Ray tracing computations in the smoothed Salt Model

Source at the surface - history file 'sal-ray1.h'

This is an example of two-parametric ray tracing in smoothed Salt Model with the most important interfaces. Velocity model is specified in the file 'sal-mod2.dat'. The model is version with the most important interfaces: the ocean bottom, the interfaces limiting the salt body, the geopressure horizon and the bottom flat interface. Generation of the selected and other smooth versions of SEG/EAGE Salt Model is described in Building the Salt Model by inversion. Model 'sal-mod2.dat' is an output of history file 'sal-inv2.h'. The name of the model output file was changed from the 'sal-m2.out' to 'sal-mod2.dat'.

Computation is performed for one shot in the depth of 20m at the centre of 21x21 receivers square located in the depth of 0m. The receiver grid step is 320m. The position of the shot above the salt crest is similar to the original SEG/EAGE Phase A Acquisition where this point is the intersection of two 3-D shot lines (Aminzadeh et al. 1997). Rays of refracted P-wave and P-waves reflected from top and bottom interfaces of salt and bottom interface are computed. Synthetic seismograms for selected profile are generated and plotted. History file 'sal-ray1.h' adds rays, receivers and source into GOCAD file.

For more information about this computation see Bucha (2003).

Sources in the well - history files 'sal-rwe1.h', 'sal-rwe2.h'

This is an example of two-parametric ray tracing in smoothed Salt Model with the most important interfaces. Velocity model is the same as in the part describing the use of history file 'sal-ray1.h'. Computation is performed for two shots in a well. One shot is located above the salt body in the depth of 900m and the other shot is located below the salt body in the depth of 2300m. The square of 21x21 receivers is located in the depth of 0m. The receiver grid step is 320m. The positions of the shot-receiver configurations are derived from the original SEG/EAGE Phase A Acquisition (Aminzadeh et al. 1997). Rays of refracted P-wave and P-waves reflected from top and bottom interfaces of salt and bottom interface are computed. Rays from the shot below the salt include multiple reflections. Synthetic seismograms for selected profile are generated and plotted. History files 'sal-rwe1.h' and 'sal-rwe2.h' add rays, receivers and shots into GOCAD file.

For more information about this computation see Bucha (2004a).

Maps of reflections - history file 'sal-mapx.h'

History file 'sal-mapx.h' computes two-point rays of the P-wave reflected from the flat bottom interface in the smoothed SEG/EAGE Salt Model. Velocity model is the same as in the part describing the use of history file 'sal-ray1.h'. Six shot-receiver configurations derived from the original Phase A Acquisition are used for generation of reflection point maps, colour coded according to amplitudes. History file 'sal-mapx.h' executes step by step six other history files 'sal-map1.h', 'sal-map2.h', 'sal-map3.h', 'sal-map4.h', 'sal-map5.h', 'sal-map6.h'. Each of these six history files can be executed separately. Shots are located in the depth of 20m and the step is 800m. Receivers are located in the depth of 0m and the grid step is 320m. Computed rays, reflection maps, receivers and sources are added into GOCAD file.

For more information about this computation see Bucha (2004b).

Comparison of finite-difference and ray-theory seismograms in the elastic SEG/EAGE Salt Model, shot number 145

The original acoustic SEG/EAGE Salt Model contains P-wave velocity values (Aminzadeh et al., 1997). House et al. (2004) calculated finite-difference seismograms in an elastic SEG/EAGE Salt Model. This elastic velocity model was supplemented by values of S-wave velocities and densities derived from the original P-wave velocity values. House et al. (2004) performed elastic numerical calculations within the Next-Generation Seismic Modeling and Imaging project. SEGY synthetic seismograms computed within Phase 1 finite-difference calculations in the elastic SEG/EAGE model are available via internet ( www.ees11.lanl.gov/projects/next-gen).

We compared finite-difference seismograms for shot number 145 and two receiver configurations, marine streamers and ocean bottom cables with ray-theory seismograms of selected elementary waves computed using the SW3D software. The smoothed SEG/EAGE Salt Model (see Building the Salt Model by inversion) with the most important interfaces was supplemented by S-waves and densities according to House et al. (2000). Gridded data for six surfaces and five complex blocks of the elastic smoothed SEG/EAGE Salt Model are specified in the file 'sal-mod3.dat'.

