GNGTS 2013 - Atti del 32° Convegno Nazionale

Results and discussions. Both surveys covered the area reported in Fig. 1 in order to obtain a detailed investigation of the surveyed site. We focus here on two almost parallel profiles along a line starting close to the lake northern shore toward the lake center (Fig. 1). In Fig. 2a the results of the Chirp sonar profile is reported. The lake floor along this profile is almost flat, slightly dipping toward south, and is marked by a weak reflection at about, on average, 3 meter of water depth, partial hidden by the direct wave. We note two main seismic units, a transparent, fine layered upper unit overlaying a lower more reflective unit, showing high- amplitude internal reflectors. The boundary between these units is a dome-shape reflector (U1), which marks a relatively high acoustic impedance contrast. In the first (northern) part of the profile, the presence of gas in the sediment hamper the penetration of the signal down to few decimeters below the lake floor. The observation that the top of gas-bearing deposits is a strong reflector (U0) might indicate early diagenesis due to fluid circulation. The upper unit is punctuated by hyperbolic reflections in the southern, more distal part of the profiles. Observation that these hyperbolas result aligned in correspondence of U0 might suggests a similar origin for such features. In Fig. 2b the results of the LCI inversion of CVES data approximately acquired along the same profile are reported. The thickness of the low resistivity water layer (average constant resistivity of 7 Ω·m) has been constrained in the inversion thanks to a contemporary acquired bathymetry profile which evidences a lake bottom slightly dipping toward south coherently with the sonar survey. Within the lake sediments it is possible to evidence two low resistivity variations: a reduced increase in resistivity (R1) along an interface which roughly matches the dome-shaped reflector U1 evidenced from the seismic reflection line; this interface can be followed also at the beginning of the profile where the penetration of the seismic signal is hampered by gas bearing sediments. Secondly a slightly more marked decrease in resistivity (R0) is noted towards the lake center with an abrupt change just after the rift of the previous dome. This second resistivity variation appears to be not noted in the seismic reflection line. Even if these are only preliminary results of the surveys and more accurate and complete comparison over the whole surveyed area, with precise location of each survey line, must be undertaken, it is possible to evidence that both of the surveys have evidenced the presence of a submerged interface within the sediments showing a dome shape that can be related to a more compacted/resistive material probably associable to the ancient shore line. The evidence of lower resistivity sediments in front of this anomaly can support this hypothesis. Moreover electrical surveys mapped this interface for the whole survey area allowing a spatial 2D reconstruction of the this layer; they have also locally evidenced the presence of sandstone formation protruding down the lake bottom near the shore. Conclusions. Preliminary results of both seismic Chirp Sonar reflection lines and CVES profiles over the survey area have evidenced their potentiality in characterizing the bottom sediments of the area around the Malpasso which is interesting both from the geological and archeological point of views. The results of the two different surveys seem to match in first approximation. However further studies are necessary to allow for an unique reconstruction of the geological setting respectful of both the surveys evidences. In this respect, accurate data Fig. 1. – Investigated area along the northern shore of the Trasimeno Lake near the Malpasso site. Evidence of the survey line along which the results of geophysical profiles are reported in Fig. 2. 188 GNGTS 2013 S essione 3.3