Long-term monitoring to inform the geophysical detection of archaeological ditch anomalies in different climatic conditions

Contrasts in electromagnetic properties between the target feature and surrounding soil are of importance for detection of archaeological features with Ground Penetrating Radar. These vary because of changing climatic conditions and soil type and are currently poorly understood. Long-term in situ monitoring of apparent relative dielectric permittivity, bulk electrical conductivity and soil temperature over two archaeological ditch features on sites with different soil types (one clay and one free draining) was employed to understand the detection dynamics and processes by which these properties change over time. Results were correlated with geotechnical properties of the soil for both archaeological ditchfills and the surrounding natural soil matrix and previously derived laboratory relationships between water content, temperature and geophysical properties to find the timing and reasons for the optimum geophysical contrasts. Monitoring included two distinct, relatively stable periods: one wet and one dry. In contrast to previous perception that there are significant differences in infiltration between the ditch and surrounding natural soil, time-lagged correlation analysis showed no significant differences in infiltration speed. The key differences between archaeological and natural soils were the amount of water held in a saturated state, the rates at which the different soils dried and the temperature. Thus, the optimum time for surveys was after a sustained period of several days of hot (>15°C) weather, which accentuates both water content and temperature contrasts. However, on freely draining sites that had a greater difference in the soil texture and therefore water holding capacity between the archaeological and natural soils, the timing is less critical.