Ultra‐shallow seismic and GPR methods applied on sand‐covered limestone in northern Jutland, Denmark

This report gives an analysis and a geophysical interpretation of an ultra‐shallow high‐resolution seismic profile recorded on sand‐covered limestone. The interpretations include velocity and structural elements characterising the limestone surface. The seismic profile consists of short offset gathers with a very limited number of traces and with a shot‐point distance along the profile giving presumed equidistant reflection points on the subsurface interfaces. By subsequent band‐pass filtering and NMO‐correction, this field set‐up is found to be sufficient to create a detailed image of the immediate subsurface down to 20 m depth on the investigation site. This recording procedure facilitates a considerable increase in profiling speed compared to the denser shot‐point recordings normally required to obtain CDP gathers. A small number of additional larger gathers is recorded in order to verify that target reflectors are within the data window and source generated noise is outside. Interval velocities are inferred from a subset of the shot gathers by means of normalised semblance analyses demonstrating that critically refracted signals are not present in the gathers. Two major reflecting interfaces are detected, one in the sand above the limestone and the other on the top of the limestone, and there is a fault in the limestone. In addition, a small depression and possible cracks/karst formations in the limestone surface are visible. Anomalies in the amplitude of a reflector in the sands along the profile can be linked with the cracks/karst formations. Multiples are formed in the sand layers above the limestone, but they can be attenuated by predictive decon‐volution. The seismic velocity field indicates progressively higher water content in the sand layers above the limestone along the profile. Georadar facies descriptions provide additional insight into the more recent geological development on the investigation site. A georadar profile recorded near the limestone fault has penetration depth and resolution high enough to yield a facies model supporting geological interpretations which include the identification of aeolian sands on top of marine beach sand deposits. Furthermore, beach foreland and buried remains of sand dunes can be identified by their structural facets as derived from the georadar interpretations. The top of the marine beach sand is identified on the seismic and the georadar profiles and may be associated with the Littorina Sea bottom in the sand layers above the limestone.