Patent Publication Number: US-9417350-B2

Title: Subsalt velocity model building

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/751,095, filed May 21, 2007, now U.S. Pat. No. 7,586,811 which claims benefit of U.S. provisional patent application Ser. No. 60/810,320, filed Jun. 2, 2006. Each of the aforementioned related patent applications is herein incorporated by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     Implementations of various technologies described herein generally relate to the field of seismic data processing, and more specifically, to prestack depth migration. 
     2. Description of the Related Art 
     The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section. 
     Seismic surveying may be used to determine structures or compositions of subsurface earth formations. For example, seismic surveying may be used to determine the presence of useful materials, such as petroleum, in the subsurface earth formations. Generally, the process of seismic surveying includes deploying an array of seismic sensors at or near the earth&#39;s surface at selected geographic positions and deploying one or more seismic energy sources at selected locations, also at or near the earth&#39;s surface. The one or more seismic energy sources may be actuated, causing seismic energy to emanate from the sources, traveling generally downwardly through the earth&#39;s subsurface until it reaches one or more acoustic impedance boundaries in the earth. The seismic energy may be reflected from the one or more impedance boundaries, where it may then travel upwardly until detected by one or more of the seismic sensors. Structures and compositions of the earth&#39;s subsurface may be inferred from the travel time of the reflected seismic energy, from the geographic position of the source to each of the sensors, and from the amplitude and phase of the various frequency components of the reflected seismic energy with respect to the energy emanating from the seismic source. 
     Structures of the earth&#39;s subsurface may be inferred from the travel time of the seismic energy from the source to the acoustic impedance boundaries and back to the seismic sensors at the surface. In order to infer depth of and the structures of subsurface earth formations from reflection seismic travel times measured at the earth&#39;s surface, it may be necessary to determine the acoustic velocity of the various formations through which the seismic energy passes. Velocities of the earth formations may vary both with respect to depth in the earth (vertically), and with respect to geographic position (laterally). Seismic data, however, are typically recorded only with respect to time. Methods known in the art for estimating velocities of the earth formations both vertically and laterally rely on inferences about the travel path geometry of the seismic energy as it travels from the seismic source to the various seismic receivers deployed at or near the earth&#39;s surface. 
     SUMMARY 
     Described herein are various techniques for a method for building a subsalt velocity model. In one implementation, the method may include determining a velocity of a sedimentary area surrounding a salt body, determining an initial velocity of a subsalt sedimentary area disposed below the salt body by interpolating the velocity of the surrounding sedimentary area with one or more boundaries defining the salt body, and updating the initial velocity of the subsalt sedimentary area. 
     In another implementation, the boundaries of the salt body may be defined by: selecting a top horizon of the salt body, performing a salt-flood migration on a subsurface velocity model containing the salt body, the sedimentary area and the subsalt sedimentary area; selecting a base horizon of the salt body; and defining the boundaries of the salt body based on the top horizon and the base horizon. 
     In yet another implementation, the initial velocity of the subsalt sedimentary area may be updated by: determining a thickness of a subsalt layer disposed directly below the salt body, determining a velocity of the subsalt layer and applying a three dimensional smoothing filter to the velocity of the subsalt layer. 
     Described herein are also various technologies for subsurface velocity model having one or more velocity values in a salt body, one or more velocity values in a top salt sedimentary area disposed above the salt body, and one or more velocity values in a subsalt sedimentary area disposed below the salt body, wherein the one or more velocity values in the subsalt sedimentary area comprises one or more velocity values in a subsalt layer disposed directly below the salt body. 
     The above referenced summary section is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description section. The summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of various technologies will hereafter be described with reference to the accompanying drawings. It should be understood, however, that the accompanying drawings illustrate only the various implementations described herein and are not meant to limit the scope of various technologies described herein. 
         FIG. 1  illustrates a subsurface velocity model in accordance with implementations of various techniques described herein. 
         FIG. 2  illustrates a flow diagram of a prestack depth migration method in accordance with implementations of various technologies described herein. 
         FIG. 3  illustrates a computer network, into which implementations of various technologies described herein may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     The discussion below is directed to certain specific implementations. It is to be understood that the discussion below is only for the purpose of enabling a person with ordinary skill in the art to make and use any subject matter defined now or later by the patent “claims” found in any issued patent herein. 
