IMAGING OF REFLECTION SEISMIC ENERGY FOR MAPPING SHALLOW FRACTURE-ZONES IN CRYSTALLINE ROCKS

The high-resolution reflection seismic technique is being used increas ingly to address geologic exploration and engineering problems. There are, however, a number of problems in applying reflection seismic tech niques in a crystalline rock environment. The reflection seismic data collected over a fractured crystalline rock environment are often char acterized by low signal-to-noise ratios (S/N) and inconsistent reflect ion events. Thus it is important to develop data processing strategies and correlation schemes for the imaging of fracture zones in crystall ine rocks. Two sets of very low S/N, high-resolution seismic data, pre viously collected by two different contractors in Pinawa, Canada, and the island of Aspo, Sweden, were reprocessed and analyzed, with specia l emphasis on the shallow reflection events occurring at depths as sha llow as 60-100 m. The processing strategy included enhancing the signa ls hidden behind large-amplitude noise, including clipped ground roll. The pre- and poststack processing includes shot f-k filtering, residu al statics, careful muting after NMO correction, energy balance, and c oherency filtering. The final processed seismic sections indicate that reflected energy in these data sets is closely related to rock qualit y in Aspo data and fracturing in Atomic Energy of Canada, Ltd. (AECL) data. The lithologic boundaries are not clearly mappable in these data . When thickness of the reflection zone is of the order of a wavelengt h, the top and bottom of the zone may be resolved. The major fracture zones in crystalline rocks correlate closely with the well-log data an d are usually characterized by very low velocity and produce low-acous tic-impedance contrasts compared to those of surrounding rocks. Becaus e the incidence angles vary rapidly for shallow-reflection geometries, segments of major fracture zones can effectively be analyzed in terms of reflectivity. Reflection images of each fracture zone were investi gated in the common-offset section, where each focused event was assoc iated with a consistent incidence angle on the reflectivity map. The c omplex attributes of the data indicate that strong reflectors at shall ow depth coincide with intensely fractured zones. These correlate well with instantaneous amplitude plots and instantaneous frequency plots. The instantaneous phase plot also identifies the major and minor frac tures.