R. Martorana

The structure of a hydrothermal system from an integrated geochemical, geophysical and geological approach: the Ischia Island case study

The complexity of volcano-hosted hydrothermal systems is such that thorough characterization requires extensive and interdisciplinary work. We use here an integrated multidisciplinary approach, combining geological investigations with hydrogeochemical and soil degassing prospecting, and resistivity surveys, to provide a comprehensive characterization of the shallow structure of the southwestern Ischias hydrothermal system. We show that the investigated area is characterized by a structural setting that, although very complex, can be schematized in three sectors, namely, the extra caldera sector (ECS), caldera floor sector (CFS), and resurgent caldera sector (RCS). This contrasted structural setting governs fluid circulation. Geochemical prospecting shows, in fact, that the caldera floor sector, a structural and topographic low, is the area where CO2-rich (>40 cm3/l) hydrothermally mature (log Mg/Na ratios 150 g m−2 d−1), is clearly captured by electrical resistivity tomography (ERT) and transient electromagnetic (TEM) surveys as a highly conductive (resistivity 10,000 mg/l) and poorly conductive meteoric-derived (TDS < 4,000 mg/l) waters are observed, respectively. We finally integrate our observations to build a general model for fluid circulation in the shallowest (<0.5 km) part of Ischias hydrothermal system.

Geophysical investigations at the Himera archaeological site, northern Sicily

In this paper we present a geophysical survey that was carried out as a research activity during field trips to the Himera archaeological site, where relics of a large Greek settlement are still buried, effected by a joint group of archaeologists and geophysicists during an educational project. Two-dimensional (2D) resistivity and ground-penetrating radar (GPR) surveys were performed to locate buried archaeological structures at this archaeological site. The results of the GPR surveys show some anomalies characterized by semi-hyperbolic shapes. In some restricted areas of the town, two grids of parallel GPR profiles were acquired while time- and depth-slices were calculated to identify the extension of the buried structures. In fact, the shapes of many anomalies recognized in the slices seem to describe the distribution of the relics (i.e. walls, roads, etc.). A 2D electrical resistivity profile showed clear anomalies, which were connected with wall-like relics. This was also obtained by using on-site calibration on partially uncovered structures, while the low background resistivity values were interpreted as representing river deposits having different moisture characteristics. Recommendations were also given to the archaeological site-excavators, following which some of the results were confirmed by subsequent archaeological excavations.

Geophysical and geotechnical investigations to support the restoration project of the Roman ‘Villa del Casale’, Piazza Armerina, Sicily, Italy

A multidisciplinary geophysical and geotechnical study, including some non-invasive geophysical applications, was carried out during the restoration of the ‘Villa del Casale’, a Roman villa discovered near Piazza Armerina (Sicily, Italy) in 1929, famous for its Roman floor mosaics. The project aims were to characterize the geology of the subsoil and provide information for solving the main building structural problems including the subsidence of some parts of the floor and the detachment of the tesserae (i.e., the tiles) of the mosaics. Another goal was the detailed study of the underground structures of the Corridor of the Great Hunt, a part of the villa strongly affected by subsidence and detachment of mosaics. The acquisition of geotechnical and geophysical surveys, including core drillings, time-domain electromagnetic (TDEM) soundings, downhole seismic logs and a seismic refraction profile, allowed the reconstruction of the geometry of geological formations below the villa to be ascertained, especially with regard to the top of the bedrock. The results suggest the presence of a buried riverbed that has been identified as the main cause of the villa floor subsidence. The study of the Corridor of the Great Hunt involved full 3D electrical resistivity tomography (ERT) and 3D ground-penetrating radar (GPR) data acquisition using 100 MHz, 400 MHz and 1600 MHz antennas, as well as the above mentioned refraction seismic profile. Data acquisition required the use of special non-invasive sensors to avoid any damage to the mosaics. The integrated inversion of all the data and the comparison between the resulting 3D resistivity model and 400 MHz GPR depth-slices allowed the identification of many shallow anomalies to be identified, including some pipes for water drainage and a reinforced concrete basement placed under the floor during a previous restoration intervention. 100 MHz GPR profiles validated the results of the seismic refraction profile model. Finally, 1600 MHz GPR depth-slices, in the most subsided zone, allowed the location of air bubbles under the mosaic to be identified. This work shows that the integrated use of different geophysical techniques for archaeological purposes, especially if constrained by direct explorations, greatly reduced the intrinsic uncertainties of each method. Moreover, customizing standard geophysical equipment to avoid any damage is essential when working on protected cultural heritage.

