MAGNETIC TECHNIQUES AND RESULTS AT A PREHISTORIC EARLY PUEBLO II VILLAGE SITE

Different magnetometers in various configurations were tested at a prehistoric Early Pueblo II cultural site in southwestern Colorado. The Geometrics model G-856 proton precession magnetic gradiometer, a Bartington Instruments dual sensor Grad 601-2 fluxgate magnetic gradiometer and the Geometrics G-858 Cesium-vapor magnetic gradiometer were used. The G-858 Cesium-vapor magnetic gradiometer was used in horizontal and vertical sensor configurations for data comparison and to investigate magnetic anisotropy. Compared to the model G-856, the Grad 601 was much easier to use and provided greater sampling of the survey grid in less time by recording two lines of data in only one traverse along the grid. Other advantages of this instrument include minimum grid layout, light-weight, easy to configure and use, and a sensitivity of 0.1nT/m. The model G-858 was also quick and efficient during magnetic surveying, and provided 0.01nT/m data sensitivity. The G-858 gradiometer was first tested in a horizontal configuration with both sensors positioned vertically 25 cm above the ground and separated by 75 cm. Data quality and resolution were comparable to the Grad 601-2 results. The G-858 instrument was also tested in a vertically stacked configuration with the top and bottom sensors positioned 1m and 25 cm above the ground, respectively. Magnetic gradient data were edited and then digitally filtered with directional filters and a low-pass Gaussian matrix filter that worked especially well to further remove high frequency noise and striping. Striping occurred because instrument noise was incorporated into the low amplitude magnetic data, such as is commonly collected at prehistoric sites. Results from the study showed that all instruments and orientations generated similar magnetic anomalies. Overall, the model G-858 in a vertical sensor configuration provided the most detailed images of the Early Pueblo II village architecture. Ruins of living structures, walls, kivas, storage pits, and hearths were among the prehistoric archaeological features detected. Anomalies related to kiva structures were more prominent than those related to wall structures, and fire pits and hearths also produced marked anomalies.