Post-stack Time Migration Research Articles

3-D prestack Kirchhoff depth migration: From prototype to production in a massively parallel processor environment

Portable, production‐scale 3-D prestack Kirchhoff depth migration software capable of full‐volume imaging has been successfully implemented and applied to a six‐million trace (46.9 Gbyte) marine data set from a salt/subsalt play in the Gulf of Mexico. Velocity model building and updates use an image‐driven strategy and were performed in a Sun Sparc environment. Images obtained by 3-D prestack migration after three velocity iterations are substantially better focused and reveal drilling targets that were not visible in images obtained from conventional 3-D poststack time migration. Amplitudes are well preserved, so anomalies associated with known reservoirs conform to the petrophysical predictions. Prototype development was on an 8-node Intel iPSC860 computer; the production version was run on an 1824-node Intel Paragon computer. The code has been successfully ported to CRAY (T3D) and Unix workstation (PVM) environments.

On: “Pulse distortion in depth migration,” M. Tygel, J. Schleicher, and P. Hubral (October 1994 GEOPHYSICS, 59, p. 1561–1569).

Tygel et al. have written an excellent and rigorous discussion of pulse distortion in seismic reflection data caused by prestack depth migration. Such distortion is easily understood by recognizing that it is more or less the same effect as normal moveout (NMO) stretch combined with frequency shifting due to poststack time migration.

Integrated seismic analysis: Kidney area, northern Alberta, Canada

Integrating various seismic data via simultaneous processing or correlation can increase the accuracy and confidence in subsurface images. To this end, a multioffset VSP survey with simultaneous surface recording was acquired in the fall of 1986 near Red Earth, northern Alberta, in the Kidney prospect area. P‐wave vibrators at three locations west of the survey well were recorded by a downhole three‐component geophone at 15 m increments between depths of 225 m and 1570 m and were recorded simultaneously on a 2900 m surface spread of vertical geophones spaced 25 m apart in line with the VSP sources and just west of the well. These data, along with existing seismic lines and well logs, correlated convincingly and were used to increase confidence in the interpretation of the potential reservoir of interest—a Keg River carbonate (KRC) unit. Tomographic procedures (constrained traveltime inversion) were developed to process simultaneously the well logs, VSP, and surface seismic data. These procedures provided a macroscopic seismic interval velocity in depth. This tomographic velocity structure (TVS) was used in poststack depth migration of an existing seismic line over the region of interest. The resulting depth‐migrated section near the well matched the synthetic seismogram (computed in depth from the sonic log) reasonably well in character and depth for most of the major reflectors. The TVS was also used in a fairly conventional processing flow which included poststack time migration, maximum‐likelihood deconvolution, seismic trace inversion, and time‐to‐depth stretching. The final processed pseudosonic section in depth bore considerable resemblance to the sonic log. About 30 m of KRC was measured from the pseudosonic section, while 35 m of KRC was interpreted from the well logs. The pseudosonic section also strongly suggested thinning or discontinuity in the KRC unit on the section around a previously proposed drilling location (1.7 km west of the well). This interpretation was confirmed by another operator’s subsequent drilling at this location, which encountered only 1 m of KRC. The predicted depth of the KRC top (about 1395 m), interpreted from the pseudosonic section at the second well location, was close to the depth (1400 m) on the well logs.