Integrated geophysical survey for 3D modelling of a coastal aquifer polluted by seawater

Geophysical surveys are carried out in the coastal area of Petrosino (south-western Sicily) to study the time evolution of seawater contamination of the coastal aquifer, probably increased due to human impact. The overexploitation of the aquifer, due to an intensive agricultural use has affected significantly the natural hydro-geochemical state of the basin. The study is based on a processing and integrated analysis of hydrogeological, geochemical and geophysical data. In particular in the last two years seasonal time-lapse electrical resistivity tomographies (ERT), new TDEM soundings and Multi-Analysis Surface Wave soundings (MASW) have been carried out. The interpretation of the total set of previously existing and new geophysical data made it possible to reconstruct a threedimensional model of the electrical resistivity of the aquifer, aimed at defining the extent and geometry of the seawater intrusion. Furthermore, the execution of a series of high-resolution timelapse electrical tomographies and a correlation analysis between geophysical measures and geochemical, geological and hydrogeological data allowed to discriminate the effects of the salt concentration in the groundwater and the porosity and saturation degree of the rock on the time variations of the measured electrical resistivity. Finally, the average porosity of the rocks forming the reservoir was determined.

From Geophysics to Microgeophysics for Engineering and Cultural Heritage

The methodologies of microgeophysics have been derived from the geophysical ones, for the sake of solving specific diagnostic and/or monitoring problems regarding civil engineering and cultural heritage studies. Generally, the investigations are carried out using different 2D and 3D tomographic approaches as well as different energy sources: sonic and ultrasonic waves, electromagnetic (inductive and impulsive) sources, electric potential fields, and infrared emission. Many efforts have been made to modify instruments and procedures in order to improve the resolution of the surveys as well as to significantly reduce the time of the measurements without any loss of information. This last point has been achieved by using multichannel systems. Finally, some applications are presented, and the results seem to be very promising and promote this new branch of geophysics. Therefore, these methodologies can be used even more to diagnose, monitor, and safeguard not only engineering buildings and/or large structures, but also ancient monuments and cultural artifacts, such as pottery, statues, and so forth.

Study And Monitoring Of Salt Water Intrusion In The Coastal Area Between Mazara Del Vallo And Marsala (South-Western Sicily)

In this chapter the study of a coastal aquifer located in South-Western Sicily (between the towns of Marsala and Mazara del Vallo) is presented, carried out using geochemical, hydro-geological and geophysical techniques. The aquifer has been over-exploited to the point of being subject to intense and worrisome salt-water intrusion.A preliminary chemical and physical characterization of the waters was carried out; this included measuring their conductivity and their chloride content. This allowed to detect the marine intrusion wedge in the coastal aquifer. A series of electromagnetic soundings, suitably calibrated by well logs, were effected in the whole area and allowed to create a 3D interpretative model of the resistivity distribution in the aquifer, thereby enabling to recognize the main intrusion directions and the pattern of the aquifer bed. Furthermore an integrated geophysical 2D section was carried out along a line roughly perpendicular to the coast, in one of the zones that is particularly involved in the intrusion phenomenon. Field measures included ERT, IP, TDEM and seismic soundings, all of which were aimed at reconstructing a highly detailed geophysical section. The seismic soundings clearly show the lateral variation between the fresh and salt water, such as the overburden and the clayey bed of the aquifer. The final target of this research is to propose an optimized management-model of underground resources. The lessons drawn from the use of different techniques for defining geophysical profiles suggest an integrated methodology to identify in detail the sea intrusion zone in aquifers. Therefore, the methodology used can be suitably extended and exported for studing and monitoring many similar Mediterranean coastal areas

