CN112305595A - Method for analyzing geologic body structure based on refracted wave and storage medium - Google Patents
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Abstract
The invention discloses a method for analyzing a geologic body structure based on refracted waves and a storage medium, wherein the method comprises the following steps: setting initial values of the underground propagation velocities of the seismic refracted waves at each geological grid point, constructing ray paths of the seismic refracted waves from the seismic source to each geological grid point, and correcting the initial values of the underground propagation velocities according to the difference value between the actual first arrival travel time of the seismic refracted waves from the seismic source to each geological grid point and the theoretical first arrival travel time of the seismic refracted waves propagating along the ray paths so as to obtain the corrected values of the underground propagation velocities of the seismic refracted waves at each geological grid point; and determining the geological quality factor of each geological grid point according to the correction value of the underground propagation speed and the geological absorption characteristic time of each ray path based on the geological quality factor inversion model. The seismic refraction wavelet analyzed by the method is easy to distinguish from other waves, so that the seismic refraction wavelet can be accurately extracted, the first arrival can be conveniently and reliably picked up, and the reliability of the subsequent geologic body structure inversion is improved.
Description
Technical Field
The invention belongs to the field of seismic data processing, and particularly relates to a method for analyzing a geologic body structure based on refracted waves and a storage medium with the method for analyzing the geologic body structure based on the refracted waves stored inside.
Background
Currently, in geophysical exploration, Seismic data are often processed and interpreted using ground Seismic reflection waves, interwell transmission waves, VSP (Vertical Seismic Profiling, i.e., Vertical Seismic profile) up-and down-going waves, and the like.
However, the following problems exist for the explanation of the processing using the surface seismic reflection wave, the interwell transmission wave, the VSP up-and down-going wave, and the like:
firstly, as the seismic information of shallow reflection is distributed in a near channel and is seriously influenced by a near surface wave, accurate seismic reflection wavelets are difficult to extract; VSP up and down waves are not easy to separate; the interwell transmitted waves are typically secondary references to surface seismic reflections.
Secondly, exploration of the structural morphology of the deep base by using seismic reflection waves can cause that the deep reflection energy becomes very weak, the reliability of effective information is also deteriorated, and small-scale geological structures such as underground thin layers and cracks cannot be well identified.
Thirdly, most of the seismic reflected wave energy is only concentrated on the near offset, and the acquisition and imaging of information at the far offset cannot be carried out.
The quality factor Q is an important parameter reflecting the energy absorption and attenuation of the underground medium to the seismic wave, can represent the characteristic attribute of the stratum, and can be used for identifying and explaining the rock property, the fault and the fluid distribution, so that the accurate estimation of the Q value has important significance for researching the lithology, the physical property, the structure, the fluid-containing property and the like of the underground medium. The geological quality factor can be calculated by Q-tomography.
There is a need for a method for analyzing a geologic body structure based on refracted waves and a storage medium having stored therein the method for analyzing a geologic body structure based on refracted waves.
Disclosure of Invention
The invention aims to solve the technical problem that the geologic body structure analysis result is inaccurate due to the fact that seismic wavelets cannot be accurately acquired.
In order to solve the technical problem, the invention provides a method for analyzing a geologic body structure based on refracted waves, which comprises the following steps:
s100, partitioning a geologic body profile map of a target reservoir into a plurality of geological grids to form a plurality of geological grid points;
s200, setting initial values of the underground propagation velocities of seismic refracted waves at each geological grid point, constructing ray paths of the seismic refracted waves from a seismic source to each geological grid point, and correcting the initial values of the underground propagation velocities according to the difference value between the actual first arrival travel time of the seismic refracted waves from the seismic source to each geological grid point and the theoretical first arrival travel time of the seismic refracted waves along the ray paths so as to obtain the corrected values of the underground propagation velocities of the seismic refracted waves at each geological grid point;
s300, based on a geological quality factor inversion model, determining geological quality factors of all geological grid points according to the corrected value of the underground propagation speed of seismic refracted waves at all the geological grid points and geological absorption characteristic time of all ray paths;
and S400, analyzing the geologic body structure of the target reservoir according to the distribution of the geologic quality factors of each geologic grid point.
