Geophysical investigations of buried Quaternary valleys in Denmark: An integrated application of transient electromagnetic soundings, reflection seismic surveys and exploratory drillings

Buried Quaternary valleys are important hydrological structures in Denmark. Geophysical and geological investigations were performed to develop an integrated interpretational methodology for a quantitative description of their structure and lithology. Three buried valleys in central eastern Jutland, Denmark were investigated using the transient electromagnetic (TEM) sounding method, two-dimensiona reflection seismic profiling, vertical seismic profiling (VSP) and analyses of data and samples from exploratory drillings. The most advantageous approach for fieldwork is the sequence of (1) resistivity mapping with TEM to identify the buried valleys, (2) reflection seismic profiling across the valleys, (3) location and drilling of exploratory drillings, and (4) vertical seismic profiling. The accuracy of the geological interpretation is improved substantially by combining the structural information from the seismic profiling with the predominantly lithological information derived from the TEM soundings. Sequential interpretation of the data sets is optimal when variations in geology are low to moderate. Strong variations may, however, lead to breakdown of the interpretation models. Buried valleys occur primarily as cut-and-fill structures filled with Quaternary deposits consisting of till, glacio-lacustrine clay/silt and meltwater sand and gravel. The seismic velocity of the tills is considerably higher (2150 m/s) than it is for the other valley-fill sediments (1750 m/s). Resistivities of the clays are distinctly less than those of sands and gravels. Hence, classification of these parameters gives indirect information about the lithology of the valley fill as well as the structure.