Figure

Sandia National Laboratories is the technical manager for a Department of Energy project directed toward selection and characterization of a potential in-situ coal gasification site in the State of Washington. Prior geophysical investigations at this site in the Centralia-Chehalis coal district included in-line and broadside refraction surveys, VLF and Turam electromagnetic surveys, in-seam seismic wave propagation, and geophysical logging of exploratory boreholes. The results of these surveys, presented at last year's SEG meeting, indicated several anomalous features which we interpreted as faulting. Detailed description of the existing structure, however, could not be determined from the earlier surveys.

A high-resolution, seismic reflection survey, reported here, was later conducted to detail the site geologic structure and, thereby, determine areal continuity of objective coal seams. Approximately 12,000 ft (3,658 m) of survey line was obtained over this 62-acre (25 ha.) site, seeking to define the first 1,000 ft (305 m) of the subsurface. A 30-ft (9 m) receiver interval was employed; a portable "shotgun" source supplied seismic energy yielding a net stack of 1,200% at 15 ft (4.6 m) CDP spacing.

Analysis of the resulting reflection data verifies the existence of faulting across the site as interpreted from earlier geophysical and borehole data. However, the geologic structure is found to be more complex than indicated by the earlier data above. A sample reflection record section is given in the figure. Here, faulting of the "Big Dirty" seam (approximately 170 msec) is evident as are offsets in both deeper and shallower seam. Correlation of prior data and those most recent lead to total site characterization.

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