D. Luzio

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.

Nonparametric Clustering of Seismic Events

In this paper we propose a clustering technique, based on the maximization of the likelihood function defined from the generalization of a model for seismic activity (ETAS model, (Ogata (1988))), iteratively changing the partitioning of the events. In this context it is useful to apply models requiring the distinction between independent events (i.e. the background seismicity) and strongly correlated ones. This technique develops nonparametric estimation methods of the point process intensity function. To evaluate the goodness of fit of the model, from which the clustering method is implemented, residuals process analysis is used.

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.

Clustering and Registration of Multidimensional Functional Data

In order to find similarity between multidimensional curves, we consider the application of a procedure that provides a simultaneous assignation to clusters and alignment of such functions. In particular we look for clusters of multivariate seismic waveforms based on EM-type procedure and functional data analysis tools.

Selection of Time Windows in the Horizontal‐to‐Vertical Noise Spectral Ratio by Means of Cluster Analysis

The selection of the elementary analysis windows in continuous noise recordings for optimal estimation of the mean horizontal‐to‐vertical spectral ratio (HVSR) curve is generally performed by visual inspection of HVSR curves considered as functions of time. Starting from full‐length records, HVSR curves are determined in consecutive time windows of appropriate lengths. Time windows with HVSR curves that are anomalous on the basis of a simple visual inspection are generally ignored in the computation of the average HVSR curve. It is often very difficult to optimize the selection of time windows to be used for the calculation of the HVSR curve representative of a site. The use of nonobjective selection criteria produces results which depend on personal opinions of the operator and for which reliability cannot be assessed with quantitative parameters. We implemented an automatic procedure, based on cluster analysis, for the determination of the optimal window subset for the computation of the average HVSR curve. The procedure is based on the application of the agglomerative hierarchical clustering, using a measure of proximity of the standard correlation between HVSR curves and, as a rule for merging clusters, the average linking criterion. The procedure has been applied to 814 measures of seismic noise, carried out for the first‐level microzonation of 20 municipalities of Eastern Sicily characterized by high seismic hazard. A critical comparison of the results obtained by the clustering procedure implemented with those previously obtained by processing the same recordings with a technique based on the visual comparison of the spectral ratios of all the analysis windows has shown that the automatic clustering procedure seems to be capable of achieving a better partitioning of a set of HVSR curves and thus provides effective help in the process of distinguishing between peaks mainly linked to the site effects and others mainly related to the source effects.

Cluster analysis to support microzonation studies

In recent times the use of microtremor techniques for subsoil investigation increased significantly. The use of HVSR (Horizontal to Vertical Spectral Ratio)technique for seismic microzoning studies allows in many case to obtain detailed reconstruction of the roof of the seismic bedrock and to identify areas with similar seismic behaviour. Two different algorithms of clusteringhave been tested on a HVSR datasets acquired for studies of seismic microzoning in various Sicilian urban centers. HVSR data were previously properly processed to extract frequency and amplitude of peaks by a code based on clustering of HVSR curves determined in sliding time windows.To select an optimal set of time windows we have implemented a cluster procedure based on Agglomerative Hierarchical Clustering algorithms.After defining the average HVSR curves a second multi-parametric clustering procedurehas been used to group peaks tobe attributed to the same origin (stratigraphic, tectonic, topographic, anthropogenic or other sources). A nonhierarchical centroid-based algorithm has been implemented. The comparison of the HVSR pattern with the information about outcropping formations allowed to assess the geological hypotheses on the heavily urbanized investigated areas.

Comparison of Different Acquisition Patterns for 2D Tomographic Resistivity Surveys

A systematic comparison is presented between some 2D resistivity models and their images by the inversion of synthetic datasets relating to three different arrays, suitable for multichannel data acquisitions (dipole-dipole (DD), Wenner-Schlumberger (WS) and multiple gradient (MG)). The goal is to study how the measurement errors affects the resolution of the tomographic models and the ability to retrieve correct information on buried targets. We considered different data acquisition patterns, gradually increasing the complexity of the combinations of potential spacing and dipolar distance. To this end we increased the number of current dipoles to obtain approximately the same amount of measures, increasing the investigation time. Results from noise-free and noisy data are discussed and compared with those from field data. The results show that: the quality of the inversion models, for a fixed noise level, depends significantly on the data acquisition pattern; the information recovery and the resolution, being equal the number of measurements, is overall better for WS and worse for DD; the decrease of sensitivity with depth is lower for particular acquisition patterns that allow to better resolve deeper targets; the MG array can be preferred because it provides comparable results, using a smaller number of current electrodes.

Tomographic techniques for Resistivity Pseudo-Section Representation

Usual construction techniques of the pseudo-sections relative to d.c. resistivity profiles strongly distort the shapes of the anomalous buried bodies, whatever electrodic array is used (Fraser, 1981; Scurtu, 1972).

ERT and IPT Surveys to Check the Integrity of the Geomembrane in the Landfill of Bellolampo (Palermo, Italy)

In landfills, changes in resistivity and chargeability can be related to the characteristics of the waste and they can be abrupt and considerable within short distances. These physical properties are function of generation, mobility and degree of saturation of the leachate, gas generation, compaction density and variability. These relationships mean that it can be possible to get an overall image of the quantity and characteristics of the waste from surface electrical measurements over the landfill. In this paper, we present and discuss the results of three electrical tomographies carried out in the landfill site of Bellolampo (Palermo, Italy). The main aim of these surveys was to check the integrity of the geomembrane. The application of the geoelectrical methods have allowed to obtain useful information to check the integrity of the geomembrane at the base of the new landfill built in the waste site of Bellolampo, unfortunately on fractured limestones and at high risk of pollution. The comparison between ERT and IPT, performed upstream, above and downstream of the landfill, allowed to identify the electrical properties of the rock, waste and leachate, and detect the possible presence of plumes of pollutant nearby and below the landfill.

Assessment of the Reliability of the SPAC Method in Urban Areas

We have carried out several long-term measures of seismic noise with triangular arrays of broad-band seismic sensors, to investigate the reliability of the SPAC method in urban areas. We investigate the stationary and anisotropy of the microtremor wave field. We have found that the anisotropy of the microtremor wave field can be evaluated analyzing the HVNSR as function the azimuth. Furthermore, we showed as strong anisotropies in the microtremors are often observable in particular at high frequencies (> 1 Hz). We have verified as microtremors can be difficult to considered as ergodic processes, and for this reason time averaging, also even over a long periods of time, cannot replace an appropriate azimuthal sampling.