waves caused by artificial excitation, utilizes the differences in elasticity and density of underground media, and infers the properties and forms of underground rock formations by observing and analyzing the propagation laws of seismic waves generated by artificial earthquakes underground.

[0003] In the process of processing and interpreting seismic data obtained from seismic exploration, using surface investigation methods to find out parameters such as the thickness and velocity of the shallow surface layer is an important step. The situation of the near-surface structure, including lithology, water content, topography, the thickness and velocity stratification caused by weathering, etc., has a very obvious impact on the quality of seismic acquisition data. Therefore, in seismic exploration, the surface structure investigation of the work area is an indispensable and important basic work, and the results of surface investigation can be applied to the design of the excitation well depth and static correction. The accuracy of surface investigation determines the accuracy of the final results of seismic exploration.

[0004] Due to the non-perfect elastic characteristics of formation media, seismic waves will undergo severe attenuation during the propagation process in the formation, especially the high-frequency energy part is significantly missing, which greatly reduces the imaging resolution and accuracy of seismic data. With the continuous deepening of oil and gas exploration, due to reasons such as complex near-surface conditions and severe weathering and erosion in the area, the energy attenuation of seismic waves during the propagation process in the formation is even more serious, which has become one of the main reasons affecting the resolution of seismic data. Therefore, using the results of surface investigation to calculate the absorption attenuation coefficient and perform Q compensation on seismic waves can effectively improve the resolution of seismic data.

[0005] Since the shallow surface layer of the earth usually changes rapidly and has strong heterogeneity, a higher point density is required for surface investigation. Therefore, a method with high precision, simple construction, low cost, preferably without additional drilling, and the instrument equipment can be reused can be popularized in actual seismic exploration. Currently, the commonly used conventional surface structure investigation methods in seismic exploration include shallow refraction method, micro-logging investigation method, etc.

[0006] The shallow refraction method uses shallow refraction waves and direct waves to study the surface structure. Generally, dozens of geophones are laid on a flat ground surface with different trace intervals. Usually, the trace interval closer to the shot point is designed to be smaller, and the interval gradually increases as the shot-receiver distance increases. Limited by the channel capacity of the instrument, the maximum offset is within 200 meters. It is applicable to areas with flat ground surface and very small dip angles of the underground interface. The advantages are that a larger array length can be designed and the trace intervals can be reasonably distributed to ensure the controlled number of channels for direct waves and refraction waves of each layer. At the same time, there is no need for drilling, and the construction cost is relatively low. However, the disadvantage is that it is not applicable to complex ground surfaces and the accuracy is relatively low.

[0007] The micro-logging method is a method of collecting seismic wave information and obtaining geophysical parameters of the near-surface or shallow underground strata by exciting in the well and receiving on the ground, or exciting on the ground and receiving with downhole geophones, or exciting in the well and receiving in the well. The micro-logging method can directly receive the up-going wave information excited or received at different depths in the well, and is less affected by the terrain, so the interpretation accuracy is high. The micro-logging method is often used in areas with complex surface structures such as severe terrain undulations, formation velocity inversion, or the existence of thin interbeds. Compared with the shallow refraction method, the micro-logging method has high investigation accuracy, but the operation process is complex, the construction efficiency is low, and the cost is high.

[0008] The disadvantages of the existing surface investigation techniques are in three aspects. First, the shallow refraction method has relatively low accuracy and is not applicable to areas with large surface undulations and large dip angles of the underground interface, so the application area is greatly limited. Second, for the purpose of reducing costs, the micro-logging investigation method with downhole receiving and surface excitation is mostly used in surface investigation. During construction, one or several geophones are placed in a shallow well to measure the seismic wave velocity of the shallow layer. Since the geophones placed in the shallow well need to be reused in different shallow wells, it is impossible to bury the geophones arranged in the shallow well underground to ensure good coupling between the downhole geophones and the formation. This incomplete or poor coupling will bring great errors and mistakes to the measurement accuracy of the seismic wave velocity in the shallow well. Due to the limited channel capacity of the shallow layer investigation instrument, 24 channels or 48 channels are commonly used in China. To achieve a high sampling density, the downhole geophones need to be moved point by point for measurement, resulting in low operation efficiency of the micro-logging and unable to arrange a large number of micro-logging in the construction area with high density. Third, since the surface seismic source of the conventional micro-logging is only excited near the wellhead, the conventional micro-logging method only measures the vertical velocity of the shallow seismic waves from the wellhead to the bottom of the well. Due to the strong longitudinal and transverse inhomogeneity of the shallow underground medium, the seismic wave velocity in the shallow layer will be anisotropic, that is, there will be an obvious difference between the vertical velocity and the horizontal velocity of the seismic waves in the formation, and this obvious difference cannot be solved by the micro-logging technology. Summary of the Invention

[0009] The object of the present invention is to provide a seismic detection system, method and its application in seismic surface investigation in view of the deficiencies existing in the prior art, and to solve at least one problem existing in the seismic surface investigation technology in the prior art.

[0010] To achieve the above object, the present invention provides a seismic detection system, comprising:

[0011] A detection well;

[0012] An optical fiber sensor, which is arranged in the detection well and / or buried in the ground;

[0013] An optical fiber data acquisition module, which is connected to one end of the optical fiber sensor and is used to provide a light source and acquire detection data;

[0014] An excitation device, which is arranged in the detection well and / or on the ground near the wellhead of the detection well and is used to form a seismic source.

[0015] Optionally, the optical fiber sensor is an armored optical fiber, and the armored optical fiber comprises an elastic rod, an optical fiber core and a shell. The optical fiber core is spirally wound around the outer periphery of the elastic rod, and the shell is wrapped outside the elastic rod around which the optical fiber core is wound.

[0016] Optionally, a part of the optical fiber sensors are arranged vertically in the detection well, and another part of the optical fiber sensors are buried in the ground and connected to the optical fiber data acquisition module.

[0017] Optionally, the optical fiber sensor arranged in the detection well is inserted into a sleeve, and the inner diameter of the sleeve matches the outer diameter of the optical fiber sensor, so that the optical fiber sensor can be coupled and the optical fiber sensor can be pulled out of the sleeve.

[0018] Optionally, the detection well is filled with a first filler, and the first filler compacts and fixes the sleeve in the detection well.

[0019] Optionally, the ground is provided with a receiving groove, and the optical fiber sensor buried in the ground is arranged in the receiving groove. The receiving groove is provided with a second filler, and the second filler buries and compacts and fixes the optical fiber sensor in the receiving groove.

