Abstract: Seismic Exploration Leading to Discovery of Trap Spring Field, Nevada

John H. Vreeland, Bert H. Berrong

Seismic exploration for Tertiary objectives in the Basin and Range province requires recognition of geologic and geophysical problems and use of appropriate field techniques for their solution. These problems are caused by surface geology and topography as well as the three-dimensional aspects of trap resolution. Normal seismic exploration using 6-fold to 12-fold COP does not have the resolving power needed to delineate traps in these complex valleys. The presence of steep dips, complex faulting patterns, velocity anomalies, and basalt flows in the Tertiary and Quaternary valley fill necessitates the use of 24-fold COP. Two-dimensional "X" geophone arrays are used to combat side-energy problems. A dynamite crew is utilized for seismic bandwidth and because large vehicles would be immobilized in much of the area. Once meaningful seismic data of high quality have been obtained in the field, the data processor must contend with the steep dips, complex faulting, and enormous velocity gradients in two dimensions.

Problems encountered in the interpretation are caused by shallow basalt flows and the thinning of the volcanic package. Determination of fault placement, fault orientation, and rock package also are difficult. Both Trap Spring and Eagle Springs fields are related in that both exhibit thinning of the ignimbrite package near the basin margin with some fault closure involved. Proper interpretation requires that in-depth geologic knowledge be applied during the data processing. Velocities determined from seismic data are almost useless to determine rock types, for there is extreme variability and an overlapping of rock velocities. The faulting in Railroad Valley is extremely complex, with numerous small faults, some of which are related to flexures in the volcanic rocks. Seismic determination of sealing versus nonsealing faults is impossible. Many of the faults mapped can be related to surface lineations, thus strongly tying seismic investigations to geomorphology.

AAPG Search and Discovery Article #90963©1978 AAPG/SEG/SEPM Pacific Section Meeting, Sacramento, California