Patent Publication Number: US-11656378-B2

Title: Seismic imaging by visco-acoustic reverse time migration

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to an approach for identifying geologic features in a subsurface formation. 
     BACKGROUND 
     In geology, sedimentary facies are bodies of sediment that are recognizably distinct from adjacent sediments that resulted from different depositional environments. Generally, geologists distinguish facies by aspects of the rock or sediment being studied. Seismic facies are groups of seismic reflections whose parameters (such as amplitude, continuity, reflection geometry, and frequency) differ from those of adjacent groups. Seismic facies analysis, a subdivision of seismic stratigraphy, plays an important role in hydrocarbon exploration and is one key step in the interpretation of seismic data for reservoir characterization. The seismic facies in a given geological area can provide useful information, particularly about the types of sedimentary deposits and the anticipated lithology. 
     In reflection seismology, geologists and geophysicists perform seismic surveys to map and interpret sedimentary facies and other geologic features, for example, identification of potential petroleum reservoirs. Seismic surveys are conducted by using a controlled seismic source (for example, a seismic vibrator or dynamite) to create a seismic wave. The seismic source is typically located at ground surface. The seismic wave travels into the ground, is reflected by subsurface formations, and returns to the surface where it is recorded by sensors called geophones. The geologists and geophysicists analyze the time it takes for the seismic waves to reflect off subsurface formations and return to the surface to map sedimentary facies and other geologic features. This analysis can also incorporate data from other sources, for example, borehole logging, gravity surveys, and magnetic surveys. 
     One approach to this analysis is based on tracing and correlating along continuous reflectors throughout the dataset produced by the seismic survey to produce structural maps that reflect the spatial variation in depth of certain facies. These maps can be used to identify impermeable layers and faults that can trap hydrocarbons such as oil and gas. 
     SUMMARY 
     Seismic images can be viewed as photographs for underground layers of rocks. Seismic images can be generated by measuring different probing elastic waves. Various images can made from the echoes of these waves that reflect different rock properties. However, as media complexity becomes more complex, inaccuracies can be introduced to velocity model in seismic data. This is because seismic imaging can be based on an acoustic assumption. This assumption is that the seismic wave signal does not have physical dispersion or amplitude dissipation except for the spherical diffusion. In reality, a property of the Earth is that it is anelastic, and seismic signals are distorted during the wave propagation because of a viscosity of the seismic environment. To compensate these distortions, an inverse-Q filter can be applied during a data processing stage of the seismic data. In some implementations, a more advanced compensation technique is to incorporate the compensation during seismic inversion and imaging. 
     This specification describes a stable visco-acoustic imaging technology based on the de-coupled time-domain complex-valued wave equation. The de-coupled visco-acoustic wave equations are applied for wavefield reconstruction, and the final seismic image accounts for effects for both attenuation and velocity dispersion. More specifically, decoupled visco-acoustic wave equations are applied for wavefield reconstruction by an imaging condition-based combination of a source wavefield generated by forward modeling and a receiver wavefield generated by combining two receiver wavefields generated from observed data to provide a final seismic image that accounts for both attenuation and velocity dispersion. 
     In a general aspect, a process for generating a seismic image representing a subsurface formation by compensating for distortions of seismic data includes receiving seismic data for the subsurface formation, including receiver wavelet data and source wavelet data. The process includes generating source wavefield data based on a forward modeling of the source wavelet data. The process includes generating receiver wavefield data that compensates for distortions in the seismic data by: applying a dispersion-only model to the receiver wavelet data to generate a first reconstructed back-propagated receiver wavefield portion; applying a dissipation-only model to the receiver wavelet data to generate a second reconstructed back-propagated receiver wavefield portion; and combining the first back-propagated receiver wavefield portion and the second back-propagated receiver wavefield portion into the receiver wavefield data. The process includes applying an imaging condition to the receiver wavefield data and the source wavefield data. The process includes generating, based on applying the imaging condition, visco-acoustic reverse time migration (VARTM) result data. The process includes, based on the VARTM result data, generating a seismic image of the subsurface formation. 
     In a general aspect, a system for generating a seismic image representing a subsurface formation by compensating for distortions of seismic data includes one or more processing devices and a memory in communication with the one or more processing devices, the memory configured to store instructions that, when executed by the one or more processing devices, cause the one or more processing devices to perform operations including receiving seismic data for the subsurface formation, including receiver wavelet data and source wavelet data. The operations include generating source wavefield data based on a forward modeling of the source wavelet data. The operations include generating receiver wavefield data that compensates for distortions in the seismic data by: applying a dispersion-only model to the receiver wavelet data to generate a first reconstructed back-propagated receiver wavefield portion; applying a dissipation-only model to the receiver wavelet data to generate a second reconstructed back-propagated receiver wavefield portion; and combining the first back-propagated receiver wavefield portion and the second back-propagated receiver wavefield portion into the receiver wavefield data. The operations include applying an imaging condition to the receiver wavefield data and the source wavefield data. The operations include generating, based on applying the imaging condition, visco-acoustic reverse time migration (VARTM) result data. The process includes, based on the VARTM result data, generating a seismic image of the subsurface formation. 
     In a general aspect, one or more non-transitory computer readable media store instructions for generating a seismic image representing a subsurface formation by compensating for distortions of seismic data, instructions configured to cause one or more processing devices that execute the instructions to perform operations comprising receiving seismic data for the subsurface formation, including receiver wavelet data and source wavelet data. The process includes generating source wavefield data based on a forward modeling of the source wavelet data. The process includes generating receiver wavefield data that compensates for distortions in the seismic data by: applying a dispersion-only model to the receiver wavelet data to generate a first reconstructed back-propagated receiver wavefield portion; applying a dissipation-only model to the receiver wavelet data to generate a second reconstructed back-propagated receiver wavefield portion; and combining the first back-propagated receiver wavefield portion and the second back-propagated receiver wavefield portion into the receiver wavefield data. The process includes applying an imaging condition to the receiver wavefield data and the source wavefield data. The process includes generating, based on applying the imaging condition, visco-acoustic reverse time migration (VARTM) result data. The process includes, based on the VARTM result data, generating a seismic image of the subsurface formation. 
     In an aspect, the systems, processes, and non-transitory computer readable media can each be configured for one or more of the following embodiments. In some implementations, forward modeling of the source wavelet data comprises applying a complex value velocity model to the source wavelet data. In some implementations, the complex velocity model comprises a generalized visco-acoustic wave Equation 
                       ∂   2       p   h         ∂     t   2         +       v   e     2     1   -   γ         ⁢     k     2     1   -   γ         ⁢     p   h         =       s   h     (   t   )       ,       wherein   ⁢           v   e       =       v   0         ω   0   γ     [     1   -     i   ⁢          tan   ⁡   (     πγ   2     )         ]         ,         
wherein S h =S+i[−i sgn(ω)S] is the inverse Fourier transform of s h (t), and wherein P h =P+i[−i sgn(ω)P], is the inverse Fourier transform of p h (t).
 
