阅读说明：本技术 非均质岩石波速场的获取方法 (Method for acquiring heterogeneous rock wave velocity field ) 是由 王智洋 吴志军 翁磊 储昭飞 于 2020-12-09 设计创作，主要内容包括：本发明涉及一种非均质岩石波速场的获取方法,该方法首先将波速场转化为能够与应力波传导时间相关的虚拟场,并通过测量几组平行路径应力波的传导时间在频域内重构虚拟场；随后将频域虚拟场转化为空间域的波速场并消除其中奇异点即得到最终重构的波速场。本发明能够利用多组脉冲穿透测量准确获取非均质岩石的波速场,该方法具有操作简单,计算量小,重构过程不基于波速场连续性假设的优点,因此能够较好地重构出非均质岩石中波速场的非连续变化,适用于非均质岩石实验室声发射实验中波速模型的获取。(The invention relates to a method for acquiring a wave velocity field of a heterogeneous rock, which comprises the steps of firstly converting the wave velocity field into a virtual field which can be related to the conduction time of stress waves, and reconstructing the virtual field in a frequency domain by measuring the conduction time of stress waves of a plurality of groups of parallel paths; and then converting the frequency domain virtual field into a wave velocity field of a space domain and eliminating singular points in the wave velocity field to obtain a finally reconstructed wave velocity field. The method can accurately acquire the wave velocity field of the heterogeneous rock by utilizing the penetration measurement of a plurality of groups of pulses, has the advantages of simple operation, small calculated amount and no wave velocity field continuity hypothesis-based reconstruction process, can better reconstruct the discontinuous change of the wave velocity field in the heterogeneous rock, and is suitable for acquiring the wave velocity model in the acoustic emission experiment of the heterogeneous rock laboratory.)

1. A method for acquiring a heterogeneous rock wave velocity field is characterized by comprising the following steps: the method comprises the following steps:

1) constructing a virtual field related to a wave velocity field and measuring projections of the virtual field at different angles;

2) reconstructing the virtual field in the frequency domain;

3) converting the virtual field reconstructed in the frequency domain into a spatial domain;

4) judging singular points possibly appearing in the reconstructed wave velocity field according to the value range of the virtual field and eliminating the influence of the singular points;

5) and calculating a reconstructed wave velocity field.

2. The method for acquiring the wave velocity field of heterogeneous rock according to claim 1, wherein:

in the step 1), the construction method of the virtual field comprises the following steps: and dispersing the wave velocity field, and constructing a virtual field by the ratio of the unit size to the corresponding wave velocity of the unit.

3. The method for acquiring the wave velocity field of heterogeneous rock according to claim 1 or 2, wherein:

in the step 1), the method for acquiring the virtual field projection comprises the following steps: the physical meaning of the virtual field shows that the stress wave propagation time measured by the pulse penetration method is the projection of the virtual field on the path.

4. The method for acquiring the wave velocity field of heterogeneous rock according to claim 3, wherein: and obtaining projections of different angles of the virtual field through measurement of multiple groups of parallel paths.

5. The method for acquiring the wave velocity field of heterogeneous rock according to claim 1, 2 or 4, wherein:

in the step 2), the method for reconstructing the virtual field in the frequency domain comprises: and performing one-dimensional Fourier transform on the virtual field space projection, and reconstructing the virtual field in the frequency domain through the equivalent relationship between the Fourier transform of the virtual field space projection and the virtual field in the frequency domain on the corresponding angle section.

6. The method for acquiring the wave velocity field of heterogeneous rock according to claim 1, 2 or 4, wherein:

in the step 3), the method for converting the virtual field reconstructed in the frequency domain into the wave velocity field in the spatial domain comprises: and converting the virtual field reconstructed in the frequency domain into a space domain through two-dimensional inverse Fourier transform, and converting the virtual field into a wave velocity field through the relation between the virtual field and the wave velocity field.

7. The method for acquiring the wave velocity field of heterogeneous rock according to claim 5, wherein:

in the step 3), the method for converting the virtual field reconstructed in the frequency domain into the wave velocity field in the spatial domain comprises: and converting the virtual field reconstructed in the frequency domain into a space domain through two-dimensional inverse Fourier transform, and converting the virtual field into a wave velocity field through the relation between the virtual field and the wave velocity field.