Preferably, the step S200 includes the steps of:
s210, setting initial values of the underground propagation speeds of seismic refracted waves at geological grid points;
s220, constructing ray paths of seismic refracted waves from the seismic source to each geological grid point, and determining theoretical first-arrival travel time of the seismic refracted waves transmitted along the ray paths according to the initial value of the underground propagation speed;
and S230, based on the underground velocity inversion model, correcting the initial value of the underground propagation velocity according to the difference value between the actual first arrival travel time of the seismic refracted wave from the seismic source to each geological grid point and the theoretical first arrival travel time of the seismic refracted wave propagating along the ray path, so as to obtain the corrected value of the underground propagation velocity of the seismic refracted wave at each geological grid point.
Preferably, the step S200 includes the steps of:
s210, setting initial values of the underground propagation speeds of seismic refracted waves at geological grid points;
s220, constructing ray paths of seismic refracted waves from the seismic source to each geological grid point, and determining theoretical first-arrival travel time of the seismic refracted waves transmitted along the ray paths according to the initial value of the underground propagation speed;
s230, based on the underground velocity inversion model, correcting the initial value of the underground propagation velocity according to the difference value between the actual first arrival travel time of the seismic refracted wave from the seismic source to each geological grid point and the theoretical first arrival travel time of the seismic refracted wave propagating along the ray path, so as to obtain the corrected value of the underground propagation velocity of the seismic refracted wave at each geological grid point;
s240, judging whether the difference value between the theoretical first-arrival travel time and the actual first-arrival travel time of the correction value corresponding to the current underground propagation speed is smaller than a preset threshold value:
if yes, go to step S300;
if not, the initial value of the underground propagation velocity is equal to the correction value of the current underground propagation velocity, and the step S220 is returned to correct the underground propagation velocity of the seismic refracted wave at each geological grid point again.
Preferably, in step S200, an initial value of the underground propagation velocity of the seismic refracted wave at each geological grid point is set so as to conform to a two-dimensional velocity distribution of a uniform variation gradient; and (4) constructing ray paths of the seismic refracted waves from the seismic source to each geological grid point by a ray tracing method.
Preferably, in step S200, the actual first arrival journey time of the seismic refracted wave from the seismic source to each geological grid point is the actual first arrival journey time of the seismic refracted wave at the geological grid point picked up on the unstacked single-shot seismic record section.
Preferably, the step S400 includes the steps of:
s410, correspondingly filling the geological quality factors of each geological grid point into a geological profile so as to obtain the structural distribution of the geological quality factors;
and S420, analyzing the geologic body structure of the target reservoir according to the distribution of the geologic quality factors in the geologic body profile.
Preferably, the subsurface velocity inversion model is as follows:
Bδv＝δt
wherein B is an underground velocity inversion coefficient matrix, δ v is a difference value between a corrected value and an initial value of the underground propagation velocity of the seismic refracted wave of each geological grid point, and δ t is a difference value between the actual first-arrival travel time and the theoretical first-arrival travel time of the seismic refracted wave of each geological grid point.
Preferably, in step S300, the geologic absorption characteristic time of each ray path is obtained by the following expression:
wherein, t*And C (f) is a stratum response frequency spectrum, and f is a seismic refraction wavelet frequency.
Preferably, in step S300, the geologic quality factor inversion model is:
wherein Q (x, z) is the geological quality factor of the geological grid point, vk(x, z) is a correction value for the subsurface propagation velocity of seismic refracted waves at a geologic grid point on the kth ray, wk(x, z) is the weight factor of the k ray, t* kGeologic absorption characteristic time, L, for the kth raykThe ray path of the k-th ray.
According to another aspect of the invention, a storage medium is provided, on which a computer program is stored, characterized in that the program, when being executed by a processor, carries out the steps of the method as described above.
