USE OF CONTINUOUS SEISMIC PROFILING TO DIFFERENTIATE GEOLOGIC DEPOSITS UNDERLYING MAJOR CANALS IN WESTERN AND CENTRAL NEBRASKA

Continuous seismic-reflection profiling (CSP) produces digital data capable of classifying subsurface lithology in a variety of marine, estuarine, riverine, and lacustrine settings. In August 2001, the U. S. Geological Survey, in cooperation with the Platte River Cooperative Hydrologic Study group, conducted a CSP survey in western and central Nebraska. CSP is useful in assessing the depth and profile of the bedrock surface underlying unconsolidated deposits and to determine the thickness of overlying unconsolidated deposits.

Typically test holes and surficial geologic mapping determine the depth and shape of the bedrock surface underlying unconsolidated deposits. However, this information is often either not in the area that needs to be defined or data locations are spaced too far apart to define adequately the bedrock surface. These problems lead to estimates of both the depth of the bedrock and the bedrock profile; however, these estimates can lead to incorrect assessments of the thickness of saturated zones used in numerical ground-water-flow models, which can accentuate inherent errors to a ground-water-flow model.

Preliminary interpretation of data collected for approximately 26 of the 90 miles surveyed using CSP has been completed, and showed that about 70 percent of the data examined so far exhibited reflectors corresponding to general unconsolidated depositional features. Alternatively, 23 percent of the data examined showed continuous definable reflectors that likely could be classified as bedrock and could be correlated with borehole drillers-log and geophysical log data.

The preliminary results indicate that the presence of gravel and boulders in unconsolidated materials beneath the canal probably absorbed and reflected most of the available acoustic energy, thereby weakening penetration of seismic signals in some areas. Most of the signals generated during data collection in seasonal canals (predominant in western Nebraska) did not penetrate the sediments adequately to identify specific geologic features, but were absorbed in the hard bed of the seasonal canals. In central Nebraska, data collected in perennial canals locally indicated reflectors corresponding to bedrock and produced the best results in the study.