Near Surface High Resolution Seismic Imaging of Glacial Deposits in Sweden at the Heby and Marsta Sites

Sammanfattning: Near surface seismic methods have the potential for mapping theoverburden and bedrock in the Nordic environment. They can provideeffective information about shallow glacial deposits and groundwaterresources in Sweden. We analysed seismic data from the Heby andMarsta sites with the aim of improving the imaging resolution andunderstanding the substructures better, developing systematicstrategies to deal with the detection of aquifers and the delineation ofsome significant boundaries. The general scheme for every case studyis composed of processing, inversion and forward modelling.Processing strategies for the Heby and Marsta have different keypoints, because of the different acquisition equipment and systems.The Heby data were acquired by dynamite and collected withconventional geophones, while the Marsta data were acquired by asledge hammer source and a 3C data landstreamer with MEMSsensors. Inversion can be used for enhancing image quality,particularly in velocity model building. Strongly undulatedsubstructures in the Heby profile cause ambiguities and conflictingdips in the stacked section. By employing the tomographic inversionresult as a starting point to implement prestack depth migration(PSDM) the final image shows better continuity at the top of bedrockand shallow layers are resolved clearer. At Marsta, vertical and radialcomponent data were used to obtain PP and PS information,respectively, to delineate the bedrock surface. Common conversionPoint (CCP) binning of the radial component identifies the surface inbetter resolution, even a 2m variation in the depth of the bedrock canbe seen. PSDM radial component data supports and complements theresult from the CCP binning. In both the Heby and Marsta cases,forward modelling was applied for testing processing workflows forthe field data using models that mimick the real substructures.Acoustic modelling was used for understanding the P-wave responsein the Heby single component data, while elastic wave modelling wasperformed to simulate the multicomponent data at Marsta. The resultsdemonstrate that a comprehensive application of seismic methods inthe near surface (shallower than 100m) can provide adequateresolution. Noteworthy is that an accurate velocity estimation plays asignificant role in the whole scheme for seismic imaging. Thestrategies for building the velocity are also meaningful in thisresearch.