     In some implementations, the dispersion-only model comprises applying a wave equation to the receiver wavelet, the wave equation comprising: 
     
       
         
           
             
               
                 
                   
                     
                       ∂ 
                       2 
                     
                     p 
                   
                   
                     ∂ 
                     
                       t 
                       2 
                     
                   
                 
                 + 
                 
                   
                     
                       v 
                       _ 
                     
                     e 
                     
                       2 
                       
                         1 
                         - 
                         γ 
                       
                     
                   
                   ⁢ 
                   
                     k 
                     
                       2 
                       
                         1 
                         - 
                         γ 
                       
                     
                   
                   ⁢ 
                   p 
                 
               
               = 
               
                 s 
                 ⁡ 
                 ( 
                 t 
                 ) 
               
             
             , 
             
               
                 where 
                 ⁢ 
                     
                 
                   
                     v 
                     _ 
                   
                   e 
                 
               
               = 
               
                 
                   
                     v 
                     0 
                   
                   
                     
                       ( 
                       
                         ω 
                         0 
                       
                       ) 
                     
                     γ 
                   
                 
                 . 
               
             
           
         
       
     
     In some implementations, the dissipation-only model comprises applying a wave equation to the receiver wavelet, the wave equation comprising: 
                       ∂   2       p   h         ∂     t   2         +         v   ⌣     e   2     ⁢     k   2     ⁢     p   h         =       s   h     (   t   )       ,       where   ⁢             v   ⌣     e       =       v   0       1   -     i   ⁢          tan   ⁡   (     πγ   2     )             ,         
where S h =S+i[−i sgn(ω)S] is the inverse Fourier transform of s h (t), and where P h =P+i[−i sgn(ω)P] is the inverse Fourier transform of p h (t).
 
     In some implementations, combining the first back-propagated receiver wavefield portion and the second back-propagated receiver wavefield portion into the receiver wavefield data comprises performing the following combination 
                   S   _     (       x   S     ,   ω     )     =         ❘   &#34;\[LeftBracketingBar]&#34;           S   2     (       x   S     ,   ω     )         S   1     (       x   S     ,   ω     )         ❘   &#34;\[RightBracketingBar]&#34;       ⁢       S   2     (       x   S     ,   ω     )         ,         
where S 1 (x S ,ω)=e (iφ) e −2α(ω)r S(x S ,ω) comprises the first reconstructed back-propagated receiver wavefield portion and wherein S 2 (x S ,ω)=e −α(ω)r S(x S ,ω) comprises the second reconstructed back-propagated receiver wavefield portion. In some implementations, the imaging condition comprises a deconvolution imaging condition.
 
     The data processing system and processes described can enable one or more of the following advantages. Generally, a seismic wave propagation has anelastic characteristics in real earth materials. In exploration geophysics, the recorded signals are significantly affected by the attenuation property in both amplitude and phase. The data processing system is able to account for the effects of attenuation and velocity dispersion to generate an accurate seismic image. This prevents distortions in a final seismic image and reduces drilling hazards. Thus, the data processing system is configured for to depth domain seismic data processing and velocity model building to overcome amplitude attenuation and velocity dispersion for seismic imaging. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic view of a seismic survey being performed to map subsurface features such as facies and faults. 
         FIGS.  2 A- 2 B  are example processes for seismic imaging using visco-acoustic reverse time migration. 
         FIG.  3    is a block diagram of an example system for identifying geologic features in a subsurface formation using seismic diffraction imaging 
         FIG.  4    shows graphs illustrating examples of wavefield reconstruction. 
         FIGS.  5 A- 5 B  show illustrations of a velocity model and attenuation model of an example environment. 
         FIG.  6    shows examples of zero-offset profiles generated using acoustic and visco-acoustic forward modeling of the environment of  FIGS.  5 A- 5 B . 
         FIG.  7    shows example data generated from reverse time migration (RTM) of the data from  FIG.  6   . 
         FIG.  8    shows a portion of the data of  FIG.  7    with a zoomed-in view. 
         FIG.  9    is a diagram of an example computing system. 
     