[0020] Optionally, the excitation device comprises a plurality of first excitation devices, and the plurality of first excitation devices are arranged at intervals vertically in the detection well.

[0021] Optionally, the first excitation device is a detonator, and the plurality of detonators are connected to an excitation cable. The upper end of the excitation cable extends to the ground and is connected to a detonator.

[0022] Optionally, the lower end of the excitation cable is wound around the outer periphery of the lowermost detonator and connected to the lower end of the optical fiber sensor in the inspection well.

[0023] Optionally, the excitation device includes a second excitation device disposed on the ground near the wellhead of the inspection well, and the second excitation device is a heavy hammer, a vibroseis or a detonator.

[0024] The present invention also provides a seismic detection method, which uses the above seismic detection system, and is characterized by including:

[0025] Setting an inspection well, and arranging an optical fiber sensor, an optical fiber data acquisition module and an excitation device;

[0026] Starting the optical fiber data acquisition module and the excitation device, forming a seismic source by the excitation device and collecting detection data by the optical fiber data acquisition module;

[0027] Extracting the moment when the seismic wave is received, and performing time-depth conversion, calculating formation velocity, formation thickness and stratification conditions.

[0028] Optionally, the performing time-depth conversion, calculating surface layer velocity, surface layer thickness and stratification conditions includes:

[0029] Calculating the formation velocity by using the following formula (1):

[0030] V i =(T i -T i-1 ) / (H i -H i-1 )

[0031] wherein, V i is the formation velocity at the i-th sampling point; T i is the moment when the seismic wave is received at the i-th sampling point; H i is the depth at the i-th sampling point;

[0032] Performing time-depth conversion by using the following formula (2):

[0033] T 0 =T*(H / sqrt(H 2 +d 2 ));

[0034] wherein, T is the reception time, that is, the reception moment minus the excitation moment; H is the excitation point depth; d is the distance between the reception point of the optical fiber sensor buried on the ground and the wellhead of the inspection well;

[0035] Converting the first arrival time at the offset to the vertical time of zero offset, and eliminating the influence of the elevation difference and the buried depth between the wellhead and the excitation point;

[0036] By using the relationship between depth and time, the time-depth curve of each layer, i.e., the vertical time-distance curve, is fitted.

[0037] The velocity of each layer and the thickness of each layer are obtained by calculating the slope of the time-depth curve and the intersection points of adjacent time-depth curves.

[0038] The converted vertical time and the corresponding depth are plotted in a time-depth coordinate system.

[0039] When different depth points are within the same velocity layer, the distribution of the points is a straight line. The slopes of the straight lines corresponding to different velocities are different. According to their distribution rules, the positions of each layer are divided. A straight line is fitted for each layer by the least squares method. The reciprocal of the slope of the straight line is the layer velocity of the medium, and the intersection point of the two straight lines is the interface of the medium.

[0040] Optionally, it further includes:

[0041] According to the characteristics of the amplitude and spectrum change of the seismic waves received by different parts of the fiber optic sensor, the seismic plane wave attenuation coefficient and Q value of the shallow subsurface are calculated and obtained by using the spectral ratio method or the centroid frequency shift method or the spectral fitting method.

[0042] Optionally, calculating and obtaining the seismic wave attenuation coefficient and Q value of the shallow subsurface includes:

[0043] The amplitude spectrum of the plane wave is calculated by using the following formula three:

[0044] B(f, t) = A(t)B(f, t 0 )exp(-πft / Q);

[0045] The amplitude ratio at different times is calculated by using the following formula four:

[0046] B(f, t i ) / B(f, t i-1 ) = (A(t i )exp(-πft i / Q)) / (A(t i-1 )exp(-πft i-1 / Q));

[0047] The logarithm of the amplitude ratio is calculated by using the following formula five:

[0048] ln(B(f, t i ) / B(f, t i-1 )) = C - (πf(t i - t i-1 ) / Q);

[0049] Among them, B(f,t) is the amplitude spectrum of a plane wave vertically propagating from the ground to underground at time t; A(t) is the coefficient of the plane wave amplitude spectrum; f is the frequency of the plane wave; t is the time; Q is the plane wave attenuation coefficient; the logarithm of different amplitude ratios is only a linear function of the frequency, and the Q value can be obtained by fitting the slope.

[0050] The present invention also provides an application of the above seismic detection method in seismic surface investigation.

[0051] The present invention provides a seismic detection system, method and its application in seismic surface investigation. The beneficial effects are as follows: The seismic detection system has an optical fiber sensor and an optical fiber data acquisition module. It uses the distributed optical fiber acoustic wave sensor technology to conduct seismic detection and then carry out seismic surface investigation. The optical fiber data acquisition module provides a light source for the optical fiber sensor and collects detection data, and can receive detection data on the ground and in the detection well. This seismic detection system combines the advantages of two seismic surface investigation techniques, namely shallow refraction and micro-logging. Under the condition of ensuring coupling, it can better receive seismic signals. At the same time, the optical fiber sensor can be recycled, reducing the construction cost and facilitating popularization and use.

[0052] Other features and advantages of the present invention will be described in detail in the subsequent specific implementation part. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] By describing the exemplary embodiments of the present invention in more detail in conjunction with the drawings, the above and other objects, features and advantages of the present invention will become more obvious. Among them, in the exemplary embodiments of the present invention, the same reference numerals generally represent the same components.

[0054] Figure 1 FIG. 1 shows a schematic structural diagram of a seismic detection system according to Embodiment 1 of the present invention.

[0055] Figure 2 FIG. 2 shows a schematic structural diagram of the armored optical fiber of a seismic detection system according to Embodiment 1 of the present invention.

[0056] Figure 3 FIG. 3 shows a schematic structural diagram of the lower end of the excitation cable of a seismic detection system according to Embodiment 1 of the present invention.

[0057] Figure 4 FIG. 4 shows a schematic structural diagram of a seismic detection system according to Embodiment 2 of the present invention.

[0058] Figure 5 FIG. 5 shows a flowchart of a seismic detection method according to Embodiment 3 of the present invention.

