CN110568487A - Active fault structure imaging method based on natural seismic waveform - Google Patents
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Abstract
A method of natural seismic waveform based active tomographic imaging, comprising: performing reverse-time reconstruction on the seismic wave elastic wave field; carrying out decoupling separation on a longitudinal wave field and a transverse wave field of the seismic wave elastic wave field; and implementing zero time delay cross-correlation imaging conditions on the decoupled longitudinal wave field and the decoupled transverse wave field, and converting the four-dimensional back-propagation wave field into imaging of a discontinuous structure in the medium. The method utilizes the coherence of a seismic wave field which is reversely propagated at a discontinuous structure in a medium to image an underground fault structure, seismic source information such as a seismic source function, a seismic source position and an earthquake-generating moment is not needed, the calculation difficulty is reduced, a three-dimensional fine structure of an active fault zone can be economically and effectively depicted, and the seismic pregnancy and the seismic rupture distribution can be researched. Meanwhile, the energy focusing degree of elastic wave reverse-time imaging is improved by adopting a grouping imaging condition, the spatial resolution is improved, the data noise and the imaging noise are reduced, and the imaging signal-to-noise ratio is improved.
Description
Technical Field
the invention relates to the technical field of active fault structure imaging, in particular to an active fault structure imaging method based on natural seismic waveforms.
Background
Earthquake frequently occurs in active fault areas, and high earthquake disaster risks are faced. The active fault three-dimensional structure with high resolution and high precision is obtained, the establishment of the cause relation between the active fracture geometric structure and the earthquake fracture subsection is facilitated, a reliable model is provided for earthquake disaster assessment, and the earthquake scientific development is promoted.
Currently, seismic methods for studying fault structures include natural seismic imaging methods and active source seismic exploration methods. Methods for studying subsurface media using natural seismic data mainly include seismic travel time imaging and background noise imaging methods. However, the seismic travel time imaging method and the background noise imaging method can only obtain smooth underground structure information, and lack fine description of the three-dimensional structure of the fault.
Active source seismic exploration is widely applied to underground structure imaging, and high-precision and high-resolution artificial seismic sections can be obtained based on a seismic migration imaging method. The active source seismic imaging method mainly comprises an artificial explosion source migration imaging method, an air gun seismic source detection method and the like. However, the cost of deploying large-scale active seismic source seismic exploration is much more expensive than passive seismic source seismic monitoring, and the use of a large-explosive-source seismic source has more limitations, and the active seismic source seismic exploration is generally arranged in a linear form, and it is difficult to obtain the change of a fault along the trend, so that the research on the underground fault structure by using the active seismic imaging method has certain limitations.
Artman et al (2010) proposes a reverse-time imaging method. The method records a reverse-time propagation seismic wave field according to a station of a natural earthquake, utilizes the coherence of the wave field at a seismic source, and applies a cross-correlation imaging condition to the wave field to position the seismic source.
discontinuities such as faults in the subsurface medium may be considered secondary sources that excite scattered waves in the propagation of seismic waves.
disclosure of Invention
Technical problem to be solved
In view of the above, the main objective of the present invention is to provide an active fault structure imaging method based on natural seismic waveforms, so as to effectively depict the three-dimensional fine structure of the active fault zone and study the seismic pregnancy and the seismic fracture distribution.
(II) technical scheme
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
A method of natural seismic waveform based active tomographic imaging, comprising: performing reverse-time reconstruction on the seismic wave elastic wave field; carrying out decoupling separation on a longitudinal wave field and a transverse wave field of the seismic wave elastic wave field; and implementing zero time delay cross-correlation imaging conditions on the decoupled longitudinal wave field and the decoupled transverse wave field, and converting the four-dimensional back-propagation wave field into imaging of a discontinuous structure in the medium.