    
    
     DETAILED DESCRIPTION 
     This specification describes a stable visco-acoustic imaging technology based on the de-coupled time-domain complex-valued wave equation. The de-coupled visco-acoustic wave equations are applied for wavefield reconstruction, and the final seismic image accounts for effects for both attenuation and velocity dispersion. Decoupled visco-acoustic wave equations are applied for wavefield reconstruction by an imaging condition-based combination of a source wavefield generated by forward modeling and a receiver wavefield generated by combining two receiver wavefields generated from observed data to provide a final seismic image that accounts for both attenuation and velocity dispersion. 
     In some implementations, the process includes generated a result for a Q compensated Reverse Time Migration transform (Q-RTM). Q is as ratio of stored energy to dispersed energy. It measures a relative energy loss per oscillation cycle. Q increases when the density and the velocity of the material of a seismic formation increases. Generally, for the processes subsequently described, a visco-acoustic reverse time migration (VARTM) approach is used instead of ARTM in order to reduce computational complexity, in some cases by at least 50%. 
       FIG.  1    is a schematic view of a seismic survey being performed to map subsurface features such as facies and faults in a subsurface formation  100 . The subsurface formation  100  includes a layer of impermeable cap rock  102  at the surface. Facies underlying the impermeable cap rocks  102  include a sandstone layer  104 , a limestone layer  106 , and a sand layer  108 . A fault line  110  extends across the sandstone layer  104  and the limestone layer  106 . 
     Oil and gas tend to rise through permeable reservoir rock until further upward migration is blocked, for example, by the layer of impermeable cap rock  102 . Seismic surveys attempt to identify locations where interaction between layers of the subsurface formation  100  are likely to trap oil and gas by limiting this upward migration. For example,  FIG.  1    shows an anticline trap  107 , where the layer of impermeable cap rock  102  has an upward convex configuration, and a fault trap  109 , where the fault line  110  might allow oil and gas to flow in with clay material between the walls traps the petroleum. Other traps include salt domes and stratigraphic traps. 
     A seismic source  112  (for example, a seismic vibrator or an explosion) generates seismic waves that propagate in the earth. Although illustrated as a single component in  FIG.  1   , the source or sources  112  are typically a line or an array of sources  112 . The generated seismic waves include seismic body waves  114  that travel into the ground and seismic surface waves  115  travel along the ground surface and diminish as they get further from the surface. The generated seismic signal at the source is typically located at the ground surface and is sometimes referred to as the ground force. 
     The velocity of these seismic waves depends properties, for example, density, porosity, and fluid content of the medium through which the seismic waves are traveling. Different geologic bodies or layers in the earth are distinguishable because the layers have different properties and, thus, different characteristic seismic velocities. For example, in the subsurface formation  100 , the velocity of seismic waves traveling through the subsurface formation  100  will be different in the sandstone layer  104 , the limestone layer  106 , and the sand layer  108 . As the seismic body waves  114  contact interfaces between geologic bodies or layers that have different velocities, each interface reflects some of the energy of the seismic wave and refracts some of the energy of the seismic wave. Such interfaces are sometimes referred to as horizons. 
     The seismic body waves  114  are received by a sensor or sensors  116 . Although illustrated as a single component in  FIG.  1   , the sensor or sensors  116  are typically a line or an array of sensors  116  that generate an output signal in response to received seismic waves including waves reflected by the horizons in the subsurface formation  100 . The sensors  116  can be geophone-receivers that produce electrical output signals transmitted as input data, for example, to a computer  118  on a seismic control truck  120 . Based on the input data, the computer  118  may generate a seismic data output, for example, a seismic two-way response time plot. 
     The seismic surface waves  115  travel more slowly than seismic body waves  114 . Analysis of the time it takes seismic surface waves  115  to travel from source to sensor can provide information about near surface features. 
     In some situations, seismic surveys include vertical seismic profiles, a technique of seismic measurements used for correlation with surface seismic data. In a vertical seismic profile, either the energy source, the detectors, or both are in a wellbore (e.g., wellbore  117 ). For example, sensors in a well borehole record reflected seismic energy originating from a seismic source at the surface. There are numerous methods for acquiring a vertical seismic profile (VSP). For example, zero-offset VSPs have sources close to the wellbore directly above the sensors; offset VSPs have sources some distance from sensors in a wellbore; and walkaway VSPs feature a source that is moved progressively farther offset from sensors in a fixed location. For example, a zero-offset VSP could be generated by an energy source located at the surface adjacent the wellbore  117  and detectors located downhole in the wellbore  117 . 
     A control center  122  can be operatively coupled to the seismic control truck  120  and other data acquisition and wellsite systems. The control center  122  may have computer facilities for receiving, storing, processing, and analyzing data from the seismic control truck  120  and other data acquisition and wellsite systems. For example, computer systems  124  in the control center  122  can be configured to analyze, model, control, optimize, or perform management tasks of field operations associated with development and production of resources such as oil and gas from the subsurface formation  100 . Alternatively, the computer systems  124  can be located in a different location than the control center  122 . Some computer systems are provided with functionality for manipulating and analyzing the data, such as performing seismic interpretation or borehole resistivity image log interpretation to identify geological surfaces in the subsurface formation or performing simulation, planning, and optimization of production operations of the wellsite systems. 
     In some embodiments, results generated by the computer systems  124  may be displayed for user viewing using local or remote monitors or other display units. One approach to analyzing seismic data is to associate the data with portions of a seismic cube representing represent the subsurface formation  100 . The seismic cube can also be display results of the analysis of the seismic data associated with the seismic survey. 
       FIGS.  2 A- 2 B  illustrate flow diagrams including example processes  220  and  230 , respectively, for seismic imaging using visco-acoustic reverse time migration.  FIG.  3    shows a data processing system  250  for determining a seismic attenuation factor for near-surface subsurface formations. The data processing system  250  of  FIG.  3    is configured to execute the processes  220  and  230  of  FIGS.  2 A- 2 B . 
       FIG.  2 A , a process  220  performed by a physical acquisition system (such as a part of system described in relation to  FIG.  1   ) is shown. One or more sources (such as source  112  in the wellbore  117  of  FIG.  1   ) are excited ( 222 ) to generate a seismic signal in the formation  100 . The seismic signal is propagated ( 224 ) through subsurface formation  100  and recorded ( 226 ) at receiver(s) (such as receivers  116  in the wellbore  117  of  FIG.  1   ). 
     As described previously, as the seismic signal propagates in the formation  100 , the signal disperses and experiences amplitude dissipation. The data processing system described in this specification is configured to model the amplitude dissipation and the dispersion to perform a correction on the received seismic signal. The data processing system uses the corrected seismic signal to generate a seismic image including a more accurate representation of the formation  100  than seismic images generated using uncorrected seismic signals. The data processing system (such as data processing system  250  of  FIG.  3   , subsequently described) uses the following visco-acoustic wave equation to model the dispersion and amplitude dissipation. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             ∂ 
                             2 
                           
