Patent Publication Number: US-10761228-B2

Title: Method to calculate acquisition illumination

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application having Ser. No. 62/438,955, filed Dec. 23, 2016, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF TECHNOLOGY 
     The present disclosure relates generally to a method for collecting and processing seismic data as well as seismic acquisition design. More specifically, the present disclosure provides a method to obtain acquisition illumination with a lower computational cost, especially when used in reverse time migration. 
     BACKGROUND 
     Seismology is used for exploration, archaeological studies, and engineering projects that require geological information. Exploration seismology techniques are used widely in the oil and gas industry in the exploration to identify prospects and types of geologic formations. In order to apply a seismic investigation of a geological area, the seismic data shall be acquired based on an optimal seismic acquisition system that defines the source and receiver locations for all the shots. With the seismic data as well as seismic processing and imaging techniques are deployed to generate subsurface images of the geologic area for interpretation. 
     Acquisition illumination is widely used to evaluate the acquisition system capability in acquiring the reflection information for subsurface structures. It also can be used to compensate the seismic image in order to minimize the effect of unbalanced illumination on subsurface structures. Acquisition illumination is a combination of effects from source and receiver locations for all the shots, subsurface structures of the model, and model properties. The calculation of acquisition illumination requires calculating both the source-side illumination and the receiver-side illumination for all the shots in the acquisition system, which typically involves a large amount of computations for production applications. 
     Reverse Time Migration (RTM) is the most advanced seismic imaging method. It is widely used in seismic processing in oil and gas industry as a standard advanced imaging method. The basic concept of RTM includes three parts, which are 1) simulating the source propagation, i.e., propagating forwards in time; 2) back-propagating the receiver records, i.e., propagating backwards in time; and 3) applying cross-correlation image condition with source and receiver wavefields at each time step during the propagation. Since the simulation of source and receiver wave propagations and applying image condition require a large amount of numerical computation, RTM is a computationally intensive process. 
     Acquisition illumination can be used to further improve imaging quality output from RTM as a compensation for the acquisition and complex structure effect. In the implementation of RTM, the source-side wavefield for each shot will be calculated. The source-side wavefield can be used to generate source-side illumination by summing up all the source-side wavefields for all the time steps. However, there is no receiver-side wavefield calculated for each receiver in RTM processing so that RTM cannot output receiver-side illumination. Accordingly, acquisition illumination during RTM processing still needs improvement. The current disclosure provides solutions to solve this issue. 
     SUMMARY 
     The present disclosure provides a method to output acquisition illumination during RTM processing. This method employs the source-side wavefield generated in regular RTM processing and therefore reduces the computational costs and time for outputting an acquisition illumination in a seismic processing production project. The solution in generating acquisition offered by this disclosure enables the utilization of acquisition illumination to compensate the seismic image output by RTM and in analyzing the seismic acquisition system. 
     The acquisition illumination calculation for a full survey typically includes calculating illumination for each single shot and summation of the illumination for all the shots to get the acquisition illumination for the survey. The illumination calculation for a single shot typically includes the calculation of source-side illumination (which can be obtained from the Green&#39;s function at the source location) and the calculation of receiver-side illumination (which are the summation of all the receivers&#39; illumination). The receiver illumination can be obtained from the Green&#39;s function at the receiver location. 
     In one of the embodiment of the current disclosure, a method for seismic acquisition illumination has the following steps. First, a plurality of source locations and a plurality of receivers are placed in a survey field. The receivers include one or more zero-offset receivers. Each zero-offset receiver is placed at one of the plurality of source locations. I.e., the location of each zero-offset receiver coincides with a source location. Conversely, a receiver that is not located at a source location is a non-zero-offset receiver. Further, at least one shot is deployed at each of the plurality of source locations. Each shot generates seismic waves that cover a volume of earth formation. The receivers collect seismic signals reflected from the volume of earth formation. 
     In addition, the source wavefield propagation is calculated for each of the plurality of source locations. The illumination of each of the plurality of source locations is calculated according to equation (4):
 
 D   s     i     ≈G   s     i     2 *Σ j=1   Ns     i   ( G   s     j     2 )  (4),
 
wherein G si  is the wavefield propagation of the source location, Ns i  the number of source locations positioned in the volume of earth formation, and G s     j    is the wavefield propagation of each source location positioned in the volume of earth formation. Finally, the illuminations of all of the plurality of source locations are summed up to obtain the illumination of the survey field.
 
     In another embodiment of this disclosure, the method for seismic acquisition illumination calculation includes the step of performing reverse time migration (RTM) of seismic signals in a seismic survey. The seismic survey deploys a plurality of shots at a corresponding number of source locations and a plurality of receivers in a survey field. The wavefield propagation for each of the plurality of source locations is calculated and assigned to be the wavefield propagation for a zero-offset receiver coincides with its corresponding source location. 
     Further, the illumination of each of the plurality of source locations is calculated according to equation (3): 
                     D     s   i       ≈       G     s   i     2     *       ∑     j   =   1       Nri     mx   *   my         ⁢     (     G     r   j     2     )                 (   3   )               
D s     i    is an illumination for a single shot which is a 3D volume having a grid size (Nx, Ny, Nz) in the xyz coordinate system, G si  and G rj  are respectively the wavefield propagation of the source location and the wavefield propagation of the receivers in the 3D volume, Nr i  is a total number of receivers located in the 3D volume, m x  and m y  are decimation factors in X- and Y-directions. Finally, illuminations of all of the plurality of source locations are summed up to obtain the illumination of the survey field.
 