Geophysical investigations in the Flumendosa River Delta, Sardinia (Italy) — Seismic reflection imaging

A geophysical investigation that included seismic-reflection surveying and time-domain electromagnetics (EM) was carried out in the Flumendosa River Delta plain in southeastern Sardinia, Italy. The objective was to improve knowledge of geologic and hydrogeologic controls on a highly productive aquifer hosted in thick Quaternary deposits and known to be affected by an extensive saltwater intrusion. The seismic reflection survey, whose results are reported here, aimed to image the Paleozoic bedrock topography and to obtain detailed structural and stratigraphic information on the sequence of largely fluvial sediments extending from the surface down to bedrock. The survey consisted of two inline profiles, nearly parallel to the coastline and 1 km inland. The sources ( 0.25 kg of explosives buried at 2 m depth) and receivers (50-Hz vertical geophones) produced a twelvefold data set with common midpoints every 2.5 m . Detailed integrated velocity analysis (constant velocity gathers, constant velocity stacks, an...

3D ERT for the Study of an Ancient Wall Covered by Precious Mosaics

The DC electrical tomography (ERT) is still a method scarcely diffused for the study of masonry of recent and ancient buildings, in spite of the very high resolution power of this methodology. The major limitation on ERT applicability on precious surfaces (like masonries or floors with fresco or mosaic covering) depends on the method of current injection: usually it is necessary to drive electrodes (small nails) into small perforations on the surfaces, to obtain sufficiently high current values (to produce good signal to noise ratio). An interesting application of the MYG array (that reduces significantly, compared to other classical arrays, the number of electrodes utilized for current injection) has been carried out on a wall of the fountain room of the Zisa Palace (Palermo, XII century A.D.), covered by a mosaic wall suffering important detachment of tesserae due to internal moisture. The results seem to suggest that ERT investigation with the MYG array has important applicative potentiality in study of precious walls or floors in artistic buildings, because of the low-invasivity and high resolution power as well as velocity of acquisition.

Integrated Geophysical Surveys in the Tarragona Cathedral

An integrated geophysical survey has been conducted at the Tarragona’s Cathedral (Catalonia, NE Spain) with the aim to detect the existence of archaeological remains of the Roman’s temple devoted to August. Many hypotheses have been proposed about its possible location, the last ones regarding the inner part of the Cathedral, which is one of the most famous temple of Spain (12th century) evolving from Romanesque to Gothic styles. A project including electrical resistivity tomography (ERT), ground probing radar (GPR) and frequency domain electromagnetics (FDEM) has been planned over one year and conducted during a week of intensive field survey. From all the methods applied, both ERT and GPR have provided some detailed information on subsoil structures. The ERT method has been applied with different techniques and arrays, ranging from standard Wenner-Schlumberger 2D sections to full 3D electrical imaging using the MYG array. Electrical resistivity data have been recorded extensively and therefore, 70.000 apparent resistivity data were available to obtain a full 3D image after an integrated inversion. In conclusion, some significant buried structures have been revealed providing conclusive information for archaeologists.

Tomographic resistivity 3D mapping: filter coefficients and depth correction

In this paper further adjustments are presented regarding the method given by Cosentino et al. (1995, 1997) to construct rapidly 3D tomographic pictures from resistivity data. The purpose of the procedure (a filtered back-projection of the experimental data) is to obtain 2D or 3D pictures which match as closely as possible the geometry of the structures which are being investigated. In order to optimize the target some problems should be solved. Firstly, one should try to detect correctly the depth of electrical discontinuities (or anomalous bodies); secondly and not less important, one should try to determine the size as weIl the shape of the structures which are buried. In this paper we study the conneetion between the resistivity of the anomalous bodies and their effect on the apparent resistivity distribution. The results suggest that is best to optimize the values of the filters using also the negative values of the influence coefficients; they give also some information which appear useful to improve the estimate of both the depth and geometry of the bodies.