Patent Application
20120253681
This disclosure relates generally to the seismic data processing, and more particularly to a method and system for minimizing the effects of shallow overburden attenuation.
Shallow overburden anomalies are known to have significant detrimental effects on seismic data quality. Such anomalies may include amplitude attenuation, frequency loss and wave front distortion as received (reflected) waves from deeper “target” levels of the subsurface travel through gas-charged channel complexes and hydrates at shallower regions. This may cause mis-positioning, dimmed amplitudes and/or lower bandwidth of the reflected seismic signals received from the target levels, thus impacting the quality of the subsurface characterization.
Conventional compensation methods for spatially-varying amplitude attenuation due to shallow bodies have been developed. See for example: “Turning ray amplitude inversion: Mitigating amplitude attenuation due to shallow gas,” SEG Annual Meeting Expanded Technical Program Abstracts with Biographies, vol. 21, pp. 2078-2081 (2002), by M. Deal, G. Matteucci, Y. Kim, and A. Romero; “Efficient compensation for attenuation effects using pseudo-Q migration,”SEG Annual Meeting Expanded Technical Program Abstracts with Biographies, vol. 27, pp. 2206-2210 (2008), by L. Bear, J. Liu and P. Traynin; “3-D tomographic amplitude inversion for compensating amplitude attenuation in the overburden,” SEG Annual Meeting Expanded Technical Program Abstracts with Biographies, vol. 27, pp. 3239-3243 (2008), by K. Xin, B. Hung, S. Birdus and J. Sun; “Compensation for the effects of shallow gas attenuation with viscoacoustic wave-equation migration,” SEG Annual Meeting Expanded Technical Program Abstracts with Biographies, vol. 21, pp. 2062-2065 (2002), by Y. Yu, R. Lu and M. Deal; and “True-amplitude prestack depth migration,” Geophysics, vol. 72, issue 3, pp. S155-S166, (June 2007), by F. Deng and G. McMechan. Successful application of these conventional methods, however, depends on the accuracy of the absolute attenuation or Q-field. Q-field estimation from amplitudes is computationally expensive and traditionally very difficult because amplitudes are affected by a number of factors such as propagation length, wavefront changes and reflectivities. Compensation methods that rely on Q-field often make simplifying assumptions such as using turning rays, limiting input data to far offsets, and weak attenuation conditions.
Other empirical compensation methods, including amplitude correction methods using spatially smoothed power sections and amplitude ratios have the potential to remove target amplitude information.
Therefore, a need exists to overcome the known shortcomings of conventional shallow overburden compensation methods. More specifically, a need exists for a shallow overburden compensation method that does not require prior knowledge of the Q-field, and which incorporates both overburden and target geology in the compensation. The compensation method should be consistent with amplitude-preserving workflows that enable improved quantitative seismic analysis for purposes of reservoir characterization.
A method is disclosed for processing seismic data corresponding to a subsurface area of interest. In accordance with an embodiment of the present invention, the method includes the steps of: determining, from the seismic data, a first amplitude attribute map at a first image depth or “layer”; determining, from the seismic data, a second amplitude attribute map at a second image depth; normalizing each of the first and second amplitude attribute maps. The normalized first and second amplitude attribute maps are used to determine a ratio map, which is then scaled and applied as scale factor map to the seismic data to compensate for effects of shallow overburden attenuation.
In accordance with another embodiment of the present invention, a corresponding system is provided processing seismic data corresponding to a subsurface area of interest. The system includes a data source containing the seismic data, and a computer processor in communication with the data source for processing the seismic data. The processor includes computer readable media having computer readable code for executing the steps of: determining, from the seismic data, a first amplitude attribute map at a first image depth; determining, from the seismic data, a second amplitude attribute map at a second image depth; normalizing each of the first and second amplitude attribute maps; determining a ratio map based on a ratio of the normalized first and second amplitude attribute maps; scaling the ratio map to generate a scale factor map; and applying the scale factor map to the seismic data to compensate for effects of shallow overburden attenuation.