                           
                             p 
                             h 
                           
                         
                         
                           ∂ 
                           
                             t 
                             2 
                           
                         
                       
                       + 
                       
                         
                           v 
                           e 
                           
                             2 
                             
                               1 
                               - 
                               γ 
                             
                           
                         
                         ⁢ 
                         
                           k 
                           
                             2 
                             
                               1 
                               - 
                               γ 
                             
                           
                         
                         ⁢ 
                         
                           p 
                           h 
                         
                       
                     
                     = 
                     
                       
                         s 
                         h 
                       
                       ( 
                       t 
                       ) 
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1.1 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   
                     
                       v 
                       e 
                     
                     = 
                     
                       
                         v 
                         0 
                       
                       
                         
                           ω 
                           0 
                           γ 
                         
                         [ 
                         
                           1 
                           - 
                           
                             i 
                             ⁢ 
                                
                             
                               tan 
                               ⁡ 
                               ( 
                               
                                 πγ 
                                 2 
                               
                               ) 
                             
                           
                         
                         ] 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1.2 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   
                     
                       S 
                       h 
                     
                     = 
                     
                       S 
                       + 
                       
                         i 
                         [ 
                         
                           
                             - 
                             i 
                           
                           ⁢ 
                              
                           
                             sgn 
                             ⁡ 
                             ( 
                             ω 
                             ) 
                           
                           ⁢ 
                           S 
                         
                         ] 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1.3 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   
                     
                       P 
                       h 
                     
                     = 
                     
                       P 
                       + 
                       
                         i 
                         [ 
                         
                           
                             - 
                             i 
                           
                           ⁢ 
                              
                           
                             sgn 
                             ⁡ 
                             ( 
                             ω 
                             ) 
                           
                           ⁢ 
                           P 
                         
                         ] 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1.4 
                   ) 
                 
               
             
           
         
       
     
     Equation 1.1 is the generalized visco-acoustic wave equation. s h  and p h  are the inverse Fourier transform of Equations 1.3 and 1.4, respectively. In these equations, v e  represents the complex velocity model, which is a function of the initial velocity ν 0 , ω represents the angular frequency, t represents time, γ is a constant, and k represents the wave number, and p represents the acoustic pressure. 
     Equation 1.2 shows the complex value velocity model to be used for wave field forward modeling. Based on Equations 1.1, 1.2, 1.3, and 1.4 (collectively Equation 1), an additional two wave equations are derived. Equations 2.1 and 2.2 (collectively Equation 2), is a dispersion-only visco-acoustic wave equation. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             ∂ 
                             2 
                           
                           p 
                         
                         
                           ∂ 
                           
                             t 
                             2 
                           
                         
                       
                       + 
                       
                         
                           
                             v 
                             _ 
                           
                           e 
                           
                             2 
                             
                               1 
                               - 
                               γ 
                             
                           
                         
                         ⁢ 
                         
                           k 
                           
                             2 
                             
                               1 
                               - 
                               γ 
                             
                           
                         
                         ⁢ 
                         p 
                       
                     
                     = 
                     
                       s 
                       ⁡ 
                       ( 
                       t 
                       ) 
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   2.1 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   
                     
                       
                         v 
                         _ 
                       
                       e 
                     
                     = 
                     
                       
                         v 
                         0 
                       
                       
                         
                           ( 
                           
                             ω 
                             0 
                           
                           ) 
                         
                         γ 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   2.2 
                   ) 
                 
               
             
           
         
       
     
     Equations 3.1 and 3.2 (collectively Equation 3) is a dissipation-only visco-acoustic wave equation. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             ∂ 
                             2 
                           
                           
                             p 
                             h 
                           
                         
                         
                           ∂ 
                           
                             t 
                             2 
                           
                         
                       
                       + 
                       
                         
                           
                             v 
                             ⌣ 
                           
                           e 
                           2 
                         
                         ⁢ 
                         
                           k 
                           2 
                         
                         ⁢ 
                         
                           p 
                           h 
                         
                       
                     
                     = 
                     
                       
                         s 
                         h 
                       
                       ( 
                       t 
                       ) 
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   3.1 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   
                     
                       v 
                       ⌣ 
                     
                     e 
                   
                   = 
                   
                     
                       
                         v 
                         0 
                       
                       
                         1 
                         - 
                         
                           i 
                           ⁢ 
                              
                           
                             tan 
                             ⁡ 
                             ( 
                             
                               πγ 
                               2 
                             
                             ) 
                           
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   3.2 
                   ) 
                 
               
             
           
         
       
     
     In each of these equations, ν̆ e  represents the complex velocity model, which is a function of the initial velocity ν 0 , ω represents the angular frequency, t represents time, γ is a constant, and k represents the wave number. 
     The format of Equation 2.1 is the same as Equation 1.1. However, the Hilbert variables s h  and p h  are replaced by s and p. This indicates that Equation 2.1 represents a model that is computationally less complex than the model shown by Equation 1.1. Equation 2.1 represents a computationally less complex model because there is no need to save complex wavefield values for computation, which is needed for computing a model using Equation 1.1. Generally, the computation and memory usage for models generated using the approach of Equation 2.1 are about 50% lower (for both computation and memory usage) than for models generated using the approach of Equation 1.1. 
     Additionally, Equation 3.1 includes a similar format as a conventional acoustic wave equation, except that Equation 3.1 includes an imaginary portion including ν̆ e . In other words, when the imaginary portion ν̆ e  is set as zero, Equation 3.1 degenerates to the acoustic wave equation. In comparison to Equations 1.1 and 2.1, Equation 3.1 can be solved by the data processing system with a spatial domain finite difference method. Thus, the solution of Equation 3.1 is less computationally expensive than the spectral domain solved used in Equations 1.1 and 2.1. Generally, if Equation 3.1 is solved in the wavenumber domain, the computational cost is the same as Equation 1.1. If Equation 3.1 is solved in spatial domain, it would be cheaper, because finite difference will be cheaper than spectral method. 
     Generally, several methods of Reverse Time Migration (RTM) are available for performing the compensation of the seismic signal. In some implementations, a Q-compensated Reverse Time Migration (Q-RTM) can be used. In some implementations, an Acoustic Reverse Time Migration (ARTM) is used. Here, a version of a Visco-Acoustic Reverse Time Migration (VARTM) is applied. Though VARTM and Q-RTM are similar, VARTM is shown for the example implementation. 
     Compared to the approaches using ARTM, a challenge for VARTM is performing a receiver wavefield back-propagation or reconstruction. The receiver wavefield back propagation is challenging because the amplitude dissipation is compensated during the back-propagation in Equations 4.1, 4.2, and 4.3 (collectively Equation 4). This process can be numerically unstable. In Equation 4.1, d h  is the inverse Fourier transform of Equation 4.3. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             ∂ 
                             2 
                           
