The image below is formed from an extract of twin probe electrical resistance data and total magnetic field data for exactly the same area (the top, left and bottom edges correspond in both cases).
Electrical resistance data (left) 30 – 120 Ohm, white high, magnetic data (right) +/- 1 nT, white positive
In this case the geology is Devensian Till, the soil fairly poorly drained and there has been ridge and furrow cultivation across the site. Why does the electrical resistance data show an enclosure whereas the magnetic data shows no sign of this?
This was not a survey pre-development, but what if it was and the archaeological curator had (as normal) only asked for magnetic survey? This site would not have been detected, at great inconvenience to all concerned later in the process. Yet this is normal practice in the UK and despite decades of geophysical wisdom that dictates surveys should always be multi-method, archaeologists keep insisting otherwise, whether as curators or ‘geophysical’ contractors.
Food for thought… how much archaeology is not being detected by the routine and exclusive use of magnetic survey pre-development? Is the rush for faster and larger magnetic surveys at the expense of proper multi-method strategies? Why do curators still, after many years of published guidance, ignore the need for multiple survey methods?
