A DEPTH CALIBRATED GPR STUDY OF A 17TH CENTURY FRONTIER HOMESTEAD

The important 17^th century Field-Bickford Garrison site in Durham, N.H. was the subject of a detailed GPR study to determine the extent and number of structures present in the complex. The data was collected with GSSI Sir20 Ground Penetrating Radar (GPR) with a 400 MHz antenna and post processed with RADAN© 5.0. Site stratigraphy consists of a 17-20cm plowzone overlying a 1 – 2m Holocene nearshore marine sequence of clays and sands resting on an irregular bedrock surface consisting of the metasedimentary Ordovician – Silurian Kittery Formation. Several GPR surveys were conducted both within the area of archaeological interest and beyond the initial site limits, ultimately generating a GPR grid of .6 ha. Survey line intervals ranged from 0.3m to 1m and were conducted over the course of several seasons allowing us to normalize the data for site conditions ranging from dry to moderately wet. Post processing included cross talk and surface effects removal, increasing signal gain, filtering, and transforms. A deconvolution function was initially applied to the records but degraded the more subtle features and consequently was not used. Although the targets consisted of foundation stones up to .4 meter in diameter, migrating the records did not improve clarity. Spotty but good archaeological control provided both the travel time to features and the type of features represented on the record. This allowed the detailed tracking of features beyond the ground limits of the archaeological survey. From the data anomalies identified the presumed foundations of the Field-Bickford homestead as well as the foundations of two to three additional structures have been located. In addition, there are anomalies that we interpret to be the cellar holes of these structures. Corroboration of the major aspects of the results has been provided both by directed archaeological test pits and by a separate EM induction survey which identified many of the larger anomalies.