Remote Sensing in Archaeology: An Explicitly North American Perspective

5. Conductivity Survey: A Survival Manual

5 Conductivity Survey: A Survival Manual R.BerleClay Earth conductivity survey, also known as electromagnetic (EM) survey, 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 [mS/m], 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, there are fortunately a number of lucid published explanations aimed specifically at archaeologists (see Bevan 1983, 1998:29–43; Frohlich and Lancaster 1986), as well as more technical discussions (McNeill 1980), that should be helpful to both the user and the manager. The following discussion builds on these and focuses on my personal experiences from almost 20 years of use with one particular EM survey instrument (Figure 5.1). This brief introduction is designed to get the first user or discouraged archaeological 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 most extensively by nonarchaeologists for a variety of geological, environmental, and agricultural applications. Because of the wide availability of EM technology in colleges and universities, in many instances an earth conductivity meter, generally wielded by a nonarchaeologist, may be the archaeologist’s 80 ~ R. Berle Clay hands-on introduction to near-surface geophysical survey. Because of different data-collecting goals, which are reflected in field techniques, there tends to be little understanding between archaeological and nonarchaeological users (with the exception of communication with the geophysicists themselves, who may or may not have archaeological interests). Again, and as another reflection of the many users, EM technology has not been built specifically for archaeology but has remained generalized and hence applicable to a wide range of geophysical interests. These problems should not stand in the way of the widespread use of EM survey in archaeology, although they do, as a product of initial field efforts that did not really seem to accomplish useful archaeological goals. 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 than with how the specific qualities of EM survey data may be incorporated in a larger strategy for collecting multiple sets of complementary geophysical data on archaeological sites. Summary Comments To begin, it is useful to outline what I see as the strong and weak points (from an archaeologist’s standpoint) of EM survey technology and the specific survey problems I wish to discuss here. Most of these points I specifically mention; all are implicit throughout my discussion. Strong Points of EM Survey 1. Fast because it involves no electrical contact with the ground 2. Can be used in a variety of ground conditions (grass/brush/tree cover, ridged ground, and so on) where other techniques may be more difficult to deploy Figure 5.1. A conductivity survey in progress. The EM38 is carried in my right hand, ca. 15 cm off the ground. It is contained in a sheath of foam insulation held together with duct tape. The data logger is carried in the left hand and manipulated with the left thumb. The operator’s attention is focused on the marked rope below the EM38. Conductivity Survey ~ 81 3. Works well in team with magnetic gradient survey 4. Can be used in dry periods as well as wet (an edge here on resistivity) 5. Can measure magnetic susceptibility (in parts per thousand), as well as earth conductivity (in mS/m), a mixed blessing 6. Can do a certain degree of vertical separation of geophysical phenomena Cautions These must be managed with field technique: 1. Temperature drift 2. Digital lag 3. Appropriate visual output and data processing 4. Metal on the operator Weak Points 1. Sensitive to a wide range of metals (from flip tops to water mains!) 2. Depends upon the presence of soil contrasts: if they don’t exist...