Evaluation of 2-D resistivity and seismic refraction methods in a complex karstic area

Electrical Resistivity Tomography (ERT) and seismic refraction investigations were carried out to assess the feasibility of geophysical investigations to map subsurface 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 in a test area near Salice Salentino village, located few kilometres north-west of Lecce (Italy). The main characteristics of this area is the high density of superficial karstic formation (dolines and sinkholes) and, therefore, the significant hazard of ground surface collapse. A selected area is presented in this paper as an example of the capabilities of the geophysical methods used. ERT profiles were acquired at the end of summ er, using long (48 electrodes, 5 m electrode spacing, 240 m profile length) and short (24 electrodes, 5 m electrode spacing, 115 m profile length) arrays. Earth models generated from field data acquired by the Wenner-Schlumberger array indicated significant horizontal and vertical resistivity variations. Seismic profiles, overlapped to ERT profiles, were acquired after one day of rain, using the 14 Hz resonance frequency vertical geophones. The resistivity distribution in the test area shows an almost horizontal stratigraphy (gently dipping to the North), with resistivity values increasing with depth. Encouraging correlations were noted between seismic refraction and ERT data in the test area. The high seismic velocity anomalies, visible on the seismic refraction sections, coincide with high-resistivity anomalies on the electric model sections and suggest the validity of a joint application of seismic refraction and ERT methods for mapping karstic cavities or karstified zones.