A METHOD FOR INVERSION OF TWO-DIMENSIONAL SEISMIC REFRACTION DATA WITH APPLICATIONS TO THE SNAKE RIVER PLAIN REGION OF IDAHO

GREGORY JAY ELBRING, Purdue University

Abstract

With the growing amount of seismic refraction data being collected in structurally complex regions, the need for a simple and rapid method for modeling the laterally inhomogeneous velocity structures of such areas has also increased. To aid in solving this problem, a method is proposed and computer code developed for the damped least-squares, or Marquardt, inversion to obtain two-dimensional velocity structure from the first arrival times of seismic refraction surveys. The velocity structure is output as discreet velocities at the intersections of an evenly- or unevenly- spaced grid.^ The inversion method is then tested on two known models under a variety of conditions to test the effects of number of sources, velocity constraints, errors in travel-time data, errors in fixed velocities, and grid spacing, in addition to examining how well the output defines the original velocity structure.^ The method is then applied to data from three seismic refraction profiles collected as a part of the 1980 Yellowstone-Snake River Plain seismic refraction experiment. The resulting velocity models exhibit several interesting features about the relationship of the SRP with the Northern Rocky Mountains to the north. Velocity structures in the Precambrian and Paleozoic sedimentary rocks north of the plain indicate no deformation related to the emplacement of the SRP, but do display significant pre-Tertiary folding and faulting. At the northwest margin of the SRP, a large normal fault or series of step faults is modeled. This fault, downthrown on the side of the plain, displays a minimum of 2 km of displacement. Lastly, a significant increase in thickness of the surface basalts of the SRP to nearly 3 km is seen as the edge of the plain is approached indicating contemporaneous deposition of the basalts with the motion on the fault or, alternatively, differential topographic subsidence and infilling with basalts due to more rapid cooling along the margins of the SRP. ^

Degree

Ph.D.

Subject Area

Geophysics

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