Shallow 3D seismic-reflection imaging of fracture zones in crystalline rock

A limited 3D seismic-reflection data set was used to map fracture zones in crystalline rock for a nuclear waste disposal site study. Seismic-reflection data simultaneously recorded along two roughly perpendicular profiles (1850 and 1060 m long) and with a 400 x 400-m receiver array centered at the intersection of the lines sampled a 680 x 960-m(2) area in three dimensions. High levels of source-generated noise required a processing sequence involving surface-consistent deconvolution, which effectively increased the strength of reflected signals, and a linear tau-p filtering scheme to suppress any remaining direct SV-wave energy. A flexible-binning scheme significantly balanced and increased the CMP fold, but the offset and azimuth distributions remain irregular; a wide azimuth range and offsets < 400 m are concentrated in the center of the survey area although long offsets (> 800 m) are only found at the edges of the site. Three-dimensional dip moveout and 3D poststack migration were necessary to image events with conflicting dips up to about 40 degrees. Despite the irregular acquisition geometry and the high level of source-generated noise, we obtained images rich in structural detail. Seven continuous to semicontinuous reflection events were traced through the final data volume to a maximum depth of around 750 m. Previous 2D seismic-reflection studies and borehole data indicate that fracture zones are the most likely cause of the reflections.