in the above scheme, the performing reverse-time reconstruction on the seismic wave elastic wave field includes: the seismic wavefield D is: d (X)r，t)＝S(Xs，t)G(Xr，Xs，t)
where t represents time, S represents a seismic source time function, G represents a Green' S function, XrRepresenting the position of a seismic station in a three-dimensional medium, Xr＝(xr，yr，zr)，xrAnd yrRepresenting the horizontal coordinates of the seismic station in meters, zrCoordinates in meters, X, representing the seismic station in the depth directionsRepresenting the position, X, of a seismic event in a three-dimensional mediums＝(xs，ys，zs)，xsand yscoordinates representing the seismic event in the horizontal direction, in meters, zscoordinates representing the seismic event in the depth direction in meters;
and (3) with the seismic wave field D as an initial condition, propagating the seismic wave field along the reverse direction of time according to the smooth velocity model of the region, namely performing reverse-time reconstruction on the wave field to obtain a reverse-thrust wave field W:
Where t denotes time, i denotes the ith station, and G is the conjugate of the green's function.
In the above solution, the performing the decoupling separation of the longitudinal wave field and the transverse wave field on the seismic wave elastic wave field includes: decoupling and separating longitudinal and transverse wave fields by adopting a divergence and rotation method:
Wherein, λ and μ represent Lame coefficients,The degree of divergence is expressed as a degree of divergence,X represents the derotation, u represents the elastic wavefield, P represents the decoupled compressional wave, and S represents the decoupled shear wave.
In the above scheme, the performing zero-delay cross-correlation imaging conditions on the decoupled longitudinal wave field and shear wave field, and the converting the four-dimensional backward wave field into the imaging of the discontinuous structure in the medium includes: after decoupling longitudinal and transverse wave fields by adopting divergence and rotation methods, implementing zero-delay cross-correlation imaging conditions on decoupled longitudinal waves P and transverse waves S, and converting four-dimensional back-propagation wave fields into imaging I of discontinuous structures in underground mediumPS：
IPS(X)＝∑t(∑iPi(X，t)∑iSi(X，t))
therein, sigmaiPi(X, t) and ∑iSiAnd (X, t) is a longitudinal wave field and a transverse wave field obtained by decoupling the reverse propagation elastic wave fields of all the stations.
In the above solution, the applying zero-delay cross-correlation imaging conditions to the decoupled longitudinal wave field and shear wave field to convert the four-dimensional backward wave field into imaging of discontinuous structures in the medium includes:
Grouping imaging conditions are used for grouping the seismic stations, different groups of passive source seismic records are used as initial conditions to obtain a plurality of back-propagation wave fields, then coherence among the wave fields of the different groups is used for imaging a medium structure, and a corresponding formula is as follows:
Wherein, giIndicating the station of the i-th group of stations,pi represents the multiplication by the number of the two,andIs a longitudinal wave field and a transverse wave field obtained by decoupling the I-th group of reverse-transmission elastic wave fields IPSIs the imaging of a discontinuity in the subsurface medium, t represents time, and X represents a three-dimensional spatial coordinate.
(III) advantageous effects
1. According to the active fault structure imaging method based on the natural seismic waveform, only the elastic wave field needs to be transmitted in a reverse time mode according to the wave equation, and the seismic source wave field is not required to be transmitted in a forward mode like active source seismic imaging, so that seismic source information such as a seismic source function, a seismic source position and an earthquake-generating moment is not required, and the calculation difficulty is reduced.
2. According to the active fault structure imaging method based on the natural seismic waveform, the elastic wave field recorded by the station is transmitted along the reverse direction of time, so that longitudinal waves and converted transverse waves can return to a discontinuous body where wave field conversion occurs at the same time, the conversion points where the converted waves occur can be imaged by utilizing the coherence of the longitudinal wave field and the converted transverse wave field at the moment, the three-dimensional fine structure of an active fault zone can be economically and effectively depicted, and the seismic pregnancy and seismic fracture distribution can be researched.
