Gian Piero Deidda

Some considerations on shallow seismic reflection surveys

Abstract High-resolution shallow seismic reflection surveys require more attention to the choice of source and configuration, receivers and recording geometry for optimizing data acquisition than conventional oil exploration surveys. Moreover, some standard processing techniques to increase signal/noise (S/N) ratio need special accuracy (for example, surgically precise removal of early-time coherent noise and iterative, small time shift static corrections). This paper compares results obtained using different sources at two test sites: explosive, cap, shotgun, hammer and weight drop. Data from experiments using geophones with different natural frequencies and using various acquisition geometries are also compared. In data processing, it is demonstrated how increasing the S/N ratio for high-resolution results requires special consideration in some common processing steps (F–K filter, first arrivals muting, elimination of air wave and static corrections). The comparison, based on shot gathers and stack sections, shows that attenuation of high frequencies by the earth is the most significant influence on the spectral properties of the data, as expected the source itself also does have some influence on frequency content, depending to some extent on surface conditions. The high-velocity explosive sources produced the highest frequency reflections and best S/N ratio, because they have higher energy related to higher burn/blast velocity and source containment then the other sources and they are used in hole (i.e. below ground surface where the air wave energy is more attenuated) but the shotgun also an explosive source was reasonably comparable to high explosive when used in hole. Special care must be taken during processing to insure artifacts are distinguished from real reflection events.

An ultrashallow SH‐wave seismic reflection experiment on a subsurface ground model

An SH‐wave seismic reflection experiment was conducted to evaluate the feasibility and cost effectiveness of reflection imaging ultrashallow targets commonly encountered in engineering, groundwater, and environmental investigations. It was carried out on a purpose‐built subsurface ground model consisting of a concrete layer, at a depth from 2.85–5 m, and a low‐velocity overburden (<80 and 150 m/s for S‐ and P‐waves, respectively), constituted of filling material, with the water table 2.60 m deep. High‐quality CDP data, acquired by using a 10‐kg sledgehammer and newly designed horizontal detectors, allowed us to obtain an extremely detailed stacked section with a minimal amount of processing. Uncertainty in determining the depth and horizontal dimensions of the concrete model was estimated to be 0.2 and 0.3 m, respectively; however, the dominant frequencies lower than 150 Hz, the low‐transmission coefficient at the upper interface, and the relatively high velocity (900 m/s) of the concrete layer prevented ...

Comparison of standard horizontal geophones and newly designed horizontal detectors

To increase the speed and efficiency of shallow seismic SH‐wave data recording and to decrease acquisition costs, we have designed, constructed, and tested a new velocity tool that detects horizontal movements better than standard horizontal geophones. A comparative evaluation of significant characteristics and field performances of this new detector was conducted through laboratory and field tests on two sets of receivers: one consisting of 24 prototypes of the new detector and the other consisting of 24 standard horizontal geophones. Laboratory measurements revealed similar behaviors of impedance curves and geophone responses for the two types of detectors, but the impedance amplitudes and the frequency‐response amplitudes of the new detector were twice those of the standard geophones. However, comparison of the horizontal‐to‐vertical (response) ratios, proved that the new detector better discriminates (6 dB on average) between the horizontal and vertical excitements. Field data proved that the energy, ...

Noninvasive characterization of the Trecate (Italy) crude-oil contaminated site: links between contamination and geophysical signals

The characterization of contaminated sites can benefit from the supplementation of direct investigations with a set of less invasive and more extensive measurements. A combination of geophysical methods and direct push techniques for contaminated land characterization has been proposed within the EU FP7 project ModelPROBE and the affiliated project SoilCAM. In this paper, we present results of the investigations conducted at the Trecate field site (NW Italy), which was affected in 1994 by crude oil contamination. The less invasive investigations include ground-penetrating radar (GPR), electrical resistivity tomography (ERT), and electromagnetic induction (EMI) surveys, together with direct push sampling and soil electrical conductivity (EC) logs. Many of the geophysical measurements were conducted in time-lapse mode in order to separate static and dynamic signals, the latter being linked to strong seasonal changes in water table elevations. The main challenge was to extract significant geophysical signals linked to contamination from the mix of geological and hydrological signals present at the site. The most significant aspects of this characterization are: (a) the geometrical link between the distribution of contamination and the site’s heterogeneity, with particular regard to the presence of less permeable layers, as evidenced by the extensive surface geophysical measurements; and (b) the link between contamination and specific geophysical signals, particularly evident from cross-hole measurements. The extensive work conducted at the Trecate site shows how a combination of direct (e.g., chemical) and indirect (e.g., geophysical) investigations can lead to a comprehensive and solid understanding of a contaminated site’s mechanisms.

