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New Tool for Old Sites

Last September the Tongass archeologists and a representative from Sealaska Heritage Institute gathered to learn about the use of ground-penetrating radar to examine archeological sites.

Traditional archaeological excavation methods ultimately result in the destruction of a site. This destruction is not an issue for sites threatened by construction or other development, since archeologists take great care to record as much information about the site as possible during the excavation. Investigative options are limited, though, for sites not affected by potential destruction.

Ground-penetrating radar offers a non-destructive way to look at the buried layers of an archaeological site. The Tongass prefers to conserve archaeological sites for future study and for the enjoyment of future generations. As technology advances, conservation ensures that sites will be around when we can maximize the amount of recovered information.

Archeological excavation strategies have changed dramatically over the last decade, especially for sites managed by the federal government. Declining budgets and increasing support for conservation archeology have resulted in less large-scale excavations, especially for those sites not threatened by destruction. In some cases it is not feasible or desirable to conduct traditional archeological excavations.

A traditional Tlingit site near Juneau, Alaska, is such an example. At the request of a local Indian tribe, the Auk Kwan, the Forest Service is attempting to locate and quantify burials within a historic cemetery site on National Forest System lands. Excavation of the burials is not an option during this investigative phase.

In the photo at the right, Forest Service archeologist Myra Gilliam and Angoon liaison Donald Frank use ground-penetrating radar at the historic cemetery site.

We looked to ground-penetrating radar technology as an alternative when members of the Auk Kwan requested information about the cemetery in regards to a possible nearby Juneau Ranger District project. To protect these sacred grounds, archeologists are using ground-penetrating radar imaging to determine the number and location of burials. The use of ground-penetrating radar will potentially ensure the protection of this important site.

The photo at the left shows one process of getting an accurate image of sub-surface features using GPR-SLICE, the tool used in this project. This sample is not from the Juneau site. Click on the image to see a larger version.

The Tongass purchased a ground-penetrating radar unit that allows us to look at archeological deposits as deep as 15 feet below the present ground surface. The GPR equipment digitally records radar reflection profiles, much like radar detects airplanes in the sky. For archeological sites, an antenna sends out radar pulses that are reflected by buried objects. The antenna and a date recorder measure the time it takes for a radar pulse to be reflected. Deeper objects take a relatively longer time for these reflections to be returned to the antenna. By examining radar reflections collected across a site, we can create three-dimensional images of the site’s buried layers.

The team set up a grid pattern over an area of the site measuring about 330 feet by 30 feet. Data collection involved pulling a case with the antenna along a horizontal line, constantly collecting radar reflections. The antenna was pulled along parallel lines spaced about one and one-half feet apart until the entire grid was covered.

The following day found us at a computer lab with our collected field data. By the end of the day we were able to input our field data and create a three-dimensional color animation of the buried parts of the site. The Juneau Ranger District intends to use this information as we consult with the Auk Kwan about future management of the area.

USDA Forest Service - Tongass National Forest
Last Modified: May 18, 2005