     In a further embodiment, m x  and m y  are integers larger than one, provided that a value of 
             Nri     mx   *   my           
is an integer of one or larger than one.
 
     In still another embodiment, 
             Nri     mx   *   my           
is a number of source locations in the 3D volume, which is also the number of zero-offset receivers in the same 3D volume.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    is the source and receiver location map for a single shot in a typical seismic acquisition system. 
         FIG. 1 b    is the sectional view of the A-A plane in  FIG. 1 a   , showing the positional relations among the source, the receivers, and the subsurface structure in a formation under investigation. 
         FIG. 2  is the source location map for all the shots in a seismic acquisition system. 
         FIG. 3  is the source and receiver location map for all the shots in a seismic acquisition system. 
         FIG. 4  is the source and decimated receiver location map for a single shot in a seismic acquisition system. 
         FIG. 5  is the source and decimated receiver location map for all the shots in a seismic acquisition system. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure provides a method involving steps that may be described and implemented in the general context of a system and computer methods to be executed by a computer. Such computer-executable instructions may include programs, routines, objects, components, data structures, and computer software technologies that can be used to perform particular tasks and process abstract data types. Software implementations of the present disclosure may be coded in different languages for application in a variety of computing platforms and environments. It will be appreciated that the scope and underlying principles of the present disclosure are not limited to any particular computer software technology. 
     Moreover, those skilled in the art will appreciate that the present disclosure may be practiced using any one or combination of hardware and software configurations, including but not limited to a system having single and/or multiple computer processors, hand-held devices, programmable consumer electronics, mini-computers, mainframe computers, and the like. The disclosure may also be practiced in distributed computing environments where tasks are performed by servers or other processing devices that are linked through a one or more data communications network. In a distributed computing environment, program modules may be located in both local and remote computer storage media including memory storage devices. 
     The Figures (FIG.) and the following description relate to the embodiments of the present disclosure by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the claimed disclosures. 
     Referring to the drawings, embodiments of the present disclosure will be described. Various embodiments can be implemented in numerous ways, including for example as a system (including a computer processing system), a method (including a computer implemented method), an apparatus, a non-transitory computer readable medium, a computer program product, a graphical user interface, a web portal, or a data structure tangibly fixed in a non-transitory computer readable memory. Several embodiments of the present disclosure are discussed below. The appended drawings illustrate only typical embodiments of the present disclosure and therefore are not to be considered limiting of its scope and breadth. 
     Reference will now be made in detail to several embodiments of the present disclosure(s), examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein. 
       FIG. 1 a    schematically illustrates a map showing the source and the receivers for a single shot seismic acquisition.  FIG. 1 b    is a sectional view along the A-A plane in  FIG. 1 a   , showing one row of receivers and the source on the surface as well as a subsurface structure  101 . Note that the source and one of the receivers are located in the same location. Such a receiver is a zero-offset receiver as its distance to the source is zero. The shot can be explosives, gas or air guns, weight drop mechanisms, vibrator systems, etc. The receivers, i.e., sensors, can be geophones for land survey or hydrophones for underwater survey. 
     In a seismic acquisition, a shot is deployed at the source location, generating a wavefield that propagates from the source to the subsurface structure  101 . The reflection from the subsurface structure  101  propagates back to the surface and is detected by the receivers, including the zero-offset receiver. The receivers convert the seismic signals to voltage signals. The voltage signals are then transmitted to a computer in a recording station (not shown) to be processed and converted into seismic data. The seismic data can be stored and transmitted, or further processed. Note that the wavefield generated by one shot covers a 3D volume and the reflection propagates to the receivers that cover that 3D volume. 
     As such, seismic acquisition from a single shot collects seismic data from the 3D volume of earth formation that the seismic wave covers. A large number of such single shots are deployed at different source locations across the survey field, each covering its own specific 3D volume of earth formation so that a large volume of earth formation is investigated one specific 3D volume at one time. 
     The acquisition illumination for a single shot can be calculated using source wavefield propagation. The source wavefield propagation can be represented by its Green&#39;s function. As such, the acquisition illumination can be defined as the square of the Green&#39;s function of source location multiply by the summation of the square of the Green&#39;s functions for all receivers. It can be expressed as equation (1):
 
 D   s     i     =G   s     i     2 *Σ j=1   Nr     i   ( G   r     j     2 )  (1)
 
