Subsurface water-filled fracture detection by borehole radar: A case history

Operating radar equipment in boreholes offers the possibility of greater depth penetration and higher resolution than is achievable with surface-based ground-penetrating radar systems. We have acquired single-hole radar reflection data in a series of vertical boreholes situated within a granite body west of Beijing, China. Geological logs demonstrate that numerous fractures intersect the boreholes. After processing the single-hole reflection data, linear reflections from many of these fractures are observed. In addition, a large number of linear features that do not intersect the boreholes are interpreted as fracture reflections; they can be traced to distances of 20-30 m from the boreholes. Information from single-hole radar reflection data allows the minimum lengths, dips and distances of planar fractures from boreholes to be estimated, but not the azimuths. To determine the azimuths, information from two or more boreholes is required. For our survey site, this was achieved for three of the fractures.