                           ⁢ 
                           
                             p 
                             h 
                           
                         
                         
                           ∂ 
                           
                             t 
                             2 
                           
                         
                       
                       + 
                       
                         
                           v 
                           e 
                           
                             2 
                             
                               1 
                               - 
                               γ 
                             
                           
                         
                         ⁢ 
                         
                           k 
                           
                             2 
                             
                               1 
                               - 
                               γ 
                             
                           
                         
                         ⁢ 
                         
                           p 
                           h 
                         
                       
                     
                     = 
                     
                       
                         d 
                         h 
                       
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   4.1 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       v 
                       e 
                     
                     = 
                     
                       
                         v 
                         0 
                       
                       
                         
                           ω 
                           0 
                           γ 
                         
                         ⁡ 
                         
                           [ 
                           
                             1 
                             + 
                             
                               i 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 tan 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     πγ 
                                     2 
                                   
                                   ) 
                                 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   4.2 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       D 
                       h 
                     
                     = 
                     
                       D 
                       + 
                       
                         i 
                         ⁢ 
                         
                             
                         
                         [ 
                         
                           
                             - 
                             i 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             sgn 
                             ⁡ 
                             
                               ( 
                               ω 
                               ) 
                             
                           
                           ⁢ 
                           D 
                         
                         ] 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   4.3 
                   ) 
                 
               
             
           
         
       
     
     In these equations, ν e  represents the complex velocity model, which is a function of the initial velocity ν 0 , ω represents the angular frequency, t represents time, γ is a constant, D represents the wave equation, and k represents the wave number. p h  is the inverse Fourier transform of Equations 1.4. 
     To overcome the challenge imparted by numerical instability, the data processing system  250  is configured to execute the process  230  for determining a seismic attenuation factor for near-surface subsurface formations using VARTM. Process  230  is shown in  FIG.  2 B . The process  230  includes performing a wavefield reconstruction by an imaging condition-based combination of a source wavefield generated by forward modeling and a receiver wavefield generated by combining two receiver wavefields generated from observed data. The recorded signal at the receiver and a numerical representation of the generated signal at the source can be used by the data processing system  250  of  FIG.  3    for performing process  230  of  FIG.  2 B . In an aspect, a data processing system  250  is configured to receive the seismic data  252  and generate seismic image data  262  of the subsurface formation using a series of modules. The data processing system  250  includes an acoustic Reverse Time Migration (RTM) engine  256  and a seismic image generation module  260 . 
     Process  230  of  FIG.  2 B  includes receiving ( 232 ), by the data processing system  250 , seismic data representing a source wavelet. This seismic signal can be stored as a portion of the seismic data  252 . The data processing system  250  is configured to perform a forward modeling ( 234 ) of the seismic signal to generate the source wavefield. For the source wavefield modeling, Equation 1 is applied to the seismic signal. The data generated using Equation 1 is the source side synthetic wavefield using a user-specified wavelet. The generated wavefield includes both dispersion and dissipation, and the data is used for the imaging condition. 
     The data processing system  250  is configured to receive ( 236 ) a seismic data representing a receiver wavelet. The data processing system  250  is configured to reconstruct ( 238 ,  240 ) the receiver wavefield in two parts. The data processing system  250  applies Equation 2 to the receiver wavelet for the receiver wavefield reconstruction. This results a back-propagated wavefield using a dissipation-only wave equation. The data processing system  250  also applies Equation 3 to the receiver wavelet for receiver wavefield reconstruction. This results in a back-propagated wavefield using a dispersion-only wave equation. For reconstructions  238 ,  240 , the parameter s is replaced by observed data d of the seismic data. 
     The data processing system  250  is configured to combine ( 242 ) the first and second reconstructed wavefields from Equations 2 and 3. As subsequently shown in Equations 5.1, 5.2, and 5.3 (collectively Equation 5), S 1  is the reconstructed wavefield using Equation 2, representing a dissipation-only back-propagated wavefield. S 2  is the reconstructed wavefield using Equation 3, representing the dispersion-only back propagated wavefield. The final reconstructed wavefield  S  includes S 2  multiplied by the absolute value of S 2 , divided by S 1 . 
     
