GROUND PENETRATING RADAR A USEFUL TOOL FOR SHALLOW SUBSURFACE STRATIGRAPHY CHARACTERIZATION

In a region such as the Salento, constituted by a flat surface with greatly expanded coverage of agricultural land, the studies performed on the stratigraphical evolution of different lithological units in the first meters of the soil has been based upon the analysis of cut - faces, quarries, cores and shallow trenches (Bossio et al., 1987; Bossio et al., 1992; Bossio et al., 1994; Bossio et al., 1998; Bossio et al., 1999; Ciaranfi et al., 1992; D'Alessandro, et al., 1994; Margiotta, 1999; Margiotta and Ricchetti, 2002; Palmentola, 1987). Data provided by such techniques are often one- or two-dimensional. They involve surveys that are time consuming and are patchy in terms of spatial coverage. Therefore the possibilities offered by the GPR to investigate the subsoil in a non-invasively way, and to obtain 3D maps of the subsurface itself, becomes of crucial importance for geologists. The successful obtained by GPR investigations on sedimentary rock stratigraphy is well documented in literature (Annan and Davis, 1989; van Overmeeren, 1998; Mills and Speece, 1997; Mokma et al., 1990; Nobes et al., 2001; Lapen et al., 1996; Baker, 1991; Beres, et al., 1995; Leucci et al., 2000; Carrozzo et al., 2000; Carrozzo et al., 2003). Since GPR holds enormous potential for such studies, it is appropriate to assess some key considerations for, i) field data acquisition, ii) raw data processing in order to enhance data display, iii) EM wave velocity measurements in order to characterize sediments response and to perform the time to depth conversion, iv) lithological interpretation of the GPR data set. This chapter attempts to give the steps required to acquired, process and interpret GPR data in a sedimentary rock environment. GPR data were acquired in the Salento peninsula in two areas located near the city of Lecce, Italy (Fig. 1). During last years lagoonal - continental and marine oligo-miocene deposits have been recognized in some areas of Salento leccese. Del Prete & Santagati (1972) described lagoonal - continental sediment underlying the well-known miocenic formation of Pietra leccese cropping out the " Vito Fazzi" hospital of Lecce. They referred this lagoonal deposit to Tortoniano (Miocene). Later, other important outcrops of these deposits were recognized near S. Maria al Bagno (Nardò, Lecce) by Bossio et al. (1992) and near Galatone (Lecce) by Colella (1994), respectively along the roadside of the Gallipoli - Lecce highway and along a cut of Sud Est railway. Moreover Barbera et al. (1993) referred to late Oligocene a shallow marine calcarenite, rich in Scutelle, cropping out in a quarry near Galatone. Recently Bossio et al. (1999) recognized, not very far away from Lecce, two different informal units referred to the Oligo - Miocene transition, the Galatone Formation (lagoonal - continental deposits) and Lecce Formation (shallow marine deposits). Bossio et al. (2000) proposed to formalize the Galatone Formation. Notwithstanding these researches, at the moment, the stratigraphical relationships between Galatone Formation and Lecce Formation in consequence of extended soil cover have not been defined. GPR measurements has been carried out to define the geometrical relationship between these two units. GPR measurements have been carrying out along a cut-face in order to assess the potential for imaging and characterising different lithological facies of this method and to choose the better antenna and set up. In this first phase some methodological aspects related to the data processing were considered. Particularly first one, although used in potential field and in seismic data processing, Discrete Wavelet Transform (DWT) based filtering procedures was used to GPR images for the particular problem of removing coherent noise (linearly and