     Here D s     i    is the illumination for a single shot that covers a 3D volume having a grid size (Nx, Ny, Nz) in a xyz coordinate system. This 3D volume can be viewed as a physical volume that the single shot covers as well as a data volume of seismic data collected from the physical volume. G s     i    is the Green&#39;s function of the source location of this 3D volume. G rj  is the Green&#39;s function of a receiver in this same 3D volume, e.g., covering an area the size of NxNy. For a single shot, Nr i  is the number of receivers used for the shot in the same 3D volume. 
       FIG. 2  illustrates an array of (9×5) shots covering the survey area. An actual survey may involve thousands of shots. The acquisition illumination for the survey is defined as the summation of acquisition illuminations of all the shots. It can be written as:
 
 D=Σ   i=1   Ns   D   s     i     (2)
 
     Here D is the acquisition illumination of the survey. Ns is the total number of shots deployed in the survey. The survey volume is the sum of all the 3D volumes covered by all the shots in the survey. 
     A conventional acquisition illumination calculation simulates the Green&#39;s function for each source and receiver in the survey, then follows equations (1) and (2) to obtain the acquisition illumination. 
     Typically a large number shots and associated receivers are employed in a survey.  FIG. 3  illustrates a grid having the array of (9×5) sources and an array of (17×9) receivers. In this case, the interval between the receivers is twice as large the interval between the source locations. Note that each source location coincides with a zero-offset receiver. 
     For a 900-square km survey, assuming one shot is deployed every 50 m in the X-direction and every 200 m in the Y-direction, and assuming a receiver interval of 25 m in both the X- and Y-directions, the total number of sources and receivers locations would be 1.44 million. Each shot generates 130,321 seismic traces. To obtain the acquisition illumination for the whole survey field using the conventional method illustrated in equations (1) and (2), the Green&#39;s function for each source and receiver are simulated. Assuming it takes 0.5 hrs to simulate one Green&#39;s function, it would take 50 servers 600 days to complete the acquisition illumination for the whole survey field, which is impractical. 
     In an embodiment of the current disclosure, in order to reduce the computational time and cost, a decimation is applied in the calculation to reduce the receiver locations used in the calculation. The decimation may reduce the number of receiver locations in the X-direction, the Y-direction, or both. With decimated receivers, the acquisition illumination for a single shot can be calculated with much small number of receivers, as shown in equation (3): 
     
       
         
           
             
               
                 
                   
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     Here D s     i    is the illumination for a single shot which is a 3D volume having a grid size (Nx, Ny, Nz) in the xyz coordinate system. G si  and G rj  are respectively the Green&#39;s functions of the source location and the Green&#39;s function of the receivers in the same 3D volume. Nr i  is the total number of receivers in the same 3D volume. m x  and m y  are the decimation factors in X- and Y-directions, respectively. 
       FIG. 4  illustrates an example in which the decimation factors m x =2 and m y =2 are applied to the case shown in  FIG. 3 . Using the 900-square km survey as an example, if the source and receivers are decimated to 50 m interval from 25 m internal, one out of four receivers, including the zero-offset receivers, are calculated for their Green&#39;s functions. Accordingly, the total computation time would be reduced to 150 days from 600 days. 
     In a further embodiment of the instant disclosure, the method of acquisition illumination is further decimated. The Green&#39;s function at a source location is the same as the Green&#39;s function of the zero-offset receiver at the same location, i.e., G s     i   =G r     i   . Therefore, the Green&#39;s function at the source location can be used to calculate both the source-side illumination and the receiver-side illumination. 
       FIG. 5 , in which each source location overlaps with a zero-offset receiver location, further illustrates this embodiment. Once all the Green&#39;s functions for source locations are calculated, the receiver-side illumination can be generated using only the zero-offset receivers. Accordingly, equation (3) can be expressed as equation (4) below:
   D   s     i     ≈G   s     i     2 *Σ j=1   Ns     i   ( G   s     j     2 )  (4)
 
G si  is the Green&#39;s function of the source which covers the 3D volume having the size (Nx, Ny, Nz). Ns i  is the number of shots deployed in the same 3D volume during the survey. G s     j    is the Green&#39;s function of each shot deployed in the same 3D volume. Illumination acquisition for the whole survey is then computed according to equation (2).
 
     Using the 900 square km-survey as an example, according the method of this embodiment, only the Green&#39;s functions of the source locations (the same as the locations of the zero-offset receivers) are calculated, i.e., N s =9,000 instead of 1.44 million, the total computation time is reduced to 37.5 days. 
     In another embodiment, when the RTM is used to process seismic data, all the source wavefields are calculated in RTM, providing the Green&#39;s function for each of the source locations in the entire survey field. These Green&#39;s functions are used to generate the acquisition illumination according to the decimated illumination equation (4), which takes practically zero computation time. 
     While in the foregoing specification this disclosure has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the disclosure is susceptible to alteration and that certain other details described herein can vary considerably without departing from the basic principles of the disclosure. In addition, it should be appreciated that structural features or method steps shown or described in any one embodiment herein can be used in other embodiments as well.