Sergio Negri

A ground-penetrating radar survey for archaeological investigations in an urban area (Lecce, Italy)

Abstract A ground-penetrating radar (GPR) survey, using mostly a 500 MHz antenna, was carried out in an urban area (Lecce, Italy) to obtain a detailed characterization of the most superficial layers, where presumably archaeological structures are buried, and to quickly identify anomalous zones for excavation. In the area, the presence of remnants of a historical Franciscan friary (built in 1432 and pulled down in 1971) and, probably, of more ancient (Roman and/or Messapic) features was expected. The geological setting (mainly wet calcarenite named “Pietra Leccese”) was not the most favourable for the application of GPR methodology because of an expected high attenuation of electromagnetic energy. The low penetration depth of the signal, not exceeding 1 m and even using a 100 MHz antenna, made it possible to obtain information only between the ground level and the top of the calcarenitic basement. Data recorded along parallel profiles, 1 m spaced, prevented the clear identification of the walls of the historical building constructed in “Pietra Leccese” blocks, because of the weak contrast in the electromagnetic parameters with respect to the hosting material. On the other hand, the analysis of the radar sections allowed for identification and reconstruction of the shape and extension of a barrel-vault cavity, subsequently confirmed by archaeological excavations. Time slice representations were used as a tool to locate other features including modern-day urban utilities and the planimetric development of the barrel-vault cavity.

Mapping the susceptibility to sinkholes in coastal areas, based on stratigraphy, geomorphology and geophysics

Sinkholes and land subsidence are among the main coastal geologic hazards. Their occurrence poses a serious threat to the man-made environment, due to the increasing density of population, pipelines and other infrastructures along the coasts, and to the catastrophic nature of the phenomena, which generally occur without any premonitory signs. To assess the potential danger from sinkholes along the coast, it is important to identify and monitor the main factors contributing to the process. This article reports a methodology based on sequential stratigraphic, hydrogeological and geophysical investigations to draw up a susceptibility map of sinkholes in coastal areas. The town of Casalabate situated in the Apulia region (southern Italy), affected by a long history of sinkhole phenomena, is here presented as an example. The approach proposed is based on sequential stratigraphical, geomorphological and geophysical surveys to identify the mechanisms of sinkhole formation and to provide a zonation of the areas in which further sinkhole phenomena may likely occur. Interpretation of the ground penetration radar and electrical tomography profiles has enabled us to identify the potentially most unstable sectors, significantly improving the assessment of the sinkhole susceptibility in the area. The proposed methodology is suitable to be exported in other coastal areas where limestone bedrock is not directly exposed at the surface, but covered by a variable thickness of recent deposits.

Geophysical and Geological Investigations in a Karstic Environment (Salice Salentino, Lecce, Italy)

Karstic forms (dolines and sinkholes) are notoriously difficult geophysical targets, and selecting an appropriate geophysical solution is not straightforward. The fundamental objective in the application of geophysical techniques to environmental studies is to assess and use the correct techniques for the investigation being undertaken. Electrical Resistivity Tomography (ERT) and Ground-Penetrating Radar (GPR) investigations were carried out, primarily to assess the feasibility of geophysical investigations to map the underground stratigraphy of shallow karstic aquifers, in order to help in the prevention of both groundwater pollution from agricultural activities and risk of ground surface collapse. This preliminary study was carried out at-two test areas (labelled area A and area B respectively) near Salice Salentino village, located few kilometers north-west of Lecce (Italy). The main characteristics of these areas are the high density of superficial karstic formation (dolines and sinkholes), the presen...

Use of Ground Penetrating Radar to Map Subsurface Moisture Variations in an Urban Area

The range of Ground Penetrating Radar (GPR) applications is very wide and diverse. GPR provides high-resolution continuos profiles, and has been used in various fields (environmental problems, geology, geotechnical engineering, archaeology etc.). This paper describes a test application of GPR in an urban area (Mesagne, Apulia, Italy) for the detection of underground water pipes and to obtain a map of the moisture in the ground around them. Some authors have proven that GPR can be used effectively, although only as a reconnaissance tool, for studies of moisture in the subsurface. Electromagnetic wave velocity in the subsurface depends on moisture via the dielectric properties of the wet material. The free volumetric water content can be determined from the dielectric constant of subsurface terrain by an empirical relation proposed by Topp et al. (1980). GPR wave velocity determination can be carried out by means of several methods. In this paper the hyperbolic anomalies caused by small inhomogeneities and ...

Karst geosites at risk of collapse: the sinkholes at Nociglia (Apulia, SE Italy)

This paper focuses on the impacts of contemporary geomorphological processes on fruition activities in a karst area of Salento (Apulia, southeastern Italy). The work illustrates the results of studies in a sinkhole field at Nociglia, in the Lecce province, recently recognized as a geosite and where shallow phreatic speleogenesis operates close to the water table level with the formation of karst caves, successive roof collapse, formation of wide caverns, and sinkhole development at the surface. All these features threaten the nearby infrastructures including a province road. Salento has a great number of active sinkholes related to natural and anthropogenic cavities. Their presence is at the origin of several problems in the built-up environment, due to increasing population growth and development pressures. In such a context, the detection of cavities, and the assessment of sinkhole hazards present numerous difficulties. To assess the potential danger from sinkholes, it is important to identify and monitor the main factors contributing to the formation process. A multi-disciplinary approach, comprising geological, geomorphological, and geophysical analyses, is necessary to obtain a comprehensive knowledge of these complex phenomena in karst areas. Geophysical methods can be of great help to monitor the processes and for identifying and mapping features related to the underground voids, by detecting contrasts in physical properties, such as density and electrical resistivity, with the surrounding sediments. At the same time, recognition of the presence of sinkholes by geophysical methods has to adapt to the local geological conditions and take advantage of the integration of the several methodologies available. The territory of Nociglia demonstrates that monitoring is essential for the safe exploitation of these type of geomorphosites.

