High resolution seismic reflection PP and PS imaging of the bedrock surface below glacial deposits in Marsta, Sweden

Multi-component high resolution seismic reflection data were acquired at the Uppsala University field test site in Marsta, Sweden, in March 2019 with the aim of obtaining an improved understanding of the subsurface structure in the area. An advantage of the site is that a number of boreholes have been drilled there for both hydro geological and geophysical purposes, allowing surface geophysics to be compared with downhole information. The presence of a low velocity layer above the water table generates significant processing challenges due to trapped waves. A further complication with the data is that the upper part of this layer was frozen at the time of the survey, resulting in the sediments just below the surface having a significantly higher velocity than those below. By analyzing and processing both the vertical and radial component data it was possible to build a velocity model that is consistent with the observed data. A strong PS converted reflection allows the bedrock to be imaged on the radial component stacks to much higher resolution than on the vertical component stacks. Both common conversion point binning and pre-stack depth migration were used to process the radial component data. We confirmed that our processing strategy was effective by generating synthetic data that were processed in a similar manner as the real data. The PS images indicate a step in the bedrock of about 2 m, with depth to it increasing from about 15 m to 17 m close to one of the boreholes. This step is not observed on the PP stacks, due to their lower resolution.