[0059] DESCRIPTION OF THE REFERENCE NUMERALS

[0060] 1. Detection well; 2. Optical fiber sensor; 3. Optical fiber data acquisition module; 4. Elastic rod; 5. Optical fiber core; 6. Shell; 7. Sleeve; 8. Detonator; 9. Excitation cable; 10. Initiator; 11. Connection part; 12. Elastic airbag. Detailed implementation mode

[0061] The preferred implementation modes of the present invention will be described in more detail below. Although the preferred implementation modes of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited by the implementation modes set forth herein. On the contrary, these implementation modes are provided to make the present invention more thorough and complete, and to fully convey the scope of the present invention to those skilled in the art.

[0062] Embodiment 1

[0063] As Figures 1 to 3 shown, the present invention provides a seismic detection system, including:

[0064] Detection well 1;

[0065] Optical fiber sensor 2, the optical fiber sensor 2 is arranged in the detection well 1 and / or buried in the ground;

[0066] Optical fiber data acquisition module 3, the optical fiber data acquisition module 3 is connected to one end of the optical fiber sensor 2, and is used to provide a light source and collect detection data;

[0067] Excitation device, the excitation device is arranged in the detection well 1 and / or on the ground near the wellhead of the detection well 1, and is used to form a seismic source.

[0068] Specifically, in order to solve at least one problem existing in the seismic surface investigation technology in the prior art, the seismic detection system provided by the present invention has an optical fiber sensor 2 and an optical fiber data acquisition module 3, and uses the distributed optical fiber acoustic wave sensor technology to perform seismic detection, and then carry out seismic surface investigation. The optical fiber data acquisition module 3 provides a light source for the optical fiber sensor 2 and collects detection data, and can receive detection data on the ground and in the detection well 1. This seismic detection system combines the advantages of two seismic surface investigation technologies, namely shallow refraction and micro-logging. Under the condition of ensuring coupling, it can better receive seismic signals. At the same time, the optical fiber sensor 2 can be recycled, reducing the construction cost and facilitating popularization and use.

[0069] In this embodiment, the detection well 1 is a shallow well. The shallow well can be drilled with a small drill at the position of the surface investigation point first, and the depth is determined according to the predicted surface layer thickness in the construction area. To save construction costs, the excitation well to be used for placing explosives in production can also be used as the detection well 1.

[0070] Furthermore, the seismic detection system can simultaneously bury fiber optic sensors 2 in shallow wells and on the ground. The fiber optic sensors 2 buried on the ground can receive refracted waves from deep formations, and the fiber optic sensors 2 in the detection well 1 can receive direct waves and formation reflection waves generated by excitation at the wellhead or in the well. The detection data collected and recorded by the fiber optic data acquisition module 3 includes seismic wave data. Using known techniques for data processing and interpretation, the arrival time of the received seismic waves is extracted and time-depth conversion is performed, and the formation velocity, formation thickness, and stratification of the surface layer can be calculated.

[0071] Data interpretation: When interpreting seismic surface survey data, first convert the first arrival time at the offset to the vertical time at zero offset, and at the same time eliminate the influence of the elevation difference and burial depth between the wellhead and the excitation point; then, through the relationship between depth and time, fit the time-depth curve (vertical time-distance curve) of each layer, and calculate the velocity and thickness of each layer based on the slope of the time-depth curve and the intersection of the time-depth curves of adjacent layers.

[0072] Calculation formula for formation velocity of fiber optic in well: V i =(T i -T i-1 ) / (H i -H i-1 ); where V i is the formation velocity at the i-th sampling point, T i is the arrival time of the seismic wave at the i-th sampling point, and H i is the depth at the i-th sampling point.

[0073] Time-depth conversion formula for ground fiber optic: T 0 =T*(H / sqrt(H 2 +d 2 )); where T is the reception time (arrival time minus excitation time), H is the excitation point depth, and d is the distance from the ground fiber optic reception point to the wellhead.

[0074] Q value calculation: Based on the characteristics of the amplitude and frequency spectrum changes of seismic waves at different depths of the fiber optic, known techniques such as the spectral ratio method, centroid frequency shift method, or spectral fitting method can be used to calculate or obtain the seismic wave attenuation coefficient and Q value of the shallow subsurface.

[0075] Optionally, the fiber optic sensor 2 is an armored fiber optic, and the armored fiber optic includes an elastic rod 4, a fiber optic core 5, and a housing 6. The fiber optic core 5 is helically wound around the outer periphery of the elastic rod 4, and the housing 6 is wrapped outside the elastic rod 4 wound with the fiber optic core 5.

[0076] Specifically, the fiber optic sensor 2 uses armored optical fiber. There is at least one ordinary single-mode optical fiber or special acoustic or vibration-sensitive optical fiber in the armored optical fiber inside the outer shell 6. The optical fiber core 5 inside the armored optical fiber is in a helical structure. The optical fiber core 5 is helically and densely wound around the elastic rod 4 to set a longer length of the optical fiber core 5 in a specific area to pick up the acoustic signals acting on it and better receive seismic signals. At the same time, the helical structure is beneficial to protecting the optical fiber core 5 and is not easily broken due to pulling. The helical structure can also make the optical fiber core 5 form a certain angle with the axis of the ground and the detection well 1, which is beneficial to better receiving seismic longitudinal wave signals.

[0077] Further, the materials of the elastic rod 4 and the outer shell 6 can be selected as needed, and rubber-like materials can be selected.

[0078] Optionally, a part of the fiber optic sensors 2 are arranged vertically in the detection well 1, and another part of the fiber optic sensors 2 are buried in the ground and connected to the fiber optic data acquisition module 3.

[0079] Specifically, for the convenience of construction and carrying, a part of the fiber optic sensor 2 is arranged in the detection well 1, and another part extends out of the wellhead and is buried in the ground. The end on the ground is connected to the fiber optic data acquisition module 3. The fiber optic data acquisition module 3 includes a light source and a modem, etc., and is an integrated structure. Connecting the fiber optic sensor 2 can provide a light source for it and collect detection data.

[0080] Optionally, the fiber optic sensor 2 arranged in the detection well 1 is threaded through the sleeve 7. The inner diameter of the sleeve 7 matches the outer diameter of the fiber optic sensor 2 so that the fiber optic sensor 2 can be coupled and the fiber optic sensor 2 can be pulled out of the sleeve 7.

