APPLICATION OF HIGH-RESOLUTION SEISMIC-REFLECTION TECHNIQUES FOR FRACTURE MAPPING IN CRYSTALLINE ROCKS

Surface reflection seismic techniques have the capability of mapping s ubsurface geological features without disturbing the rock mass. They a lso have an added capability of penetrating to a much deeper depth tha n any other geophysical technique, including the ground probing radar. However, the successful application of reflection seismic techniques in crystalline rocks has in general been more difficult than in sedime ntary basins, because of the irregular geometry and low acoustic imped ance contrasts across geological boundaries. In this paper, we describ e the imaging of fracture zones in crystalline rocks. Effective data p rocessing, carefully modified from the conventional approaches, was ap plied on two high-resolution field data previously collected by differ ent contractors. The strategy included enhancement of the signal hidde n under the large-amplitude scattering noise, through pre- and post-st ack processing such as shot f-k filtering, residual statics and carefu l muting after NMO correction. Two sets of low S/N test data from Cana da and Sweden are analyzed in this research. The reflected energy in t hese data sets appeared to be more closely related to fracturing than to lithologic boundaries. The major fracture zones at shallow depth ha ve been mapped with the desired resolution and can be correlated to th e available well-log and seismic crosshole tomographic data. Once the surface waves were removed, shallow reflectors in the fracture zones c ould be identified and analyzed on the field records. Focusing analysi s of the seismic image was performed in the constant-offset section to investigate the trends of major fracture zones. The complex attribute s were also analyzed to determine whether they could be applied to the shallow fracture zones. Instantaneous frequency plots outline the int ense fracturing zone and instantaneous phase plots identify the major and minor fractures, and other coherent events with different dip atti tudes which interfere with each other.