In accordance with another embodiment of the present invention, an article of manufacture is provided that includes a computer readable medium having a computer readable code embodied therein adapted to execute a method for seismic data processing. The method includes the steps of: determining, from the seismic data, a first amplitude attribute map at a first image depth; determining, from the seismic data, a second amplitude attribute map at a second image depth; normalizing each of the first and second amplitude attribute maps; determining a ratio map based on a ratio of the normalized first and second amplitude attribute maps; scaling the ratio map to generate a scale factor map; and applying the scale factor map to the seismic data to compensate for effects of shallow overburden attenuation.
Advantageously, the present invention incorporates both overburden and target geology and allows for lateral and vertical scaling based on amplitude effects of the shallow attenuating bodies. Laterally-varying scale factors corresponding to different offsets/angles are applied to boost attenuated amplitudes within dim-out zones while preserving the non-attenuated amplitudes outside the dim-out zones. Furthermore, the method of the present invention is a straight-forward approach that corrects for attenuation based on amplitude ratios only without distinguishing scattering from inelastic attenuation, or taking into account converted waves, multiple energy or Q dependence on frequency.
A detailed description of the present invention is made with reference to specific embodiments thereof as illustrated in the appended drawings. The drawings depict only typical embodiments of the invention and therefore are not to be considered to be limiting of its scope.
a and 5b illustrates exemplary angle dependent and offset dependent implementations in accordance with the present invention.
The present invention 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 invention 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 invention are not limited to any particular computer software technology.
Moreover, those skilled in the art will appreciate that the present invention may be practiced using any one or combination of hardware and software configurations, including but not limited to a system having single and/or multi-processer computer processors system, hand-held devices, programmable consumer electronics, mini-computers, mainframe computers, supercomputers, and the like. The invention may also be practiced in distributed computing environments where tasks are performed by servers or other processing devices that are linked through one or more data communications networks. In a distributed computing environment, program modules may be located in both local and remote computer storage media including memory storage devices.
Also, an article of manufacture for use with a computer processor, such as a CD, pre-recorded disk or other equivalent devices, may include a computer program storage medium and program means recorded thereon for directing the computer processor to facilitate the implementation and practice of the present invention. Such devices and articles of manufacture also fall within the spirit and scope of the present invention.
Referring now to the drawings, embodiments of the present invention will be described. The invention 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 computer readable medium, a computer program product, a graphical user interface, a web portal, or a data structure tangibly fixed in a computer readable memory. Several embodiments of the present invention are discussed below. The appended drawings illustrate only typical embodiments of the present invention and therefore are not to be considered limiting of its scope and breadth.
As is shown in
It should be appreciated that although the modules 110-118 are illustrated in
The data storage 102 may include electronic storage media for storing seismic data. The storage media may be integrally coupled with the system 100, i.e., substantially non-removable, and/or removably connectable to the system 100 via, for example, a port (e.g., USB port, a firewire port, etc.) or a drive (e.g., a disk drive, etc.). The data storage 102 may include one or more of optically readable storage media (e.g., optical disks, etc.), magnetically readable storage media (e.g., magnetic tape, magnetic hard drive, floppy drive, etc.), electrical charge-based storage media (e.g., EEPROM, RAM, etc.), solid-state storage media (e.g., flash drive, etc.), and/or other electronically readable storage media. The electronic storage 102 may store software algorithms, information determined by the processor 108, information received via the user interface 104, information received from the information resources 106, and/or other information that enables the system 100 to function as described herein to execute the method described below with reference to
Seismic data stored by electronic storage 102 may include source wavefield data and receiver wavefield data. The seismic data may also include individual or multiple traces of seismic data (e.g., the data recorded on one channel of seismic energy propagating through the geological volume of interest from a source), offset stacks, angle stacks, azimuth stacks and/or other data.
The user interface 104 is configured to provide an interface between the system 100 and a user through which the user may provide information to and receive information from the system 100. This enables data, results, and/or instructions and any other communicable items, collectively referred to as “information,” to be communicated between the user and the system 100. As used herein, the term “user” may refer to a single individual or a group of individuals who may be working in coordination. Examples of interface devices suitable for inclusion in the user interface 104 include one or more of a keypad, buttons, switches, a keyboard, knobs, levers, a display screen, a touch screen, speakers, a microphone, an indicator light, an audible alarm, and/or a printer. In one embodiment, the user interface 104 actually includes a plurality of separate interfaces.