[0081] Specifically, the fiber optic sensor 2 in the detection well 1 is lowered into the well using the sleeve 7. To ensure the smooth recovery of the fiber optic sensor 2 received in the well, a connectable sleeve 7 made of PVC material or alloy material is used. Each section of the sleeve 7 is 1 - 2 meters long, which is convenient for carrying and construction. The sleeve 7 uses screw threads or bayonets and can be connected end to end until the bottom of the well according to the well depth. In this embodiment, the inner diameter of the sleeve 7 is slightly larger than the outer diameter of the armored optical fiber, ensuring good coupling, being able to receive high-quality seismic waves, and at the same time protecting the armored optical fiber against the shock wave generated by the explosion of the detonator 8. After the construction is completed, the armored optical fiber can be smoothly pulled out.

[0082] Optionally, the detection well 1 is filled with a first filler, and the first filler compacts and fixes the sleeve 7 in the detection well 1.

[0083] Specifically, the first filler can be sand, cuttings, soil, etc., which are used to fill and compact the detection well 1. More preferably, the gaps in the detection well 1 are filled with water to ensure good wave impedance coupling between the excitation cable 9, the sleeve 7, and the formation, and to ensure the excitation and reception effect.

[0084] Optionally, the ground is provided with a receiving groove, and the optical fiber sensor 2 buried in the ground is arranged in the receiving groove. A second filler is arranged in the receiving groove, and the second filler buries and compacts the optical fiber sensor 2 in the receiving groove.

[0085] Specifically, the length of the optical fiber sensor 2 buried in the ground is determined according to the terrain around the inspection well 1 and its shallow burial depth. The length is preferably not less than 1.2 times the burial depth of the formation to be measured. The burial method is to excavate a receiving groove, place the optical fiber sensor 2, and then backfill and compact.

[0086] Optionally, the excitation device includes a plurality of first excitation devices, and the plurality of first excitation devices are arranged at intervals in the vertical direction in the inspection well 1.

[0087] Specifically, when well excitation is adopted, a plurality of first excitation devices are provided, and the plurality of first excitation devices are arranged at intervals in the vertical direction in the inspection well 1. The spacing can be set to one excitation point per 1 meter.

[0088] Optionally, the first excitation device is a detonator 8. A plurality of detonators 8 are connected to the excitation cable 9, and the upper end of the excitation cable 9 extends to the ground and is connected to a detonator 10.

[0089] Specifically, using the detonator 8 as the first excitation device, the detonator 10 arranged on the ground is sequentially connected to a plurality of detonators 8 through the excitation cable 9. The excitation cable 9 is lowered into the inspection well 1, and each detonator 8 is excited sequentially from bottom to top during excitation.

[0090] Optionally, the lower end of the excitation cable 9 is wound around the outer periphery of the lowermost detonator 8 and is connected to the lower end of the optical fiber sensor 2 in the inspection well 1.

[0091] Specifically, the lower end of the excitation cable 9 forms a connecting portion 11. The connecting portion 11 is connected to the bottom of the optical fiber sensor 2 and is wound around the lowermost detonator 8. There are two benefits. One is that the optical fiber sensor 2 and the excitation cable 9 can be inserted to the bottom of the inspection well 1 through the casing 7, and under the action of the weight of the detonator 8, the lower end of the optical fiber sensor 2 can be pulled down to form a drooping effect, which is beneficial to straightening the optical fiber sensor 2 and ensuring the accuracy of the excitation position of the detonator 8 and the receiving position of the optical fiber sensor 2. The other is that the connecting portion 11 can be blown off when the detonator 8 explodes, which is convenient for removing the optical fiber sensor 2.

[0092] This seismic detection system uses distributed fiber optic acoustic sensing technology for seismic detection, and then realizes seismic surface investigation, integrating the advantages of two surface investigations, namely shallow refraction and micro-logging. It includes armored optical fibers installed in the detection well 1 and buried on the ground. The armored optical fibers are connected to a fiber optic data acquisition module 3 with a distributed fiber optic sensing modulation and demodulation instrument, and excitation can be carried out around the wellhead or in the well of the detection well 1; in this embodiment, excitation in the well is adopted. The armored optical fibers buried on the ground are buried in a shallow groove, that is, a receiving groove. After the armored optical fibers installed in the detection well 1 are protected by a casing 7, they are filled and compacted with water-containing sand and gravel; the armored optical fibers buried on the ground can extract and calculate the refraction wave travel times from each seismic source point in the well to each sampling point, and calculate the velocity and thickness of the deep formation; the armored optical fibers installed in the detection well 1 can calculate the average velocity of the formation around the well according to the distance from each seismic source point to the sampling point, the travel time of the direct wave, and the waveform, and then divide the formation according to the velocity change situation, determine the thickness of each layer, and perform shallow surface tomography; at the same time, the seismic wave attenuation coefficient and Q value of the shallow part of the underground can be calculated or obtained, and static correction and seismic wave absorption attenuation compensation can be carried out to improve the processing accuracy of surface seismic data.

[0093] This seismic detection system has four main advantages:

[0094] Firstly, it integrates the advantages of shallow refraction, single-well micro-logging, and double-well micro-logging. The armored optical fibers buried on the ground can receive refraction wave signals from deep layers, solving the defect that the traditional micro-logging survey method cannot calculate the velocity and thickness below the bottom of the well; the armored optical fibers installed in the detection well 1 can simultaneously receive seismic waves excited from the ground and in the well, achieving the effect of shallow refraction + double-well micro-logging with one well.

[0095] Secondly, the distributed fiber optic acoustic sensing technology is continuous sampling, with no length limitation, and there is no need for multiple up-and-down or chasing shot excitations. The excitation wavelets have good consistency. At the same time, it solves the problems of limited channel capacity of conventional surface investigation instruments in the past, low sampling rate, and low interpretation accuracy when encountering complex formations or formations with velocity inversion.

[0096] Thirdly, the armored optical cable adopted is slender and uniform, facilitating formation coupling and carrying. The process of sleeving with a casing 7 and lowering it into the well ensures good coupling and can also be taken out smoothly. It is tensile and wear-resistant and convenient to carry; it solves the problems of the traditional geophone being bulky and difficult to couple in the well.

[0097] Fourthly, the reusable armored optical fibers save construction costs and are convenient for popularization and use. The length of the armored optical fibers is not limited, sampling is sufficient, and there is no need for multiple up-and-down or chasing shot excitations, reducing the number of excitation points; it can be carried out in production wells without increasing the well depth or in double wells for investigation, greatly saving production costs.

