Seismic Shear Wave Investigations for Fracture Mapping in Bedrock Controlled Hydrology

C. Richard Bates

Geophysical techniques that have been applied in the past for mapping shallow fractures that control local hydrogeology have included resistivity surveys, heat flow, ground penetrating radar, magnetometry, and seismics. However, few attempts have been made to map shallow fractures using the full potential of shear wave technologies in a similar manner to deeper oil and gas studies. With the increased power of engineering seismographs, and PC processing packages, the oil and gas technology is finally available and is now cost effective for use in shallow hydrogeological fracture studies. This paper describes in further detail the theoretical background and practical application oi shear waves as a tool for fracture mapping. Specific case histories demonstrate the potential of the technique. A horizontally polarized shear wave refraction survey conducted in Florida showed a bedrock variation in velocity with sheaf wave orientation of 21 % from which the regional fracture trend was predicted. In Colorado a common offset shear wave refraction survey identified are anomalous region near a leaking hydrocarbon tank which was correlated with a

major fracture zone. At both sites fracture analysis was critical to ground water evaluation. Surface and borehole shear wave surveys measure properties over greater bulk volumes than drilling programs, thus in the case of fractures which represent only a small volume compared to the whole rock mass the potential for encountering the fractures is greatly increased. By using shear wave techniques in hydrological studies, a saving in both time and money can be realized in overall site characterization and in the locating of wells.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California