Ultrasonic logging across unconformities — outcrop and core logger sonic patterns of the Early Triassic Middle Buntsandstein Hardegsen unconformity, southern Germany

Abstract

The recognition of unconformities in thick and monotonous sandstone successions of consistent grain size is often difficult in locally exposed surface outcrop sections and also poorly constrained by changes of petrophysical properties across the boundary surfaces. Here, a mobile ultrasonic device is tested as a time-efficient handheld logging tool to investigate relative changes in sandstone cementation and porosity across the Early Triassic Middle Buntsandstein Hardegsen unconformity in southern and central Germany. A persistent offset of 18–20% sonic transit time occurs in all tested sections across the unconformity. This offset is not restricted to single peaks but clearly separates the different sonic patterns of the Hardegsen and Solling Formations below and above the unconformity. Integrated thin section analysis, porosity–permeability measurements and gamma-density logging suggest that these changes in sonic transit time are due to major variations in sandstone cementation and the formation of secondary porosity associated with the unconformity. Field-based ultrasonic measurements thus provide a useful addition to established outcrop logging methods and may help in the non-destructive in-situ identification of porosity/cementation heterogeneities in sandstones, in the localization of unconformities and in the correlation of borehole measured sequence boundaries to outcrop analogs.