3. The active fault structure imaging method based on the natural seismic waveform provided by the invention adopts the grouping imaging condition to improve the energy focusing degree of elastic wave reverse-time imaging so as to improve the spatial resolution, reduce data noise and imaging noise and improve the imaging signal-to-noise ratio.
4. compared with an artificial exploration method, the active fault structure imaging method based on the natural seismic waveform is lower in cost and higher in imaging resolution compared with natural seismic travel time imaging and noise surface wave imaging. The method can economically and effectively obtain the three-dimensional fine structure of the active fault zone, is beneficial to researching the earthquake pregnancy and the earthquake rupture distribution, and has important theoretical and practical significance.
Drawings
FIG. 1 is a schematic diagram of the principle of natural seismic waveform based active tomographic imaging in accordance with an embodiment of the present invention;
FIG. 2 is a flow diagram of a method for natural seismic waveform based active tomographic imaging in accordance with an embodiment of the present invention;
FIG. 3 is a schematic diagram of a two-dimensional borehole microseismic observation system in accordance with an embodiment of the present invention;
FIGS. 4a to 4d are graphs showing the results of two-dimensional elastic wave reverse time imaging according to the embodiment of the present invention;
FIG. 5 is a Parkfield area station and seismic profile, Calif., of the United states, in accordance with an embodiment of the present invention;
FIG. 6 is a Vp velocity profile across the SAFOD normal to the fault from Parkfield area double difference seismic travel time imaging according to an embodiment of the present invention;
FIG. 7 is a schematic diagram of active source seismic imaging through a San Andrea fault SAFOD drilling section, Calif., in accordance with an embodiment of the present invention;
FIG. 8 is a Parkfield areal three-dimensional reverse-time imaging result in accordance with an embodiment of the invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to specific embodiments and the accompanying drawings.
The invention provides an active fault structure imaging method based on natural seismic waveforms, which utilizes the coherence of a seismic wave field propagated in a reverse time at discontinuous structures in a medium to image a subsurface fault structure. The imaging method only needs to reversely propagate the elastic wave field according to the wave equation, and does not need to forward transmit the seismic source wave field like the active source seismic imaging, so that seismic source information such as a seismic source function, a seismic source position and a seismic origin moment is not needed.
The invention provides an active fault structure imaging method based on natural seismic waveforms, which is an imaging method by utilizing elastic wave information. The seismic waves generate wave field conversion phenomena at the discontinuous part of the underground medium, and longitudinal waves and converted shear waves (or shear waves and converted longitudinal waves) start from the discontinuous part and are recorded by a seismic station. The elastic wave field recorded by the station is propagated along the reverse direction of time, so that the longitudinal wave and the converted transverse wave (or the transverse wave and the converted longitudinal wave) can also return to the discontinuous body where the wave field conversion occurs, and the conversion point where the converted wave occurs (namely the discontinuous part of the underground medium) can be imaged by utilizing the coherence of the longitudinal wave field and the transverse wave field at the moment.
The imaging principle is as shown in fig. 1, fig. 1 is a schematic diagram of the imaging principle of an active fault structure based on natural seismic waveforms according to an embodiment of the invention, and a five-pointed star represents a seismic source; circles represent discontinuities in the medium; triangles represent seismic stations; the solid line represents the propagation path of the seismic wavefield used in the imaging method proposed by the present invention, i.e. the longitudinal waves and converted shear waves (or shear waves and converted longitudinal waves) excited by the discontinuity in the medium; the dashed lines represent the propagation paths of seismic wavefields that are not used in the imaging method.