     Since many prolific reservoirs have been discovered in sediments below salt bodies, exploration in sediments below the salt bodies have become of special importance to hydrocarbon exploration. However, due to the sharp velocity and acoustic impedance contrasts between the salt bodies and the sediments surrounding the salt bodies, updating of the subsalt velocity and thus imaging of the subsalt sediments have been difficult. Generally, the subsalt velocity may be updated using vertical scan, vertical update or tomographic update techniques. In velocity scan, multiple velocity models may be used to migrate target lines and the velocity that provides the most coherent image and the flattest image gathers may be selected as the final velocity model. In vertical update, the velocity correction may be obtained from residual velocity analysis to optimize the flatness of reflection events in gathers. In tomographic update, the events in migrated image gathers may be picked and the residual velocity may be estimated to flatten the gathers using an optimization process which involves ray-tracing with the existing model. However, any of these techniques often generates inaccurate results, particularly where the salt bodies have rugose structures. Accordingly, implementations of various techniques described herein are directed to enhance subsalt velocity updates by improving the signal to noise ratio in the post-migration subsalt seismic gathers. 
     One or more implementations of various techniques for building a subsalt velocity model will now be described in more detail with reference to  FIGS. 1-3  in the following paragraphs. 
       FIG. 1  illustrates a subsurface velocity model  100  in accordance with implementations of various techniques described herein. The subsurface velocity model  100  may include the velocity in a salt body  10  and the velocity in a sedimentary area  20  surrounding the salt body  10 . The surrounding sedimentary area  20  may include a top salt sedimentary area  80  and subsalt sedimentary area  70 . The top salt sedimentary area  80  is the sedimentary area directly above the salt body  10 . The subsalt sedimentary area  70  is the area below the salt body  10 . The sedimentary area  20  may include sand and shale. The subsurface velocity model  100  may further include a velocity in a subsalt layer  30  disposed directly below the salt body  10 . The creation of the subsalt layer  30  will be described in more detail with reference to  FIG. 2 . The subsalt layer  30  may be part of the subsalt sedimentary area  70 . 
       FIG. 2  illustrates a flow diagram of a prestack depth migration method  200  in accordance with implementations of various technologies described herein. It should be understood that while the operational flow diagram  200  indicates a particular order of execution of the operations, in some implementations, certain portions of the operations might be executed in a different order. 
     At step  210 , the velocity of the surrounding sedimentary area  20  may be determined. The velocity of the surrounding sedimentary area  20  may be determined by performing a number of migrations and velocity updates on the prestack seismic gathers. Migration may be defined as a sequence of signal processing steps that accurately images and positions subsurface geologic structures. In one implementation, an accurate subsurface velocity model is essential for migration, especially for prestack depth migration. Velocity update may be defined as a method to correct the velocity after a migration. The velocity updates may be performed using velocity scan, vertical update or tomography techniques. The migrations and velocity updates on the prestack seismic gathers may be repeated until the image of the surrounding sedimentary area  20  is coherent and has a predetermined sharpness and/or the events of the prestack seismic gathers for the surrounding sedimentary area  20  are substantially flat. 
     At step  220 , the top horizon  40  of the salt body  10  may be selected. 
     At step  230 , a salt-flood migration may be performed on the subsurface velocity model  100  to determine the base horizon  50  of the salt body  10 . The salt-flood migration may involve extending the top horizon  40  of the salt body to the maximum depth  60  of the subsurface velocity model  100  and flooding the subsurface velocity model  100  with the velocity inside the salt body  10 , i.e., salt velocity, which may be determined from lab data. 
     At step  240 , the base horizon  50  of the salt body  10  may be selected. 
     At step  250 , the top horizon  40  and the base horizon  50  may be used to define the boundaries of the salt body  10 . In one implementation, in addition to the top horizon  40  and the base horizon  50 , the salt body  10  may be defined by overhang horizons on the top horizon  40  or the base horizon  50  (not shown). 
     At step  260 , the initial velocity of the subsalt sedimentary area  70  may be determined by interpolating the velocity of the surrounding sedimentary area  20 , which was determined at step  210 , with the boundaries defining the salt body  10  and other well information. 
     Steps  270 - 290  may be directed to updating the velocity of the subsalt sedimentary area  70 . Initially, the velocity of the subsalt sedimentary area  70  at a given depth would be substantially the same as the surrounding sedimentary velocity. It may be assumed that the presence of the salt body  10  may cause a decrease in the velocity of the subsalt sedimentary area  70 . 
     Steps  270 - 280  may be directed to determining the subsalt layer  30 . Although the subsalt layer  30  may be described as a single subsalt sedimentary layer, it should be understood that in some implementations the subsalt layer  30  may include more than one subsalt sedimentary layer. 
     At step  270 , the thickness of the subsalt layer  30  may be determined according to the following expression: t=a*T, where t is the thickness of the subsalt layer  30 , a is a parameter and T is the thickness of the salt body  10 . The thickness of the subsalt layer  30  may vary spatially, i.e., along the base horizon  50  of the salt body  10 . Parameter a may be a constant or a mathematical function of the spatial location and shape of the salt body  10 . In this manner, the thickness of the subsalt layer  30  may be proportional to the thickness of the salt body  10 . In one implementation, parameter a may be selected based on data obtained from wells that penetrate salt bodies. 
