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Electrical and electromagnetic methods often require the determination of the amplitude of a periodic signal,produced by controlled sources and masked by natural and artificial noise. Since noise is usually random and uncorrelated, several techniques based on a stacking procedure and spectral decomposition are applied. In thispaper we revise some of these procedures based on Fourier analysis. We propose a technique which uses «super-averaged»functions obtained from the average of the results of the Fourier analysis from the whole data set.

I propose a fast method for constructing pseudosections of apparent resistivity from geoelectrical data collected for deep studies with continuous polar dipole-dipole arrays. Once a vertical section is fixed, each value of apparent resistivity is assigned to a point on the section and finally pseudosections are obtained by interpolation. This allows the geophysicist to represent a large amount of data in a fast and simple way, to perform a qualitative interpretation and to facilitate the quantitative interpretation.

We carried out three electrical soundings, with continuous polar dipole-dipole array, and three magnetotelluric stations at Gran Sasso d'Italia, in the area of Campo Imperatore. Geoelectrical data show the existence of a resistive background with exceptionally high values of resistivity (greater than 25000 ? m) at a depth of about 250 m. The magnetotelluric data were used to constrain the thickness of this resistive layer, below which we find an intermediate conductive structure and a deeper resistive background. The geological interpretation of this result is still open to discussion, but we can exclude that the shallow resistive layer consists of Cretaceous limestones, since their resistivity has been estimated to be about 5000 ? m.

Eleven MT soundings and two dipole-dipole electrical soundings were carried out in the area of the Po plain south of the town of Padua. MT soundings were performed in the frequency band from 100 to 1/64 Hz; DC soundings were achieved, respectively, with a 2000 and 4000 m length of the array. The electrical interpretative models show a predominant monoclinalic tectonic behaviour with an evident deepening of the resistive basement to ward NE. The joint use of the two electrical techniques defined the geological and structural setting up to a depth of 2 km in a highly conductive area.

We present some results of a geoelectrical investigation program conducted in the Northern Apennines, namely in the Val d'Aveto and Bobbio window and surrounding areas. Field activity included the execution of more than 50 vertical electrical soundings with continuous polar dipole-dipole spread. We image the geometries of some deep geological structures; in particular we found a resistive background, whose resistivity is different along the geoelectrical profiles. In our interpretation the resistive background consists of subligurid and tuscan units underlying the alloctone Ligurid units in the area surrounding the Val d'Aveto and Bobbio window. The resistive background was not found, at least at the same depths, toward north-east. Therefore, the geoelectrical survey revealed the position of the front of the subligurid and Tuscan nappes toward the plain for a depth of about one kilometer.

The structure of active and inactive volcanoes can be explored with electric and electromagnetic surveys. We test the actual applicability of prospecting methods that employ both stationary and time-varying fields, using mathematical models both for layered and complex structures. The geometry and the resistivities of the geological structures which are considered have been taken from real case studies reported in the literature. In particular we analyse the sensitivity of different methods to conductive and resistant bodies.

Some deep DC electrical soundings, performed in alpine and apenninic areas with the continuous polar dipole-dipole spread, show apparent resistivity curves with positive slopes. Measured values of apparent resistivity reach 30000 Wm. Applying the "surface charges" method we developed three dimensional mathematical models, by means of which we can state simple rules for determining the minimum extensions of the deep resistive bodies, fundamental information for a more precise interpretation of the field results.