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Shallow High-Resolution Seismic Reflection to Delineate Upper 400 m around a Collapse Feature in Central Kansas
Shallow seismic reflection techniques successfully delineated reflectors altered by localized dissolution of the 400-m-deep and 125-m-thick Permian Hutchinson Salt Member in central Kansas. The Hutchinson Salt Member underlies ~120,000 km2 of central Kansas and north central Oklahoma. Three nominal 48-fold seismic reflection profiles were acquired around a gradually subsiding sinkhole in central Kansas. Seismic images of the altered salt interval possess more detail and greater resolution potential than do profiles from previous high-resolution seismic reflection attempts to map dissolution altered zones for appraisal of risk, rate, and extent of future surface subsidence. The common depth point (CDP) stacked sections possess a dominant frequency of ~100 Hz at 400 m and therefore a vertical resolution potential of ~3 m. Roof failure and staggered subsidence of units overlying the salt is evident and suggestive of continued subsidence at nonuniform rates. The interpreted presence of both normal and reverse faulting within this 90,000-m2 area is indicative of at least two unique episodes of subsidence, each controlled by dramatically different stress environments. The prominent reflections from within the 125-m-thick salt are from anhydrite layers laterally continuous over several tens of kilometers. These anhydrite layers act as markers on seismic sections to identify clearly the shape and preferential orientation of the disturbed area within the salt. Roof failure and collapse within the salt interval are acoustically represented by a chaotic zone, likely indicative of a rubble area. The >15 msec of relative difference in subsidence between the shallowest reflection and the Stone Corral Anhydrite reflection (an interval of ~250 m) along the southern and eastern portions of this sinkhole suggests subsidence will likely continue in these areas.
A low fold, minimal deployment three-dimensional (3-D) survey designed around common offset and shallow seismic reflection techniques was acquired to evaluate the potential of minimal deployment 3-D and to evaluate better the lateral variability of the interpreted faults at this sinkhole. Significant potential exists for this technique in delineating specific targets critical to shallow site characterization. The high cost and high technological nature as well as many assumptions appropriate for deeper petroleum problems are not realistic or feasible for near surface applications. This data set demonstrates the effectiveness and potential as well as some of the problems of minimal deployment 3-D surveys to delineate major structural features.
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