Conductivity (EM) Survey: A Survival Manual

Summary

This report/manual focuses on the techniques and uses of the Conductivity survey, also known as the electromagnetic survey (EM) and how it measures the ability of the soil to conduct an electric current. The value, measured in siemens, is the reciprocal of resistivity (to convert to resistivity in ohm meters, divide the conductivity, in millisiemens per meter, into one thousand (Bevan 1983:51)). This said, there is considerable difference in the way earth conductivity and earth resistivity are measured. Although the theory behind EM survey is considerably more complex than the theory behind resistivity, fortunately there are a number of lucid published explanations aimed specifically at archaeologists (cf Bevan 1983, 1998:29-43, Frohlich and Lancaster 1986) as well as more technical discussions (McNeil 1980) which should be helpful to both the user and the manager. The following discussion builds on these and focuses on the personal experiences of the author from almost 20 years of use with one particular EM survey instrument.

This brief introduction is designed to get the first user, or discouraged user (I find there are many of these) into (or back into) the field collecting useful EM data on archaeological sites. In the United States, one of the problems with doing EM survey is that the technology is used extensively by non-archaeologists for a variety of applications. Because of different field techniques and goals, there tends to be little communication between archaeological and non-archaeological users (with the exception of communication between the geophysicists themselves, who may or may not have archaeological interests). Again, EM technology has not been built specifically for archaeology but has remained generalized, hence applicable to a wide range of geophysical interests. However, these problems should not stand in the way of the widespread use of EM survey in archaeology, although they do.

Finally, I always think in terms of using EM survey in concert with magnetic survey, exploiting the specific advantages of the different survey methodologies (Clay 2001). Therefore I am less concerned with the strengths or weaknesses of EM survey in contrast to another form of survey technology in archaeology, but rather with how the specific qualities of EM survey data may be incorporated in a larger strategy for collecting geophysical data on archaeological sites.

This Resource is Part of the Following Collections

• Cultural Resource Analysts, Inc (CRAI) Archaeological Reports

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Individual & Institutional Roles

Contact(s): Charles Niquette

Project Director(s): Charles Niquette

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