S.Te.G.A., Via Monte Bianco 5, 01100 VITERBO (ITALY) -Fax +39 761 228191 The purpose of this presentation is to show the great power of resolution of Time Domain Electromagnetics Methods in volcanics areas, in comparison with classic electric methods. We performed a hydrogeological study aimed to water supply, in an area layîng to the north-east of Bolsena Lake, the largest Italian volcanic lake. The schematic geological succession is represented by volcariics of the Vulsini apparatus (tuffs,

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... vas and scoriae), resting on a sedimentary substratum (clays and turbidites). Volcanics represent the main regional aquifer, thanks to good porosity and fracturation of rocks, while sedimentary substratum is impermeable, so that it constitutes the aquiclude. Volcanic aquifer feeds Bolsena Lake and some important springs. The structural characters of sedimentary substratum strongly determine underground drainage: the most productive areas are located above "structural low" (where impenneable sub stratum is deeper), thanks to the greater thickness of the aquifer. Geophysical methods are very useflil to detect the most promising hydrogeological areas; in fact the volcanic aquifer is more resistive than sedimentary substratum, so it is possible to reconstruct the geometry of the aquiclude top. For this reason electrical methods has been widely used for hydrogeological investigations; but sometimes these methods provided poor resuits: a well excavated on the ground of a previous geophysical survey, showed pooi roductivity. This is due to the fact that electric soundings are not always able to resolve aquiclude; other geophysicists interpreted the conductive layer, which lays under a resistive unit, as the impermeable substratum, but they openly forced geophysical data: it is too much optimistic to interpret the maximum of the curve, placed at about AB = 1000 m, as the top of the sedimentary substratum; according to this model, the aquiclude should be rest at a depth of about 400 m, which is clearly an overestimated depth of exploration for SEV. Our SEV also show a maximum at AB= 400-500 m (see Fig. 1 ), but a more realistic interpretation detects a conductive layer at a depth of aboi.tt 130 m, it is cannot be the aquiclude, because it is too much shallow. The same conductive layer is detected by TDEM 209