Archaeo-Physics, LLC - geophysical survey and subsurface mapping

Case Study

Fort Phil Kearny

19th Century Military Site, Northern Wyoming

In August 1998 Dr. Lewis Somers of Archaeo-Physics, LLC and Absaraka Cultural Resource Consultants, Inc. (Sheridan, Wyoming), working under contract for LTA, Inc. (Laramie, Wyoming), conducted a geophysical survey of portions of the Fort Phil Kearny Historic Site, in northern Wyoming.

Fort Phil Kearny was one of two forts constructed in 1866 to protect Euro-American interests along the Bozeman Trail (located in present-day Wyoming and Montana). Geophysical survey is one phase of a multi-component project that also includes archaeological investigations of four corners of the fort stockade and the subsequent reconstruction of these features. Geophysical survey was utilized to guide placement of archaeological units and as a management tool for guiding future work at the site.

The Wyoming Department of State Parks & Cultural Resources administers approximately half of the fort site; the remaining half is located on private property. Since very little evidence of the fort exists on the surface, the State developed a master plan that includes provisions for archaeological research and limited site reconstruction for interpretation purposes. The State was awarded a Transportation Enhancement Activities (ISTEA) grant to perform geophysical survey across the state owned portion of the fort site; conduct archaeological excavation of portions of the stockade wall; reconstruct portions of four stockade corners, a gun bastion, and an officer's gate; and place interpretive signs across the site.

Magnetic gradiometer map of historic fort
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Standard deviation plot shows the distribution of metal artifacts
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The images above show two different plots of magnetic field gradient data collected over a portion of Fort Phil Kearny, a 19th Century military site in northern Wyoming.

The top image displays the magnetic field gradient data with minimal processing (zero mean traverse, interpolation of data points, data clipped to +/-30nT). Strong bipolar (having both a positive and negative component) fields which approach or exceed the limits of displayed data are presumed to be associated with substantial brick, iron, or steel objects. More subtle variation may be associated with nails or other small metal objects, or disturbed, burned or organically enriched soils.

The bottom image displays data that has been subjected to a standard deviation filter. Each data point is replaced by the log-weighted standard deviation of data within a one-meter radius.

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