Calculation of pre-Zechstein Bouguer anomaly in northwest Germany

The gravitational influence of the sedimentary sequence from the surface to the base of the Zechstein (Late Permian) in northwest Germany has been calculated. The structure of the sedimentary sequence is known from interpretation of seismic reflection data. Corrections were ca1culated by taking the differences between the densities of the sedimentary rocks and an assumed density of 2.65 g cm-3 for the pre-Zechstein. Thus a new map showing an improved Bouguer anomaly was obtained, which must essentially depend upon the density distribution below the Zechstein formation. There are several uncertainties: limited data on the densities of the geological formations: deep Tertiary troughs; large Keuper (Upper Triassic) thicknesses; salt domes derived from the Zechstein; and Rotliegendes (Early Permian) troughs in which the base of the Zechstein is at depths of about 10 km. These all cause problems in the determination of the corresponding influences on the gravity. The map of the improved Bouguer anomaly was produced by forward modelling which involved variation of the density values.