[0098] Embodiment 2

[0099] As shown Figure 4 in the figure, the present invention provides a seismic detection system, comprising:

[0100] a detection well 1;

[0101] an optical fiber sensor 2, which is arranged in the detection well 1 and / or buried in the ground;

[0102] an optical fiber data acquisition module 3, which is connected to one end of the optical fiber sensor 2 and is used to provide a light source and collect detection data;

[0103] an excitation device, which is arranged in the detection well 1 and / or on the ground near the wellhead of the detection well 1 and is used to form a seismic source.

[0104] Specifically, to solve at least one problem existing in the seismic surface investigation technology in the prior art, the seismic detection system provided by the present invention has an optical fiber sensor 2 and an optical fiber data acquisition module 3, and adopts the distributed optical fiber acoustic wave sensor technology to conduct seismic detection, and then carry out seismic surface investigation. The optical fiber data acquisition module 3 provides a light source for the optical fiber sensor 2 and collects detection data, and can receive detection data on the ground and in the detection well 1. This seismic detection system integrates the advantages of two seismic surface investigation technologies, namely shallow refraction and micro-logging. Under the condition of ensuring coupling, it can better receive seismic signals. At the same time, the optical fiber sensor 2 can be recycled, reducing the construction cost and facilitating popularization and use.

[0105] In this embodiment, the detection well 1 is a shallow well. The shallow well can first be drilled with a small drill at the position of the surface investigation point, and the depth is determined according to the predicted surface layer thickness in the construction area. To save construction costs, the excitation well to be filled with explosives during production can also be used as the detection well 1.

[0106] Furthermore, this seismic detection system can simultaneously bury the optical fiber sensor 2 in the shallow well and on the ground. The optical fiber sensor 2 buried on the ground can receive the refracted waves from the deep formation, and the optical fiber sensor 2 in the detection well 1 can receive the direct waves and formation reflection waves generated by excitation at the wellhead or in the well. The detection data collected and recorded by the optical fiber data acquisition module 3 includes seismic wave data. Using known technologies for data processing and interpretation, extracting the arrival time of the received seismic waves, and performing time-depth conversion, the formation velocity, formation thickness, and stratification conditions of the surface layer can be calculated.

[0107] Data Interpretation: When interpreting seismic surface survey data, first convert the first arrival time at the offset to the vertical time at zero offset, and at the same time eliminate the influence of the elevation difference and burial depth between the wellhead and the shot point; then, through the relationship between depth and time, fit the time-depth curve (vertical time-distance curve) of each layer, and obtain the velocity and thickness of each layer according to the slope of the time-depth curve and the intersection point of the time-depth curves of adjacent layers.

[0108] Calculation formula for formation velocity of fiber optic in well: V i =(T i -T i-1 ) / (H i -H i-1 ); where V i is the formation velocity at the i-th sampling point, T i is the moment when the seismic wave is received at the i-th sampling point, and H i is the depth at the i-th sampling point.

[0109] Time-depth conversion formula for surface fiber optic: T 0 =T*(H / sqrt(H 2 +d 2 ))), where T is the receiving time (the moment of reception minus the moment of shot), H is the depth of the shot point, and d is the distance between the surface fiber optic receiving point and the wellhead.

[0110] Q-value calculation: According to the characteristics of the amplitude and frequency spectrum changes of seismic waves at different depths of the fiber optic, known techniques such as the spectral ratio method, centroid frequency shift method, or spectral fitting method can be used to calculate or obtain the seismic wave attenuation coefficient and Q-value of the shallow subsurface.

[0111] Optionally, the fiber optic sensor 2 is armored fiber optic, and the armored fiber optic includes an elastic rod 4, a fiber optic core 5, and a housing 6. The fiber optic core 5 is helically wound around the outer periphery of the elastic rod 4, and the housing 6 is wrapped outside the elastic rod 4 around which the fiber optic core 5 is wound.

[0112] Specifically, the fiber optic sensor 2 uses armored fiber optic. There is at least one ordinary single-mode fiber or special acoustic or vibration-sensitive fiber inside the housing 6 of the armored fiber optic; the fiber optic core 5 inside the armored fiber optic is of a helical structure. The fiber optic core 5 is helically and densely wound on the elastic rod 4 in order to set a longer length of the fiber optic core 5 in a specific area to pick up the acoustic signals acting on it and better receive seismic signals; at the same time, the helical structure is beneficial to protecting the fiber optic core 5 and is not easily broken due to pulling; the helical structure can also make the fiber optic core 5 form a certain angle with the axis of the ground and the detection well 1, which is beneficial to better receiving seismic P-wave signals.

[0113] Furthermore, the materials of the elastic rod 4 and the housing 6 can be selected as needed, and rubber-like materials can be selected.

[0114] Optionally, a part of the optical fiber sensor 2 is vertically arranged in the inspection well 1, and the other part of the optical fiber sensor 2 is buried in the ground and connected to the optical fiber data acquisition module 3.

[0115] Specifically, for the convenience of construction and portability, a part of the optical fiber sensor 2 is arranged in the inspection well 1, and the other part extends out of the wellhead and is buried in the ground. The end on the ground is connected to the optical fiber data acquisition module 3. The optical fiber data acquisition module 3 includes a light source and a modem, etc., and is an integrated structure. Connecting the optical fiber sensor 2 can provide it with a light source and collect detection data.

[0116] In this embodiment, the armored optical fiber can be directly lowered into the inspection well 1.

[0117] Optionally, the inspection well 1 is filled with a first filler, and the first filler compacts and fixes the casing 7 in the inspection well 1.

[0118] In this embodiment, the first filler uses an elastic airbag 12 or a water bag. Inflating or filling the armored optical fiber with water when it is lowered into the well can push and compact the armored optical fiber against the well wall of the inspection well 1.

[0119] Optionally, a receiving groove is provided on the ground, and the optical fiber sensor 2 buried in the ground is arranged in the receiving groove. A second filler is provided in the receiving groove, and the second filler buries and compacts and fixes the optical fiber sensor 2 in the receiving groove.

[0120] Specifically, the length of the optical fiber sensor 2 buried in the ground is determined according to the topography around the inspection well 1 and its shallow burial depth. The length is preferably not less than 1.2 times the burial depth of the formation to be measured. The burial method is to excavate a receiving groove, put the optical fiber sensor 2 in, and then backfill and compact.

[0121] Optionally, the excitation device includes a second excitation device, and the second excitation device is arranged on the ground near the wellhead of the inspection well 1. The second excitation device is a heavy hammer, a vibroseis or a detonator 8.

