Reprocessing of high-resolution seismic data for imaging of shallow groundwater resources in glacial deposits, SE Sweden

Reprocessing of high-resolution seismic reflection data over groundwater-bearing glacial deposits near Heby, southeastern Sweden, improved the images of near-surface structure at this site. Post-stack time migration and pre-stack depth migration were tested and compared to determine the improvements on imaging an undulating bedrock surface. The pre-stack depth migration image displays better continuity of the dipping structures within the glacial sediments and provides a more detailed topography of the bedrock reflector. First-arrival picks were used to define an initial model for input into tomographic inversion. The tomography result then formed the basis for building the migration velocity model. The final pre-stack depth migration image shows a strong reflection at around 35 m elevation (about 9 m below the surface) that can be correlated to a thin (0.2 m) hard silt layer. The upper 20 m of overburden is interpreted to consist of clay, and the seismic images show weaker sub-horizontal reflections within this unit, except for the strong silt reflection, consistent with our modelling results. Below 20 m, sand/gravel sediments are present and overlay the bedrock. Forward modelling based on the pre-stack depth migration image and subsequent processing shows that pre-stack depth migration provides a higher resolution image compared with post-stack time migration. Our study shows that pre-stack depth migration is preferable to post-stack time migration even for the shallow near-surface seismic data acquired at Heby, and that integrating tomography, migration, modelling and geological information provides a better understanding of the structure which these groundwater resources are contained in.