Geophysical prospecting at archaelogical sites with some examples from northern Greece

The application of geophysical techniques to archaeology is well established. They are used to provide information for planning excavations, either at the outset or while work is in progress, so that the excavations may proceed more quickly and therefore more cheaply. In this paper the role of the geophysics is first discussed, and illustrated with a generalised flow chart to show the steps which should be followed in the investigations. Then several examples of magnetic and electrical surveys, mainly in northern Greece, are presented. Special attention is given to misleading factors which can confuse the interpretation of the data: geological factors, the effects of recent human activities such as the dumping of rubbish and undocumented excavations from the last century; and complicated situations where many features of interest may be buried close to or beneath each other. For the sake of completeness, other geophysical techniques which have been used in archaeological searches are briefly reviewed. Referring to Fig. 1, the first step after undertaking an archaeological project is to consider carefully the archaeological aspects. For a particular site, the era of occupation is usually known or suspected from historical evidence. It is therefore possible to guess the types of structures and remains for which excavation will be taking place. The ancient quarries are usually known for a particular location, so the construction materials which are likely to have been used are known, and their physical properties can be considered with respect to geophysical surveying techniques. The corresponding properties of the overburden may be estimated from geological and geomorphological data, and if a rough estimate of burial depth can be made it is possible to predict the size of geophysical anomalies with respect to background noise. Aerial photos, if they exist, information from the records of local authorities, and information provided by local inhabitants can all be of great importance. Field inspeetion is always necessary before undertaking any geophysical survey. If the site is in an area which is being investigated for the first time, then it is recommended that some field tests be made. Laboratory measurements of the susceptibility and conductivity of the soil and other samples can help in assessing the applicability of geophysical techniques and, later, in the interpretation of results. In some cases it may be desirable to gather samples for geochemical analysis during the field inspection, for example to measure the concentration of organic gases from concealed coal mines. The procedures described above may be regarded as the preliminary stage. It is now necessary to decide whether geophysical techniques should be employed or not. If so, then the acquisition parameters must be chosen for optimum detectability and resolution of buried features. After the survey has commenced, and preferably before it has been completed, the initial results should be considered together with the information gleaned during the preliminary stage. The results need to be sufficiently clear and precise to help the archaeologist decide on excavation procedures.