Investigations of andesitic volcanic debris terrains: Part I — Geophysical

Abstract

This article contains the findings of geophysical investigations conducted at the site of the proposed international airport at Melville Hall, Commonwealth of Dominica, West Indies. The main geophysical method used was seismic refraction; additionally, ground-penetrating radar, resistivity sounding, and resistivity tomographic imaging were performed at some of the sites, but only the results of the seismic refraction and electrical resistivity sounding contributed to the development of the model presented. Analysis of the seismic data shows a gradual increase in velocity with depth, for which a model has been determined. Ancillary models or predictions of porosity, density, and natural compaction with depth are given, based on the basic seismic model. The seismic velocity model allows an engineer with a topographic map of the area to predict the volume of rock/soil in each rippability class and to project the rate of ripping. The results of the 1D resistivity soundings strongly suggest a nearly homogeneous material with resistivity variations that can be explained by moisture and low-level lithological variations. Therefore, based on integrated seismic and resistivity results, our model is a thick ash/boulder conglomerate with velocity increasing with depth, mainly from overburden stress compaction. In other words, the material at depth is not well-cemented rock. As the overburden stress is removed, the material essentially expands and loosens to produce a lower seismic velocity, similar to present surface conditions. The soil/rock can therefore be removed with little or no ripping or blasting.