FIG. 2 is a flow diagram of a method for natural seismic waveform based active tomographic imaging in accordance with an embodiment of the present invention, the method comprising the steps of:
S11, performing reverse-time reconstruction on the seismic wave elastic wave field;
At Xs＝(xs，ys，zs) A seismic event occurs, wherein Xsrepresenting the position, x, of a seismic event in a three-dimensional mediumsand ysCoordinates representing the seismic event in the horizontal direction, in meters, zsThe coordinates of the seismic event in the depth direction are expressed in meters. The seismic wavefield D is placed at Xr＝(xr，yr，zr) Of seismic station, wherein XrRepresenting the position of a seismic station in a three-dimensional medium, xrAnd yrrepresenting the horizontal coordinates of the seismic station in meters, zrRepresenting the coordinates of the seismic station in the depth direction in meters. Seismic wavefield zrCan be expressed as:
D(Xr，t)＝S(Xs，t)G(Xr，Xs，t) (1)
where t represents time, S represents a source time function, and G is a Green' S function.
And (3) with the seismic wave field D as an initial condition, propagating the seismic wave field along the reverse direction of time according to the smooth velocity model of the region, namely performing reverse-time reconstruction on the wave field to obtain a reverse-thrust wave field W:
Where t denotes time, i denotes the ith station, and G is the conjugate of the green function.
Various numerical calculation methods can be used to solve the conjugate of the green's function, i.e. calculate the elastic wavefield starting from the largest time and going to the smallest time direction. The accumulation of i represents the simultaneous use of the waveform recordings of all the seismic stations for the reverse-time reconstruction of the seismic wavefield.
S12, performing longitudinal wave field and transverse wave field decoupling separation on the seismic wave elastic wave field;
specifically, according to waveform recording of a natural earthquake, a seismic wave elastic wave field is calculated from the maximum time to the minimum time along the reverse direction of time, and then longitudinal waves and transverse waves are decoupled and separated at each time step.
In this embodiment, the divergence and rotation methods may be used to decouple and separate the longitudinal and transverse wave fields:
wherein, λ and μ represent Lame coefficients,The degree of divergence is expressed as a degree of divergence,X represents the derotation, u represents the elastic wavefield, P represents the decoupled compressional wave, and S represents the decoupled shear wave.
S13, implementing zero-delay cross-correlation imaging conditions on the decoupled longitudinal wave and transverse wave by utilizing the coherence of the longitudinal wave field and the transverse wave field, and converting the four-dimensional counter-propagating wave field into imaging of a discontinuous structure in a medium;
The elastic wave field recorded by the station propagates along the time reverse direction, so that the longitudinal wave and the converted shear wave (or the shear wave and the converted longitudinal wave) can also return to the discontinuous body where the wave field conversion occurs, and the conversion point where the converted wave appears (namely the discontinuous part of the underground medium) is imaged by utilizing the coherence of the longitudinal wave field and the shear wave field at the moment. After longitudinal and transverse wave decoupling is carried out by adopting a divergence and rotation method, zero-delay cross-correlation imaging conditions are implemented on the decoupled longitudinal wave P and transverse wave S, and a four-dimensional back-propagation wave field is converted into an imaging I of a discontinuous structure in a mediumPS：
IPS(X)＝∑t(∑iPi(X，t)∑iSi(X，t)) (4)
Therein, sigmaiPi(X, t) and ∑iSiAnd (X, t) is a longitudinal wave field and a transverse wave field obtained by decoupling the reverse propagation elastic wave fields of all the stations.
Aiming at sparseness (relative to artificial seismic exploration) and irregularity of natural seismic stations, the invention provides a grouping imaging condition to improve the energy focusing degree of elastic wave reverse-time imaging so as to improve the spatial resolution of the elastic wave reverse-time imaging and reduce imaging noise. Compared with the traditional cross-correlation imaging problem condition, the grouping imaging condition groups the seismic stations, obtains a plurality of back propagation wave fields by taking different groups of passive source seismic records as initial conditions, and then images the medium structure by utilizing the coherence among the wave fields of the different groups, wherein the corresponding formula is as follows:
Wherein, giindicating the ith station, pi indicates multiplication,AndIs a longitudinal wave field and a transverse wave field obtained by decoupling the I-th group of reverse-transmission elastic wave fields IPSIs undergroundimaging of discontinuities in the medium, t representing time, and X representing three-dimensional spatial coordinates.
