USING CROSS-HOLE GPR REFLECTIONS TO IMPROVE TOMOGRAPHIC IMAGING AND HYDROGEOLOGIC INTERPRETATION

Tomographic radar or seismic surveys consist of acquiring cross-hole data, then inverting first arrival travel times for the velocity structure or inverting direct arrival amplitudes for the attenuation character of the subsurface. However, the data in tomography also may contain valuable information in the form of up-going and down-going reflections from material property boundaries between the wells. Although the reflections are difficult to see in unprocessed records, they can provide important constraints on the interpretation of the tomographic image. For example, tomographic inversions commonly rely on regularizing techniques, such as seeking smooth solutions for the velocity structure, to overcome shortcomings in the acquisition geometry of the experiment and noise in the data. We use reflection information to delineate bound aries in a tomographic image that otherwise would be blurred or removed in processing. We present cross-hole radar reflection data acquired between wells at the Boise Hydrogeophysical Research Site (BHRS), a research wellfield located on a gravel bar adjacent to the Boise River. The site is underlain by ~20m of coarse (cobble and sand), unconsolidated fluvial deposits with a shallow (2-3 m) water table. Stratigraphy interpreted from core and porosity logs at the BHRS suggest a layered sequence and variations in facies and hydraulic parameters within layers. We are exploring ways to supplement direct geologic and hydrologic data at wells with geophysical imaging between wells that can be related to structure of distinct sedimentary bodies (layers, facies within layers) and to parameter distributions within them. In this regard, strong radar reflections in radar tomography data between wells at the BHRS are observed from the water table, a sand channel, and a clay underlying the cobbles and sands. Other, weaker reflections correlate with porosity changes between the sand and clay; we are investigating the relationship of these reflections to previously interpreted units from porosity logs, and to core and hydraulic properties.