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Technically, five different types of geophysical survey use Electromagnetic means to measure properties of the ground. These are Frequency Domain Electromagnetic surveys, time domain electromagnetic surveys (pulse induction metres), GPR, metal detecting and Magnetic Susceptibility (MS) surveys using field induction. This section deals with Frequency Domain EM survey, and where ‘EM’ or

Electromagnetic survey is referred to in the text, it is this type of survey that is being referred to. However, all types of electromagnetic survey rely on the way the ground responds to the propagation of EM waves. The exact properties of the soil being measured depends on the frequency of the EM waves induced (Gaffney & Gater 2003, 43- 44).

6.4.1 Physical principles

A solenoid (coil of wire around a magnetically susceptible core) either generates an alternating current in the presence of a time varying magnetic field, or creates a time varying magnetic field when an alternating current is passed through the coil.

Frequency Domain EM survey exploits this physical principle by using two such coils.

One creates an EM field, which in turn excites the miniature magnets/ coils in the ground, and creates both eddy currents and excites magnetic susceptibility (see Figure 6.2). These create their own small electrical and magnetic fields which in turn affect an EM field being generated by a second, receiving coil; these mild perturbations are measured in different parts of the signature; the quadrature and inphase components (Clark 1996, 36). The alternating current produced by the transmitting coil produces a response in the ground that is detected by the receiving coil that is proportional to the conductivity of the ground. The magnetic susceptibility (MS) can also be measured because ‘while the rate of change of the magnetic field measured in the receiver is proportional to the conductivity, the magnetic signal is related to the strength of the magnetic properties of the soil’ (Gaffney & Gater 2003, 43).

There are numerous potential coil combinations, with very complex field geometry. In some EM instruments, the coils are perpendicular but oriented 55 degrees from horizontal, which means the transmitting and receiving field cancel each other out, leaving just the perturbations in the latter, which are output to a logging device.

Arrangements also exist with one coil horizontal and one vertical.

In other instruments, the coils are coplanar but used either vertically or horizontally oriented with respect to the plane being surveyed. In these instruments, additional electronics is needed to ‘cancel’ the induction signal from the recorded response.

Changes in the quadrature component of the signal relate directly to the conductivity of the ground; and given that the instrument operates over a known volume of soil, this can be directly expressed in mS/m. The inphase responds to the magnetic susceptibility of the material surveyed and is expressed as that response in ppt of the primary magnetic field. It is not an absolute measurement, so caution needs to be taken when comparing measurements between different surveys. It is also not directly comparable with laboratory measurements made using the Bartington MS2B, which produces results in SI units (Dearing 1999), as an expression of the volume specific magnetic susceptibility.

Uptake of this type of survey has not been as great as Clark tentatively predicted (1996, 34), perhaps due to the complexities of the physics involved and the problems of interpretation associated with the complicated relationship between the two measurements, and changes in the instruments sensitivity over depth that make interpretation challenging (see Figure 6.3).

The technique has had a lot of attention in continental Europe, particularly France, with a very great deal of work done on EM survey for archaeology by Professor Tabbagh (1986), notably where an EM survey over a peat environment allowed the detection of a Bronze Age trackway indirectly, by locating the hoards that had been placed along its length. Increasingly common is the use of larger scale surveys with levels of resolution too low for archaeological features to be detected, but greater depth penetration to characterise landscapes as part of culture resource management (CRM) investigations (Carey et al. 2006; Bates et al. 2007; Conyers et al. 2008).

6.4.2 Detection capabilities and limits

The detection capabilities of these systems vary a great deal and are largely a function of the array size, and the frequency of the EM signal the coils are producing. This means they can be employed (as discussed above) to measure changes on geological

scales, right down to archaeological ones, and laboratory measurements of even smaller changes.

In practice, the most commonly employed instrument for archaeological purposes is the Geonics EM38 (in various permutations). It has a coil separation of 1m, with coplanar coils that can measure both the quadrature and inphase components of the response that give the conductivity and magnetic susceptibility. Generally speaking with the coils held vertical to the ground surface the EM38 is most sensitive at about 0.3 to 0.5m deep, and less sensitive to surface changes, which can be advantageous where there is a disturbed ploughzone adding noise to conventional surveys. With the coils horizontal to the ground, the instrument is most sensitive at the surface. This is the case for both the quadrature and inphase response, but the inphase response is complicated; the sensitivity curve goes negative at about 0.5m deep for a time; this means a positive anomaly at that depth might produce a negative response, or just be cancelled out by this effect. Understanding this complex response is key to making good use of this instrument, and this has been a barrier to the wider uptake of the technique (see Figure 6.3).

The Geonics EM31 is a 4m array that has been successfully employed in geoarchaeological studies and locating larger structures, but generally has poor resolution (greater than 1m), of little use to accurately image archaeological scale anomalies.

One major advantage of EM survey is that, unlike resistivity survey, it does not rely on being able to overcome a contact resistance to inject electrical current into the ground, so it can be used in very dry environments, situations with standing water, and where there are other problems with the surface conditions for resistivity survey.

The instruments are usually self contained; the EM38 can be handled by one person very effectively, as can the EM31. Larger systems may require a towing vehicle or other arrangements, but for instruments commonly used on archaeological surveys, one person is sufficient to operate the system, and there are no trailing wires, as there is with resistivity survey.

Another advantage, when the equipment is built to do so, is that both conductivity and MS can be measured in one survey, rather than two, saving on operator time and fatigue.

These systems should, in theory at least, be better at detecting lenses of MS

enhancement and thin layers of material than a gradiometer, which would only see the edges of such a layer and not the spread of it, as it measures rates of change rather than the total field effects (Gaffney & Gater 2003, 44).

6.4.3 Known conflicts and issues

As the technique relies on EM induction, it can suffer in highly conductive environments or where there is a lot of saline water present as the EM energy is conducted away rather than setting up reciprocal fields and eddy currents.

There can also be problems when the survey includes objects or deposits that are very conductive or very magnetic; the effect on the induced fields can be so great that a response appears in the other part of the signal; i.e. highly conductive objects show up as MS anomalies and vice versa. This further complicates the response of the

instrument.

Modern conductive rubbish in the topsoil can cause problems with spiking and signal-leak.

6.4.4 Instrumentation employed

This research used a Geonics EM38B; a 1m coplanar coil instrument that allows both the quadrature and inphase to be measured at the same time. The instrument was used with a Polycorder data logger, and on some of the sites a modified snowboard was used as a sled to survey with the instruments’ coils in the horizontal orientation without damaging or triggering the adjustment dials and reading trigger button.