for the cross-correlation imaging condition represented by equation (4), the anti-propagation longitudinal or shear wave fields of all stations are added; whereas for the grouped imaging conditions represented by equation (5), the back-propagation longitudinal or shear wave fields of different station groups are multiplied. The energy of the wavefield at the transition point where the converted wave occurs (i.e., the discontinuity of the subsurface medium) can be increased by multiplying the wavefields of different station groups, so that the tolerance of the grouped imaging conditions to the sparse seismic stations is better than that of the cross-correlation imaging conditions. Meanwhile, station multiplication can reduce data noise and imaging noise and improve the imaging signal-to-noise ratio.
the method provided by the invention is used for carrying out three-dimensional fine structure imaging on a SanAndrea Fault (SAFZ for short) in California. FIG. 3 is a schematic diagram of a two-dimensional borehole microseismic observation system in accordance with an embodiment of the present invention. The fracture region on the east side of the event is imaged by using 6 forward simulated seismic event waveform records and the imaging method provided by the invention by adopting a zero-delay cross-correlation imaging condition (formula 4), as shown in fig. 4 a. Also, by the imaging method provided by the invention, 15 stations are divided into 2, 3 and 5 groups, and imaging is carried out by adopting the grouped imaging conditions, and the results show that the new imaging conditions can image the crack region more clearly and have higher signal-to-noise ratio, as shown in fig. 4 b-d.
the seismic waveform data used in the present invention was 560 earthquakes recorded by 68 temporary ground stations deployed in the Parkfield centered on SAFOD wells, as shown in fig. 5, which is a Parkfield station and seismic profile in california, usa, according to an embodiment of the present invention. Wherein the triangle: a seismic station; dot: drilling location of the SAFOD; solid line: saint Andrews fault; gray dots distributed along the saint anderless fault: and (5) natural earthquake.
Velocity profiles (Zhang et al, 2009) through SAFOD perpendicular to the fault obtained by the Parkfield area double difference seismic travel time imaging are shown in fig. 6, and fig. 6 is a Vp velocity profile through SAFOD perpendicular to the fault obtained by the Parkfield area double difference seismic travel time imaging according to the embodiment of the invention. Where the thick solid lines represent SAFOD drilling locations and the black dots represent natural seismic events.
therefore, the speed imaging result can only obtain a smooth speed structure in the Parkfield area, and the fine description of the fault structure is lacked. In order to study the fine fault structure near the SAFOD well, the area adopts expensive artificial seismic exploration, and the imaging result detects near-vertical reflecting surfaces (Bleibinhaus et al 2007) inside and outside the fault through an active source exploration and wave field backscattering imaging method, as shown in FIG. 7. The San Andreas fault is located at about X-2.0 as shown in fig. 7.
According to the active fault structure imaging method based on the natural seismic waveform, the Saint Anderson fault structure of a Parkfield area (figure 5) is imaged according to the waveform records of 120 seismic events distributed on the depth of 3.6-6km recorded by 68 seismic stations, the superposition imaging result of 120 seismic events is shown in figure 8, and white dots represent the natural seismic events. As can be seen from fig. 8, the nearly vertical main fault at X ═ 2km has a clearly identifiable structure. In addition, a nearly vertical secondary fault is visible at 0 km. Comparing the imaging results of the existing artificial seismic migration profile in the area with that of FIG. 7, the main fault with X being 2km and the secondary fault with X being 0km are true and credible. It can be concluded that it would be advantageous to study the seismic structure by using more seismic events distributed at different depths to obtain a clearer and more reliable fault structure.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention, and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (5)
1. A method of imaging an active fault structure based on natural seismic waveforms, comprising:
performing reverse-time reconstruction on the seismic wave elastic wave field;
Carrying out decoupling separation on a longitudinal wave field and a transverse wave field of the seismic wave elastic wave field; and
and (3) implementing zero time delay cross-correlation imaging conditions on the decoupled longitudinal wave field and shear wave field, and converting the four-dimensional back propagation wave field into imaging of discontinuous structures in the medium.
