Evaluation of Fault Scarp at Harlan County Lake, Harlan County, Nebraska using High Resolution Seismic Reflection Surveying

Richard D. Miller
Donald W. Steeples

SUMMARY
Shallow high resolution seismic techniques were used in an attempt to delineate the subsurface expression of a fault-like displacement exposed in a loess cliff on the Harlan County Lake, a reservoir in south-central Nebraska. Regardless of data-processing flows to compensate for a very irregular bedrock surface, it appears one or more faults may have offset consolidated rocks between 70 and 500 m of depth immediately north of the scarp expression in the wave cut shoreline. Seismic reflection data in a CDP format were acquired along the base of the cliff within 20m of the scarp. The seismic line ran along the water’s edge, which for the past three years was at the base of the cliff as the Corps of Engineers maintained the water at near record levels for irrigation and flood control. Preliminary walkaway tests at the site included using several different types of sources and source configurations which were limited by access problems and extremely wet near-surface conditions. Test data were evaluated for resolution, signal-to-noise ratio, depth range of imaging, and optimum equipment and parameters for effective profiling of a fault with offset as small as a couple meters. Based on CDP processed data and on exposure studies of the fault plane, the fault seen in loess at the surface does not connect in an obvious way to the faults in the subsurface. The subsurface faulting could be primarily normal or reverse, although the reverse interpretation seems to fit the seismic reflection data better.
INTRODUCTION
Shallow high resolution seismic techniques possess the necessary resolution potential within the saturated loess that borders Harlan County Lake to detect any abrupt displacement of around 3 m or more in otherwise continuous reflectors at depths between 70 and 500 m (Miller et al., 1992; Miller and Steeples, 1991). Shallow P-wave reflection surveys have routinely been successful in imaging faulted rock at shallow depths (< 100 m) as well as within overlying unconsolidated sequences (Miller et al. 1992; Treadway et al. 1988; Myers et al. 1987). A single 48-fold CDP seismic-reflection line was acquired along the face of the east-facing cliff marking the western perimeter of Bone Cove (Figures 1, 2). The data were collected to take advantage of the multifold redundancy and therefore the noise suppression potential of CDP data processing techniques (Mayne, 1962). The line was located so equal portions of the profile were north and south of the surface trace of the fault in the cove. The abnormally high reservoir level limited the potential lateral extent of the seismic profiles. Meaningful correlation of two-way travel time reflections on CDP stacked sections to specific geologic units requires an accurate average velocity that only a surface-to-borehole or borehole-to-surface acoustic survey can provide, so our results are lacking absolute depth accuracy.

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Figure 2. The wave-cut exposure of the loess and slip planes. The arrows indicate the location of the slip planes. The left (south) side isdownthrown, offsetting the buried soils of the Gilman Canyon Formation by 1.8 m.

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