The use of microgravity for cavity characterization in karstic terrains

Abstract

Microgravity is the interpretation of changes in the subsurface density distribution from the measurement of minute variations in the gravitational attraction of the Earth. As a technique, it is particularly suited to the investigation of subsurface structures, mapping of geological boundaries and, most importantly in this case, the location and characterization of voids or cavities. Gravity variations due to the geological/petrophysical changes associated with fracturing and changes in pore composition are superimposed upon much larger variations due to elevation, latitude, topography, Earth tides and regional geological variations. However, these external changes can be modelled or monitored with sufficient accuracy to be removed from the data. With the recent development of high-resolution instruments, careful field acquisition techniques and sophisticated reduction, processing and analysis routines, anomalies as small as 10 microgal can be detected and interpreted effectively. This paper describes the ‘state-of-the-art’ application of the microgravity technique for the detection and characterization of karstic cavities in a variety of limestone terrains, including the Carboniferous Limestone of the United Kingdom and Eire and the coral limestones of the Bahamas. The case study examples show how the recorded gravity anomalies have revealed the location of density variations associated with underground cave systems and, ultimately, provided information on their depths, shapes and morphology from a combined analysis of their spectral content, characteristic gradient signatures and modelling responses. In addition, mass deficiencies have been estimated, directly from the anomaly map, by the use of Gauss9s theorem without any prior knowledge of the exact location, or nature, of the causative bodies.