       
         
           
             
               
                 
                   
                     
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     Here, i is the imaginary unit, ω is the angular frequency, α is a constant, and r is a coordinate. Generally, after the source and receiver wavefields at each image location are reconstructed, a deconvolution imaging condition is applied ( 244 ) by the data processing system  250  for the final image process. A VARTM result is generated ( 246 ). Generally, this result is similar to a Q-RTM result for generating the seismic image. From this VARTM, the data processing system  250  is configured to generate ( 248 ) a seismic image using the VARTM result. 
       FIG.  4    shows graphs  270  and  280  illustrating examples of wavefield reconstruction using Equations 1, 2, and 3 as described above. Graph  270  of  FIG.  4    shows each of S 1 , S 2 , and  S , and the source wavelet. As shown in graph  270 , the values of the wavefield reconstruction  S  for travel times vs. amplitudes closely approximate the source wavefield. The source wavelet and observed data are shown with an offset of 1.5 km. A shown in graph  280 , the theoretical result matches the observed numerical result. The line  282  in graph  280  is the input data for back propagation (amplified 100 times for this visualization). As stated previously, after the source and receiver wavefields at each image location are reconstructed, a deconvolution imaging condition is applied by the data processing system  250  for the final image process. 
     Generally, the deconvolution imaging condition is defined in Equation 6: 
     
       
         
           
             
               
                 
