A method and system for analyzing relative changes in ground stress

Abstract

The present invention discloses a method and system for analyzing relative changes in geostress, comprising: step S1, acquiring piezomagnetic geostress monitoring data; step S2, deriving additional stress from the piezomagnetic geostress monitoring data using the least squares method; and step S3, deriving current stress based on the change in the additional stress. The technical solution of the present invention can effectively determine the relative changes in the stress field and reveal the relationship between regional stress field activity and earthquakes.

Description

Technical Field

[0001] The present invention belongs to the technical field of crustal stress monitoring, and in particular relates to a method and system for analyzing relative changes in crustal stress. Background Art

[0002] The development and occurrence of earthquakes are intrinsically linked to changes in the crustal stress field. The significance of studying seismic activity by measuring relative changes in ground stress is obvious, and this is also a concept proposed by Professor Li Siguang as a possible approach to earthquake prediction. Current technology for observing dynamic changes in the crustal stress-strain field can be summarized as "deep drilling, multi-component, and broadband." Comprehensive observations of multiple parameters of rock mass stress and deformation are conducted within a single deep borehole, facilitating comparison and data quality verification. Accurately recording the accumulation of crustal strain fields over time periods ranging from seconds to months allows for the study of their amplitude and rate of change, understanding the accumulation and release of crustal tectonic deformation energy, and exploring its relationship to tectonic activity and earthquake occurrence.

[0003] In 1991, Wang Lianjie, Pan Lizhou, and others published the monograph "Ground Stress Measurement and Its Application in Engineering," which systematically discussed the basic theory, measurement methods, and instrumentation of ground stress measurement, and provided examples of engineering applications. Since my country began exploring the use of borehole stress-strain observation technology for earthquake prediction, there have been many successful cases. Although a mature theoretical framework has not yet been established and much of the work is based on experience and speculation, the recurring and stable occurrence of various precursory features before earthquakes is encouraging. It may reveal something intrinsic and fundamental about the source process and deserves further study. However, current analysis methods for observational data are inadequate to reveal the relationship between regional stress field activity and earthquakes. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method and system for analyzing relative changes in ground stress, which can effectively obtain the relative change law of the stress field and reveal the relationship between regional stress field activity and earthquakes.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for analyzing relative changes in ground stress, comprising:

[0007] Step S1, obtaining piezoresistance monitoring data;

[0008] Step S2: obtaining additional stress by least square method based on the piezomagnetic in-situ stress monitoring data;

[0009] Step S3: obtaining the current stress according to the change of the additional stress.

[0010] Preferably, in step S1, the piezomagnetic in-situ stress monitoring data is obtained by a piezomagnetic in-situ stress monitoring probe; wherein, the four measuring elements in different directions of the piezomagnetic in-situ stress monitoring probe are arranged in sequence at intervals of 45°.

[0011] Preferably, in step S2, the stress component of the additional stress is obtained according to the piezoresistance monitoring data, namely:

[0012]

[0013] Where i = 1, 2, 3, 4,

[0014] The maximum principal stress for calculating additional stress is:

[0015] The minimum principal stress for calculating additional stress is:

[0016] Calculate the maximum principal stress σ of the additional stress H and σ x The angle γ is:

[0017] Preferably, in step S3, the measured ground stress value at the depth closest to the installation depth of the piezoresistive ground stress monitoring probe is used as the initial stress state, and the change of the additional stress is superimposed to obtain the current stress.

[0018] The present invention also provides a system for analyzing relative changes in ground stress, comprising:

[0019] Piezomagnetic stress monitoring instrument, used to obtain piezomagnetic ground stress monitoring data;

[0020] The data processing and analysis device is used to obtain additional stress through the least square method based on the piezoresistive stress monitoring data; and to obtain the current stress based on the change of the additional stress.

[0021] Preferably, the piezomagnetic stress monitoring instrument obtains piezomagnetic ground stress monitoring data through a piezomagnetic ground stress monitoring probe; wherein, the four measuring elements in different directions of the piezomagnetic ground stress monitoring probe are arranged in sequence at intervals of 45°.

[0022] Preferably, the additional stress is obtained by the data processing and analysis device as follows:

[0023] According to the piezoresistance monitoring data, the stress components of the additional stress are obtained, namely:

[0024]

[0025] Where i = 1, 2, 3, 4,

[0026] The maximum principal stress for calculating additional stress is:

[0027] The minimum principal stress for calculating additional stress is:

[0028] Calculate the maximum principal stress σ of the additional stress H and σ x The angle γ is:

[0029] Preferably, the data analysis device is used to take the measured ground stress value at a depth closest to the installation depth of the piezoresistive ground stress monitoring probe as the initial stress state, and superimpose the change of the additional stress to obtain the current stress.

