Airborne Gradiometer Survey For The Proposed San Andreas Fault Drill Site

William Marsh Rice University, Houston TX

Investigators

Abstract

0132137 Talwani Airborne gravity gradiometry is a technology that has only recently been developed, but which has enormous potential. It is capable of making absolute gravity gradient measurements of great precision very rapidly over almost any kind of terrain, and at a relatively modest cost. It could well revolutionize measurements involving gravity for various types of studies focused on the shallow crust, and because the instrument determines absolute values, for time lapse studies as well. The former could include, for example, studies involving the detection of blind faults, determination of the thickness of sedimentary fill or of basalt flows and lithologic contacts in densely vegetated areas as well as environmental and mineral exploration related problems. The latter could help trace fluid flow in the shallow crust or magma movement in volcanoes, as well as other movements of material within the shallow crust on the human time scale. Only two airborne gradiometers are actually operational today. Both are based on Lockheed Martin's GGI gradiometer and have been adapted for airborne work jointly by Lockheed Martin and BHP. It is clear that this technology is in its infancy and some preliminary experimentation is necessary in order to test its suitability for crustal work. This project involves an aerial gradiometer survey of the proposed drill site for the San Andreas Fault Observatory at Depth (SAFOD), a component of EAR's Earthscope Initiative. SAFOD will involve drilling a 4 km deep hole which will intersect the San Andreas Fault near Parkfield, CA. In addition to helping determine the deep structure at the proposed drill site, the fact that the gradiometer makes absolute measurements can be put to another use. It is quite possible that in a time lapse experiment, that is by making two measuremenbts over the identical space separated in time, changes in the subsurface can be monitored. These changes could be the motion of fluids or changes associated with an earthquake. For the latter experiment, it is obviously important to establish a base line measurement before the earthquake. If a number of measurements are made it may be also possible to see if fluid motions in the shallow subsurface are reliable precursors to earthquakes. ***