History files 's145se1.h',..., 's145se8.h', summarize parameters and execute programs that calculate simulation of marine survey for one shot and eight receiver lines (each history file computes rays and seismograms for the shot and one receiver line). History files 's145oe1.h',..., 's145oe6.h', summarize parameters and execute programs that calculate simulation of ocean bottom cable survey for one shot and six receiver lines. History files add rays, receivers and shots into GOCAD file.

Finite-difference seismograms are read and plotted using the Seismic Unix programs suswapbytes, supswigp and psbbox (Cohen & Stockwell, 2004). The commands and parameters used for figures in the paper by Bucha (2005) are in files 'sefdsu.sh' for marine streamers and 'oefdsu.sh' for ocean bottom cables. Plotting of ray-theory seismograms was performed in two ways. Seismograms generated from all the computed elementary waves are at first converted from GSE format to SEGY format by program 'gse2segy.for', read and plotted using the Seismic Unix programs segyread, supswigp and psbbox for better comparison with finite-difference seismograms. The maximum amplitude at each trace is scaled to a given constant. The commands and parameters used for figures are in files 'se1rtsu.sh',..., 'se8rtsu.sh' for marine streamers and 'oe1rtsu.sh',..., 'oe6rtsu.sh' for ocean bottom cables. Seismograms of individual elementary waves distinguished by colours are plotted by program 'sp.for'.

For more information about this computation see Bucha (2005).

Comparison of finite-difference seismograms and ray-theory travel times in the elastic SEG/EAGE Salt Model, shot 145

Bucha (2005) presented comparison of finite-difference and ray-theory seismograms in the elastic SEG/EAGE Salt Model only for two receiver configurations: simulated marine streamer survey running in East direction and simulated ocean-bottom cable with cables aligned East-West. Here we compare finite-difference seismograms and ray-theory travel times for all five receiver configurations computed for shot 145 (East, South, West marine streamer surveys, North-South and East-West ocean-bottom cables). The plots of travel times are more clear than plots of seismograms of individual elementary waves presented by Bucha (2005). Travel times of individual elementary waves distinguished by colours are plotted by program 'sp.for'. Plotting of travel times was added to history files 's145se1.h',..., 's145se8.h', for marine streamer survey in East direction and to history files 's145oe1.h',..., 's145oe6.h', for ocean bottom cable survey in East-West direction.

For more information about this computation see Bucha (2006).

References

Aminzadeh, F., Brac, J., Kunz, T. (1997): 3-D Salt and Overthrust Models. SEG/EAGE 3-D Modeling Series No.1., Soc. Explor. Geophysicists, Tulsa.

Bucha, V. (2003): Ray tracing computations in the smoothed Salt Model. In: Seismic Waves in Complex 3-D Structures, Report 13, pp. 239-246, Dep. Geophys., Charles Univ., Prague.

Bucha, V. (2004a): Ray tracing computations in the smoothed Salt Model. Part 2: Sources in the well. In: Seismic Waves in Complex 3-D Structures, Report 14, pp. 27-34, Dep. Geophys., Charles Univ., Prague.

Bucha, V. (2004b): Ray tracing computations in the smoothed Salt Model. Part 3: Maps of reflections. In: Seismic Waves in Complex 3-D Structures, Report 14, pp. 35-42, Dep. Geophys., Charles Univ., Prague.

Bucha, V. (2005): Comparison of finite-difference and ray-theory seismograms in the elastic SEG/EAGE Salt Model, shot number 145. In: Seismic Waves in Complex 3-D Structures, Report 15, pp. 189-206, Dep. Geophys., Charles Univ., Prague.

Bucha, V. (2006): Comparison of finite-difference and ray-theory travel times in the elastic SEG/EAGE Salt Model, shot 145. In: Seismic Waves in Complex 3-D Structures, Report 16, pp. 29-46, Dep. Geophys., Charles Univ., Prague.

Cohen, J.K. & Stockwell, Jr.J.W. (2004): CWP/SU: Seismic Un*x Release No. 38: a free package for seismic research and processing, Center for Wave Phenomena, Colorado School of Mines.

House, S., Larsen, S., Hoelting, C., Marfurt, K., Wiley, R. (2004): Next-Generation Seismic Modeling and Imaging project: summary of elastic modeling results. Exp. Abstr., Soc. Expl. Geophys. 74th Ann. Meet., pp. 2201-2204.