REDUCING UNCERTAINTY IN THE HAZARD OF VOLCANIC DISRUPTION OF THE PROPOSED YUCCA MOUNTAIN RADIOACTIVE WASTE REPOSITORY USING A HIGH-RESOLUTION AEROMAGNETIC SURVEY TO DETECT BURIED BASALTIC VOLCANOES IN ALLUVIAL-FILLED BASINS

Yucca Mountain lies within the southern Great Basin, a region characterized by infrequent episodes of small-volume basaltic volcanism during the past five million years. An expert elicitation convened in 1995-1996 by the Department of Energy (DOE) estimated the mean hazard of volcanic disruption of the repository as slightly greater than 10^-8 dike intersections per year with an uncertainty of about two orders of magnitude. Basaltic volcanism within 20 km of Yucca Mountain is associated with alluvial-filled structural basins. Boreholes penetrating alluvium have encountered three buried basalt flows at depths of 100 to 400 meters and ranging in age from 3.8 to 11.3 Ma. Previous aeromagnetic surveys suggest that as many as 20 additional magnetic anomalies near Yucca Mountain may represent buried basaltic volcanoes. Sensitivity studies based on the expert-elicitation framework indicate that if all magnetic anomalies represent buried volcanoes, mean estimates of the hazard would increase by about 40%, which is insignificant compared to the range of uncertainty. If additional, undetected buried volcanoes exist in the alluvial basin to the east of Yucca Mountain, the mean hazard could increase by an order of magnitude. In order to reduce the uncertainty in hazard estimates and assess earlier estimates, DOE is sponsoring 1) a new high-resolution, helicopter-borne aeromagnetic survey, completed in mid-2004, 2) drilling of selected anomalies based on survey results to better characterize the number, location and age of buried volcanoes, scheduled for late 2004, and 3) an update to the 1996 expert elicitation to reassess the hazard estimate when new data for buried volcanoes are available.

The new aeromagnetic survey provides much higher spatial resolution than previous surveys and covers an area of 865 km², including Yucca Mountain and adjacent basins to the east, west and south. 16,000 km of data lines were flown at a spacing of 60 meters and instrument altitude of 30 meters. Principal instrumentation was a total-field magnetometer and a broadband (450 Hz – 30 kHz) electromagnetic sensor. The high resolution of the survey allows detection of igneous features smaller than previously detectable, better interpretation of the source of anomalies, and better interpretation of the relationship between volcanism and faulting.