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. Geological Survey Fractured Rock Research Site at Mirror Lake, in Grafton County, New Hampshire. The study was conducted to evaluate the benefits of applying multi-offset seismic processing techniques to borehole-radar reflection surveys in fractured rock. The multi-offset reflection surveys were conducted in conjunction with a saline tracerinjection experiment. During injection, a sodium chloride (NaCl) solution was continuously pumped into a hydraulically conductive zone that was isolated by specially constructed, reusable, PVC straddle packers suspended from PVC casing. Eight common-offset borehole reflection profiles were collected within the PVC-sleeved portion of the borehole before and during the tracer injection. The offset between the transmitter and receiver antennas ranged from 6.4 to 9.9 m (meters). The common offset data were filtered, sorted into common distance-point (CDP) gathers, normal move-out (NMO) corrected, and stacked to produce a zero-offset borehole CDP profile. Comparison of the common-offset and CDP profiles indicates that multi-offset data acquisition and CDP processing; (1) increases the resolution of reflectors near the borehole, (2) decreases the effects of direct wave coupling, antenna ringing, and system noise, and (3) improves the clarity of difference images used to identify the effects of saline tracer on reflections from transmissive fractures. INTRODUCTION Single-hole, borehole-radar reflection surveys typically are conducted using commonoffset methods (fixed distance between the transmitter and receiver) (fig. 1) (Olsson and others, 1992; Haeni and others, 1993; Gaylor and others, 1994; Lane and others, 1994; Hansen and Lane, 1995; Lane and others, 1996). Although single-hole borehole-radar reflection surveys can provide information about the location, extent, and orientation of fractures and fracture zones that are within the radar range, the relatively large minimum transmitter-receiver offset makes it difficult to image near-borehole structures. In addition, reflections from structures close to the borehole are frequently obscured by the direct wave that propagates between the transmitter and receiver and by antenna ringing phenomena. Use of multi-offset data acquisition and processing methods can provide a means to (1) suppress direct arrival and instrument noise in order to