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

Abstract

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 [Formula: see text] long) and with a [Formula: see text] receiver array centered at the intersection of the lines sampled a [Formula: see text] 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 [Formula: see text] filtering scheme to suppress any remaining direct [Formula: see text]-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 [Formula: see text] are concentrated in the center of the survey area although long offsets [Formula: see text] 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°. 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 [Formula: see text]. Previous 2D seismic-reflection studies and borehole data indicate that fracture zones are the most likely cause of the reflections.