Detection of resistive features using towed slingram electromagnetic induction instruments

Abstract

Slingram frequency domain electromagnetic (FDEM) instruments allow simultaneous measurement of both magnetic susceptibility and electrical conductivity, which should justify their widespread use in archaeological surveying. However, this is not the case and their application remains quite limited due to: (i) a lack of knowledge about the role of coil orientation and spacing in terms of the detection abilities for archaeological features (especially for resistive bodies); and (ii) a lack of instrumentation specifically designed for shallow targets.We present here a test of a new version of the CS60 instrument (VCP coil configuration and 0.6 m intercoil spacing) for shallow depth resistive feature detection. This experiment was undertaken on the Roman site of Vieil-Evreux where a complete series of control resistivity and radar data was obtained. Detection of buried Roman walls was successful, in accordance with what can be expected from three-dimensional modelling.This confirms that the application of this type of instrument in archaeological surveys merits to be extended significantly.

Prospection of two circular Bronze Age ditches with multi-receiver electrical conductivity sensors (North Belgium)

“…The anomalies and inverted profiles of this platform corresponded well with static array measurements. The maps demonstrated that the receiver configurations with the shallowest depth sensitivity of the electrical conductivity sensors produced the most pronounced anomalies for this target (similar results for shallow depth targets were obtained by Thiesson et al, 2009). The other receiver configurations could be used to interpret the natural soil layering and in the case of the ER-sensor applied for 2-D inversion.…”

“…Although both methods investigate the s a , the measurements can differ considerably (Linford, 1998). A first factor is the spatial sensitivity of both methods, which depends on the configuration (separation, arrangement) of the electrodes and the coils (Keller and Frischknecht, 1966;Tabbagh, 1986;Thiesson et al, 2009). Secondly, ER is more sensitive to low-conductivity contrasts and FDEM more sensitive to high-conductivity contrasts (Tabbagh, 1985).…”

3D Reconstruction of Buried Structures from Magnetic, Electromagnetic and ERT Data: Example from the Archaeological Site of Phaistos (Crete, Greece)

“…As a magnetic method, FDEM is a very fast technique able to provide information about the occurrence of buried anthropical structures in terms of geophysical contrasts with their host rocks. In archaeological research, the FDEM method (Thiesson et al, 2009;Saey et al, 2012;Simon et al, 2012;De Smedt et al, 2014) is generally preferred to the magnetic method for investigating sites where the magnetic susceptibility contrasts are too low to be detected, while high electrical resistivity contrasts exist. This is particularly true in the presence of wall remains made of high-resistivity limestone and embedded in a highly conductive environment, such as soil or clayey sediments, which are characterized by a large capacity to retain water.…”

Slingram EMI prospection: Are vertical orientated devices a suitable solution in archaeological and pedological prospection?

“…These experiments were achieved using the CS60 apparatus, a prototype device developed and manufactured in our lab (Job et al, 1995;Thiesson et al, 2009), in both VCP and VVCP configurations and a reference survey in each case.…”

“…The application of this class of instruments is significant for a large panel of subsurface studies based on ground electrical conductivity measurements (Frischknecht et al, 1991), especially in pedology (Brus et al, 1992;Corwin et al, 2006) and started to be used in archaeological context (Scollar et al, 1990;Simpson et al, 2009a,b;Thiesson et al, 2009). This technique may also be used in magnetic susceptibility studies.…”