Compared with the prior art, one or more embodiments in the above scheme can have the following advantages or beneficial effects:
1) the seismic refraction wavelet analyzed by the method is easily distinguished from other waves and near channel interference is avoided, so that the seismic refraction wavelet can be accurately extracted, the first arrival can be conveniently and reliably picked up, and the reliability of the subsequent geologic body structure inversion is improved;
2) according to the method, the propagation process of the seismic refraction wavelets in the multiple geological grids is analyzed, and due to the fact that the seismic refraction wavelets are high in coaxial energy and good in traceability, speed imaging can be conducted on small-scale geological structures such as underground thin layers and cracks, the small-scale geological structures can be identified conveniently and reliably, and explanation is further provided for details of the geological structure;
3) the seismic refraction wavelet has the characteristic of wide angle in the propagation process of a plurality of geological grids, can acquire and image information at a far offset distance, and provides a more complete explanation for the inversion of a geological body structure.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
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The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention and not to limit the invention. In the drawings:
FIG. 1 shows a flow diagram of a method for analyzing geologic body structures based on refracted waves according to an embodiment of the present invention;
FIG. 2 is a flow chart illustrating the specific steps of a method for analyzing the geologic body structure based on refracted waves according to a first embodiment of the present invention;
FIG. 3 is a flow chart illustrating another specific step of the method for analyzing geologic body structure based on refracted waves according to the second embodiment of the present invention;
FIG. 4 shows data patterns of a test work area in a fourth embodiment of the present invention;
figure 5 shows a layered initial velocity model created by time-lapse tomography in a fourth embodiment of the present invention,
the reference numbers in the drawings are: 1-refracted wave, 2-reflected wave, 3-water wave.
Detailed Description
The following detailed description of the embodiments of the present invention will be provided with reference to the drawings and examples, so that how to apply the technical means to solve the technical problems and achieve the technical effects can be fully understood and implemented. It should be noted that, as long as there is no conflict, the embodiments and the features of the embodiments of the present invention may be combined with each other, and the technical solutions formed are within the scope of the present invention.
The method acquires the underground propagation velocity of each geological grid point according to the travel time tomography principle, and the travel time tomography principle is a process for establishing an underground medium velocity structure by taking travel time as a starting point. The speed inversion process belongs to a nonlinear inversion problem, and the nonlinear problem can be linearized by solving the problem by adopting an iterative method. After the linear method is converted, an initial velocity model is provided, and the path length and the theoretical first arrival travel time can be calculated by the shortest path ray tracing method. And secondly, obtaining the residual error of the speed model according to the selected regularization criterion and the travel time difference. And finally, gradually updating the speed structure model according to the residual error of the speed until the residual error is within an acceptable enough range, namely obtaining a real underground speed model, namely obtaining the underground propagation speed of each geological grid point. And obtaining the Q value of each geological grid point according to the underground propagation speed of each geological grid point and the Q tomography principle. The Q-tomography principle requires two-dimensional or three-dimensional ray tracing. The Q tomography principle comprises two steps: in the first step, the travel time t associated with Q is determined by using the waveform and amplitude spectrum ratio of the relevant wave. And secondly, performing Q value inversion according to the t value obtained in the first step. The inversion needs to grid the model, and after the velocity model is obtained, the value of v at each grid point corresponds to a Q value to be obtained.
Specifically, the method adopts a spectral ratio method of a frequency domain to carry out Q value estimation and tomography on a refracted wave ray propagation area.
Example one
In order to solve the technical problems in the prior art, embodiments of the present invention provide a method for analyzing a geologic body structure based on refracted waves.
