M.J. Senos Matias

A geophysical and hydrogeological study of aquifers contamination by a landfill

Abstract Landfills are the classical solution for waste disposal. During the last years there has been a growing concern about the effect of landfills in public health, because leaching water can contaminate nearby aquifers. Hydrogeological studies are very important to investigate and remedy these contamination problems but an integrated use of both geophysical and hydrological methods can be even more effective. In this paper, the groundwater contamination by a landfill, located on very porous and permeable formations, is assessed by both geophysical and hydrogeological techniques. Bearing in mind the geological and hydrogeological characteristics of the area, electrical methods were chosen to carry out the geophysical survey. At first, electromagnetic methods were used and a ground conductivity survey was carried out to determine the contamination plume. Then several resistivity soundings were proposed and two resistivity pseudo-sections obtained so that a more detailed study of the contamination was accomplished. From the geophysical data interpretation, strategically located boreholes were drilled, water samples obtained and chemical analysis carried out. Finally, the overall results are integrated and discussed as well as the merits and disadvantages of the techniques that have been employed.

Square array anisotropy measurements and resistivity sounding interpretation

Abstract The use of geophysics in shallow investigations, such as in environmental, geotechnical and hydrogeological studies requires the development of fast, reliable, high-resolution field and interpretation techniques. When resistivity methods are used, limitations can be expected if ground inhomogeneities and anisotropy are present. In these cases, 1D surveys are not suitable and 2D approaches need geological information so that adequate survey line orientation is chosen. On the other-hand, 3D techniques could prove to be expensive and time-consuming, both in the field and at the interpretation stages. Nonconventional electrode arrays, such as the square array, have been proposed and could provide useful information on local inhomogeneities and anisotropy. However, interpretation material for these arrays is still difficult to obtain. In this paper, it will be shown how conventional resistivity sounding curves can be used to derive ground anisotropy parameters when square arrays are applied. At first, two-layer sounding model curves are interpreted with traditional 1D resistivity curves and estimates for the model apparent resistivity, anisotropy, and strike are derived. Then, several field curves for two, three and four layers are fully interpreted, and it is demonstrated how to obtain depths, apparent resistivity, strike and apparent anisotropy estimates for concealed earth layers.

Some neotectonic indicators in quarternary formations of the northwest coastal zone of Portugal

Abstract In the coastal zone of northwest Portugal, between the Minho River and the Serra da Boa Viagem/Mondego Cape, evidence of structural (overthrust, tilting), sedimentological (contorted bedding, slumping) and chronological (diachronous bedding) features have been detected during the last years, which seem to indicate the occurrence of neotectonic deformation in Quaternary deposits. Recently, geophysical campaigns were initiated in order to locate the presence of tectonic accidents that could be responsible for the observed field evidence. To complement the geophysics, 45 m deep cores were drilled at some points of the area. Till now, just preliminary data are available. However, they confirm the post-Pleistocene neotectonic activity in the coastal zone of northwest Portugal.

An investigation into the use of geophysical methods in the study of aquifer contamination by graveyards

Graveyards are the traditional way of dealing with the deceased and often their location is unsuitable so that there is a potential contamination danger for local aquifers and soils. Such a contamination is complex as organic, inorganic and biological pollutants will be present. This problem has been studied by direct methods, i.e. boreholes. However borehole location can be difficult and so it is proposed here to use geophysical multiparameter surveys, as well as hydrochemistry studies, to investigate the contamination in the vicinity of two graveyards located in very porous and permeable formations. Based on the geophysical interpretation, two shallow boreholes were drilled and groundwater sampling confirmed local aquifer contamination. The geophysical response is shownto depend on the water table depth, and when this is close to the ground surface, it is demonstrated that geophysical methods can be used to map contamination plumes originated in the graveyards and, therefore, assist borehole location.

Electrical strike imaging and anisotropy diagnosis from surface resistivity measurements

Field resistivity measurements are often affected by the inhomogeneity and anisotropy of subsurface formations. The crossed square array is an electrode array particularly suited to investigating these problems. Herein, its use to obtain concealed formations electrical strike pseudosections will be shown. Field and 2D data modelling will be discussed and images of computed strike in depth and space will be presented and interpreted. Data modelling was carried out by using theoretical solutions based on the application of image theory to simple 2D outcropping models. More complex 2D concealed model solutions were obtained by physical analogue modelling. Furthermore, it has also been difficult to distinguish ground inhomogeneity and lateral resistivity variations from anisotropy in field data. Hence, it will also be shown that with crossed square array data anisotropy analysis it is possible to distinguish fracture anisotropy and structural anisotropy from media that conform closely to real anisotropy situations.

