Antonino D'Alessandro

Simultaneous seismic wave clustering and registration

In this paper we introduce a simple procedure to identify clusters of multivariate waveforms based on a simultaneous assignation and alignment procedure. This approach is aimed at the identification of clusters of earthquakes, assuming that similarities between seismic events with respect to hypocentral parameters and focal mechanism correspond to similarities between waveforms of events. Therefore we define a distance measure between seismic curves in R^dd>=1, in order to interpret and better understand the main features of the generating seismic process.

A Low Cost Customizable Micro-ROV for Environmental Research - Applications, Advances and Challenges

Summary Several shallow submarines areas are characterized by considerable releases of hydrocarbon gases, which, during earthquakes or volcanic activity, may suddenly and violently occur. In some instances, changes in chemical and physical properties of gas emissions, which are generated by geothermal systems connected to the volcanoes’ plumbing system, are important precursors of volcanic activity and useful indices for the study of the geothermal field. Furthermore, some superficial structures such faults, landslide slopes or archaeological relics should be studied with such a high detail that geophysical and bathymetric surveys are unable to provide. In response to the needs increasingly felt of a completely customizable low cost tool for geological/geophysical/geochemical investigation of shallow submarine system, in the framework of the ROVER project, starting from an international project called OpenROV, is being implemented a micro-ROV for multidisciplinary environmental investigation. In this paper, we will describe the project progress, with particular reference to the first tests conducted in pool and in sea water, the geophysical/geochemical instrumentation of which will be equipped and the automatic control algorithms that are being developed for the semi-automatic navigation.

Drones - New technologies for geophysics?

Drones, also referred as Unmanned Aircraft Systems (UAS), Unmanned Aerial Vehicles (UAV) or Remotely Piloted Aircraft (RPA), are aircraft without pilot on board. USA is today the leading country in the field of UAS but recently, seven European countries have pledged to launch joint programs for drones, with the aim of producing the next generation of UAS before 2020. According to some studies on the potential market of UAS in Europe, the geoscience sector is definitely one of the most promising. In this paper, we bring a short description and classification of drones, according to their sized and shaped, with particular attention to the advantages and drawbacks of each type in reference to the possible employment in geosciences applications. Currently the drones are used for geological and topographic mapping, coastal control, landslide inspections, etc., and are capable to integrate geophysical sensors like magnetic, electromagnetic, infrared, radar, natural gamma ray sensors and etc. UAS technology have several advantage over conventional airborne geophysics like resolution, accurancy, cost, etc. We are witnessing the birth of a new branch of aviation, which could be profitably applied to geophysics measurements.

Centroid-based Cluster Analysis of HVSR Data for Seismic Microzonation

atasets acquired for studies of seismic microzoning in various urban centers of Sicilian towns, have been used to test clustering analysis through a non-hierarchical centroid-based algorithm. In this context clustering techniques may be useful to identify areas with similar seismic behaviour through HVSR data. Centroid-based algorithms generally require the number of clusters, k, and the initial centroid coordinates to be specified in advance. This aspect is considered to be one of the biggest drawbacks of these algorithms. The proposed algorithm doesn’t limit the number of k clusters and choose the initial centroids automatically from the data set. Azimuthal variation of the H/V peaks was also taken into account. Finally different partitions obtained using the centroid-based algorithm were superimposed on the geological map of the analyzed sites to identify possible correlations with geology and topography. The obtained results underline how the most appropriate clustering algorithm for a particular site often needs to be chosen experimentally. In fact in many cases the choice of the partition is strongly linked to the choice of parametric distance and to geological knowledge, while in other cases, the results showed similar results regardless of a priori choices.

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.

Seismo-stratigraphic model of La Bandita area in the Palermo Plain (Sicily, Italy) through HVSR inversion constrained by stratigraphic data

Ambient noise Horizontal-to-Vertical Spectral Ratio (HVSR) technique is commonly used approach to obtain 1D models of the shear-wave velocity in the shallow surface of an investigated area. However, obtained models can have a wide margin of uncertainty if inversions have not been appropriately constrained by detailed stratigraphic information. An application of HVSR inversion constrained by lithostratigraphic data is presented in order to verify the effectiveness of this technique for purposes of geological and geophysical reconstruction of a sedimentary basin in a densely urbanized area. This is often the case of seismic microzonation studies, in which almost all the information derives from near surface stratigraphic drillings, since other geophysical methods are logistically difficult to carry out. In our work, we used stratigraphic constraints derived from 93 superficial bore-holes whose depth rarely exceeds 30 m. In an area called La Bandita, located in Palermo Plain (Sicily, Italy), a geophysical survey was performed by means of 55 microtremor recordings. Part of these was distributed randomly, while others very close to the available stratigraphic perforations. The reconstruction of the stratigraphy in the studied area has been obtained by a review of the main stratigraphic sequences and by a consequent stratigraphic three-dimensional modelling. HVSR curves have been interpreted taking care the thicknesses of the near surface successions derived by the stratigraphic 3-D model. The results, in terms of vertical profiles of the shear-wave velocity, have been interpolated to obtain a 3D seismic model. This has been used to extract basic information to identify and reconstruct the seismic bedrock and the main geological boundaries that were not directly identifiable by means of only stratigraphic logs. It results that the bedrock is affected by a fault system that generated adjacent depressions where Quaternary successions deposited.

Low-cost underwater navigation systems by multi-pressure measurements and AHRS data

This paper deals with accurate navigation for underwater remotely operated vehicles. A feasibility study for a multi-pressure device to be mounted on an ROV is presented. The device can provide accurate estimates of a ROV orientation and angular speed. It is based on the well-known total pressure principle, also used in a Pitot tube, and allows reconstruction of static and dynamic pressures, which in turn provide good estimates of the ROVs orientation and rotational speed, respectively. An appealing feature of the proposed device is its ability to provide accurate estimates even for low-speed movements.

Micro-ROV instrumental customization and canister design under pressure

Remotely Operated Vehicles are very useful tools for submarine exploration. We have customized a Micro-Class ROV for shallow marine geophysical and geochemical research. Due to our specific requirements, micro-ROV frame, fairing and pressure canisters must be of lightweight, resistant and non-magnetic material. Based on its mechanical and optical properties, for the realization of the micro-ROV pressure canister, the PMMA was chosen. By means of FEM analysis, we have investigated four pressure canister geometries under different external pressures. We have showed that the maximum depths that should be considered in a possible micro-ROV customization are that obtained with Von Mises yield stress criterion. By means of FEM analysis we have identified a linear relationship both between stress and water depth and strain and water depth.

Mechanical aspects in a tide gauge station design

In this work the mechanical design of a tide gauge station is presented. The structure consists of a surge pipe anchored to a pier and partially immersed in seawater. Despite such conceptual simplicity, this design required a careful analysis under different points of view to ensure a long life in the hostile environment in which the station will be installed and to guarantee an optimal sea level monitoring. The structure of the tide gauge station was 3D modeled with the main purposes of simplifying the production process and the installation phase, and materials were chosen to withstand the aggressiveness of the marine environment. A modal analysis of the surge pipe was performed to avoid dangerous resonances; the static and fatiguing loads were considered in the structural dimensioning. This document, though dealing with a specific case, could be considered as a guideline suitable for the design and the realization of other tide gauge stations.