[0122] Specifically, when wellhead excitation is adopted, a heavy hammer, a small vibroseis or a detonator 8 can be used as the second excitation device, and the second excitation device is arranged on the ground near the wellhead.

[0123] In this embodiment, the production excitation shallow well can be used as the inspection well 1 for construction, which will not damage the shallow well and does not require the use of the casing 7, further simplifying the construction difficulty and reducing the construction cost.

[0124] Embodiment 3

[0125] As Figure 5 shown, the present invention also provides a seismic detection method, which uses the seismic detection system in Embodiment 1, and is characterized by including:

[0126] A detection well 1 is set up, and an optical fiber sensor 2, an optical fiber data acquisition module 3 and an excitation device are arranged.

[0127] The optical fiber data acquisition module 3 and the excitation device are started, and a seismic source is formed by the excitation device and detection data is acquired by the optical fiber data acquisition module 3.

[0128] The arrival time of the received seismic wave is extracted, and time-depth conversion is performed, and the formation velocity, formation thickness and stratification conditions are calculated.

[0129] Specifically, this seismic detection method uses the above-mentioned seismic detection system. First, the detection well 1, the optical fiber sensor 2, the optical fiber data acquisition module 3 and the excitation device are arranged. The detection well 1 is a shallow well. A small drill is first used to drill a hole at the surface survey point position, and the depth is determined according to the predicted surface thickness in the construction area. To save construction costs, the excitation well to be filled with explosives during production can also be used. After setting up the above-mentioned seismic detection system, the optical fiber data acquisition module 3 and the excitation device can be started. A seismic source is formed by the excitation device and detection data is acquired by the optical fiber data acquisition module 3. The arrival time of the received seismic wave is extracted using the detection data, time-depth conversion is performed, and the formation velocity, formation thickness and stratification conditions can be calculated and obtained.

[0130] Optionally, performing time-depth conversion, calculating the surface velocity, surface thickness and stratification conditions includes:

[0131] The formation velocity is calculated using the following formula (1):

[0132] V i =(T i -T i-1 ) / (H i -H i-1 )

[0133] where V i is the formation velocity at the i-th sampling point; T i is the arrival time of the received seismic wave at the i-th sampling point; H i is the depth at the i-th sampling point;

[0134] Time-depth conversion is performed using the following formula (2):

[0135] T 0 =T*(H / sqrt(H 2 +d 2 ));

[0136] where T is the receiving time, that is, the receiving time minus the excitation time; H is the excitation point depth; d is the distance from the receiving point of the optical fiber sensor 2 buried on the ground to the wellhead of the detection well 1;

[0137] Convert the first arrival time at the offset to the vertical time at zero offset, while eliminating the influence of the elevation difference and burial depth between the wellhead and the shot point;

[0138] Through the relationship between depth and time, fit the time-depth curves of each layer, that is, the vertical travel-time curves;

[0139] Obtain the velocity and thickness of each layer according to the slope of the time-depth curve and the intersection of the time-depth curves of adjacent layers;

[0140] Plot the converted vertical time and the corresponding depth in the time-depth coordinate system;

[0141] When different depth points are within the same velocity layer, the distribution of the points is a straight line, and the slopes of the straight lines corresponding to different velocities are different. According to their distribution rules, divide the positions of each layer. Each layer is fitted with a straight line by the least squares method. The reciprocal of the slope of the straight line is the layer velocity of the medium, and the intersection of the two straight lines is the interface of the medium.

[0142] Optionally, it further includes:

[0143] According to the characteristics of the amplitude and spectrum change of the seismic waves received by different parts of the fiber optic sensor 2, use the spectral ratio method or the centroid frequency shift method or the spectral fitting method to calculate and obtain the seismic plane wave attenuation coefficient and Q value of the shallow subsurface.

[0144] Optionally, calculating and obtaining the seismic wave attenuation coefficient and Q value of the shallow subsurface includes:

[0145] Use the following formula three to calculate the amplitude spectrum of the plane wave:

[0146] B(f, t) = A(t)B(f, t 0 )exp(-πft / Q);

[0147] Use the following formula four to calculate the amplitude ratio at different times:

[0148] B(f, t i ) / B(f, t i-1 ) = (A(t i )exp(-πft i / Q)) / (A(t i-1 )exp(-πft i-1 / Q));

[0149] Use the following formula five to calculate the logarithm of the amplitude ratio:

[0150] ln(B(f, t i ) / B(f, t i-1 )) = C - (πf(t i - t i-1 ) / Q);

[0151] Among them, B(f,t) is the amplitude spectrum of the plane wave vertically propagating from the ground to the underground at time t; A(t) is the coefficient of the plane wave amplitude spectrum; f is the frequency of the plane wave; t is the time; Q is the plane wave attenuation coefficient; the logarithm of different amplitude ratios is only a linear function of the frequency, and the Q value can be obtained by fitting the slope.

[0152] In summary, in this embodiment, the specific implementation steps of the seismic inspection method are as follows:

[0153] Step 1: The inspection well 1 needs to be drilled first with a small drill at the position of the surface investigation point, and the depth is determined according to the predicted surface thickness of the construction area. To save construction costs, the excitation well where explosives are to be placed during production can also be used.

[0154] Step 2: The armored optical fiber buried on the ground is determined according to the terrain around the inspection well and the shallow burial depth. The length is 1.2 times the burial depth of the formation to be measured. The burial method is to excavate a receiving groove, put the armored optical fiber in and then compact it; the armored optical fiber set in the inspection well is lowered into the well using the sleeve 7 to ensure that the armored optical fiber received in the well can be recovered smoothly. The connectable sleeve 7 made of PVC material or alloy material, with a length of 1-2 meters per section, is convenient for carrying and construction; the sleeve 7 adopts a screw thread or a bayonet, and can be connected end to end until the bottom of the well according to the well depth; the diameter of the sleeve 7 is slightly larger than the diameter of the armored optical fiber, which can ensure good coupling, receive high-quality seismic waves, and at the same time can protect the armored optical fiber against the shock wave generated by the explosion of the detonator 8. After the construction is completed, the armored optical fiber can also be pulled out smoothly.

