Geophysical and Geological Investigations in a Karstic Environment (Salice Salentino, Lecce, Italy)

Abstract

Karstic forms (dolines and sinkholes) are notoriously difficult geophysical targets, and selecting an appropriate geophysical solution is not straightforward. The fundamental objective in the application of geophysical techniques to environmental studies is to assess and use the correct techniques for the investigation being undertaken. Electrical Resistivity Tomography (ERT) and Ground-Penetrating Radar (GPR) investigations were carried out, primarily to assess the feasibility of geophysical investigations to map the underground stratigraphy of shallow karstic aquifers, in order to help in the prevention of both groundwater pollution from agricultural activities and risk of ground surface collapse. This preliminary study was carried out at-two test areas (labelled area A and area B respectively) near Salice Salentino village, located few kilometers north-west of Lecce (Italy). The main characteristics of these areas are the high density of superficial karstic formation (dolines and sinkholes), the presence of numerous water supply wells, the intense use of fertilizers and pesticides in agriculture (mainly in the numerous vineyards for production of fine wines) and, therefore, the significant risk of groundwater contamination and ground surface collapse. Selected areas are presented in this paper as an example of the capabilities of the geophysical methods used. A series of two-dimensional resistivity profiles collected in the two areas reveal substantial differences in depth of investigation and resolution using Wenner-Schlumberger and dipole-dipole data collection techniques. ERT profiles were acquired using short ([Formula: see text] electrode spacing, [Formula: see text] profile length) arrays in the area A as well as long ([Formula: see text] electrode spacing, [Formula: see text] profile length) arrays. Earth models generated from field data acquired by the dipole-dipole array reveal more detail but lower depth of investigation than Wenner-Schlumberger array. The dipole-dipole array uniquely imaged isolated high resistivity bodies while Wenner-Schlumberger array consistently indicated significant horizontal and vertical resistivity variations. GPR profiles, which overlapped the ERT profiles, were acquired using the [Formula: see text] (center frequency) antenna for shallow high-resolution structural images. Distinctly different results were observed at the two locations. Encouraging correlations were noted between GPR and ERT data in the area A. The diffraction hyperbolic anomalies, visible on the radar sections, coincide with high-resistivity anomalies on the electric model sections and this suggests the validity of a joint application of GPR and ERT methods for mapping karstic cavities or karstified zones. The resistivity distribution in area B shows an almost horizontal stratigraphy (gently dipping to the north), with resistivity values increasing with depth. The high absorption of the electromagnetic energy, noted in the GPR data, is probably due to the resistivity values below [Formula: see text] in the first [Formula: see text] in depth. These results suggest that GPR might be unsuccessful in the area B.