Ground Penetrating Radar Survey along the streets of York City Centre Roman York: beneath the streets GPR Report 3

Abstract

5 ha of the streets of York city centre were surveyed using a step-frequency GPR system, dragged along the street surface at low speed. This was the first large-scale trial of its kind in the UK focused on testing the observability of archaeological features in urban environments. The GPR frequency ranged from 200-3000 MHz, which in theory meant that it should be good for capturing data from the surface down to 3m or more, though anything other than the largest archaeological features would probably lose resolution beyond 2m. The survey registered details about the road makeup, services, and utilities. It had been imagined that the density of modern utilities might make seeing the archaeology between or through them impossible. However, it was noted that while dense in some areas, in many parts of the city there was plenty of space between linear features to observe any potential phenomena. Unfortunately, no identifiably Roman features were discovered. The problem was not caused by utilities, but significant signal attenuation, with few features visible below about one meter. No archaeology was observed below this depth, even when the survey was above known archaeological features. To investigate if this was due to soil ground conditions or equipment, two areas were resurveyed using a 250 MHz detector which had worked well on open grass areas within the city (Reports 1 & 2 in this series), but it too suffered from significant signal attenuation below 1 m. This may relate to the road makeup, and beneath that the attenuation from clay and sedimentary deposits. This report is primarily of use for understanding methodological issues, from the archaeological point of view. Nonetheless, the data in the repository may be of use to anyone commissioning PAS 128 Utilities surveys in the streets in York city centre.