It is to be understood that other communication techniques, either hard-wired or wireless, are also contemplated by the present technology as the user interface 104. For example, the present technology contemplates that the user interface 104 may be integrated with a removable storage interface provided by the electronic storage 102. In this example, information may be loaded into the system 100 from removable storage (e.g., a smart card, a flash drive, a removable disk, etc.) that enables the user to customize the implementation of the system 100. Other exemplary input devices and techniques adapted for use with the system 100 as the user interface 104 include, but are not limited to, an RS-232 port, RF link, an IR link, modem (telephone, cable or other). In short, any technique for communicating information with the system 100 is contemplated by the present technology as the user interface 104.
Optional information resources 106 may include one or more additional sources of information, including but not limited seismic data. By way of non-limiting example, one of information resources 106 may include a field device used to acquire seismic data from a geological volume of interest, or databases or applications for providing “raw” and/or processed seismic data, including but not limited to pres-stack and post-stacked seismic data, and other information derived therefrom related to the geologic volume of interest. Other information may include velocity models, time horizon data, etc.
Referring again to
Similarly, the seismic data is used to determine a second amplitude attribute map at a second “target” image depth, step 204.
Next, the method 200 of the present invention includes the step 206 of normalizing each of the shallow and target layer amplitude attribute maps to a reference value. The reference value can be, for example, the average, maximum or minimum amplitude at the corresponding layer. Additional thresholding or “clipping” of one or both of the normalized amplitude attribute maps is performed to ensure the resulting scale factor map values do not boost amplitudes outside dim zones. For example, in the case of a shallow layer amplitude attribute map where the attribute is normalized to an average value, normalized amplitude attribute values having a value less than 1 can be set to a value of 1. In the case of a target layer amplitude attribute map where the attribute is an normalized to an average value, normalized amplitude attribute values having a value greater than 1 can be set to a value of 1.
Following the normalization step 206, a ratio map is determined based on a ratio of the normalized first and second amplitude attribute maps, step 208. Optionally, ratio map values having a value less than 1 can be set to a value of 1 to ensure resulting scale factor map values do not boost amplitudes outside dim zones. The ratio map is then scaled according to Equation 1, step 210, to derive the scale factor at any x,y location:
Scale Factor (x,y)=Ratio Map Amplitudes (x,y)/(Amin*Amax); (Equation 1)
where Amin is the minimum amplitude from the target layer amplitude attribute map and Amax is the maximum amplitude from the ratio map. The scale factor map (i.e., scaled ratio) characterizes the differential attenuation (dQ) (i.e., attenuation between shallow and target layers) at any given (x,y) location. The scale factor map determined in accordance with step 210 is equivalent to the inverse of differential attenuation (1/dQ), and therefore the method of the present invention does not require prior knowledge of absolute Q values.
Optionally, scale factors having a value greater than 1 can be set to a value according to Equation 2:
Scale Factor (x,y)=1+(Ratio Map Amplitudes (x,y)−1)/(Amin*Amax). (Equation 2)
Next, step 212 of the present method includes the step of applying the scale factor map to the seismic data to compensate for effects of shallow overburden attenuation. Application to CDP gathers is now considered to illustrate the step 212 of the present invention.
In the case of CDP gathers, corresponding ray paths may sample different areas of shallow overburden. As such, the total ray path that is to be compensated includes shot-side and receiver-side contributions. The amplitude for any given trace (CDP gather) can be restored by multiplying shot and receiver scale factors and the original trace. With reference to
For pre-stack angle dependent seismic data, the equations provided below with reference to
surf_offset=tan φ*0.5*vave2*t2; (Equation 3)
atten_offset=tan φ*0.5*(vave2*t2−vave1*t1); (Equation 4)
where φ is a nominal angle of the stacked seismic data, vave1 is an average velocity at the attenuating layer, vave2 is an average velocity at the target layer, t1 is a two-way time (down-going and up-going rays) at the attenuating layer, and t2 is a two-way time at the target layer.
