Abstract
Abstract. High-resolution reflection seismic methods are an established non-destructive tool for engineering tasks. In the near surface, shear-wave reflection seismic measurements usually offer a higher spatial resolution in the same effective signal frequency spectrum than P-wave data, but data quality varies more strongly. To discuss the causes of these differences, we investigated a P-wave and a SH-wave seismic reflection profile measured at the same location on the island of Föhr, Germany and applied seismic reflection processing to the field data as well as finite-difference modelling of the seismic wave field. The simulations calculated were adapted to the acquisition field geometry, comprising 2 m receiver distance (1 m for SH wave) and 4 m shot distance along the 1.5 km long P-wave and 800 m long SH-wave profiles. A Ricker wavelet and the use of absorbing frames were first-order model parameters. The petrophysical parameters to populate the structural models down to 400 m depth were taken from borehole data, VSP (vertical seismic profile) measurements and cross-plot relations. The simulation of the P-wave wave-field was based on interpretation of the P-wave depth section that included a priori information from boreholes and airborne electromagnetics. Velocities for 14 layers in the model were derived from the analysis of five nearby VSPs (vP =1600–2300 m s-1). Synthetic shot data were compared with the field data and seismic sections were created. Major features like direct wave and reflections are imaged. We reproduce the mayor reflectors in the depth section of the field data, e.g. a prominent till layer and several deep reflectors. The SH-wave model was adapted accordingly but only led to minor correlation with the field data and produced a higher signal-to-noise ratio. Therefore, we suggest to consider for future simulations additional features like intrinsic damping, thin layering, or a near-surface weathering layer. These may lead to a better understanding of key parameters determining the data quality of near-surface shear-wave seismic measurements.
Highlights
Near-surface geophysical methods constitute a nondestructive means to investigate the shallow subsurface
To discuss the causes of these differences, we investigated a P-wave and a SH-wave seismic reflection profile measured at the same location on the island of Föhr, Germany and applied seismic reflection processing to the field data as well as finite-difference modelling of the seismic wave field
The petrophysical parameters to populate the structural models down to 400 m depth were taken from borehole data, vertical seismic profiles (VSPs) measurements and cross-plot relations
Summary
Near-surface geophysical methods constitute a nondestructive means to investigate the shallow subsurface. There has been a lot of progress in its evolution, and it can be applied routinely for simple subsurface structures, especially in marine environments, its application for near-surface studies is still experimental (Romdhane et al, 2011; Groos et al, 2012; Plessix, 2012). This has to do with the importance of a good starting model for the inversion process and with the large parameter space in the near surface. To assess the lower signal-to-noise ratio of the recorded shearwave field data, we simulated SH-wave seismograms on the basis of this model to comprehend quality variations in the SH-wave reflection field data
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