[0030] The present invention obtains additional stress by the least squares method based on the piezomagnetic stress monitoring data; obtains the current stress based on the change of the additional stress; the present invention realizes the rapid analysis and automatic output of the piezomagnetic stress monitoring data, and can quickly obtain the stress evolution process; at the same time, it realizes the correlation analysis of the relative change of stress and the correlation with seismic activity, especially the stress change before and after a strong earthquake, including pre-earthquake abnormal changes, co-seismic stress changes and post-earthquake adjustment process. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0032] Figure 1 This is a flow chart of the method for analyzing relative changes in ground stress of the present invention;

[0033] Figure 2 Schematic diagram of the arrangement of piezomagnetic ground stress monitoring probes in four different directions. DETAILED DESCRIPTION

[0034] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0035] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] Example 1:

[0037] like Figure 1 As shown, an embodiment of the present invention provides a method for analyzing relative changes in ground stress, including:

[0038] Step S1, obtaining piezoresistance monitoring data;

[0039] Step S2: obtaining additional stress by least square method based on the piezomagnetic in-situ stress monitoring data;

[0040] Step S3: obtaining the current stress according to the change of the additional stress.

[0041] As an implementation method of the embodiment of the present invention, in step S1, the piezomagnetic in-situ stress monitoring data is obtained by using a piezomagnetic in-situ stress monitoring probe; wherein, the four measuring elements of the piezomagnetic in-situ stress monitoring probe are arranged in sequence at intervals of 45°, such as Figure 2 shown.

[0042] As an implementation method of the present invention, based on the deformation of the drill hole under the plane stress state of elastic mechanics, the ground stress drill hole is regarded as a circular hole in an infinite thin plate. We are concerned with the radial displacement μ of the hole wall during the deformation process of the drill hole under the action of external force. Assume that there is a circular hole in an infinite elastic thin plate. At infinity, the thin plate is subjected to a uniformly distributed tensile force in a certain direction with an intensity of σ. H , and the uniformly distributed tension in the perpendicular direction, with an intensity of σ h The displacement of a point A on the hole wall can be expressed as follows:

[0043]

[0044] Where E is the elastic modulus of the infinite elastic plate; a is the drilling radius; α is σ H The angle to point A is positive if it is counterclockwise and negative if it is clockwise. The displacement direction μ away from the hole center is positive, and negative if it is opposite. We can further obtain:

[0045]

[0046] According to the principal stress σ H , σ h and stress component σ x , σ y , τ xy Relationship:

[0047]

[0048] make

[0049] 1+2cos2θ i =C 1θi

[0050] 1-2cos2θ i =C 2θi

[0051] 4sin2θ i =C 3θi

[0052] i=1,2……n

[0053] where γ is σ x to σ H The angle of θ is σ x Angle to the measuring element.

[0054] The above formula can be rewritten as:

[0055]

[0056] make

[0057]

[0058] According to the least squares method, a set of solutions is found from many possible solutions that minimizes the sum of squares of residual errors (i.e., the deviations between the observed values ​​and the regression line). This set of solutions is the most reliable and closest to the true value of the measured stress at that point. When it is the most reliable solution, it must satisfy the following conditions:

[0059] Aσ=G

[0060] in:

[0061]

[0062] σ=[σ x σ y τ xy ] T

[0063]

[0064] Right now:

[0065] σ=A -1 G

[0066] Furthermore, in step S2, the stress component of the additional stress is obtained based on the piezomagnetic in-situ stress monitoring data, namely:

[0067]

[0068] Where i = 1, 2, 3, 4,

[0069] The maximum principal stress for calculating additional stress is:

[0070] The minimum principal stress for calculating additional stress is:

[0071] Calculate the maximum principal stress σ of the additional stress H and σ x The angle γ is:

[0072] As an implementation method of the embodiment of the present invention, in step S3, the measured ground stress value at the depth closest to the installation depth of the piezoresistive ground stress monitoring probe is used as the initial stress state, and the change of the additional stress is superimposed to obtain the current stress.

[0073] Furthermore, the measured ground stress value at the depth closest to the installation depth of the monitoring probe is taken as the initial stress value σ H1 , σ h1 , β1, which are the directions of maximum principal stress, minimum principal stress and maximum principal stress, and let σ H1 The angle between the plane and the x-axis is γ1. According to the two-dimensional plane stress tensor conversion formula, the stress component σ in the rectangular coordinate system xoy can be calculated. x0 , σ y0 , τ xy0 .

[0074]

[0075] Let Δσ x Δτ xy Δσ y is the change of additional stress, the stress component σ of the current stress, x ,σ, y , τ, xy :

[0076]

[0077] Then according to σ, x ,σ, y , τ, xy , calculate the maximum principal stress σ of the current stress, H , minimum principal stress σ, h and the maximum principal stress σ, H With σ, x The angle γ,, the direction of the maximum principal stress β, = π / 2-γ,.

[0078] The present invention can quickly obtain additional stress and current stress, and then analyze the correlation between the relative change of ground stress and seismic activity.