Referring to fig. 2, the method for analyzing the geologic body structure based on refracted waves of the present embodiment includes the following steps:
s100, partitioning a geologic body profile map of a target reservoir into a plurality of geological grids to form a plurality of geological grid points;
s200, setting initial values of the underground propagation velocities of seismic refracted waves at each geological grid point, constructing ray paths of the seismic refracted waves from a seismic source to each geological grid point, and correcting the initial values of the underground propagation velocities according to the difference value between the actual first arrival travel time of the seismic refracted waves from the seismic source to each geological grid point and the theoretical first arrival travel time of the seismic refracted waves along the ray paths so as to obtain the corrected values of the underground propagation velocities of the seismic refracted waves at each geological grid point;
specifically, the step S200 includes the steps of:
s210, setting initial values of the underground propagation speeds of seismic refracted waves at geological grid points;
s220, constructing ray paths of seismic refracted waves from the seismic source to each geological grid point, and determining theoretical first-arrival travel time of the seismic refracted waves transmitted along the ray paths according to the initial value of the underground propagation speed;
and S230, based on the underground velocity inversion model, correcting the initial value of the underground propagation velocity according to the difference value between the actual first arrival travel time of the seismic refracted wave from the seismic source to each geological grid point and the theoretical first arrival travel time of the seismic refracted wave propagating along the ray path, so as to obtain the corrected value of the underground propagation velocity of the seismic refracted wave at each geological grid point.
S300, based on a geological quality factor inversion model, determining geological quality factors of all geological grid points according to the corrected value of the underground propagation speed of seismic refracted waves at all the geological grid points and geological absorption characteristic time of all ray paths;
and S400, analyzing the geologic body structure of the target reservoir according to the distribution of the geologic quality factors of each geologic grid point.
Example two
In order to solve the above technical problems in the prior art, an embodiment of the present invention further provides another specific implementation of a method for analyzing a geologic body structure based on refracted waves.
Referring to fig. 3, the method for analyzing the geologic body structure based on refracted waves of the present embodiment includes the following steps:
s100, partitioning a geologic body profile map of a target reservoir into a plurality of geological grids to form a plurality of geological grid points;
here, the geologic profile may be drawn according to a target reservoir, which is the geologic body to be analyzed. The geologic profile is parameterized by a uniform grid, which is of constant size in each direction.
S200, setting initial values of the underground propagation velocities of seismic refracted waves at each geological grid point, constructing ray paths of the seismic refracted waves from a seismic source to each geological grid point, and correcting the initial values of the underground propagation velocities according to the difference value between the actual first arrival travel time of the seismic refracted waves from the seismic source to each geological grid point and the theoretical first arrival travel time of the seismic refracted waves along the ray paths so as to obtain the corrected values of the underground propagation velocities of the seismic refracted waves at each geological grid point;
specifically, the step S200 includes the steps of:
s210, setting initial values of the underground propagation speeds of seismic refracted waves at geological grid points;
specifically, an initial velocity model of seismic refracted waves propagating in a target reservoir is obtained, and initial values of the underground propagation velocities of the seismic refracted waves at each geological grid point are extracted from the initial velocity model; the initial velocity model is a two-dimensional velocity distribution with a uniform variation gradient, and the initial value of the underground propagation velocity of the seismic refracted wave at each geological grid point is set so as to be in accordance with the two-dimensional velocity distribution with the uniform variation gradient. The initial velocity model is derived from empirical data and is related to the attributes of the geologic volume. For example, the initial estimation can be performed according to the actual first arrival travel time of the seismic refracted wave at each geological grid point, and in the invention, the two-dimensional velocity distribution with uniform variation gradient is given according to the measured known parameters such as the depth of the submarine topography, the acquisition range and the like.
S220, constructing ray paths of seismic refracted waves from the seismic source to each geological grid point, and determining theoretical first-arrival travel time of the seismic refracted waves transmitted along the ray paths according to the initial value of the underground propagation speed;
specifically, ray paths of seismic refracted waves from a seismic source to geological grid points are constructed through a ray tracing method. In particular, the forward process of travel and ray path adopts a ray tracing forward method of Vidale (1988,1990), which is not described herein.
