Too old for Modern Exploration? Old Torsion Balance Observations Revisited

Abstract

Although gravity gradiometry was among the first geophysical methods used successfully in applied geophysics (Eötvös torsion balance), the technology fell from favour after the 1930s. Here we present a new analysis of torsion balance measurements that were made at this time to detect salt domes in the Northwest German Basin. The study was aimed at assessing methods for interpretation of torsion balance observations. The approach is to synthesize classical procedures with computer based numerical methods, taking horizontal gradients Wxz and Wyz as input parameters. First we give an overview of torsion balance operational details and then discuss the interpretation of torsion balance data. A total of 39 maps at 1:25,000 scale covering the Northwest German Basin have been digitised. These maps provide some 40,000 torsion balance measurements, but no field books or field forms are available. The gradients were digitised from old paper maps, then reprocessed and recalculated. 3D modelling of Wxz, Wyz and other components of the Eötvös tensor provides detailed insight into the geometry of salt domes and provides additional constraints for modelling the Bouguer gravity field. 3D forward modelling is conducted using the IGMAS program package. This package allows the calculation of potential, gravity and its components, and the Eötvös tensor components. Results show that knowledge of the second derivatives of the potential could fundamentally change the role of gravity field measurements in underground investigations, not only for resource exploration, but also for investigations of large fault systems like the Dead Sea Transform.