Double Ditch and Fort Clark State Historic Sites, in North Dakota, and a variety of instruments.
(1) The utility of a twin coil device, the Geonics Ltd. EM38B, is examined for recording magnetic susceptibility (MS), the ability of a material to be magnetized, because these instruments have not been greatly used for this purpose. Results are compared against a well understood single coil instrument, the Bartington MS2D. A variety of archaeological features were surveyed by both instruments, permitting visual and quantitative comparisons of the mapped results. The EM38B parallels or surpasses the MS2D data in quality, and its greater speed of survey and depth penetration often make it preferable as a field instrument.
(2) How subsurface magnetism forms anomalies recorded by magnetometry on the surface is not well understood. Three former excavations were re-opened at Double Ditch that bisect common archaeological features (a storage pit, fortification ditch, and house floor). Magnetic stratigraphy was measured on the exposed profiles with three MS meters: the Exploranium KT-9, the Bartington MS2F, and the MS2D. Mathematical models of the measurements were then compared to the shapes of anomalies recorded at the surface by magnetometry that demonstrate how these surface anomalies are formed.
(3) Anomalies revealed by magnetometry are generally of two types, thermoremanent and induced. The former arise by intense burning, while the latter result from materials of high MS. Both look the same to a magnetometer, which measures the sum of all magnetism. Distinguishing hearths (thermoremanent) from storage pits (induced) is critical to interpretation, but difficult because both generate circular of similar sizes and magnitudes. Magnetometry surveys are used to measure total magnetism followed by MS surveys that measure only induced magnetism. A statistical technique, regression, removes the common correlation between the data sets, exposing the remainder which point to thermoremanent anomalies comprised mainly of hearths.
(4) The presence of such nomadic groups as the Dakota and Crow is well-documented at Fort Clark, yet magnetic surveys have seldom been carried out in hunting-gathering camps. A variety of surveys reveal enhanced MS and magnetic anomalies in areas known to have been occupied by these groups that point to areas of cooking, fire building, food processing, and the accumulation of refuse.
This was made possible through Grant MT-2210-11-NC-13 from the National Center for Preservation Technology and Training (NCPTT).