Gravity field separation and mapping of buried quaternary valleys in Lolland, Denmark using old geophysical data

Abstract

In this paper we utilise the old industrial data for planning new surveys. The overall purpose is a detailed mapping of possible aquifers for the island of Lolland, Denmark. This is done through detection and modelling of the buried quaternary valleys, which either can serve as potential aquifers or potential aquifer barriers. The present paper deals only with one aspect of a larger study; namely a case story leading to the detection of unknown buried valleys and the first attempts to model them in 3D from gravity and seismics. Also, the emphasis here is not on any theoretical or even methodological developments (we use slightly modified standard methods), but rather on the sequence of investigations and decisions leading to the focused fieldwork and modelling. In our work, the gravity data play a crucial role. Surprisingly, the choice of simple modelling methods (the geological stripping followed by a simple high-pass filtering) seems to work for this particular area. A number of previously unknown shallow geological features is clearly seen in the residual signal and were later confirmed by new surveys. This work consists of a number of steps (some of which are not described here even though we use the results): the construction of depth models from industrial seismic data for known geological units; geological stripping with mass density adjustment; high-pass filtering of the residual gravity signals; vibroseis surveys; microgravity surveys along seismic lines; the first modelling of buried valleys.