And S230, based on the underground velocity inversion model, correcting the initial value of the underground propagation velocity according to the difference value between the actual first arrival travel time of the seismic refracted wave from the seismic source to each geological grid point and the theoretical first arrival travel time of the seismic refracted wave propagating along the ray path, so as to obtain the corrected value of the underground propagation velocity of the seismic refracted wave at each geological grid point.
Specifically, the subsurface velocity inversion model is as follows:
Bδv＝δt
wherein, B is an underground velocity inversion coefficient matrix, δ v is the difference value between the underground propagation velocity and the initial velocity of each geological grid point, and δ t is the difference value between the actual first-arrival travel time and the theoretical first-arrival travel time of the seismic refracted wave of each geological grid point. Here, B is applied to a specific matrix of δ v by δ t column vector as the matrix of the underground velocity inversion coefficients, and when δ t and δ v are both column vectors of n × 1, B is a matrix of n × n.
Specifically, the actual first arrival travel time of seismic refracted waves at geological grid points is picked up on unstacked single shot seismic record sections. The method comprises the steps of collecting first-arrival refracted waves during travel based on artificial source wide-angle seismic data collected by Nissan islands in the Sumen ansha area, selecting a time window with a proper width to completely intercept needed waveforms, and manually intercepting the data in the step, wherein the quality of the intercepted data in the step determines the reliability of a subsequent inversion step.
S240, judging whether the difference value between the theoretical first-arrival travel time and the actual first-arrival travel time of the correction value corresponding to the current underground propagation speed is smaller than a preset threshold value:
if yes, go to step S300;
if not, the initial value of the underground propagation velocity is equal to the correction value of the current underground propagation velocity, and the step S220 is returned to correct the underground propagation velocity of the seismic refracted wave at each geological grid point again.
Here, the current corrected value of the subsurface propagation velocity is the corrected value of the subsurface propagation velocity of the seismic refracted wave at each geological grid point in step S230.
The following illustrates the multiple correction process of the subsurface propagation velocity of seismic refracted waves at each geological grid point:
the method comprises the steps that firstly, the actual first arrival travel time of seismic refraction waves from a seismic source to each geological grid point is set to be T, the ray path constructed for the first time is s0, and the initial value of the underground propagation speed is v 0;
secondly, forward modeling a theoretical first arrival travel time T0 of seismic refracted waves propagating along s0 according to a ray path s0 constructed for the first time and an initial value v0 of the underground propagation velocity, and obtaining a first corrected value v1 of the underground propagation velocity according to the initial value v0 of the underground propagation velocity, the theoretical first arrival travel time T0 and the actual first arrival travel time T;
thirdly, judging whether the difference value between the theoretical first-arrival travel time T and the actual first-arrival travel time T of the seismic refracted wave positively evolved according to the ray path s0 and the first correction value v1 of the underground propagation speed along s0 meets the precision; if yes, the first correction value v1 of the underground propagation velocity is the final correction value of the underground propagation velocity, and step S300 is executed; if not, returning to the step S200, reconstructing a ray path S1, forward-playing T1 of the theoretical first-arrival travel time of the seismic refracted wave propagating along S1 according to the ray path S1 and the first correction value v1 of the underground propagation speed, and obtaining a second correction value v2 of the underground propagation speed for T according to the initial value v1 of the underground propagation speed, the theoretical first-arrival travel time T1 and the actual first-arrival travel time;
fourthly, judging whether the difference value between the theoretical first-arrival travel time T and the actual first-arrival travel time T of the seismic refracted wave positively played along s1 according to the ray path s1 and the second correction value v2 of the underground propagation speed meets the precision; if yes, the second correction value v2 of the underground propagation velocity is the final correction value of the underground propagation velocity; if the seismic refracted wave does not meet the requirement, a ray path s2 is reconstructed, a theoretical first-arrival travel time T2 of the seismic refracted wave propagating along s2 is forward calculated according to the ray path s2 and a second correction value v2 of the underground propagation speed, and a third correction value v3 of the underground propagation speed is obtained according to an initial value v2 of the underground propagation speed, the theoretical first-arrival travel time T2 and the actual first-arrival travel time T;
and analogizing in sequence until the difference between the theoretical first-arrival travel time and the actual first-arrival travel time corresponding to the correction value of the current underground propagation velocity meets the precision.