                   
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     Here, S(x S ,x,ω) is the frequency domain source wavefield generated from Equation 1, and R(x S ,x,ω) is the frequency domain receiver wavefield generated from Equations 2, 3, and 5.3 
       FIGS.  5 A- 5 B  show illustrations  290 ,  302  of a velocity model and attenuation model of an example environment. The velocity model includes a four-layer model including layers  294 ,  296 ,  298 , and  300 . The model includes a lens body  292  inside the first layer which has strong attenuation property, shown in the attenuation data  304  of model  302 . This is an example environment for testing the methodology described in relation to  FIG.  5 B . 
       FIG.  6    shows examples of zero-offset profiles  310  and  320  generated using acoustic and visco-acoustic forward modeling of the environment of  FIGS.  8 A- 8 B . The data processing system  250  is configured to perform acoustic and visco-acoustic forward modeling. A plurality of shots ( 200  shots in this example) are simulated for the visco-acoustic model. The zero-offset profiles  310  and  320 . From the comparison, both dispersion and amplitude dissipation of signal caused by the strong attenuation lens body  292  are shown. 
       FIG.  7    shows example data generated from reverse time migration (RTM) of the data from  FIG.  6    are shown. The VARTM and ARTM are performed by the data processing system  250  on these data  310  and  320 . The graphs  330 ,  340 , and  350  show the migration results of the VARTM and the ARTM. Graph  330  is the ARTM result using the acoustic data as input. Graph  340  is the VARTM result using visco-acoustic data as input. As shown, graphs  330  and  340  represent two results that are very similar. Graph  350  shows ARTM using the visco-acoustic data. Graph  350  shows image distortion at the lower boundary of the lens body and also the layers below the lens body. 
       FIG.  8    shows a portion  350  of the data of graph  330  of  FIG.  7    with a zoomed-in view. The portion  350  of the data of graph is shown by dashed line  332  in graph  330 . The consistent results between the VARTM results  352   a - o  using visco-acoustic data and black dashed line ARTM result  354   a - o  using acoustic data validate the invented VARTM method. The traces  556   a - o  represent acoustic RTM using the visco-acoustic data. 
       FIG.  9    is a block diagram of an example computing system  400  used to provide computational functionalities associated with described algorithms, methods, functions, processes, flows, and procedures described in the present disclosure, according to some implementations of the present disclosure. The illustrated computer  402  is intended to encompass any computing device such as a server, a desktop computer, a laptop/notebook computer, a wireless data port, a smart phone, a personal data assistant (PDA), a tablet computing device, or one or more processors within these devices, including physical instances, virtual instances, or both. The computer  402  can include input devices such as keypads, keyboards, and touch screens that can accept user information. Also, the computer  402  can include output devices that can convey information associated with the operation of the computer  402 . The information can include digital data, visual data, audio information, or a combination of information. The information can be presented in a graphical user interface (UI) (or GUI). 
     The computer  402  can serve in a role as a client, a network component, a server, a database, a persistency, or components of a computer system for performing the subject matter described in the present disclosure. The illustrated computer  402  is communicably coupled with a network  424 . In some implementations, one or more components of the computer  402  can be configured to operate within different environments, including cloud-computing-based environments, local environments, global environments, and combinations of environments. 
     At a high level, the computer  402  is an electronic computing device operable to receive, transmit, process, store, and manage data and information associated with the described subject matter. According to some implementations, the computer  402  can also include, or be communicably coupled with, an application server, an email server, a web server, a caching server, a streaming data server, or a combination of servers. 
     The computer  402  can receive requests over network  424  from a client application (for example, executing on another computer  402 ). The computer  402  can respond to the received requests by processing the received requests using software applications. Requests can also be sent to the computer  402  from internal users (for example, from a command console), external (or third) parties, automated applications, entities, individuals, systems, and computers. 
     Each of the components of the computer  402  can communicate using a system bus  404 . In some implementations, any or all of the components of the computer  402 , including hardware or software components, can interface with each other or the interface  406  (or a combination of both), over the system bus  404 . Interfaces can use an application programming interface (API)  414 , a service layer  416 , or a combination of the API  414  and service layer  416 . The API  414  can include specifications for routines, data structures, and object classes. The API  414  can be either computer-language independent or dependent. The API  414  can refer to a complete interface, a single function, or a set of APIs. 
     The service layer  416  can provide software services to the computer  402  and other components (whether illustrated or not) that are communicably coupled to the computer  402 . The functionality of the computer  402  can be accessible for all service consumers using this service layer. Software services, such as those provided by the service layer  416 , can provide reusable, defined functionalities through a defined interface. For example, the interface can be software written in JAVA, C++, or a language providing data in extensible markup language (XML) format. While illustrated as an integrated component of the computer  402 , in alternative implementations, the API  414  or the service layer  416  can be stand-alone components in relation to other components of the computer  402  and other components communicably coupled to the computer  402 . Moreover, any or all parts of the API  414  or the service layer  416  can be implemented as child or sub-modules of another software module, enterprise application, or hardware module without departing from the scope of the present disclosure. 
     The computer  402  includes an interface  406 . Although illustrated as a single interface  406  in  FIG.  17   , two or more interfaces  406  can be used according to particular needs, desires, or particular implementations of the computer  402  and the described functionality. The interface  406  can be used by the computer  402  for communicating with other systems that are connected to the network  424  (whether illustrated or not) in a distributed environment. Generally, the interface  406  can include, or be implemented using, logic encoded in software or hardware (or a combination of software and hardware) operable to communicate with the network  424 . More specifically, the interface  406  can include software supporting one or more communication protocols associated with communications. As such, the network  424  or the hardware of the interface can be operable to communicate physical signals within and outside of the illustrated computer  402 . 
     The computer  402  includes a processor  408 . Although illustrated as a single processor  408  in  FIG.  17   , two or more processors  408  can be used according to particular needs, desires, or particular implementations of the computer  402  and the described functionality. Generally, the processor  408  can execute instructions and can manipulate data to perform the operations of the computer  402 , including operations using algorithms, methods, functions, processes, flows, and procedures as described in the present disclosure. 
     The computer  402  also includes a database  420  that can hold data (for example, seismic data  422 ) for the computer  402  and other components connected to the network  424  (whether illustrated or not). For example, database  420  can be an in-memory, conventional, or a database storing data consistent with the present disclosure. In some implementations, database  420  can be a combination of two or more different database types (for example, hybrid in-memory and conventional databases) according to particular needs, desires, or particular implementations of the computer  402  and the described functionality. Although illustrated as a single database  420  in  FIG.  17   , two or more databases (of the same, different, or combination of types) can be used according to particular needs, desires, or particular implementations of the computer  402  and the described functionality. While database  420  is illustrated as an internal component of the computer  402 , in alternative implementations, database  420  can be external to the computer  402 . 
     The computer  402  also includes a memory  410  that can hold data for the computer  402  or a combination of components connected to the network  424  (whether illustrated or not). Memory  410  can store any data consistent with the present disclosure. In some implementations, memory  410  can be a combination of two or more different types of memory (for example, a combination of semiconductor and magnetic storage) according to particular needs, desires, or particular implementations of the computer  402  and the described functionality. Although illustrated as a single memory  410  in  FIG.  17   , two or more memories  410  (of the same, different, or combination of types) can be used according to particular needs, desires, or particular implementations of the computer  402  and the described functionality. While memory  410  is illustrated as an internal component of the computer  402 , in alternative implementations, memory  410  can be external to the computer  402 . 
     The application  412  can be an algorithmic software engine providing functionality according to particular needs, desires, or particular implementations of the computer  402  and the described functionality. For example, application  412  can serve as one or more components, modules, or applications. Further, although illustrated as a single application  412 , the application  412  can be implemented as multiple applications  412  on the computer  402 . In addition, although illustrated as internal to the computer  402 , in alternative implementations, the application  412  can be external to the computer  402 . 
     The computer  402  can also include a power supply  418 . The power supply  418  can include a rechargeable or non-rechargeable battery that can be configured to be either user- or non-user-replaceable. In some implementations, the power supply  418  can include power-conversion and management circuits, including recharging, standby, and power management functionalities. In some implementations, the power-supply  418  can include a power plug to allow the computer  402  to be plugged into a wall socket or a power source to, for example, power the computer  402  or recharge a rechargeable battery. 
     There can be any number of computers  402  associated with, or external to, a computer system containing computer  402 , with each computer  402  communicating over network  424 . Further, the terms “client,” “user,” and other appropriate terminology can be used interchangeably, as appropriate, without departing from the scope of the present disclosure. Moreover, the present disclosure contemplates that many users can use one computer  402  and one user can use multiple computers  402 . 