8. The method for acquiring the wave velocity field of heterogeneous rock according to claim 1, 2, 4 or 7, wherein:

in the step 4), the method for eliminating the singular points possibly existing in the reconstructed wave velocity field comprises the following steps: when a singular point appears in the reconstructed wave velocity field, the reconstructed virtual field needs to be converted into the wave velocity field after being subjected to low-pass filtering.

9. The method for acquiring the wave velocity field of heterogeneous rock according to claim 8, wherein: the method comprises the following specific steps:

in the step 1), constructing a virtual field related to a wave velocity field and measuring projections of the virtual field at different angles:

the virtual field w and the wave velocity field v are constructed in the following relation:

wherein h represents a small displacement in the wave velocity field, namely the size of a unit in the discrete wave velocity field, and the virtual field w represents the time required for the stress wave to pass through the small displacement or the unit;

the time of propagation of the stress wave, as measured using the pulse penetration method, is expressed as:

where δ represents the pulse function, there is:

the xcos theta + ysin theta-rho represents a straight line path from the excitation point to the receiving point of the stress wave; the propagation time of the stress wave measured by the pulse penetration method is the projection of the virtual field w on the projection path; when the path angles are the same, the curve formed by the projection points is the projection g of the virtual field w under the angle;

based on the above formula, to obtain the projection of the virtual field w at a certain specific angle, only a pulse penetration method is used for measurement on a group of parallel paths corresponding to the angle, and the stress wave travel time is recorded and curve fitting is performed;

in the step 2), reconstructing the virtual field in the frequency domain:

the resulting projection of the virtual field in the spatial domain is transformed into the frequency domain by a fourier transform, i.e.:

wherein W represents a two-dimensional Fourier transform of a virtual field W; reconstructing the virtual field in the frequency domain based on the relationship between the one-dimensional Fourier transform of the virtual field projection expressed by the formula and the two-dimensional Fourier transform of the virtual field;

in the step 3), the virtual field reconstructed in the frequency domain is converted into a spatial domain:

converting a virtual field W represented in the frequency domain into the spatial domain by a two-dimensional inverse Fourier transform, comprising:

wherein | ω | is a ramp filter; because the function is not integrable in an infinite domain, a Hamming window function h needs to be added in the integrand to limit an integration interval; to this end the virtual field w is represented in the spatial domain by the following equation:

wherein

In the step 4), singular points possibly appearing in the reconstructed wave velocity field are judged according to the value range of the virtual field, and the influence of the singular points is eliminated;

in the step 5), a reconstructed wave velocity field is calculated: and substituting the reconstructed virtual field after filtering into a formula to obtain a final result of the reconstructed wave velocity field.

Technical Field

The invention relates to the technical field of rock mechanics experiments, in particular to a method for acquiring a wave velocity field of a heterogeneous rock.

Background

The acoustic emission event positioning is the core of an acoustic emission monitoring technology, and calculates the space coordinates and the occurrence time of an acoustic emission event by using related data such as waveforms, trigger time and the like recorded by an acoustic emission monitoring system. Accurate acoustic emission positioning is of great significance in analyzing damage mechanisms of rocks. And the accuracy of acoustic emission event positioning depends greatly on the quality of the wave velocity model of the rock specimen.

Currently, in acoustic emission localization of rock materials, a single wave velocity model is typically employed, i.e. assuming that the material is continuous, homogeneous and isotropic. In reality, however, the wave velocity field of the rock material may exhibit non-continuous, non-uniform characteristics due to internal contamination or local damage. The complexity of ignoring the true wave velocity field of rock material can seriously impact the accuracy of acoustic emission event localization. Meanwhile, because the internal structure of the rock is invisible, the difficulty of directly acquiring a relatively accurate wave velocity field in the rock is high.

The existing method for acquiring the wave velocity structure of the heterogeneous rock only comprises a one-dimensional wave velocity reconstruction method, a travel-time inversion method and a waveform inversion method. Wherein, the one-dimensional approximation of the real wave velocity model obtained by the one-dimensional wave velocity reconstruction method is not enough to meet the requirement of accurate acoustic emission positioning of the heterogeneous rock material. The travel time inversion method and the waveform inversion method have high requirements on the quality of an initial wave velocity model, and both methods are based on the assumption that the wave velocity field is continuously derivable everywhere. The use effect of the heterogeneous rock material which is difficult to probe internally and has discontinuous wave velocity structure is not ideal. Therefore, the method for obtaining the high-quality wave velocity model of the heterogeneous rock material by using the simple measurement method is significant to research, and the positioning precision of the acoustic emission source can be ensured to a great extent.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to provide the method for acquiring the wave velocity field of the heterogeneous rock, which is convenient to operate and good in reliability, and is suitable for accurately positioning the acoustic emission event in the rock mechanics experiment.