Integration of high resolution optical satellite imagery and geophysical survey for archaeological prospection in Hierapolis (Turkey)

An in-depth analysis of the site of Hierapolis (Turkey) has been carried out using high resolution remotely sensed imagery integrated with geophysical survey and direct field data in order to detect evidence of archaeological features and environmental setting. High resolution panchromatic and multispectral images acquired by different satellites were used in this study. These images, acquired also in stereoscopic configuration, were digitally processed and visually interpreted. Geophysical data were collected in some areas of interest using different surveying methodologies (electric, radar). Besides the important scientific implications, the integration of remote sensing and geophysics in archaeology provided a useful tool for the optimization of landscape and cultural heritage management. All data collected were integrated in a Geographic Information System (GIS) to produce comprehensive maps of the site depicting archaeological and environmental features.

The geothermal potential of the underground of the Salento peninsula (southern Italy)

The low-enthalpy geothermal energy systems exploit the underground as a heat reservoir, by coupling it with highly efficient heat pumps: in winter, the energy stored in the subsoil can be transferred to the surface to heat buildings, whereas in summer the surplus heat in buildings can be transferred back to the ground providing a cooling action. Despite the high costs for installation, low-enthalpy geothermal systems have high energy efficiency and, therefore, contribute to energy saving, to the use of renewable energy sources, to environmental sustainability (the effects on the hydrogeological cycle are often negligible) and to the reduction in fuel consumption and in gas emissions. In this paper, a simple methodology is proposed to assess the geothermal potential of the underground of the Salento peninsula (southern Italy) at the regional scale, by calculating the equivalent thermal performance of the subsoil, which expresses the thermal power that is potentially extractable and useful to heat or cool a building. Since the possibility of exploiting the underground as a heat reservoir depends on the physical properties and water saturation of rocks, previous detailed studies on the hydrostratigraphic setup of the region are the basis for this analysis of geothermal potential. The equivalent thermal performance depends on the thermal efficiencies of different lithological units, whose ranges of values, for heating only, are provided in literature. For this case study, a stochastic approach has been applied: several combinations of thermal efficiencies were randomly selected from the intervals of reference values for every hydrostratigraphic formation and the corresponding minimum depths below the ground surface that must be reached to get the desired equivalent thermal performance are computed. The results show that the underground of the Salento peninsula has great potentialities for heating of buildings, except for some coastal areas because the saltwater intrusion phenomenon could cause problems for the realization of low-enthalpy geothermal energy systems.

GPR, VES and refraction seismic surveys in the karstic area “Spedicaturo” near Nociglia (Lecce, Italy)

The present work shows some results of an early ground-penetrating radar (GPR) study performed in a hazardous karstic area near the village of Nociglia in the central part of the Salento peninsula (Apulia, southern Italy), after a sudden subsidence event occurred on March 13th, 1996. The geophysical study, also involving vertical electric sounding (VES) and refraction seismics, is part of a multidisciplinary research project, aimed at determining the climatic, geological, morphological and hydrogeological conditions of the area in order to assess geological hazards and to prevent groundwater pollution due to agricultural activities. GPR transects led to the identification of some shallow subsurface structures including a known cave and probable karstified bedding joints, weakened shallow layers, fractures, fault zones, and burrows. The VES and seismic surveys resulted in a better understanding of the geology of the area and supported the interpretation of the GPR data.

Conceptualization and characterization of a coastal multi-layered aquifer system in the Taranto Gulf (southern Italy)

In some Mediterranean karst areas, groundwater is often the only available supply for fresh water. Besides the contamination induced by human activities, coastal aquifers often suffer from the saltwater intrusion phenomenon, which can be enhanced by both extensive withdrawals and climatic changes. Establishing an effective set of regulatory and management measures to ensure the sustainability of coastal aquifers requires a deep knowledge about natural and anthropic stresses involved in groundwater dynamics. In this regard, a prior conceptualization of aquifer systems and a deeper characterization of balance terms through mathematical modeling are of paramount importance. In this paper, previous works on the three-dimensional hydrostratigraphic characterization and two-dimensional groundwater flow modeling at the regional scale for the Salento peninsula (southeastern Italy) are shown to provide a fundamental basis for an overall conceptualization of the multi-layered aquifer system of the Taranto Gulf (northwestern Salento). This is done by identifying source terms and providing a preliminary estimate of different balance terms, based on available data and information. Then, a further characterization will be provided using three-dimensional groundwater flow models and calibration techniques based on the solution of an inverse problem. As a result the hydraulic features of the deep karst aquifer can be mapped; the stresses due to human water-demanding activities and the occurrence of the saltwater intrusion phenomenon were analyzed and possible weaknesses of the conceptual and of the preliminary numerical model pointed out.

Integrated analysis of geological and geophysical data for the detection of underground man-made caves in an area in southern Italy

In Cutrofiano, in the southern part of the Salento Peninsula, Apulia, Italy, a Pleistocene calcarenitic sequence was quarried by digging extensive networks of galleries along the geological succession most suitable for the quarrying activity. These caves represent a potential hazard for the built-up environment due to the occurrence of underground instability that may propagate upward and eventually reach the surface, causing sinkholes. In this work we propose integrated interdisciplinary methods for cavities detection. The methodology was applied at a test area located along a major road near Cutrofiano using geological and electrical-resistivity tomography and microgravity geophysical methods.