     Implementations of the subject matter and the functional operations described in this specification can be implemented in digital electronic circuitry, in tangibly embodied computer software or firmware, in computer hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Software implementations of the described subject matter can be implemented as one or more computer programs. Each computer program can include one or more modules of computer program instructions encoded on a tangible, non-transitory, computer-readable computer-storage medium for execution by, or to control the operation of, data processing apparatus. Alternatively, or additionally, the program instructions can be encoded in/on an artificially generated propagated signal. The example, the signal can be a machine-generated electrical, optical, or electromagnetic signal that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. The computer-storage medium can be a machine-readable storage device, a machine-readable storage substrate, a random or serial access memory device, or a combination of computer-storage mediums. 
     The terms “data processing apparatus,” “computer,” and “electronic computer device” (or equivalent as understood by one of ordinary skill in the art) refer to data processing hardware. For example, a data processing apparatus can encompass all kinds of apparatus, devices, and machines for processing data, including by way of example, a programmable processor, a computer, or multiple processors or computers. The apparatus can also include special purpose logic circuitry including, for example, a central processing unit (CPU), a field programmable gate array (FPGA), or an application specific integrated circuit (ASIC). In some implementations, the data processing apparatus or special purpose logic circuitry (or a combination of the data processing apparatus or special purpose logic circuitry) can be hardware- or software-based (or a combination of both hardware- and software-based). The apparatus can optionally include code that creates an execution environment for computer programs, for example, code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of execution environments. The present disclosure contemplates the use of data processing apparatuses with or without conventional operating systems, for example, LINUX, UNIX, WINDOWS, MAC OS, ANDROID, or IOS. 
     A computer program, which can also be referred to or described as a program, software, a software application, a module, a software module, a script, or code, can be written in any form of programming language. Programming languages can include, for example, compiled languages, interpreted languages, declarative languages, or procedural languages. Programs can be deployed in any form, including as stand-alone programs, modules, components, subroutines, or units for use in a computing environment. A computer program can, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data, for example, one or more scripts stored in a markup language document, in a single file dedicated to the program in question, or in multiple coordinated files storing one or more modules, sub programs, or portions of code. A computer program can be deployed for execution on one computer or on multiple computers that are located, for example, at one site or distributed across multiple sites that are interconnected by a communication network. While portions of the programs illustrated in the various figures may be shown as individual modules that implement the various features and functionality through various objects, methods, or processes, the programs can instead include a number of sub-modules, third-party services, components, and libraries. Conversely, the features and functionality of various components can be combined into single components as appropriate. Thresholds used to make computational determinations can be statically, dynamically, or both statically and dynamically determined. 
     The methods, processes, or logic flows described in this specification can be performed by one or more programmable computers executing one or more computer programs to perform functions by operating on input data and generating output. The methods, processes, or logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, for example, a CPU, an FPGA, or an ASIC. 
     Computers suitable for the execution of a computer program can be based on one or more of general and special purpose microprocessors and other kinds of CPUs. The elements of a computer are a CPU for performing or executing instructions and one or more memory devices for storing instructions and data. Generally, a CPU can receive instructions and data from (and write data to) a memory. A computer can also include, or be operatively coupled to, one or more mass storage devices for storing data. In some implementations, a computer can receive data from, and transfer data to, the mass storage devices including, for example, magnetic, magneto optical disks, or optical disks. Moreover, a computer can be embedded in another device, for example, a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a global positioning system (GPS) receiver, or a portable storage device such as a universal serial bus (USB) flash drive. 
     Computer readable media (transitory or non-transitory, as appropriate) suitable for storing computer program instructions and data can include all forms of permanent/non-permanent and volatile/non-volatile memory, media, and memory devices. Computer readable media can include, for example, semiconductor memory devices such as random access memory (RAM), read only memory (ROM), phase change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), and flash memory devices. Computer readable media can also include, for example, magnetic devices such as tape, cartridges, cassettes, and internal/removable disks. Computer readable media can also include magneto optical disks and optical memory devices and technologies including, for example, digital video disc (DVD), CD ROM, DVD+/−R, DVD-RAM, DVD-ROM, HD-DVD, and BLURAY. The memory can store various objects or data, including caches, classes, frameworks, applications, modules, backup data, jobs, web pages, web page templates, data structures, database tables, repositories, and dynamic information. Types of objects and data stored in memory can include parameters, variables, algorithms, instructions, rules, constraints, and references. Additionally, the memory can include logs, policies, security or access data, and reporting files. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
     Implementations of the subject matter described in the present disclosure can be implemented on a computer having a display device for providing interaction with a user, including displaying information to (and receiving input from) the user. Types of display devices can include, for example, a cathode ray tube (CRT), a liquid crystal display (LCD), a light-emitting diode (LED), and a plasma monitor. Display devices can include a keyboard and pointing devices including, for example, a mouse, a trackball, or a trackpad. User input can also be provided to the computer through the use of a touchscreen, such as a tablet computer surface with pressure sensitivity or a multi-touch screen using capacitive or electric sensing. Other kinds of devices can be used to provide for interaction with a user, including to receive user feedback including, for example, sensory feedback including visual feedback, auditory feedback, or tactile feedback. Input from the user can be received in the form of acoustic, speech, or tactile input. In addition, a computer can interact with a user by sending documents to, and receiving documents from, a device that is used by the user. For example, the computer can send web pages to a web browser on a user&#39;s client device in response to requests received from the web browser. 
     The term “graphical user interface,” or “GUI,” can be used in the singular or the plural to describe one or more graphical user interfaces and each of the displays of a particular graphical user interface. Therefore, a GUI can represent any graphical user interface, including, but not limited to, a web browser, a touch screen, or a command line interface (CLI) that processes information and efficiently presents the information results to the user. In general, a GUI can include a plurality of user interface (UI) elements, some or all associated with a web browser, such as interactive fields, pull-down lists, and buttons. These and other UI elements can be related to or represent the functions of the web browser. 
     Implementations of the subject matter described in this specification can be implemented in a computing system that includes a back end component, for example, as a data server, or that includes a middleware component, for example, an application server. Moreover, the computing system can include a front-end component, for example, a client computer having one or both of a graphical user interface or a Web browser through which a user can interact with the computer. The components of the system can be interconnected by any form or medium of wireline or wireless digital data communication (or a combination of data communication) in a communication network. Examples of communication networks include a local area network (LAN), a radio access network (RAN), a metropolitan area network (MAN), a wide area network (WAN), Worldwide Interoperability for Microwave Access (WIMAX), a wireless local area network (WLAN) (for example, using 402.11 a/b/g/n or 402.20 or a combination of protocols), all or a portion of the Internet, or any other communication system or systems at one or more locations (or a combination of communication networks). The network can communicate with, for example, Internet Protocol (IP) packets, frame relay frames, asynchronous transfer mode (ATM) cells, voice, video, data, or a combination of communication types between network addresses. 
     The computing system can include clients and servers. A client and server can generally be remote from each other and can typically interact through a communication network. The relationship of client and server can arise by virtue of computer programs running on the respective computers and having a client-server relationship. 
     Cluster file systems can be any file system type accessible from multiple servers for read and update. Locking or consistency tracking may not be necessary since the locking of exchange file system can be done at application layer. Furthermore, Unicode data files can be different from non-Unicode data files. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular implementations. Certain features that are described in this specification in the context of separate implementations can also be implemented, in combination, in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations, separately, or in any suitable sub-combination. Moreover, although previously described features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. 
     Particular implementations of the subject matter have been described. Other implementations, alterations, and permutations of the described implementations are within the scope of the following claims as will be apparent to those skilled in the art. While operations are depicted in the drawings or claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed (some operations may be considered optional), to achieve desirable results. In certain circumstances, multitasking or parallel processing (or a combination of multitasking and parallel processing) may be advantageous and performed as deemed appropriate. 
     Moreover, the separation or integration of various system modules and components in the previously described implementations should not be understood as requiring such separation or integration in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Accordingly, the previously described example implementations do not define or constrain the present disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of the present disclosure. 
     Furthermore, any claimed implementation is considered to be applicable to at least a computer-implemented method; a non-transitory, computer-readable medium storing computer-readable instructions to perform the computer-implemented method; and a computer system comprising a computer memory interoperably coupled with a hardware processor configured to perform the computer-implemented method or the instructions stored on the non-transitory, computer-readable medium. 
     While this specification contains many details, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features specific to particular examples. Certain features that are described in this specification in the context of separate implementations can also be combined. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple embodiments separately or in any suitable sub-combination. 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the data processing system previously described. Accordingly, other embodiments are within the scope of the following claims.