F. Almeida

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.

Examples of the Use of Non-Invasive Techniques for the Evaluation of Stone Decay in Portugal

Today experts agree that precise damage diagnosis is the key to comprehensive characterization, interpretation, rating and prediction of stone damage. It provides vital information for monument conservation and sustainable preservation. Better understanding of the stones used in monuments and the factors, processes and characteristics involved in stone decay is therefore essential to the sustainable preservation of cultural heritage. A frequent and major obstacle to studying stone decay in monuments is the impossibility of touching or obtaining samples for study in the laboratory or even in-situ. The aim of this paper is to present the results of three non-invasive geophysical (3-D electrical resistivity and seismic refraction) and geochemical (soluble salts typology and distribution) techniques, that were used to diagnose stone damage in case studies involving Portuguese cultural heritage. Different techniques were applied based on decay typology or observed phenomena.

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.

An All In Resistivity and Seismic Portable Field System

Summary A data acquisition system based on Lab View virtual instruments was developed to allow both electrical resistivity and seismic wave velocity measurements with no need for switching equipments and standard multicables. This is achieved by simply using eight electrodes as connectors for four geophones when changing from electrical to seismic methods. The Pole Pole resistivity array was adopted using the eight electrodes for potential measurements while the current electrode was manually displaced thus using less cable. The tomographic refraction method was based on four common receiver gathers. Each receiver gather consists of 16 shots one metre apart fired manually This apparatus enables the use of a simple, lightweight and also cheap field system with a good spatial seismic and electrical parameter resolution for multipurpose site characterization. A controlled experiment was done to validate the system.

Exploring New Ways of Integration, Visualization and Interaction with Geotechnical and Geophysical Data

The work presented in this paper aims at exploring new ways of integrating, visualizing and interacting with geotechnical and geophysical data that may be more rich and interactive than those offered by most current Geographic Information Systems (GIS). Some visualization techniques enabling simultaneous visualization of the several data types available in our case study are proposed. Moreover, methods were developed to guide experts while defining layers and other relevant geological structures. The work is still in an early stage and is main goal has been assessing the validity and adequacy of the proposed techniques to the specific geotechnical and geophysical data under consideration.

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.

Impact shear wave generator for dynamic shallow soil characterisation

P and S waves are used to determine Poisson ratios that are useful for micro zoning in seismic engineering studies. P waves are easily generated by many vertical impact sources. On the other hand S waves are commonly obtained by a polarised sideways impact on a heavy beam perpendicular to the line of geophones (MOONEY, 1973). Any S wave source always produces some P wave component that can be extracted from the paired field data. Each pair consists of two opposite polarity records generated by inverted directional impacts. Recently developed sources are based on a vibration sweep pulse (GHOSE et al, 1996) requiring dedicated hardware and processing software and thus making it impossible to use with standard engineering seismographs.

Time domain induced polarisation experiments on gasoline contaminated samples

Recently environmental studies have used geophysical methods to delineate oil contaminated sites (Vanhala, 1997). Organic liquids have a high resistivity and thus induction methods are not weil adapted to fine contaminated areas. On the other hand Induction Polarisation has been widely recognised as a useful method for the detecting organic electrical response In field ( Vanhala et al, 1992) and in laboratory (Vanhala et Soininen. 1995) experiments. The later author had a frequency domain approach (SIP) to the problem. On the other hand our initial work carried out in the time domain (Sumner, 1976) and is still in a laboratory phase. In order to study the induced polarisation effect in gasoline contaminated soil samples a GPIB controlled system was developed to carry out electrical measurements as it is shown in Fig.l. This System is able 10 obtain measurements at a rate of ten samples per second. Specially developed porous ceramic electrodes were built to obtain suitable electrical current contacts. Non polarizable electrodes SilverlSilver Chloride were used to measure potential.

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.

A comprehensive resistivity and GPR investigation in a Roman thermal site

The village of Vizela, Northwest of Portugal, has a important Roman thermal site. Excavations related with sewage system development detected a Roman occupation level at the level of 99m, that was in danger of being damaged. In order to design a suitable sewage system that would preserve the Roman findings a Geophysical survey was carried out.