Feasibility Study of the Seismic Reflection Method in Amargosa Desrt, Nye County, Nevada

This report presents a detailed description of a field trial comparing explosive and Vibroseis (Trademark of Conoco, Inc.) methods for acquiring deep crustal seismic reflection profiles in Nye County, Nevada. The field work was performed during January, 1988, in anticipation of a regional deep crustal reflection survey to be conducted as part of broad geological investigation of a proposed high-level nuclear waste facility in southwestern Nevada. Field trials were conducted at two sites within the Amargosa Desert, located in the Basin and Range province just to the east of Death Valley, California. Extensive testing of the Vibroseis method was performed to establish optimal acquisition parameters for the Vibroseis source. On completion of this exhaustive suite of trials, a 60-fold Vibroseis profile along a 27-km-long line was acquired using a 12-km-long symmetric split spread. Single 91-kg (200 lb.) explosive charges were fired in 76 m (250 ft.) deep shot holes along the Vibroseis profile to generate a coincident singlefold explosive source reflection profile. Within the upper crust (upper 5 s two-way travel time) the explosive and Vibroseis common-shot records are nearly equivalent. The 60-fold Vibroseis section for the upper 5 s is of high-quality and clearly shows structures related to extension inferred from coincident seismic refraction and gravity profiles. Below 5 s, however, the final sections for the coincident explosive and Vibroseis sources clearly document that the single-fold explosive source section provides a higher-quality image of the lower crust (5-10 s) due to the higher seismic energy levels produced by the explosive sources, the virtual absence of the air wave arrival, and the reduction of the ground roll arrival on the explosive data. The explosive source profile reveals subhorizontal, discontinuous reflections between 5 to 10 s. Based on older, and limited seismic refraction data, the crust was believed to be about 35 km thick. On the reflection profiles presented here, the base of the crust is defined only by the absence of high-amplitude reflected arrivals below 9 to 10 s.