LESSONS IN MAGNETOMETER AND ELECTRICAL RESISTIVITY SURVEYS OF ARCHAEOLOGICAL SITES

Magnetometers and electrical resistivity surveys have been used for decades to map archaeological sites, and the relatively simple operational procedures of both methods often permit novices to collect valid geophysical data. However, reviews of manuscripts prepared by investigators who are far more accomplished in the area of archaeology than in geophysical field methods reveal that data quality frequently suffers when important details are overlooked. For example, a proton precession profile with a sensor less than 1 meter from the surface maps features not detected by an identical sensor about 2 meters above the surface. When time is of the essence, east-west profiles 2 meters apart with measurements taken at 1 meter intervals along each profile generate anomaly maps almost as detailed as maps based on a 1-meter grid, but many anomalies fade if a 2-meter or sparser grid is employed. The use of low-capacity rechargeable dry cells without magnetic cases eliminates most false anomalies caused by variations in sensor-control unit geometry, but use of new alloys in fabricating clothing, jewelry and eyeglass frames has resulted in new noise sources from previously nonmagnetic items. Electrical resistivity surveys frequently utilize profiles with a fixed electrode spacing rather than soundings. Because subsurface variations in material properties at archaeological sites frequently occur both laterally and vertically, sounds must be used to supplement the more rapidly collected profile data, especially during the reconnaissance phase at the new site. Examples from field work in northern Ohio and Honduras will be used to illustrate some lessons learned in mapping geophysical anomalies at archaeological sites.