[0079] Example 2:

[0080] An embodiment of the present invention also provides a system for analyzing relative changes in ground stress, including: a piezomagnetic stress monitoring instrument and a data processing and analysis device; selecting a suitable location in a tectonic activity area or an engineering area for vertical hole drilling, selecting a complete section to install a piezomagnetic stress monitoring instrument based on a comprehensive analysis of core and well logging, installing the piezomagnetic ground stress monitoring probe of the piezomagnetic stress monitoring instrument in a borehole in the crustal rock mass so that the measuring element is well coupled with the borehole rock wall, and continuously recording the ground stress data output by the measuring element. The ground stress data is sent to the data processing and analysis device in real time through a GPS transmission system. Among them, the piezomagnetic stress monitoring instrument is used to obtain piezomagnetic ground stress monitoring data; the data processing and analysis device is used to obtain additional stress by the least squares method based on the piezomagnetic ground stress monitoring data; and at the same time, the current stress is obtained based on the change in the additional stress.

[0081] As an implementation method of an embodiment of the present invention, the piezomagnetic stress monitoring instrument obtains piezomagnetic ground stress monitoring data through a piezomagnetic ground stress monitoring probe; wherein, the four measuring elements in different directions of the piezomagnetic ground stress monitoring probe are arranged in sequence at 45° intervals.

[0082] As an implementation method of an embodiment of the present invention, the additional stress is obtained by a data processing and analysis device as follows:

[0083] According to the piezoresistance monitoring data, the stress components of the additional stress are obtained, namely:

[0084]

[0085] Where i = 1, 2, 3, 4,

[0086] The maximum principal stress for calculating additional stress is:

[0087] The minimum principal stress for calculating additional stress is:

[0088] Calculate the maximum principal stress σ of the additional stress H and σ x The angle γ is:

[0089] As an implementation method of an embodiment of the present invention, the data analysis device is used to use the measured ground stress value at a depth closest to the installation depth of the piezoresistive ground stress monitoring probe as the initial stress state, and superimpose the change of the additional stress to obtain the current stress.

[0090] Taking a monitoring station as an example, the four-component piezomagnetic stress sensor monitoring probe is first calibrated. The first step is indoor calibration, converting the instrument's frequency readings into stress readings. The second step is reference value calibration. In addition to the monitoring elements at different locations, the piezomagnetic stress monitoring probe also features a suspended element. This element is suspended in the air, not in direct contact with the borehole wall, and its value fluctuations are affected by external non-stress factors. This element serves as a reference value for calibration. The changes in the four-component monitoring data are then determined. Since stress monitoring data records relative changes over the monitoring period, they are referred to as additional stress. Based on the aforementioned basic theory, the changes in the additional stress state can be determined from the stress monitoring data at this station. To characterize the current stress field, in-situ absolute in-situ stress measurements (such as those using hydraulic fracturing) are required after drilling is completed and before the monitoring instrument is installed. The measured stress value at the depth closest to the monitoring probe's installation depth is used as the initial stress state, and the additional stress changes are superimposed to form the current stress state. If the monitoring period is long enough, a stress evolution diagram for the monitoring station can be obtained. The stress evolution diagram can be used to interpret the stress changes during the monitoring period, whether compression is accumulated or tension is released, whether the direction of the principal stress axis is deflected, etc. These are of great significance for further research on tectonic stress fields, fault activities and earthquakes.

[0091] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for analyzing relative changes in ground stress, characterized in that: include: Step S1, obtaining piezoresistance monitoring data; Step S2: obtaining additional stress by least square method based on the piezomagnetic in-situ stress monitoring data; Step S3, obtaining the current stress according to the change of the additional stress; In step S1, piezomagnetic in-situ stress monitoring data is obtained by using a piezomagnetic in-situ stress monitoring probe, wherein four measuring elements in different directions of the piezomagnetic in-situ stress monitoring probe are arranged in sequence at intervals of 45°; In step S2, the stress component of the additional stress is obtained based on the piezoresistance monitoring data, namely: Among them, i=1,2,3,4, θ is σ x Angle to the measuring element; The maximum principal stress for calculating additional stress is: The minimum principal stress for calculating additional stress is: Calculate the maximum principal stress σ of the additional stress H and σ x The angle γ is: In step S3, the measured ground stress value at the depth closest to the installation depth of the piezoresistive ground stress monitoring probe is taken as the initial stress state, and the change of the additional stress is superimposed to obtain the current stress.

2. A ground stress relative change analysis system, characterized in that: include: Piezomagnetic stress monitoring instrument, used to obtain piezomagnetic ground stress monitoring data; A data processing and analysis device is used to obtain additional stress by least square method based on the piezoresistive ground stress monitoring data; and to obtain current stress based on the change of the additional stress; The piezomagnetic stress monitoring instrument obtains piezomagnetic ground stress monitoring data through a piezomagnetic ground stress monitoring probe. The four measuring elements of the piezomagnetic ground stress monitoring probe are arranged in sequence at 45° intervals. The additional stress obtained by the data processing and analysis device is as follows: According to the piezoresistance monitoring data, the stress components of the additional stress are obtained, namely: Among them, i=1,2,3,4, θ is σ x Angle to the measuring element; The maximum principal stress for calculating additional stress is: The minimum principal stress for calculating additional stress is: Calculate the maximum principal stress σ of the additional stress H and σ x The angle γ is: The data analysis device is used to take the measured ground stress value at the depth closest to the installation depth of the piezoresistive ground stress monitoring probe as the initial stress state, and superimpose the change of the additional stress to obtain the current stress.

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