Regularized solution of a nonlinear problem in electromagnetic sounding

Non destructive investigation of soil properties is crucial when trying to identify inhomogeneities in the ground or the presence of conductive substances. This kind of survey can be addressed with the aid of electromagnetic induction measurements taken with a ground conductivity meter. In this paper, starting from electromagnetic data collected by this device, we reconstruct the electrical conductivity of the soil with respect to depth, with the aid of a regularized damped Gauss–Newton method. We propose an inversion method based on the low-rank approximation of the Jacobian of the function to be inverted, for which we develop exact analytical formulae. The algorithm chooses a relaxation parameter in order to ensure the positivity of the solution and implements various methods for the automatic estimation of the regularization parameter. This leads to a fast and reliable algorithm, which is tested on numerical experiments both on synthetic data sets and on field data. The results show that the algorithm produces reasonable solutions in the case of synthetic data sets, even in the presence of a noise level consistent with real applications, and yields results that are compatible with those obtained by electrical resistivity tomography in the case of field data.

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...

Synergical Use of Passive and Active Methods to Reconstruct the Subsoil in Urban Areas

In urban areas it is often difficult to carry out not only direct but also indirect investigations, such as geophysical surveys. In fact, because of traffic noise, power lines and pipelines it is not possible to obtain reliable and repetitive measures using seismic, magnetic, electrical and electromagnetic methods. In addition, the asphalt and the paving of the sidewalks don’t facilitate the placing of geophones and electrodes. So only a few methods can usefully be applied. The gravimetric method in particular makes it possible to reconstruct maps of vulnerability that can be a useful document for the planning of appraisal or the prevention of risk of collapse for buildings. Finally the gravimetric maps represent a basic document for municipal urban development plans. Recently a seismic passive method was also developed that uses a 3-component seismic system called Tromino, which enables us to measure over time for periods of several hours, the spatial components of acceleration and velocity produced by natural tremors and vehicular traffic. Recently seismic land streamer devices have also been produced, that allow us to make profiles of seismic refraction on unpierceable areas and also to drag the system along a profile increasing its length.

Using gravimetric method in territorial planning

In territorial planning, the knowledge of shallow underground conditions is of primary importance. In fact, shallow subsurface structures, including geological, archaeological and other man made underground structures, can largely influence the process of making planning decisions. Town-planning means first off all, planning the transportation system, subway lines, underground parking, and therefore hydrogeology, geothechnical conditions and archaeological sites location should be know very well, In this context, geophysical methods can be very useful tools but most of them cannot be used properly due to severe obstacles connected with noise level, places accessibility and budget limitations. Among the geophysical methods, gravity seems to be the less influenced by these problems. Although gravity has been traditionally used for regional and large scale investigations, several authors have recently demonstrated its potentiality in engineering surveys, both in standard (Arzi, 1975; Butler, 1991; Nozaki and Kanemori, 1996; Yule et al., 1998) and vertical gradient measurements (Fajklewicz, 1976; Butler, 1984; Klingele et al., 1991; Sambuelli and Ranieri, 1995). The aim of this communication is to provide an additional case history which further witnesses the convenience of small scale gravity surveys in urban areas and that they can provide important information for town-planning engineers.

Geophysical investigations unravel the vestiges of ancient meandering channels and their dynamics in tidal landscapes

Whether or not one can detect relict signatures of the past imprinted in current landscapes is a question of the utmost theoretical and practical relevance for meandering tidal channels, owing to their influence on the morphodynamic evolution of tidal landscapes, a critically fragile environment, especially in face of expected climatic changes. Unravelling the sedimentary patterns of ancient channels is an expensive process that usually requires high resolution sediment coring. Here we use a novel inversion process of multi-frequency electromagnetic measurements to reveal the signature and characterize the dynamics of a salt-marsh paleo-meander in the Venice Lagoon. We show that the ancient meander migrated laterally while vertically aggrading, developing a peculiar bar geometry which is less common in analogous fluvial meanders. The observed point-bar dynamics and the associated architectural geometry are consistent with remote sensing and borehole data and contrast with current assessments of tidal meander morphodynamics mediated from classical fluvial theories. In addition, the proposed technique, rapid and non-invasive, bears important consequences for detecting buried stratal geometries and reconstructing the spatial distribution of ancient sedimentary bodies, providing quantitative data for the description of landscape evolution in time.

Grid/Cloud Computing as New Paradigm for Collaborative Problem Solving and Shared Resources Management in Environmental Sciences

Cloud computing is establishing worldwide as a new high-performance computing paradigm that offers formidable possibilities to industry and science. In this chapter, we discuss the impact of this paradigm change to scientific research. After introducing the basic concepts of cloud computing and discussing briefly the recent technological developments that led to its emergence, two cloud computing portals are presented as examples for a trend that will drastically change the way in which scientific research will be done in the future. We will discuss and exemplify why a cloud computing science portal offers, besides the obvious advantages over locally installed software packages such as computing power and storage capacity, completely new possibilities for scientific research, collaboration, education, and presentation of reproducible results.