     At step  280 , the velocity of the subsalt layer  30  may be determined according to the following expression: v=(Vtss+b*Vbss)/(1+b), where v is the velocity of the subsalt layer  30 , Vtss is the velocity of the top salt sedimentary area  80 , b is a parameter and Vbss is the initial velocity in the subsalt sedimentary area  70 , which was determined at step  260 . In this manner, the velocity of the subsalt layer  30  may be a simple weighted average of the velocity of the top salt sedimentary area  80  and the initial velocity of the subsalt sedimentary area  70 . In one implementation, the velocity of the subsalt layer  30  may be assumed to be vertically constant along the base horizon  50 . Like the thickness of the subsalt layer  30 , the velocity of the subsalt layer  30  may also vary spatially, i.e., along the base horizon  50  of the salt body  10 . Parameter b may be a constant or a mathematical function. In one implementation, parameter b may be selected based on data obtained from wells that penetrate salt bodies. 
     At step  290 , a three dimensional smoothing filter may be applied to the velocity of the subsalt layer  30 . In one implementation, the three dimensional smoothing filter may be applied without the base horizon  50  of the salt body  10 . In such an implementation, the velocity of the subsalt layer  30  may be finalized with the base horizon  50  of the salt body  10 . Such finalization may include various steps, such as tying the horizons in all directions to minimize the amount of gap between top salt and base salt horizons, applying another smoothing filter, gridding and the like. 
     Although various subsalt velocity model building implementations have been described with reference to prestack depth migration, it should be understood that these various subsalt velocity model building implementations may be used in prestack and poststack time migration, poststack depth migration, time-to-depth stretch, pore pressure prediction, stratigraphic imaging and interpretation, and the like. 
       FIG. 3  illustrates a computing system  300 , into which implementations of various technologies described herein may be implemented. The computing system  300  may include one or more system computers  330 , which may be implemented as any conventional personal computer or server. However, those skilled in the art will appreciate that implementations of various technologies described herein may be practiced in other computer system configurations, including hypertext transfer protocol (HTTP) servers, hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. 
     The system computer  330  may be in communication with disk storage devices  329 ,  331 , and  333 , which may be external hard disk storage devices. It is contemplated that disk storage devices  329 ,  331 , and  333  are conventional hard disk drives, and as such, will be implemented by way of a local area network or by remote access. Of course, while disk storage devices  329 ,  331 , and  333  are illustrated as separate devices, a single disk storage device may be used to store any and all of the program instructions, measurement data, and results as desired. 
     In one implementation, seismic data from the receivers may be stored in disk storage device  331 . The system computer  330  may retrieve the appropriate data from the disk storage device  331  to process seismic data according to program instructions that correspond to implementations of various technologies described herein. The program instructions may be written in a computer programming language, such as C++, Java and the like. The program instructions may be stored in a computer-readable medium, such as program disk storage device  333 . Such computer-readable media may include computer storage media and communication media. Computer storage media may include volatile and non-volatile, and removable and non-removable media implemented in any method or technology for storage of information, such as computer-readable instructions, data structures, program modules or other data. Computer storage media may further include RAM, ROM, erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other solid state memory technology, CD-ROM, digital versatile disks (DVD), or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the system computer  330 . Communication media may embody computer readable instructions, data structures, program modules or other data in a modulated data signal, such as a carrier wave or other transport mechanism and may include any information delivery media. The term “modulated data signal” may mean a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media may include wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. Combinations of any of the above may also be included within the scope of computer readable media. 
     In one implementation, the system computer  330  may present output primarily onto graphics display  327 , or alternatively via printer  328 . The system computer  330  may store the results of the methods described above on disk storage  329 , for later use and further analysis. The keyboard  326  and the pointing device (e.g., a mouse, trackball, or the like)  325  may be provided with the system computer  330  to enable interactive operation. 
     The system computer  330  may be located at a data center remote from the survey region. The system computer  330  may be in communication with the receivers (either directly or via a recording unit, not shown), to receive signals indicative of the reflected seismic energy. These signals, after conventional formatting and other initial processing, may be stored by the system computer  330  as digital data in the disk storage  331  for subsequent retrieval and processing in the manner described above. While  FIG. 3  illustrates the disk storage  331  as directly connected to the system computer  330 , it is also contemplated that the disk storage device  331  may be accessible through a local area network or by remote access. Furthermore, while disk storage devices  329 ,  331  are illustrated as separate devices for storing input seismic data and analysis results, the disk storage devices  329 ,  331  may be implemented within a single disk drive (either together with or separately from program disk storage device  333 ), or in any other conventional manner as will be fully understood by one of skill in the art having reference to this specification. 
     While the foregoing is directed to implementations of various technologies described herein, other and further implementations may be devised without departing from the basic scope thereof, which may be determined by the claims that follow. Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.