1998-04

1998-04

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New techniques of ground-penetrating radar (GPR) acquisition and computer processing were tested at buried archaeological sites in the American Southwest. The GPR method of geophysical mapping uses surface antennas to transmit high frequency radar pulses into the ground and then measure the amplitudes and reception times as they are reflected from subsurface features. These reflection data are recorded digitally on a computer and can then be processed and interpreted using visual and computer methods. Many millions of individual reflections can be recorded in one survey grid, necessitating special data manipulation and interpretation techniques that can be performed only by a computer.

A number of different acquisition and processing methods were tested at buried archaeological site covered with different sediments and soils. Sites were chosen for study if they had been excavated before the survey, or were to be excavated soon after. Sites that had little or no surface expression were especially targeted because these are the types of sites that are most in danger from construction and development projects.

A number of newly developed data processing methods were employed to process the data. The most effective technique was the amplitude slice-map method, which can process and map many millions of reflection amplitudes within a survey in three dimensions. Resulting maps produced images of “slices” in the ground, similar to standard arbitrary levels in archaeological excavations. The difference between amplitude slices and excavation levels is that the radar slices are mapping features that reflect radar energy instead of archaeological materials. With good velocity information exact depths of each slice can be determined. The slice-map method was combined with more standard data processing and visual interpretation techniques to produce accurate subsurface maps that could be tested by excavation data.

In southern Arizona, near Tucson, numerous pit structures buried in terrace alluvium were discovered and mapped at the Valencia Hohokam Site. In the Four Corners region buried features including a Chaco period Great Kiva and other pit Structures were mapped by GPR and later confirmed through excavation near Bluff, Utah.

At some sites that were tested, GPR surveys did not successfully identify buried archaeological features. These failed surveys highlight both geological and methodological problems including soil conditions, surface disturbance and. equipment calibration that may be avoided or ameliorated in future GPR surveys.

Techniques of radar data acquisition were refined and data processing methods developed to produce high quality images. The slice-map method was applied to all data sets in order to produce images that could be interpreted while still in the field.

This research was made possible through Grant MT-2255-6-NC-15 from the National Center for Preservation Technology and Training (NCPTT).

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