The broad aim of the geophysical survey is to complement and inform the results of
archaeological survey, by providing a rapid and effective means of assessing the subsurface
conditions and the nature and preservation of archaeological deposits, which underlie
complex surface distributions. Within the fenced archaeological site, which partly corresponds
to the ancient agora and is not included in our intensive surface survey, our purpose is
a) to define the limits and shape of the agora,
b) to trace monumental structures, some of which are reported by Pausanias, and
c) to identify its design principles and dimensions by ancient measuring units.
The definition of the agora area can help us establish the size of the insula, which we will also check by surveying other areas of the plateau.
Selective geophysical survey alongside the field walking teams in the wider urban context (outside the agora area) contributes directly to the interpretation and analysis of the intensive surface survey results by assessing the degree and range of representation of subsurface remains to surface assemblages. In addition, geophysical data add to the surface evidence by mapping structures and anthropogenic features not visible above ground. For example, in areas with traces of intensive habitation and activity, geophysical survey could give us a picture of the layout of a typical Sikyonian house or workshop, and beyond that of an entire quarter. Ultimately, we will produce a map which synthesizes the geophysical and other forms of evidence, and will represent in as much detail as possible the archaeology of the plateau, including artifact densities, architectural remains and subsurface structures. The creation of such a map is made all the more important today by the rapid expansion of new home construction across the plateau which threatens many well preserved deposits.
Geophysical survey involves both electrical and magnetic methods over selective squares, 20x20 m or smaller, oriented along the cardinal points (N-S, E-W). Two instruments are used: one for the electrical resistivity (Geoscan RM15 along with the Multiplex MPX15) in a Twin Probe array with different mobile probe spacing, which allows the mapping of different depths at the same time, and a gradiometer (Geoscan FM36, Fluxgate gradiometer) for the measurement of the vertical gradient of the magnetic field of the earth. The distance between the electrodes is set to 0.5 m, and the gradiometer is held 0.5 m from ground surface. The depth of the geophysical survey ranges from 1.5 m to ground level. Measurements are taken every 1 m throughout the area. In processing geophysical data all measurements are brought to the same reference system using grid and line equalization. Subsequently, selective de-spiking and compression of dynamic range are applied in order to remove overly high and low measurements. The last stage comprises the analysis of the geophysical irregularies (anomalies), their mapping and interpretation.