[0155] Step 3: The excitation cable 9 is made according to the depth of the inspection well. Usually, the detonator 8 excitation method is adopted, with one excitation point every 1 meter; the upper part of the excitation cable 9 is connected to the initiator 10, and the bottom of the tail end has an extended connecting part 11 connected to the bottom of the armored optical fiber. The connecting part 11 is wound around the last detonator 8, which is convenient for inserting both the armored optical fiber and the detonator 8 cable to the bottom through the sleeve 7 to ensure the accuracy of the excitation position and the receiving position; and it can also make the connecting part 11 break when the detonator 8 explodes, which is convenient for taking out the armored optical fiber.

[0156] Step 4: Use sand, cuttings, soil, etc. to fill and compact the shallow well, and fill the gaps with water to ensure good wave impedance coupling between the excitation cable 9, the sleeve 7 and the formation, and ensure the excitation and receiving effects.

[0157] Step 5: Use the initiator 10 to sequentially excite the detonators 8 in the order from the bottom to the top, and at the same time the optical fiber data acquisition module 3 collects and records the detection data.

[0158] Step 6: Use known technologies to process and interpret the detection data, extract the moment when the seismic wave is received by the optical fiber, and perform time-depth conversion, and the formation velocity, thickness and stratification conditions can be calculated.

[0159] Calculation formula for formation velocity of optical fiber in well: V i =(T i -T i-1 ) / (H i -H i-1 );where V i is the formation velocity at the i-th sampling point, T i is the arrival time of the seismic wave received at the i-th sampling point, and H i is the depth at the i-th sampling point.

[0160] Time-depth conversion formula for surface optical fiber: T 0 =T*(H / sqrt(H 2 +d 2 )); where T is the reception time (arrival time minus excitation time), H is the excitation point depth, and d is the distance between the surface optical fiber reception point and the wellhead.

[0161] When interpreting seismic surface survey data, first convert the first arrival time at the offset to the vertical time at zero offset, and at the same time eliminate the influence of the elevation difference and buried depth between the wellhead and the excitation point; then, through the relationship between depth and time, fit the time-depth curve (vertical time-distance curve) of each layer, and obtain the velocity and thickness of each layer according to the slope of the time-depth curve and the intersection point of the time-depth curves of adjacent layers.

[0162] The time-depth interpretation method is to plot the converted vertical time and the corresponding depth in the time-depth coordinate system. When different depth points are within the same velocity layer, the distribution of points is a straight line, and the slopes of the straight lines corresponding to different velocities are different. According to its distribution law, the positions of each layer are divided, and a straight line is fitted for each layer by the least squares method. The reciprocal of the slope of the straight line is the layer velocity of the medium, and the intersection point of the two straight lines is the interface of the medium.

[0163] Step 7: According to the characteristics of the amplitude and spectrum changes of the seismic waves received at different parts of the armored optical fiber, known techniques such as the spectrum ratio method, the centroid frequency shift method, or the spectrum fitting method can be used to calculate and obtain the seismic wave attenuation coefficient and Q value of the shallow underground.

[0164] Amplitude spectrum of plane wave: B(f, t) = A(t)B(f, t 0 )exp(-πft / Q);

[0165] Amplitude ratio at different times: B(f, t i ) / B(f, t i-1 ) = (A(t i )exp(-πft i / Q)) / (A(t i-1 )exp(-πft i-1 / Q));

[0166] Logarithm of amplitude ratio: ln(B(f, t i )) / B(f, t i-1 )) = C - (πf(t i - t i-1 )) / Q);

[0167] B(f, t) is the amplitude spectrum of a plane wave vertically propagating from the ground to underground at time t, A(t) is the coefficient of the plane wave amplitude spectrum, f is the frequency of the plane wave, t is the time, and Q is the attenuation coefficient of the plane wave. The logarithm of different amplitude ratios is only a linear function of frequency, and the value of Q can be obtained by fitting the slope.

[0168] Example 4

[0169] This example uses the seismic detection system in Example 2. This seismic detection method does not require the use of detonator 8 for excitation in detection well 1. At the ground near the wellhead of detection well 1, detonator 8, drop hammer or small vibrator can be used for excitation. Only armored cable needs to be lowered into detection well 1, and elastic airbag 12 or water bag is used to compact and fix it. After inflating or filling with water in the well, the armored optical fiber can be pushed against the well wall. This seismic detection method can use the production-induced shallow well as detection well 1 for construction, without damaging the shallow well and without the need to use casing 7, further simplifying the construction difficulty and reducing the construction cost. Other implementation steps are the same as those in Example 3 above.

[0170] Example 5

[0171] The present invention also provides an application of the above seismic detection method in seismic surface investigation.

[0172] Specifically, the above earthquake detection method utilizes the above earthquake detection system and adopts the distributed fiber acoustic sensing technology. The principle of the distributed fiber acoustic sensing technology (Distributed fiber Acoustic Sensing, abbreviated as DAS) is to obtain the changes in the physical quantities to be detected (such as sound, vibration, etc.) by detecting the phase changes of the Rayleigh scattered light at each point along the sensing fiber; the incident light interferes with the backward scattered light at a certain point along the fiber, and the changes in sound or vibration at this point will cause a linear change in the phase of the interference light. Therefore, by demodulating the phase of the interference light at this point at the receiving end, the change amount of sound or vibration can be determined; due to the continuous distribution of the fiber in space, the changes in physical quantities occurring at any point in space can be quantitatively detected, thereby realizing distributed sensing. The DAS measurement process is that the laser emits light pulses along the fiber, and some light interferes with the incident light in the pulse in the form of backward scattering. After the interference light is reflected back, the backward scattered interference light returns to the signal processing device, and at the same time, the vibration acoustic wave signals along the fiber are brought to the signal processing device; since the speed of light remains constant, the measurement results of the acoustic wave vibration per meter of the fiber can be obtained. With its advantages of long distance, high spatial resolution, passive anti-interference, etc., the DAS technology can be used in the surface investigation field of seismic exploration; its advantages are that the fiber length is not limited, the sampling rate is high, it can solve the problems of limited carrier capacity and low sampling rate of shallow seismic channels; there is no need to repeatedly lift and re-excite, solving the problem of inconsistent excitation wavelets; the fiber or armored fiber is relatively slender and uniform up and down. After being buried in the well and ensuring good coupling, it can be smoothly taken out and reused by engineering methods, solving the disadvantages of conventional geophones being bulky, difficult to ensure coupling, or unable to be taken out after landfill.

