Compilation of a Resistivity Atlas of Danish lithologies based on direct resistivity measurements and wireline logging data

Abstract

Electrical conductivity, or its inverse, the resistivity, is an important geophysical property within groundwater mapping. It is known to correlate empirically to lithology, primarily through clay minerals and pore water ions. Although, in Denmark, geoelectric and electromagnetic surveys have been carried out for decades, no systematic, nationwide study on the relationship between resistivity and lithology has been carried out. We present a procedure for generating a resistivity atlas based on resistivity measurements, which can be related directly to specific and well described soil samples. Data are obtained from archives, literature and the Danish national databases. The procedure implies a restricted use of wireline logging data in combination with direct measurements on samples, resulting in resistivity distributions for specific lithologies or geological formations. The use of documented high-quality data ensures reliable results, reflecting actual resistivity of a specific lithology. This procedure is illustrated on clay till. The resistivity variations obtained for this lithology seems to be related to real compositional variations, which reflect the process of forming the clay till. Our procedure is likely to provide equally reliable results for other main lithologies. Future detailed studies, in particular on sediments with low clay content, should consider resistivity differences related to the degree of saturation and variations in the formation water resistivity.