Soil geochemical methods in archaeo-geophysics: Exploring a combined approach at sites in Scotland

This article explores the application of geophysical and soil geochemical methods to detect archaeological features in three traditionally 'difficult' survey environments in Scotland: wind-blown sands (Bay of Skaill, Orkney), clay (Chesterhall Parks Farm, Lanarkshire) and glacial drift deposits (Forteviot, Perthshire). The results presented here are part of the first research project that systematically tested a combined approach using geophysical and soil characterisation to understand the proxy responses of known archaeological features. First, a range of geophysical techniques [earth resistance, magnetometry, frequency domain electromagnetics (FDEM) and ground-penetrating radar (GPR)] was employed over archaeological targets. Second, the different geophysical results were considered with respect to soil chemical concentrations (total phosphate and multi-element analysis), texture, pH, conductivity, organic matter content and magnetic susceptibility from archaeological deposits, topsoil and subsoil samples. This study demonstrates that, by focusing on the responses of single archaeological features and assessing their physical and chemical signatures, soil composition and processes involved in the 'history' of buried features are of importance in improving our understanding of the reasons behind their detection with geophysical means. For example, at the cropmark site at Forteviot, chemical transformations can be triggered by organic matter accumulation and increased water retention within prehistoric ditch deposits and can have an effect on the type of magnetic contrast. In addition, chemical concentrations revealing anthropogenic organic materials can explain the enhanced conductivity of theoretically impervious features, as illustrated at the Bay of Skaill site. The study also provide insights into how a particular soil environment may affect different geophysical techniques such as the masking effects of heterogeneous glacial drift deposits or deep windblown sands. These types of survey environments characterise many archaeologically rich areas in Scotland, where integrated strategies, such as the one used in this investigation, are the best option to maximise detection of subsurface features and provide confident and augmented interpretations.