The Detection of Subsurface Voids by Gravimetry

Résumé du livre

The detection of voids in the near subsurface of the earth by gravimetric methods has b en studied in theory and practice and is presented. The gravitational anomalies on the surface that would be caused by subsurface voids of different sizes and depths are described in a theoretical treatment derived from elementary laws. Instructions are supplied on how to interpret observed negative gravity anomalies in terms of the voids that give rise to them. A set of charts is provided which giv s dir ctly and conv rsely the probable size and depth of voids from the magnitude and shape of observed anomalies. Since the interpretation problem does not have a unique solution, the validity of interpretation methods depends on the geologic environment and o th skill of the interpreter, and the limitations imposed by this dependence are xamined. Field experiments, in two different locations are reporte . Negative anomalies were observed in both locations, one due to a known void and others ue either to actual voids or to spaces in the earth where heavy material is replaced by lighter material, thus changing the suitability of the location for construction purposes. The effects of rough surface terrain, and the effectiveness of methods used for correcting it, are shown. A map, and discussion, of areas in the United States where shallow voids may exist is furnished. (Author).