2. The method of claim 1, wherein said reverse time reconstructing the seismic wavefield comprises:
The seismic wavefield D is: d (X)r，t)＝S(Xs，t)G(Xr，Xs，t)
where t represents time, S represents a seismic source time function, G represents a Green' S function, XrRepresenting the position of a seismic station in a three-dimensional medium, Xr＝(xr，yr，zr)，xrand yrrepresenting the horizontal coordinates of the seismic station in meters, zrCoordinates in meters, X, representing the seismic station in the depth directionsRepresenting the position, X, of a seismic event in a three-dimensional mediums＝(xs，ys，zs)，xsAnd ysCoordinates representing the seismic event in the horizontal direction, in meters, zsCoordinates representing the seismic event in the depth direction in meters;
and (3) with the seismic wave field D as an initial condition, propagating the seismic wave field along the reverse direction of time according to the smooth velocity model of the region, namely performing reverse-time reconstruction on the wave field to obtain a reverse-thrust wave field W:
Where t denotes time, i denotes the ith station, and G is the conjugate of the green's function.
3. the method of claim 1, wherein said decoupling the seismic wavefield for the compressional and shear wavefields comprises:
Decoupling and separating longitudinal and transverse wave fields by adopting a divergence and rotation method:
Wherein, λ and μ represent Lame coefficients,The degree of divergence is expressed as a degree of divergence,Representing the derotation, u the elastic wavefield, P the decoupled compressional, and S the decoupled shear.
4. the method of claim 3, wherein the applying zero-delay cross-correlation imaging conditions to the decoupled compressional and shear wave fields, and wherein converting the four-dimensional anti-propagation wave field into imaging of discontinuities in the medium comprises:
after decoupling longitudinal and transverse wave fields by adopting divergence and rotation methods, implementing zero-delay cross-correlation imaging conditions on decoupled longitudinal waves P and transverse waves S, and converting four-dimensional back-propagation wave fields into imaging I of discontinuous structures in underground mediumPS：
IPS(X)＝∑t(∑iPi(X，t)∑iSi(X，t))
therein, sigmaiPi(X, t) and ∑iSiAnd (X, t) is a longitudinal wave field and a transverse wave field obtained by decoupling the reverse propagation elastic wave fields of all the stations.
5. The method of claim 4, wherein said applying zero-delay cross-correlation imaging conditions to the decoupled longitudinal and shear wave fields to convert a four-dimensional back-propagation wave field into imaging of discontinuities in the medium comprises:
grouping imaging conditions are used for grouping the seismic stations, different groups of passive source seismic records are used as initial conditions to obtain a plurality of back-propagation wave fields, then coherence among the wave fields of the different groups is used for imaging a medium structure, and a corresponding formula is as follows:
Wherein, giindicating the ith station, pi indicates multiplication,Andis a longitudinal wave field and a transverse wave field obtained by decoupling the I-th group of reverse-transmission elastic wave fields IPSIs the imaging of a discontinuity in the subsurface medium, t represents time, and X represents a three-dimensional spatial coordinate.
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| CN111257928A (en) * | 2020-02-12 | 2020-06-09 | 中国科学技术大学 | Seismic source positioning method and positioning system |
| CN111257928B (en) * | 2020-02-12 | 2021-08-13 | 中国科学技术大学 | Seismic source positioning method and positioning system |
| CN113885079A (en) * | 2021-08-23 | 2022-01-04 | 中国石油大学(华东) | Elastic wave field decoupling-based high-precision multi-azimuth reverse-time seismic source imaging method |
| CN113805228A (en) * | 2021-09-23 | 2021-12-17 | 西安科技大学 | Ground micro-seismic positioning method based on surface wave frequency dispersion |
| CN113805228B (en) * | 2021-09-23 | 2024-01-30 | 西安科技大学 | Ground microseism positioning method based on surface wave dispersion |
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