OPTIMIZING MAGNETOMETER DATA QUALITY AND COVERAGE AT ARCHAEOLOGICAL SITES

Geophysical surveys remain greatly underutilized as integral components of archaeological projects in North America. After serving as referee for a number of manuscripts reporting on geophysical surveys of archaeological sites, I suspect that some archaeologists lost confidence when geophysical results have proven less useful than anticipated. For example, cultural noise, insufficient data density, and less-than-optimum sensitivity can degrade total magnetic field maps generated by G-856 proton precession magnetometers. Low-capacity (2 amp-hr) NiCd “D” cells lack magnetic cases, rendering the console virtually undetectable 2 meters from the sensors, carried by a carefully demagnetized assistant. To maximize coverage, measurements at 1-meter intervals along parallel east-west transects at 2 meters apart provide adequate data density for prehistoric NW Ohio sites, although a 1-meter grid is recommended if time permits. The vertical gradiometer configuration with the uppermost sensor no higher than 1.5 m above the surface works well. Each survey must be supported by a local magnetic base station if anomalies on the 2 nT (nanoTesla) level are to be mapped. Low-capacity “D” cells lose power after 1400 or so readings (700 gradiometer stations – about 3 hours) so replacements are needed for a full day of field work. Because the base station console does not change position with respect to the sensor, alkaline “D” cells or 4 amp-hr NiCd cells provide suitable base station power. Data collected should be processed, displayed and a preliminary interpretation attempted before setting up for another day of measurements. Case histories presented include magnetometer surveys of four northwest Ohio sites with mapped anomalies including fire pits, storage pits, accumulations of garbage, and holes that once held large posts. A magnetometer survey at another NW Ohio site identified soil disturbed by poorly documented excavations performed decades ago. Applications of electrical resistivity to mapping archaeological sites will also be discussed.