Next, the scale factor map is used to look up source and receiver scale factors sca_sou and sca_rec, respectively, at attenuating layer x and y locations (atten_sou_x, atten_sou_y, atten_rec_x, atten_rec_y) in accordance with Equations 5-8 below, where φ is azimuth as shown in
atten_sou_x=CDP_x−atten_offset*sin φ; (Equation 5)
atten_sou_y=CDP_y−atten_offset*cos φ; (Equation 6)
atten_rec_x=CDP_x+atten_offset*sin φ; (Equation 7)
atten_rec_y=CDP_y+atten_offset*cos φ; (Equation 8)
where φ azimuth from north of the seismic coordinate system (i.e., Inline).
Note, the above set of Equations 5-8 can be expressed in terms of Inline and Xline coordinates using Equation 9 and nominal CDP spacing, where the nominal CDP spacing is the average distance between CDP locations:
CDP_offset=atten_offset/CDP_spacing. (Equation 9)
Therefore, for a given Inline coordinate, the scale factor map is used to look up source and receiver scale factors sca_sou and sca_rec, respectively, at Inline and Xline coordinates in accordance with Equations 10-13 below:
atten_sou=Inline−CDP_offset; (Equations 10)
atten_rec=Inline+CDP_offset; (Equations 11)
atten_sou=Xline—CDP_offset; (Equations 12)
atten_rec=Xline+CDP_offset. (Equations 13)
Next, scale factors sca_sou and sca_rec are selected from the scale factor map corresponding to locations/coordinate as determined via Equations 5-8 or 10-13, and applied to each of the pre-stack (or post-stack) traces in accordance with Equation 14 (x, y, t), or Equation 15 (Inline, Xline, t), to compensate for shallow overburden effects. An additional time-varying weighting term is included to ensure that scale factors are not applied above or at the attenuating layer:
Scaled Trace (x,y,t)=Trace (x,y,t)*sqrt(sca_sou*sca_rec)*Weight(t); (Equation 14)
Scaled Trace (Inline,Xline,t)=Trace (Inline,Xline,t)*sqrt(sca_sou*sca_rec)*Weight(t). (Equation 15)
In accordance with another embodiment of step 212, the following input data is required for an offset-dependant implementation of step 212: the scale factor map derived in accordance with steps 202-210 of the present method at the attenuating layer; average velocity map at attenuating and target layers; time horizon of attenuating layer; time horizon of target layer; migrated gathers with trace header values: CDP x-location, CDP y-location, Inline number, and Xline number; and time gate application.
Next, the attenuation offset according to Equation 4 is modified using straight ray approximation in accordance with Equation 16, where vave1, t1,vave2, and t2 are obtained at CDP_x and CDP_y locations:
atten_offset=surf_offset*(vave2*t2−vave1*t1)/vave2 * t2; (Equation 16)
where vave1 is an average velocity at the attenuating layer, vave2 is an average velocity at the target layer, t1 is a two-way time (down-going and up-going rays) at the attenuating layer, and t2 is a two-way time at the target layer.
Scale factors sca_sou and sca_rec are then selected from the scale factor map corresponding to locations as determined below by Equations 5-8.
The scale factors selected from the scale factor map that the computed x-y locations are then applied to each of the pre-stack (or post-stack) traces in accordance with Equation 17 (x, y, t domain). An additional time-varying weighting term is included to ensure that scale factors are not applied above or at the attenuating layer;
Scaled Trace (x,y,t)=
Trace (x,y,t)*sqrt(sca_sou*sca_rec)*Weight(t). (Equation 17)
As such, a map-based, target-oriented, angle/offset-varying overburden attenuation correction method and system has been disclosed. The present invention has advantages over conventional, empirical compensation methods in that the attenuation compensation is based solely upon a computed scaled ratio map (scale factor map) of shallow bright amplitudes to deep attenuated amplitudes corresponding to attenuated zones in deeper intervals. The scale factor map, of for example as shown by 604 in
Notwithstanding that the present invention has been described above in terms of alternative embodiments, it is anticipated that still other alterations, modifications and applications will become apparent to those skilled in the art after having read this disclosure. For example, it is to be understood that the present invention contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment. It is therefore intended that such disclosure be considered illustrative and not limiting, and that the appended claims be interpreted to include all such applications, alterations, modifications and embodiments as fall within the true spirit and scope of the invention.
