Archaeological prospecting by geomagnetic and resistivity surveys

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

“…Whilst this is in itself indicative of greater water contents, laboratory tests showed Ditchfill2 on the till site (0.8 m depth) had very low ARDP values at the same water content in comparison to the other soils on site, making these differences even greater than they appear. Previous experiments on earth resistance contrast (e.g., Clark, 1980, 1996) suggested that water can infiltrate the ditch with greater ease due to lower density in comparison to the surrounding soils, although it should be noted that the temporal resolution of these surveys was limited to monthly and so processes were inferred between measurements based on rainfall data. One key finding from this data is that neither of the studied sites appeared to show significant differences in infiltration patterns and behaviour between the archaeological and natural soils, with major wetting fronts affecting both at roughly the same time with the exception of the initial wetting fronts, which moved slower in the natural soil at greater depths, most likely due to differences in the sizes of the empty pore spaces.…”

“…One commonly held conception about why archaeological ditches have different geophysical properties to the natural soil around them is that there are differences in the infiltration speed following rainfall events between the archaeological ditch and the natural soil profiles (i.e., water can enter the more porous soil within the ditch quicker) (e.g. Clark, 1980, 1996; Schmidt, 2013). To test this assumption and highlight differences in infiltration patterns between the archaeological soil and the SSM profiles for fine and coarse grained soils, short periods of data taken immediately after a rainfall event were chosen.…”

“…However, these almost always represent a single moment in time and do not account for seasonal effects. Seasonal variations in an archaeological feature's visibility and the underlying EM properties have been assessed using repeated geophysical surveys, usually using electrical resistance methods (al Chalabi & Rees, 1962; Clark, 1980; Cott, 1997; Hesse, 1966; Parkyn et al, 2011) and more rarely GPR (e.g., Rogers et al, 2012). Three main limitations exist with these approaches: The temporal resolution (usually monthly) is insufficient to detect specific infiltration, drying or temperature change events, which change the ground conditions and water content contrasts between the archaeological feature and the natural soil, leading them to be inferred or assumed between surveys.…”