Preparing for the future: A reappraisal of archaeo-geophysical surveying on Irish National Road Schemes 2001-2010

Abstract

This document reviews Legacy Data generated from 10 years’ worth of road scheme activity in Ireland to determine how archaeological geophysical surveys could be carried out on national roads in the future. The geophysical surveys were carried out by several different contractors across a range of challenging field conditions, geologies, weather and seasons. The research is based upon the results of linear schemes but also has validity for wider approaches. The findings of this research are based upon the compilation of all terrestrial archaeological geophysical surveys carried out on behalf of the National Roads Authority (NRA), a review of the success or otherwise of those surveys in comparison with ground-observed excavations and in combination with experimental surveys that tested previously held assumptions or knowledge to determine best practice methods for the future. The use and success of geophysical surveys in Ireland differ quite significantly from those in the UK, from where many of the methods of assessment were derived or adapted. Many of these differences can be attributed to geology. Ireland has a very high percentage of Carboniferous limestone geology, overlain mostly by tills and frequent occurrences of peat. These soils can reduce, to some extent, the effectiveness of magnetometer surveys; the most frequently used geophysical technique in Ireland. However, magnetometer data can be maximised in these cases by increasing the spatial resolution to produce effective results. An increase in spatial resolution is also effective generally, for enhancing the chances of identifying archaeological features by discriminating between archaeological and geological anomalies as well as increasing anomaly definition and visualisation of small and subtle archaeological features. Seasonal tests have determined that Irish soils are generally suitable for year round earth resistance assessments although some counties in the southeast of the country may experience very dry soils at the surface during some periods of the year. A variety of sampling strategies were used in the past, however it is now apparent that detailed assessments across the full length and width of a proposed road corridor are the most appropriate form of geophysical investigation. Magnetometer surveys are generally suitable for most Irish soils and geologies, although exceptions apply in areas of near-surface igneous deposits, deep peat and alluvial soils; however magnetometer surveys are not capable of identifying all types of archaeological features and other methods will be required for a full evaluation. Analysis of the Legacy Data has determined that in general the NRA archaeological geophysical surveys were historically used in a very positive way on road schemes. The range of features assessed or identified account for most types of archaeological sites in Ireland. These have provided a significant archive of case studies that will be of benefit to future archaeological geophysical research and will help to protect the globally dwindling archaeological resource that is threatened by development-led or commercially driven projects.