Deep electromagnetic sounding in Central Nevada

Abstract

Sixteen s h a l l ow and deep c o n t r o l 1 ed source e l ectromagnetic soundings were performed i n Buena V i s t a Valley, near Winnemucca, Nevada, t o investigate an intra-basement conductor previously detected w i t h magnetotell urics. The survey was c a r r i e d o u t with t h e LBL EM-60 system using a remote magnetic reference f o r 1 ow-f requency geomagnetic noise cancel 1 ation, 100-m-and 2.8-km-diameter t r a n s m i t t e r 1 oops, and a minicomputer for i n-f i e l d processing. EM soundings were made a t distances from 0.5 t o 30 km from three loops over t h e frequency range 0.02 t o 500 Hz. Data were i n t e r p r e t e d by means o f 1-D inversions and the r e s u l t i n g layered models were pieced together t o y i e l d an approximate 2-0 geoelectric model along the N-S a x i s o f the valley. The EM soundings and one MT sounding show a 3 t o 7 oh-m zone a t a depth o f four t o seven km. The conductor appears t o be deepest a t the northern end o f the v a l l e y and shallowest beneath a basement r i d g e t h a t seems t o d i v i d e Buena V i s t a Valley i n t o two basinal structures. Similar intra-basement conductors are a1 so reported 50-75 m i l e s south i n the Carson Sink-Fa1 l o n areas, suggesting a comnon source, probably r e l a t e d t o an anomalously hot, t h i n crust.