And updating the underground propagation speed along with the continuous reduction of the value of delta t in multiple iterations until the delta t reaches a specified precision range, for example, the ratio of the delta t to the actual first arrival travel time is less than 0.1%, and obtaining the underground propagation speed, namely the final inversion result.
In addition, in this embodiment, inversion iteration is performed from the theoretical travel time and the picked actual travel time, and after the iteration is completed, drawing may be performed by a GMT (general Mapping tools) drawing script, so as to obtain a schematic diagram of the formation velocity structure.
In addition, in steps S100 to S240, the local formation velocity structure and ray path are preferably obtained by using a refracted wave first-arrival travel time tomography method.
S300, based on a geological quality factor inversion model, determining geological quality factors of all geological grid points according to the corrected value of the underground propagation speed of seismic refracted waves at all the geological grid points and geological absorption characteristic time of all ray paths;
specifically, the geological absorption characteristic time t of each ray path is obtained by the method proposed by Wilcock (1995). In the time domain, the seismic signals can be represented in the form of a convolution:
x(t)＝s(t)*c(t)*i(t)
where t is time, s (t) is the source signal, c (t) is the formation response, and i (t) is the instrument response.
Accordingly, in the frequency domain, the frequency spectrum C of the formation response may be obtained by dividing the frequency spectrum X of the seismic signal by the frequency spectrum S of the source signal and the frequency spectrum I of the instrument response:
wherein C (f) is the formation response spectrum, X (f) is the received seismic signal spectrum, S (f) is the source signal spectrum, I (f) is the instrument response spectrum, and f is the frequency.
Assuming that the time window for the spectral measurement has only one phase and only frequency-dependent attenuation in the propagating component of the seismic refracted sub-wave, C can be expressed as:
C(f,s)＝G(s)exp[-2πft(s)]
where s is the path of the wave and G is the divergence of the wave, which is a constant.
t may be obtained by plotting ln c (f) in the frequency domain and then performing linearization.
Wherein, t*And C (f) is a stratum response frequency spectrum, and f is a seismic refraction wavelet frequency.
Therefore, the geologic absorption characteristic time is obtained by the following expression:
wherein, t*For geological absorption characteristic time, C (f) is the formation response spectrum, f is the seismic refractonThe wave frequency.
In particular, the Q-tomography principle, i.e. t*The relation with Q can be specifically expressed as:
wherein Q (x, z) is the geological figure of merit of the geological grid point and v (x, z) is the subsurface propagation velocity of the geological grid point.
With the known velocity v (x, z), the inversion of Q (x, z) can be performed using travel time t.
This Q-solving process can be expressed by the following expression:
where A is the coefficient matrix is the quantity associated with the ray path and the grid intersection; lambda is a constraint parameter; l is Laplace operator; t is the column vector of t.
The geologic quality factor inversion model after dispersion is as follows:
wherein Q (x, z) is the geological quality factor of the geological grid point, vk(x, z) is the subsurface propagation velocity of the geological grid point on the kth ray, wk(x, z) is the weight factor of the k ray, t* kGeologic absorption characteristic time, L, for the kth raykThe ray path of the k-th ray.
The Q value is solved by using a least square method in the above linear equation set established based on the linear relationship between the t value and the Q value model corresponding to all the rays.
And S400, analyzing the geologic body structure of the target reservoir according to the distribution of the geologic quality factors of each geologic grid point.
In the second embodiment, the accuracy of the correction value of the current underground propagation velocity is ensured by controlling the accuracy of the difference between the theoretical first-arrival travel time and the actual first-arrival travel time corresponding to the correction value of the current underground propagation velocity in an iterative manner.
