Roberto Balia

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 ...

Geophysical approach to the environmental study of a coastal plain

In the study of coastal plains affected by soil and water salination, a knowledge of several geological aspects, such as structural features, depth to basement, stratigraphy of sedimentary cover, relationships between the phreatic aquifer and underlying aquifers, and the latters structure, is basic to gaining an adequate understanding of both the causes and possible evolution of salination. In this framework, geophysical techniques can play a very important role.To improve the available geophysical information about the Muravera coastal plain, Sardinia, Italy, which is affected by severe soil and water salination, previously acquired electrical resistivity, reflection seismic and gravity data have been reprocessed, and a new seismic reflection survey has been conducted. Moreover, in order to give better support to the geological and hydrogeological interpretation of geophysical data, three boreholes were drilled. Reprocessed electrical data indicate the presence of a wide, electrically homogeneous low‐re...

A geophysical approach to the enviromental study of a coastal plain. Journal of Applied Geophysics, September 2003, Volume 68, Iusse 5, pp. 14461459 BALIA R.,GAVAUDO? E.,ARDAU F. & GHIGLIERI G. (2003)

In the study of coastal plains affected by soil and water salination, a knowledge of several geological aspects, such as structural features, depth to basement, stratigraphy of sedimentary cover, relationships between the phreatic aquifer and underlying aquifers, and the latters structure, is basic to gaining an adequate understanding of both the causes and possible evolution of salination. In this framework, geophysical techniques can play a very important role.To improve the available geophysical information about the Muravera coastal plain, Sardinia, Italy, which is affected by severe soil and water salination, previously acquired electrical resistivity, reflection seismic and gravity data have been reprocessed, and a new seismic reflection survey has been conducted. Moreover, in order to give better support to the geological and hydrogeological interpretation of geophysical data, three boreholes were drilled. Reprocessed electrical data indicate the presence of a wide, electrically homogeneous low‐re...

Gravity survey and interpretation of bouguer anomalies in the campidano geothermal area (Sardinia, Italy)

A gravity survey of the Campidano geothermal fields and surrounding region was conducted in 1981. It covered an area of 1900 km2 and included 952 uniformly distributed stations. The Bouguer anomaly is generally negative within the Campidano graben, reaching −10 mgal in the central zone, whereas a positive Bouguer anomaly prevails outside the graben, exceeding 20 mgal in several areas. The gravity data were interpreted using spectral analysis and two-dimensional models, to determine the thickness of sediments and andesitic volcanics within the graben. The total thickness of these formations reaches 3000 m in the centre, but is reduced elsewhere, especially towards the sides of the graben. The thermal springs on both the eastern and western sides of the graben are associated with residual positive anomalies and are near very steep gradients in the Bouguer anomaly.

A NEW GEOPHYSICAL CONTRIBUTION TO THE STUDY OF THE CAMPIDANO GEOTHERMAL AREA (SARDINIA, ITALY)

This paper reports that in order to achieve a better definition of the geothermal potential in the area of the Campidano graben (Sardinia, Italy), where there are some thermal springs, a multi-methodological geophysical survey has been combined with recent surface and subsurface geological information. New gravity and magnetic stations, distributed over the northern part of the area (Campidano of Oristano), have been measured to complement already published data for the central-southern part of the graben. Deep dipole-dipole geoelectrical soundings have also been performed along two profiles perpendicular to the strike of the graben. The geological interpretation of the geophysical data allows us to conclude that no shallow, exploitable, geothermal reservoir exists in the survey area.

A magnetovariational study in Sardinia

Abstract A study of geomagnetic field time variations with a magnetometric array was performed in Sardinia during the summer of 1988. Nine fluxgate magnetometers with digital acquisition systems were employed to cover the main structural features of the island. The recordings do not show significant variations in the amplitudes of the horizontal components of the bay-like events, whereas the vertical component amplitudes show a strong southward increase. Fourier maps were drawn at significant periods, and induction arrows and ΔZ ΔH pseudosections were derived.

Assessment of cover-collapse sinkholes in SW Sardinia (Italy)

Abstract The SW part of Sardinia has been afflicted, in recent years, by several cover-collapse sinkholes mostly occurring in low-density population areas. The study area, that lies in the Iglesiente–Sulcis region, is characterized by the cropping out of the Palaeozoic basement related to the South European Hercynian chain, covered with Tertiary–Quaternary sediments. The main rock types that crop out are Palaeozoic metasandstones, metadolostones, metalimestones, shales and metaconglomerates, and Tertiary–Quaternary fluvial–lacustrine continental sediments. The combined application of several geophysical techniques, integrated with boreholes and geotechnical as well as hydrogeological measurements, proved to be very useful and promising in defining in detail the geological context in which each sinkhole has formed. Moreover, the gravity method, even when used alone, proved to be very effective in detecting the regional geological structures to which sinkholes are related. Eventually, the historical analysis of phenomena, the geological knowledge of the Iglesiente–Sulcis area and the results of properly designed geophysical surveys allows the most probable areas for cover-collapse sinkholes to occur in the future to be determined. In fact, this research pointed out that the depth of the sediment-covered Palaeozoic bedrock is one of the major constraints in delimiting hazardous areas, leading to the construction of a preliminary hazard map. This map shows a belt of high risk, and also suggests the areas in which further geophysical and geotechnical investigations should be carried out to estimate the depth of the bedrock.

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.

Seismic reflection imaging of an ultrashallow interface from a P-wave data set with a poor signal-to-noise ratio

It is well known that pitfalls are commonly encountered in the acquisition and processing of shallow reflection data. Although they can often lead to misinterpretations, the obsession with these difficulties can generate an excessively pessimistic attitude and ultimately lead to rejecting data that contain genuine reflections. The authors revisited a data set acquired during an ultrashallow P-wave reflection experiment conducted in December 1996 on a subsurface model characterized by one main, complex interface. The model had been purposebuilt to control acquisition and processing decisions, and to contribute to clarifying at least some of the critical aspects inherent in the ultrashallow seismic reflection method. For the experiment, the acquisition geometry was purposely designed regardless of the known characteristics of the model, considering only a target depth of less than ten metres. The data, which had been acquired with CMP techniques, were generally characterized by a poor signal-to-noise ratio and had been considered completely useless. Initially, inexperience in acquiring and, above all, processing ultrashallow reflection data, in conjunction with misevaluations, had led to disappointing failures. Velocity analysis on reflection data had proved very difficult and standard processing sequences were inadequate. However, in this case a good result was obtained by modelling the near-surface velocity using direct-wave traveltimes analysis and by adopting a simple but segregated processing sequence, which proved necessary due to significant velocity gradients and large relative variations of the reflector depth. Time-to-depth conversion afforded a more than acceptable result.