Surface‐to‐tunnel seismic tomography studies at Yucca Mountain, Nevada

Abstract

A surface‐to‐tunnel seismic survey was conducted to estimate fracture intensity and distribution in the proposed nuclear waste repository area at Yucca Mountain, Nevada. A 5‐km‐long source line and a 3‐km‐long receiver line were located on top of Yucca Mountain ridge and inside the Exploratory Study Facility (ESF) tunnel, respectively. Numerical modeling showed that the first arrival P waves are mainly propagated in the repository horizon at depth. Therefore two‐dimensional travel time inversions were performed after the actual source locations on the ridge were projected onto the layer of interest at depth. Static corrections were applied to absorb the scatter in the new source locations and to correct for variations in source and receiver coupling and travel time shifts. Zero‐offset amplitude variations of the first arrivals across the receiver line inside the tunnel showed strong correlation with the fracture density determined by measurements along the tunnel wall. Wave guidance by subhorizontal fractures is suggested as a possible source for this phenomenon. Tomographic inversion of the travel time data revealed a low‐velocity zone in the south central area of the repository horizon. Conversion of the velocity tomography results to fracture‐density tomograms showed good correlation with fracture intensity mapped along tunnel walls. The fractured zone extends over a large area in the southern region of the potential repository, in particular, southwest of the intensely fractured zone encountered in the ESF tunnel. The rock in the northern section of the repository appears more competent compared to the highly fractured areas in the south.