EXAMPLE III
To solve the technical problems in the prior art, a third embodiment of the present invention further provides a storage medium, on which a computer program is stored, where the computer program implements the steps of the method when being executed by a processor.
It should be noted that the method for implementing the program stored on the storage medium is the same as the method for analyzing the geologic body structure based on the refracted wave in the first embodiment, and therefore, the method for implementing the program stored on the storage medium in this embodiment is not described herein again.
Example four
The method for analyzing the structure of the geologic body based on the refracted waves is further described in the following with reference to specific application examples, and the geologic body with the X-axis range of 0-120km and the Z-axis range of 0-20km is subjected to structural analysis so as to show the longitudinal and transverse changes of the Q value of the sedimentary layer and the crust below the sedimentary layer. Fig. 4 shows the data form of the test work area in the fourth embodiment of the present invention, in which the forms of the refracted wave 1, the reflected wave 2, and the direct water wave 3 of the data can be clearly seen. As can be seen from FIG. 4, the seismic refraction wavelet analyzed by the method is easily distinguished from other waves and avoids near channel interference, so that the seismic refraction wavelet can be accurately extracted, the first arrival can be conveniently and reliably picked up, the reliability of subsequent geologic body structure inversion is improved, and the seismic refraction wavelet has the characteristic of wide angle in the transmission process of a plurality of geological grids, can acquire and image information at a far offset distance, and provides a more complete explanation for the inversion of the geologic body structure. Fig. 5 shows a laminar initial velocity model created by time-lapse tomography in the fourth embodiment of the present invention, and arrows in fig. 5 indicate directions in which the laminar initial velocity changes from small to large.
Firstly, obtaining an initial velocity model and a ray path of seismic refracted waves of a selected test work area by adopting a refracted wave first-arrival travel time tomography method; secondly, obtaining a t value related to the Q value by using the slope of the spectral ratio of the refracted wave first arrival to the water layer direct arrival (namely the seismic source signal spectrum); and finally, establishing a linear equation set based on the linear relation between the t value and the Q value model corresponding to all the rays, and solving the Q value model by using a least square method.
The Q tomography results show longitudinal and lateral variations in Q values present in the sediment layer and its underlying crust at depths of 5-12 km. Ranging from X-0 km to X-120 km, Q-1The overall decrease in vertical direction, especially below 10km (marine crust region) shows a lower Q-1Values show that seismic attenuation of the ocean crust is much less than that of the sediment layer. In the range of X120 km to X200 km, both the sediment layer and the marine crust show a lower Q-1Value in the transverse direction with X<The 120km area is in sharp contrast. Therefore, the method for analyzing the geologic body structure based on the refracted wave can effectively and reliably invert the Q value distribution of the underground structure, and provides powerful scientific basis for subsequent geological interpretation.
Although the embodiments of the present invention have been described above, the above description is only for the convenience of understanding the present invention, and is not intended to limit the present invention. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (10)
1. A method for analyzing geologic body structures based on refracted waves, comprising:
s100, partitioning a geologic body profile map of a target reservoir into a plurality of geological grids to form a plurality of geological grid points;
s200, setting initial values of the underground propagation velocities of seismic refracted waves at each geological grid point, constructing ray paths of the seismic refracted waves from a seismic source to each geological grid point, and correcting the initial values of the underground propagation velocities according to the difference value between the actual first arrival travel time of the seismic refracted waves from the seismic source to each geological grid point and the theoretical first arrival travel time of the seismic refracted waves along the ray paths so as to obtain the corrected values of the underground propagation velocities of the seismic refracted waves at each geological grid point;
s300, based on a geological quality factor inversion model, determining geological quality factors of all geological grid points according to the corrected value of the underground propagation speed of seismic refracted waves at all the geological grid points and geological absorption characteristic time of all ray paths;
and S400, analyzing the geologic body structure of the target reservoir according to the distribution of the geologic quality factors of each geologic grid point.
