Geophysics at Portus

The next stage of fieldwork involved two complementary elements: sub-surface mapping and the systematic collection of surface finds. When considered together these enable us to understand the spatial organisation of buried buildings and, to some degree, their chronology and function. During this phase of the Portus Project resistivity and magnetometry surveys were undertaken. Resistivity survey measures small scale variations in the electrical resistance of the soil: thus features such as walls show up as bands of higher electrical resistance. Magnetometry measures minor variations in the magnetism of the soil, again enabling features such as walls, roads hearths and kilns to be identified under appropriate conditions. In each case the key to success lies in being able to survey substantial areas using consistent methods of data collection, and mapping the results precisely in relation to the topographic information.

A 30 x 30 m survey grid was laid out with data 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 was deployed to explore areas of particular interest or uncertainty. The instruments used were a Geoscan RM15 (for resistivity survey) and FM36 (for magnetometry), and the data processed and viewed in Geoplot and InSite software packages. These programmes provide a variety of means for processing and plotting the results to maximise their visual quality.

The image shows the results of the magnetometry survey at Portus. 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 Trajanic Port 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.