Seismic tomography studies of cover thickness and near-surface bedrock velocities

Reflection seismic imaging of the uppermost kilometer of crystalline bedrock is an important component in site surveys for locating potential storage sites for nuclear waste in Sweden. To obtain high-quality images, refraction statics are calculated using first-break traveltimes. These first-break picks may also be used to produce tomographic velocity images of the uppermost bedrock. In an earlier study, we presented a method applicable to data sets where the vast majority of shots are located in the bedrock below the glacial deposits, or cover, typical for northern latitudes. A by-product of this method was an estimate of the cover thickness from the receiver static that was introduced to sharpen the image. We now present a modified version of this method that is applicable for sources located in or on the cover, the general situation for nuclear waste site surveys. This modified method also solves for 3D velocity structure and static corrections simultaneously in the inversion process. The static corrections can then be used to estimate the cover thickness. First, we test our tomography method on synthetic data with the shot points in the bedrock below the cover. Next, we develop a strategy for the case when the sources are within the cover. The method is then applied to field data from five crooked-line, high-resolution reflection seismic profiles ranging in length from 2 to 5 km. The crooked-line profiles make the study 2.5 dimensional regarding bedrock velocities. The cover thickness along the profiles varies from 0 to 15 m. Estimated thickness of the cover agrees well with data from boreholes drilled near the profiles. Low-velocity zones in the uppermost bedrock generally correlate with locations where reflections from the stacked sections project to the surface. Thus, the method is functional, both for imaging the uppermost bedrock velocities as well as for estimating the cover thickness.