2. The method according to claim 1, wherein the step S200 comprises the steps of:
s210, setting initial values of the underground propagation speeds of seismic refracted waves at geological grid points;
s220, constructing ray paths of seismic refracted waves from the seismic source to each geological grid point, and determining theoretical first-arrival travel time of the seismic refracted waves transmitted along the ray paths according to the initial value of the underground propagation speed;
and S230, based on the underground velocity inversion model, correcting the initial value of the underground propagation velocity according to the difference value between the actual first arrival travel time of the seismic refracted wave from the seismic source to each geological grid point and the theoretical first arrival travel time of the seismic refracted wave propagating along the ray path, so as to obtain the corrected value of the underground propagation velocity of the seismic refracted wave at each geological grid point.
3. The method according to claim 1, wherein the step S200 comprises the steps of:
s210, setting initial values of the underground propagation speeds of seismic refracted waves at geological grid points;
s220, constructing ray paths of seismic refracted waves from the seismic source to each geological grid point, and determining theoretical first-arrival travel time of the seismic refracted waves transmitted along the ray paths according to the initial value of the underground propagation speed;
s230, based on the underground velocity inversion model, correcting the initial value of the underground propagation velocity according to the difference value between the actual first arrival travel time of the seismic refracted wave from the seismic source to each geological grid point and the theoretical first arrival travel time of the seismic refracted wave propagating along the ray path, so as to obtain the corrected value of the underground propagation velocity of the seismic refracted wave at each geological grid point;
s240, judging whether the difference value between the theoretical first-arrival travel time and the actual first-arrival travel time of the correction value corresponding to the current underground propagation speed is smaller than a preset threshold value:
if yes, go to step S300;
if not, the initial value of the underground propagation velocity is equal to the correction value of the current underground propagation velocity, and the step S220 is returned to correct the underground propagation velocity of the seismic refracted wave at each geological grid point again.
4. The method according to any one of claims 1 to 3, wherein in step S200, the initial value of the subsurface propagation velocity of the seismic refracted wave at each geological grid point is set so as to conform to a two-dimensional velocity distribution of a uniform variation gradient; and (4) constructing ray paths of the seismic refracted waves from the seismic source to each geological grid point by a ray tracing method.
5. The method of any of claims 1 to 3, wherein the actual first-arrival travel time of the seismic refracted waves from the seismic source to each geological grid is the actual first-arrival travel time of the seismic refracted waves at the geological grid picked up on the unstacked single-shot seismic record section in step S200.
6. The method according to any of claims 1 to 3, wherein said step S400 comprises the steps of:
s410, correspondingly filling the geological quality factors of each geological grid point into a geological profile so as to obtain the structural distribution of the geological quality factors;
and S420, analyzing the geologic body structure of the target reservoir according to the distribution of the geologic quality factors in the geologic body profile.
7. A method according to claim 2 or claim 3, wherein the subsurface velocity inversion model is as follows:
Bδv＝δt
wherein B is an underground velocity inversion coefficient matrix, δ v is a difference value between a corrected value and an initial value of the underground propagation velocity of the seismic refracted wave of each geological grid point, and δ t is a difference value between the actual first-arrival travel time and the theoretical first-arrival travel time of the seismic refracted wave of each geological grid point.
8. The method according to claim 1, wherein in step S300, the geologic absorption characteristic time of each ray path is obtained by the following expression:
wherein, t*And C (f) is a stratum response frequency spectrum, and f is a seismic refraction wavelet frequency.
9. The method of claim 1, wherein in step S300, the geologic quality factor inversion model is:
wherein Q (x, z) is the geological quality factor of the geological grid point, vk(x, z) is a correction value for the subsurface propagation velocity of seismic refracted waves at a geologic grid point on the kth ray, wk(x, z) is the weight factor of the k ray, t* kGeologic absorption characteristic time, L, for the kth raykThe ray path of the k-th ray.
10. A storage medium on which a computer program is stored which, when being executed by a processor, carries out the steps of the method according to any one of claims 1 to 9.
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