Use Of A Multi-Offset Borehole-Radar Reflection Method In Fractured Crystalline Bedrock At Mirror Lake, Grafton County, New Hampshire

Multi-offset, single-hole, borehole-radar reflection surveys were conducted at the U.S.<br>Geological Survey Fractured Rock Research Site at Mirror Lake, in Grafton County, New<br>Hampshire. The study was conducted to evaluate the benefits of applying multi-offset seismic<br>processing techniques to borehole-radar reflection surveys in fractured rock.<br>The multi-offset reflection surveys were conducted in conjunction with a saline tracerinjection<br>experiment. During injection, a sodium chloride (NaCl) solution was continuously<br>pumped into a hydraulically conductive zone that was isolated by specially constructed, reusable,<br>PVC straddle packers suspended from PVC casing. Eight common-offset borehole reflection<br>profiles were collected within the PVC-sleeved portion of the borehole before and during the<br>tracer injection. The offset between the transmitter and receiver antennas ranged from 6.4 to 9.9<br>m (meters). The common offset data were filtered, sorted into common distance-point (CDP)<br>gathers, normal move-out (NMO) corrected, and stacked to produce a zero-offset borehole CDP<br>profile.<br>Comparison of the common-offset and CDP profiles indicates that multi-offset data<br>acquisition and CDP processing; (1) increases the resolution of reflectors near the borehole, (2)<br>decreases the effects of direct wave coupling, antenna ringing, and system noise, and (3)<br>improves the clarity of difference images used to identify the effects of saline tracer on<br>reflections from transmissive fractures.