MAGNETIC PROSPECTING IN ARCHAEOLOGY: A CALCULUS OF THE DETECTION OF PRECONTACT ARCHAEOLOGICAL FEATURES WITH MAGNETOMETRY

Abstract

Magnetometers are sensitive to minute changes in the earth’s magnetic field, allowing for the detection of, and mapping of, certain archaeological features. The ability of a magnetometer to detect and map an archaeological feature requires first that the feature be magnetic, but also that the magnetometer is recording data at a density appropriate to interact with the feature’s anomalous magnetic field. The size of the feature’s magnetic anomaly cannot be known prior to survey, as it depends on (among other things) the feature’s actual size and composition, so some estimation is involved in the design of magnetometer surveys to allow for these unknowns. Decisions about data density, or survey resolution, may depend on a priori knowledge of the discipline, site type, and expected or sought-for features, but also may depend on state and federal archaeological standards. Data density standards in magnetometry are rarely standardized in formal State Historic Preservation Office documents, and despite the industrialization of many facets of archaeological practice, archaeological geophysics has remained a ‘craft’ – the discipline lacks universally accepted density standards for data collection.This lack of standardization provides opportunity for, and implies a need for, evaluation of existing recommendations for magnetometry in archaeology in specific scenarios, leading to the development of a research question: what are the effects of altered transect-interval widths on precontact, thermoremanent archaeological feature detection and location in magnetometry? This research represents an application of technical geophysical theory as a craft, in exploring the technical and subjective tension between the patchwork series of state recommendations, the academy, and the practitioner. This research question is explored by evaluating if these features would have been detected and located across all ‘recommended’ transect intervals, regardless of applicable state guidelines. This thesis examines the effects of artificially reduced data density on two detected, ground-truthed, and mapped archaeological features originally detected during a high resolution survey with 0.25-meter transect interval separation. In this way, 0.50-meter and 1.0-meter transect interval separations are also evaluated, which in some states, literature, and scenarios, are accepted survey parameters. Additionally, detection of a feature has limited utility if the goal of the geophysical survey is to facilitate excavation of the feature (location) to assist with assessment of a site under the framework of the NRHP during work within the context of CRM. To this end, two levels of effort (soil core removal and shovel test excavation) are tested against the results of the interval analysis, in order to determine if the lower resolution survey results would result in the location of each feature during multiple ground-truthing scenarios.