[0173] In summary, the distributed fiber acoustic sensing technology can be applied to the surface investigation field of seismic exploration to realize a surface investigation method with high-density sampling, high precision, simple operation, and low cost of repeated use, laying a foundation for obtaining high-quality seismic data. In the surface investigation field of seismic exploration, the fiber sensing technology has not been applied to seismic surface investigation at present. The present invention makes full use of the advantages of the fiber to solve the technical defects of shallow refraction and micro-logging, and at the same time overcomes the difficulties of poor push-type coupling, inability to take out buried geophones, and high cost.

[0174] The embodiments of the present invention have been described above. The above description is exemplary and not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations are obvious to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments.

Claims

1. An earthquake detection system, characterized in that, it includes: a detection well; an optical fiber sensor, which is arranged in the detection well and / or buried in the ground; an optical fiber data acquisition module, which is connected to one end of the optical fiber sensor and is used to provide a light source and acquire detection data; an excitation device, which is arranged in the detection well and / or on the ground near the wellhead of the detection well and is used to form a seismic source.

2. The earthquake detection system according to claim 1, characterized in that, the optical fiber sensor is an armored optical fiber, and the armored optical fiber includes an elastic rod, an optical fiber core and a shell. The optical fiber core is spirally wound around the outer periphery of the elastic rod, and the shell is wrapped outside the elastic rod around which the optical fiber core is wound.

3. The earthquake detection system according to claim 1, characterized in that, a part of the optical fiber sensors are arranged vertically in the detection well, and another part of the optical fiber sensors are buried in the ground and are connected to the optical fiber data acquisition module.

4. The earthquake detection system according to claim 3, characterized in that, the optical fiber sensor arranged in the detection well is threaded through a sleeve, and the inner diameter of the sleeve is matched with the outer diameter of the optical fiber sensor so that the optical fiber sensor can be coupled and the optical fiber sensor can be pulled out of the sleeve.

5. The earthquake detection system according to claim 4, characterized in that, the detection well is filled with a first filler, and the first filler compacts and fixes the sleeve in the detection well.

6. The earthquake detection system according to claim 1, characterized in that, the ground is provided with a receiving groove, the optical fiber sensor buried in the ground is arranged in the receiving groove, and a second filler is arranged in the receiving groove. The second filler buries and compacts and fixes the optical fiber sensor in the receiving groove.

7. The earthquake detection system according to claim 1, characterized in that, the excitation device includes a plurality of first excitation devices, and the plurality of first excitation devices are arranged vertically at intervals in the detection well.

8. The earthquake detection system according to claim 7, characterized in that, the first excitation device is a detonator, and the plurality of detonators are connected to an excitation cable. The upper end of the excitation cable extends to the ground and is connected to a detonator.

9. The earthquake detection system according to claim 8, characterized in that, the lower end of the excitation cable is wound around the outer periphery of the lowermost detonator and is connected to the lower end of the optical fiber sensor arranged in the detection well.

10. The earthquake detection system according to claim 1, characterized in that, the excitation device includes a second excitation device, and the second excitation device is arranged on the ground near the wellhead of the detection well. The second excitation device is a heavy hammer, a vibroseis or a detonator.

11. An earthquake detection method, which uses the earthquake detection system according to any one of claims 1-9, characterized in that, it includes: setting up a detection well and arranging an optical fiber sensor, an optical fiber data acquisition module and an excitation device; Start the fiber optic data acquisition module and the excitation device, use the excitation device to form a seismic source, and use the fiber optic data acquisition module to collect detection data; Extract the time when the seismic wave is received, and perform time-depth conversion, calculate the formation velocity, formation thickness, and stratification situation.

12. The seismic detection method according to claim 11, characterized in that, the performing time-depth conversion, calculating the surface velocity, surface thickness, and stratification situation includes: Calculating the formation velocity using the following formula (1): V i = (T i - T i-1 ) / (H i - H i-1 ) Among them, V i is the formation velocity at the i-th sampling point; T i is the arrival time of the seismic wave at the i-th sampling point; H i is the depth at the i-th sampling point. Performing time-depth conversion using the following formula (2): T 0 = T * (H / sqrt(H 2 + d 2 )); wherein, T is the reception time, that is, the reception moment minus the excitation moment; H is the excitation point depth; d is the distance between the receiving point of the fiber optic sensor buried on the ground and the wellhead of the detection well; Convert the first arrival time at the offset to the vertical time at zero offset, and at the same time eliminate the influence of the elevation difference and buried depth between the wellhead and the excitation point; Through the relationship between depth and time, fit the time-depth curve of each layer, that is, the vertical time-distance curve; Obtain the velocity and thickness of each layer according to the slope of the time-depth curve and the intersection point of the time-depth curves of adjacent layers; Plot the converted vertical time and the corresponding depth in the time-depth coordinate system; When different depth points are within the same velocity layer, the distribution of the points is a straight line, and the slopes of the straight lines corresponding to different velocities are different. According to its distribution law, divide the positions of each layer. Each layer is fitted with a straight line by the least squares method. The reciprocal of the slope of the straight line is the layer velocity of the medium, and the intersection point of the two straight lines is the interface of the medium.

13. The seismic detection method according to claim 12, characterized in that, further comprising: According to the characteristics of the amplitude and spectrum change of the seismic wave received by different parts of the fiber optic sensor, use the spectrum ratio method or the centroid frequency shift method or the spectrum fitting method to calculate and obtain the seismic plane wave attenuation coefficient and Q value of the shallow underground.

14. The seismic detection method according to claim 13, characterized in that, calculating and obtaining the seismic wave attenuation coefficient and Q value of the shallow underground includes: Calculating the amplitude spectrum of the plane wave using the following formula (3): B(f, t) = A(t)B(f, t 0 ) exp(-πft / Q); Calculating the amplitude ratio at different times using the following formula (4): B(f, t i ) / B(f, t i-1 ) = (A(t i ) exp(-πft i / Q)) / (A(t i-1 ) exp(-πft i-1 ); Calculating the logarithm of the amplitude ratio using the following formula (5): ln(B(f, t i ) / B(f, t i-1 )) = C - (πf(t i - t i-1 )) / Q; wherein, B(f,t) is the amplitude spectrum of the plane wave vertically propagating from the ground to the underground at time t; A(t) is the coefficient of the plane wave amplitude spectrum; f is the frequency of the plane wave; t is the time; Q is the plane wave attenuation coefficient; the logarithm of different amplitude ratios is only a linear function of the frequency. By fitting the slope, the Q value can be obtained.

15. Application of the seismic detection method according to any one of claims 11-14 in seismic surface investigation.

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