In order to achieve the above object, the method for acquiring a wave velocity field of a heterogeneous rock provided by the present invention is characterized in that: the method comprises the following steps:

1) constructing a virtual field related to a wave velocity field and measuring projections of different angles of the virtual field;

2) reconstructing the virtual field in the frequency domain;

3) converting the virtual field reconstructed in the frequency domain into a spatial domain;

4) judging singular points possibly appearing in the reconstructed wave velocity field according to the value range of the virtual field and eliminating the influence of the singular points;

5) and calculating a reconstructed wave velocity field.

Preferably, in step 1), the method for constructing the virtual field includes: and dispersing the wave velocity field, and constructing a virtual field by the ratio of the unit size to the corresponding wave velocity of the unit.

Further, in the step 1), the method for acquiring the virtual field projection includes: the physical meaning of the virtual field can be known, and the stress wave propagation time measured by a pulse penetration method is the projection of the virtual field on the path;

further, the projections of different angles of the virtual field are obtained by measurements of multiple sets of parallel paths.

Further, in step 2), the method for reconstructing the virtual field in the frequency domain includes: and performing one-dimensional Fourier transform on the virtual field space projection, and reconstructing the virtual field in the frequency domain through the equivalent relationship between the Fourier transform of the virtual field space projection and the virtual field in the frequency domain on the corresponding angle section.

Further, in the step 3), a method for converting the virtual field reconstructed in the frequency domain into a wave velocity field in the spatial domain includes: and converting the virtual field reconstructed in the frequency domain into a space domain through two-dimensional inverse Fourier transform, and converting the virtual field into a wave velocity field through the relation between the virtual field and the wave velocity field.

Further, in the step 4), the method for eliminating the singular points possibly existing in the reconstructed wave velocity field includes: when a singular point appears in the reconstructed wave velocity field, the reconstructed virtual field needs to be converted into the wave velocity field after being subjected to low-pass filtering.

Further, the specific steps are as follows:

in the step 1), a virtual field related to a wave velocity field is constructed and the projections of the virtual field at different angles are measured

The structural virtual field w has the following relationship with the wave velocity field v.

Where h represents a small amount of displacement in the wave velocity field, i.e. the size of a cell in the discrete wave velocity field, and the virtual field w represents the time required for the stress wave to pass through the small amount of displacement or cell.

The time of propagation of the stress wave, as measured using the pulse penetration method, can be expressed as:

where δ represents the pulse function, there is:

the xcos theta + ysin theta-rho represents a straight line path from the excitation point to the receiving point of the stress wave. It can be seen that the propagation time of the stress wave measured by the pulse penetration method is the projection point of the virtual field w on the projection path. When the path angles are the same, the curve formed by these projection points is the projection g of the virtual field w at the angle.

Based on the above formula, to obtain the projection of the virtual field w at a certain angle, it is only necessary to measure the projection on a group of parallel paths corresponding to the angle by using a pulse penetration method, record the stress wave travel time, and perform curve fitting.

In the step 2), the virtual field is reconstructed in the frequency domain

The resulting projection of the virtual field in the spatial domain is transformed into the frequency domain by a fourier transform, i.e.:

where W represents the two-dimensional fourier transform of the virtual field W. Based on the relationship between the one-dimensional Fourier transform of the virtual field projection and the two-dimensional Fourier transform of the virtual field, which is expressed by the above formula, the virtual field can be reconstructed in the frequency domain.

In the step 3), the virtual field reconstructed in the frequency domain is converted into the space domain

Converting a virtual field W represented in the frequency domain into the spatial domain by a two-dimensional inverse Fourier transform, comprising:

where | ω | is a ramp filter. Since the function is not integrable, a hamming window function h needs to be added to the integrand to limit the integration interval. To this end the virtual field w can be represented in the spatial domain by the following equation:

wherein

In the step 4), singular points possibly appearing in the reconstructed wave velocity field are judged according to the value range of the virtual field, and the influence of the singular points is eliminated

According to the physical meaning of the virtual field, the value of the virtual field is always greater than 0. In fact, the non-continuous function is approximated by using a fourier series in steps 2) and 3), so that the virtual field calculation results have a certain oscillation. Especially in regions where the wave velocity changes rapidly, the oscillations can be particularly intense. Meanwhile, in the formula, the virtual field appears at the position of the denominator. If the 0 point is included in the oscillation region, singular points appear in the reconstructed wave velocity field. When this occurs, it is necessary to perform a filtering process using a low-pass filter on the virtual field before using the formula.

