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3.10.1 Principles of electromagnetic systems

The family of electromagnetic (EM) instruments, including metal detectors, currently has relatively little impact on archaeological geophysics in the UK. Metal detectors and EM systems work by using electromagnetic fields generated by passing an alter-nating current through a transmitter coil. This electromagnetic field induces small electrical currents (termed eddy currents) into conducting targets thus creating sec-ondary electromagnetic fields that are detected in a second receiving coil. Soil will also conduct electricity allowing EM systems to map subsurface changes that are largely due to moisture differences and so produce results that can be similar to earth resistivity survey. In metal detectors the frequency of the transmitting source is around 40kHz whereas, in more general EM survey instruments, this is reduced to around 14kHz.

This lower frequency allows better transmission of electromagnetic fields in soils and also has the added effect of allowing such systems to detect changes in magnetic sus-ceptibility as well as conductivity. This means that that an EM instrument can, in theory at least, substitute both for a magnetometer, and also for a deeper penetrating resistivity instrument when its coils have their axes vertical. Equally, it can substitute for a magnetic susceptibility meter, and also a shallower penetrating resistivity instrument when the axes of its coils are horizontal. The instrument electronics can extract the appropriate magnetic susceptibility and electrical conductivity (the reciprocal of

resistivity) component of the signal received in the detection coil. For a more complete discussion of EM systems including comparisons to other techniques in an archaeological context see Clark (1996) and Cole et al. (1995).

Metal detectors have their place in locating small metal items around and within graves and other forensic scenes but can only detect to a relatively shallow depth (approximately 20cm for a shell case). In archaeology their main use has been to provide portable find distributions more effectively under the portable antiquities recording schemes. Metal detectors have also demonstrated their worth in work on battlefield archaeological surveys from such disparate sites in time and space as the medieval battle of Towton (Sutherland and Schmidt 2003) to the Battle of the Little Big Horn (Scott and Connor 1997). Because targets only have to be able to conduct an electric current, EM instruments will detect all types of metals, unlike magnetometers which will only detect ferrous items. Good electrical conductors such as gold, silver and copper are obvious targets for the treasure hunting enthusiast often to the detriment of some archaeological sites.

Unfortunately for these users, EM instruments of all kinds will also respond to far more ubiquitous conductors such as iron and steel. However, because iron and steel have high magnetic susceptibilities (unlike the non-magnetic metals gold, silver and copper), they also create a magnetic effect in the presence of the metal detector’s electromagnetic field. As mentioned above, it is possible to differentiate electronically between the magnetic and conducting effects and this ability is employed in most metal detector systems to provide a discrimination facility to help decide on whether a conducting target is ferrous or non-ferrous. While such a facility can be useful, it should be noted that such discrimination systems are not wholly reliable due to the complexities of the responses from combinations of material, to materials in close association, or to the shape and orientation of objects. One metal detection manual candidly reminds users that ‘the only 100% reliable discriminator is called a shovel.’ (Rowan 1991: 20).

The shape, orientation and state of preservation can also affect detection: objects that are in good condition lying flat in the ground and disk- or ring-shaped will be detected more easily than a corroded, thin object (e.g. a knife) orientated vertically in the ground.

This is because of the greater difficulty in inducing eddy currents into such a shape from a coil placed flat on the ground directly above the target.

As with many active geophysical techniques, increasing the depth of detection rapidly leads to a loss of resolution and therefore diminished sensitivity to smaller targets. With metal detectors, the size of the search coil influences depth of detection, with larger coils giving greater depth while progressively reducing the ability of the detector to pick up smaller targets. Many higher quality systems have a range of interchangeable, and in some cases multiple-sized but integrated search coils; some deeper ‘hoard hunting’

systems use configurations similar to those used in the more sophisticated ground con-ductivity EM systems described below, but use the higher frequencies more appropriate for metal detecting.

Like all geophysical instruments, the ability to use this equipment effectively relies on the familiarity of the user with both the instrumentation and the response of that particular system to specific targets. In forensic and archaeological situations it is obviously important that the coverage is objectively systematic and thorough, and this can only be ensured by setting down an appropriate gridding system for survey, using a full coverage sweep pattern as advocated by Davenport (2001a: 111). Choice of

instrument, coil size, whether to employ discrimination in the selection of targets, and search methodology are all decisions that must be made by someone who is informed enough about the technique. A metal detector is a specialised variety of EM instrument, each make and model having individual strengths and weaknesses. Although metal detectors have been advocated for use in locating and recovering bullets from the base of graves (e.g. Morse et al. 1983) recent work by Vingerhoets (2004) shows that in some soils the depth of penetration of hand gun rounds that miss or pass through only soft tissue can be far greater than the detection depth of metal detectors.

3.10.2 Grave detection with electromagnetic systems

EM instruments proper were originally designed as ground conductivity meters. For both archaeological and forensic use, the 1m coil separation instrument such as the Geonics EM38 would appear to be the instrument of choice, although the Geonics EM31 has documented forensic success by Nobes (2000) and is illustrated in Davenport (2001a: Figure 5.3). However, Clark (1996: 34) comments that the resolution of the EM31 with its 3.66m coil separation was likely to be too low for archaeological use and it would follow that its usefulness for detecting single graves would therefore be limited. The Nobes publication shows an anomaly near (but apparently not over) a location where human remains were eventually found by extensive digging, and it is unclear whether, had the survey been used, it would have led to the discovery of the remains in question. Even the relatively close 1m coil separation of an EM instru-ment causes problems in resolving many archaeological features of comparable dimensions to a grave, this being exacerbated by its anisotropic response which ideally requires the averaging of two orthogonal readings at each survey point to reduce this problem. This issue is noted by Davenport (2001a: 81), who suggests that the most efficient field methodology is to survey a grid in one orientation and then repeat it in the other.

EM instruments will detect electrical conductivity contrasts that will arise from either the differing porosity of a grave fill compared to the surrounding medium (drier or wetter) or the presence of the decaying cadaver (wetter and conductivity-enhancing due to the decay products) in a similar way to earth resistivity (Figure 3.6). They are continuous reading non-contact systems that can be used over hard surfaces or in very dry conditions and so are more flexible in use than resistivity systems that require the insertion of electrodes into the ground to make a good electrical contact. Their ability to detect metals of all kinds allows EM instruments to also act as metal detectors for medium to large targets. Although large metal objects in the survey area can cause survey problems, the effect is less than that encountered when using magnetometers.

There are a number of makes and designs of EM instruments that need to be considered for both archaeological and forensic applications, depending on circumstance (below).

3.10.3 Recommendations

Because of the lack of published results of using EM instruments in forensic contexts, any recommendations can only be preliminary. Ideally, for single adult grave detection, a 0.5m coil separation instrument would seem ideal but until one is made available a 1m instrument would seem to be the most appropriate. An instrument that allows

both conductivity and magnetic susceptibility changes to be exploited should be employed in this mode. For shallow burials the horizontal dipole mode may turn out to be better than the deeper-seeking vertical mode. Readings should be taken at 0.5m intervals (or less if used in continuous reading mode) and for the highest quality results two surveys should be performed orthogonally to one another and the results merged.

In searching for mass graves, an instrument with a greater coil separation would be more suitable because of the greater depth penetration. However, if areas of burning are sought, then in a similar way to employing a topsoil magnetic susceptibility survey (above), an EM system surveying in horizontal dipole mode with a reading interval of 0.5 or less could be used to try to locate the seat of a fire.