Vertical-gradient microgravity and electrical-resistivity tomography geophysical surveys were performed over a shallow cave in the Italian Armetta Mountain karst area, close to the Liguria-Piedmont watershed. The aim of this study was to test the geophysical response of a known shallow cave. The shallowest portion of the cave exhibits narrow passages and, at about 30 meters below the entrance, a fossil meander linking two large chambers, the target of the geophysical survey. The integrated results of the two surveys show a clear geophysical response to the cave. The surveys exhibited high resistivity values and a negative gravity anomaly over the large cave passages. This work confirms the ability of these geophysical techniques to give the precise location of the voids, even in complex environments. The application of these techniques can be successful for site surveying where the presence of hollows may be expected.

S U M M A R YWe have used free-air gravity satellite data from GEOSAT and ERS-1 missions to compile a Bouguer gravity map of the Mediterranean Sea. The complete Bouguer correction has been applied by using the method of Parker, that acts in the Fourier domain and permits the exact evaluation of the gravity contribution from an highly sampled topographic model of the land. The density used for the Bouguer reduction has been obtained from the gravity data set itself, by using two different optimization methods that have given the same optimal result of 2400 kg m −3 . We have studied the radial power spectrum of the data, choosing the optimal Bouguer density from its slope, as the one which minimizes the fractal dimension of the resulting gravity map. The second approach consists of studying the correlation between topography and Bouguer anomaly by spatial cross-plots for a significant subset of the data. Both these approaches are aimed at reducing the short-wavelength effects of topography in the gravity map, but in the past they have been traditionally used alternatively since they gave different optimization values, especially the second method that seems to ignore large-wavelength isostatic effects. Actually, we have revisited both the methodologies, proposing slight modifications to make their efforts compatible. Their coincident results confirm their validity of application and give reliability to the recovered value of the Bouguer optimal density. Moreover, modifying the second approach allows us to compile a sort of normalized correlation map, which we propose in this paper, defining the 2-D isostatic setting of the investigated region without introducing any further lithospheric model. The final result is a revised Bouguer map compiled using a grid with a resolution of 2 min, that is useful for large-scale geological studies and gives important information about the compensation mechanism of the Mediterranean Sea: in a direct way we have found that the overall region seems to be in a complete isostatic equilibrium apart from the young basins of Tyrrhenian Sea and Aegean Sea, confirming previous similar results.

Capitale-Dipartimento VI "Pianificazione territoriale generale"-Servizio 3 "Geologico, difesa del suolo e protezione civile in ambito metropolitano",

We analyse a microgravity data set acquired from two spring LaCoste & Romberg gravity meters operated in parallel at the same site on Etna volcano (Italy) for about two months (August-September 2005). The high sampling rate acquisition (2 Hz) allowed the correlation of short-lasting gravity fluctuations with seismic events. After characterizing the oscillation behavior of the meters, through the study of spectral content and the background noise level of both sequences, we recognized fluctuations in the gravity data, spanning a range of periods from 1 s to about 30 s dominated by components with a period of about 15÷25 s, during time intervals encompassing both local seismic events and large worldwide earthquakes. The data analyses demonstrate that observed earthquake-induced gravity fluctuations have some differences due to diverse spectral content of the earthquakes. When local seismic events which present high frequency content excite the meters, the correlation between the two gravity signals is poor (factor < 0.3). Vice versa, when large worldwide earthquakes occur and low frequency seismic waves dominate the ensuing seismic wavefield, the resonance frequencies of the meters are excited and they react according to more common features. In the latter case, the signals from the two instruments are strongly correlated to each other (up to 0.9). In this paper the behaviors of spring gravimeters in the frequency range of the disturbances produced by local and large worldwide earthquakes are presented and discussed.

The Gulf of Manfredonia and the Tavoliere Plain have experienced intense human colonization since the neolithic times. There is past evidence in the study area of a Roman‐age settlement historically known as Salapia, probably located close to the salt‐production ponds. In particular, several portions of a partly submerged ancient pier‐like structure were found close to Torre Pietra town. This structure has been interpreted as a portion of a dragged‐ship channel that connected the town of Salapia to the sea. In this article we discuss the results of a high‐resolution geophysical prospecting of the Margherita di Savoia offshore archaeological site (Puglia, Italy). We collected magnetic gradiometric and bathymetric data to map the submerged remains of Salapia harbour. We found evidence of an offshore projection of a Torre Pietra pier‐like structure, identifying the direction of the ship channel and possibly the location of the harbour's defensive post. Copyright © 2012 John Wiley & Sons, Ltd.