In the step 5), a reconstructed wave velocity field is calculated

And substituting the reconstructed virtual field after filtering into a formula to obtain a final result of the reconstructed wave velocity field.

The invention has the following advantages and beneficial effects:

the invention provides a method for acquiring a wave velocity field of heterogeneous rock, which is mainly used for solving the problem of low precision caused by introducing a continuity assumption in the inversion calculation of the wave velocity field of the heterogeneous rock material.

Drawings

FIG. 1 is a flow chart of the method of the present invention;

FIG. 2 is a schematic diagram of projections in various directions obtained by experimental acquisition and curve fitting in an embodiment of the present invention;

FIG. 3-a is an overview of a virtual field (wave velocity field) transformed from the frequency domain to the spatial domain in an embodiment of the present invention;

3-b are characteristic line data results of the transformation of the virtual field (wave velocity field) from the frequency domain to the spatial domain in the embodiment of the invention;

3-c are the amplification results of the characteristic line data of the virtual field (wave velocity field) converted from the frequency domain to the spatial domain in the embodiment of the invention;

FIG. 4-a is an overview of the virtual field (wave velocity field) after low-pass filtering according to an embodiment of the present invention;

fig. 4-b shows the result of the virtual field (wave velocity field) characteristic line data after low-pass filtering processing in the embodiment of the invention.

Detailed Description

The technical scheme of the invention is further specifically described by the following embodiments and the accompanying drawings.

Taking a self-made heterogeneous rock test piece as an example, the wave velocity field is obtained according to the technical scheme of the invention content.

The test piece main body is granite, the size is 200mm 30mm, and the wave speed is 5128 m/s. The center of the test piece was a hole of 100mm by 30mm in size cut out by a water jet knife and filled with cement mortar. The cement mortar is prepared from 325 Portland cement and river sand according to the proportion of 1:1.5, and the wave speed is 2030m/s after the complete curing.

As shown in fig. 1, the method for acquiring the wave velocity field of the heterogeneous rock of the invention comprises the following specific steps:

step 1): constructing a virtual field related to a wave velocity field and measuring projections of the virtual field at different angles:

the wave velocity field is dispersed according to the resolution of 600 × 600, and when the pulse penetration method is used for measuring the projection points, the projection measurement is carried out once every 5 degrees, and the distance between the measurement points in the same projection is 5 mm. The projection of the virtual field of the test piece in 0 to 45 degrees is constructed by a polygon fitting method based on Hough transform, and the projection of the virtual field of the test piece in 0 to 180 degrees is obtained through symmetry (as shown in FIG. 2).

Step 2): reconstructing the virtual field in the frequency domain:

the resulting projections are subjected to univariate discrete fourier transforms, each one-dimensional fourier transform being part of a virtual-field two-dimensional fourier transform.

Step 3): transforming a virtual field reconstructed in the frequency domain into the spatial domain

The fourier transform of each projection is multiplied by the filter function | ω | and then by a hamming window function, followed by a one-dimensional discrete inverse fourier transform and integration. The above operations can obtain the virtual field (wave velocity field) reconstruction results as shown in fig. 3-a to 3-c. After the wave velocity field is converted into a reconstructed wave velocity field, the difference between the reconstructed result except the singular point and the actually measured wave velocity is not great.

Step 4): judging singular points possibly appearing in the reconstructed wave velocity field according to the value range of the virtual field and eliminating the influence of the singular points

As can be seen from fig. 3-a, the point 0 is included in the value range of the virtual field, and when the virtual field is converted into the wave velocity field, singular points (as shown in fig. 3-b) appear. To eliminate the singular points, the virtual field shown in fig. 3-a needs to be filtered using a gaussian low-pass filter. The processed virtual field is shown in fig. 4-a, and it can be seen that the value range is obviously narrowed and no more 0 point is included.

Step 5): calculating a reconstructed wave velocity field

The virtual field after low-pass filtering is converted into a wave velocity field, and it can be seen that in the final wave velocity field reconstruction result, i.e. fig. 4-b, the original singular points have been well suppressed.

The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be able to cover the technical solutions and the inventive concepts of the present invention within the technical scope of the present invention.