Geophysics – The Survey
In undertaking the survey, a 30 x 30 m survey grid was laid out over all those parts of the site surveyed by us, and located using a Total Station. Within each square data are collected every 0.5 m along a sequence of traverses 1 m apart. This produces 1800 measurements per 30 m square over the survey area. Resistivity survey and magnetometry often provide complementary results but as magnetometry can generally be undertaken more rapidly, our work at Portus generally only used this technique, although resistivity has been deployed to explore areas of particular interest or uncertainty. It is important to appreciate that geophysical survey methods only work where there are significant differences between the magnetic properties (in the case of Portus) or electrical resistance of the underlying archaeological features and the surrounding soil matrix. The instruments used for our own surveys are the commercially available Geoscan RM15 (for resistivity survey) and FM36 (for magnetometry). Data are logged automatically and can be directly down-loaded to portable computers and thus processed and viewed using commercially available Geoplot and InSite software packages. These programmes provide a variety of means for processing and plotting the results to maximise their visual quality.
Results of the Magnetometry Survey undertaken at Portus
The above image shows the results of the magnetometry survey at Portus, and should be looked at in conjunction with the plan showing the main topographical features of the site under Topographical Survey, and interpretative phase plans for the Pre-Trajanic Port, Trajanic Port and Late Antique Port under Roman Background. The dark lines and blotches represent high magnetic anomalies which represent the presence of buried walls, buildings and canals. The results thus reveal the presence of a range of buried buildings around the hexagonal Porto di Traiano at the centre of the image, as well as a canal, road, aqueduct and associated buildings between it and the river Tiber on the right hand side of the image.