A geophysical survey on the archaeological site of Mugardos (NW Iberian peninsula)

Abstract Archaeological indications near Mugardos (Ferrol, NW Spain) suggest the existence of a Roman settlement. In fact, in the area were found pavements, walls with north-south and east-west orientations and some structures that endured heating. These remains are covered by soil, more than 1 m thick, and lie over schists. In order to determine the archaeological potential of the area and to delimit future excavations a geophysical survey, consisting of a joint resistivity and magnetic survey, was planned and carried out. The square array of electrodes was used and the data are discussed as apparent square array resistivity maps and azimuthal inhomogeneity ratio ( AIR ) maps. The magnetic survey included total field measurements using sensor heights of 0.30 and 2.30 m above the ground, so that a magnetic gradient could be computed. A combined interpretation of both resistivity and magnetic data is discussed. Later excavations have confirmed the geophysical interpretation.

Odd-even Pole-pole Array and 3D Resistivity Surveys in Urban and Historical Areas

Modern and historical buildings may show some degree of subsidence resulting from foundation deterioration and local geological conditions. Hence, buildings stability can be affected and restoration plans must be envisaged. Resistivity methods have been used to investigate local conditions, providing 3D images of the soil under man made structures and hence contributing to the delimitation of hazardous areas and pathologies. However these techniques require the deployment of a grid of electrodes, which can be difficult to accomplish because of physical limitations and of the buildings nature that cannot be damaged. To overcome these problems special arrays have been used (L, Corner, Square arrays, etc). Here in it is proposed to use the “Odd-Even Pole-Pole Array” to study the ground under a contemporary building and under a high historical value XIV century Abbey, both showing evidence of subsidence. Field data quality is also addressed and it is proposed to identify low quality data to be expunged so that modelling is improved. It is also shown how to estimate resistivity values from data quality tests, to carry out further zonation, locate hazardous areas and to enhance modelling.

Sedimentary structure of the Nazaré coastal dunes (Portugal)

The internal structure of costal dunes located south of Nazaré was analyzed using Ground Penetrating Radar. These coastal dunes comprise stabilized dunes, located in the inner part of the dune field, and foredunes, both separated by a dry dune slack. The radargram analysis allowed the identification of five bounding surfaces that define six main aeolian sand units. The Units I, II, III and IV correspond to the progradation of foredunes seaward (NW), while Units V and VI represent the migration of a parabolic dune and a blowout, respectively, to southeast. A strong reflector separating Units I, II and III from Unit V is interpreted as a paleosoil, suggesting a break in aeolian sedimentation of unknown duration, but with sufficient time for soil development. The absent of the paleosoil over the Unit IV leave some doubts in relation to its relative age, although is indicative of the occurrence of an erosive event. Radar stratigraphic analysis provided a relative chronology of units that will be used as a framework for selecting sampling points for future absolute dating and sedimentological studies.

Detecting oil-contaminants by time domain induced polarisation

Oil products constitute one of the main types of organic contaminants in the soil. In order to understand the effects caused by oil contamination the polarization mechanisms in soillorganic mixtures must be understood.

Some examples of the use of GPR in engineering and environment at the university of Aveiro, Portugal

Ground Probing Radar techniques can be used to analyse projects at shallow depths of investigation (Reynolds,1997) At the Department of Geosciences, Universidade de Aveiro, Portugal, GPR techniques have been widely used in Archaeology, Geotechnics, Hydrogeology and Environrnental problems. However, these techniques should be always used in conjunction with other geophysical methods so that a better interpretation is proposed, as overall experience in GPR data processing and interpretation is developing rapidly. In Archaeological studies GPR has been used widely to identify potential escavation areas. Therefore an Archaeological application of GPR in conjunction with seismic refraction from Praca do Infante, Oporto (North Portugal) is discussed. GPR was used to delineate bedrock topography and depth, as well as to locate diffractions from potential buried structures. GPR interpretation is supported by seismic refraction data interpreted with the Generalized Reciprocal Method (Palmer, 1980) and Intercept Time Method (Adachi, 1954). The overall geophysical model was tested and ajusted using local mechanical soundings. The infonnation was integrated using a GIS system.