Ranieri, G.

Time Domain Electromagnetic (TDEM) methods have proven to be efficacious in many studies involving environmental and engineering problems, producing better results than the traditional galvanic techniques. In fact, the major advantage of this method is that the soundings can be performed in a relatively short time and in a small place. TDEM sounding appears to be particularly useful for discriminating between layers having low resistivity, but interpretative limitations arise when intermediate and deep resistive layers occur. So the method appears complementary to the traditional electrical soundings. This method has been experimentally tested in well characterised test sites where geophysical investigations (reflection seismic, SEV, IP, gravimetry) and drillings, as well as chemical and isotopic analyses had already been conducted. The results confirm that the method is practical, economic and perfectly reliable, not only at large depths, but also in shallow research, using fast acquisition devices working in the range from 4 ms to some ms.

The paper deals with some aspects of the planning and the interpretation of the IP and resistivity measurements in the San Priamo coastal plane (Sardinia, Italy). The plane presents some hydrogeological problems related to previous flowages and salt water intrusion from the sea. The geophysical results verified the good resolution of the joint elaboration of the IP and the resistivity soundings, in order to delineate some hydrogeological aspects of the San Priamo coastal plane. In particular, the technique was suitable for detecting salt water intrusion in a fresh acquifer, with good reliability.

A Spectral Induced Polarization (SIP) survey was carried out in a mining test site in Sardinia (Italy). Measurements were developed along a profile by using an axial dipole-dipole array with 10 AB positions and 6 MN positions for cach AB. The amplitude and phase spectra of the apparent resistivity were acquired in the 0.25-4096 Hz frequeney range. The results obtained through the processing and inversion step seem to confirm that, with respect to the classical TD/FD Induced Polarization, SIP allows better discrimination of some important characteristics of mineral deposits such as mineral content and grain size.