When the time is right: the impact of weather variations on the contrast in earth resistance data

The successful detection of archaeological features through electrical methods of survey requires a contrast in electrical conductivity between these features and their surrounding soil matrix. Several studies have attempted to identify the 'right time' to do earth resistance surveys, but with limited success. Practitioners need a simple method of using widely available precipitation data for the forecasting of good electrical contrast in archaeological and electrical surveys. By applying a simplified soil and moisture model to seasonal survey series in different years and at different sites, we show how an index formed from the last 15 days of precipitation and its monthly average can be used as a predictor for contrast in many archaeological situations.