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Intensive Remote Sensing Survey for Black Ramp Extension Project
Lawson Army Airfield, Fort Benning, Georgia

Geoarcheology Research Associates

October 13, 2000

The following report summarizes the results of an initial phase of remote sensing at the Lawson Army Airfield in Fort Benning, Georgia. Objectives of the project were to determine whether or not undisturbed subsurface features, including Native American burials, may be present along a tract that is proposed for an extension of an aircraft parking apron designated as Black Ramp. A previously identified site known as Kasita has already been documented at Lawson Army airfield. The configuration of known archeological sites as well as the project survey area in the vicinity of Black Ramp is shown in Figure 1. The Scope of Work (SOW) called for Ground Penetrating Radar (GPR) survey of approximately 34 acres to detect "....all human burials and significant features that may exist within the survey area." Under provisions of the SOW alternative remote sensing techniques could be applied if their utility was judged to be more efficient and appropriate to conditions obtaining in the vicinity of the airfield. The SOW called for ground location of features at a resolution of 5 m or better.

Fieldwork was conducted by a team from Geoarcheology Research Associates (GRA) between September 25-30 in conjunction with Panamerican Consultants, Incorporated (PCI). Remote sensing equipment included a Geometrics, Inc. Magmapper G-858  (Cesium Magnetometer) and a Noggin 250 Ground Penetrating Radar (GPR) system that logs the analog signal to an attached computer. The remote sensing work was performed in conjunction with Dr. Rob Sternberg of Franklin & Marshall College.

Results

Initial work consisted of gridding the site survey areas in preparation for keying the remote sensing walkovers. Mapping work was undertaken by PCI utilizing a Geodimeter System 422 total station. Five (5) discrete tracts were identified for testing as shown in Figure 1: these were "Brown Ramp"; "Open Area North"; "Open Area South"; "Retention Pond"; and "15 Hold".

Remote sensing efforts began with a series of traverses on the asphalt tarmac of "Brown Ramp" with the GPR. The initial passes consisted of a series of test transects along 1 meter profiles (running E-W) on several N-S lines. The effort was overly labor intensive since complete coverage would require between 0.5 to 1.0 m spacing on profiles and field print outs showed substantial anomalies resulting from modern construction. It was concluded that the GPR would be an excellent method for field testing results obtained for the more comprehensive magnetometer study.

Magnetometer transects were implemented in the southwestern portion of the area identified as "Open Area South" (Figure 1). After initial instrument calibration and testing, the magnetometry survey proved to be ideal for generating field maps of anomalies. Here again, however, 1 m spacings were labor intensive and required extensive mapping. At the conclusion of the first two (2) days of magnetometer survey an approximate 3% sample of the entire project area had been covered. Combined with the necessary mapping and grid preparations, the first four (4) days of project work demonstrated that coarser interval testing would be required to complete the coverage within the allocated time and budget frameworks. During the final two (2) days of field work, the transect interval for magnetometer coverage was expanded to 5 meters. At this level of measurement, the entire "Open Area South" tract was surveyed. The revised spacing and investigative strategy accounted for an expansion in areal coverage from approximately 3% to nearly 25% of the entire project area.

In the field it was possible to generate a provisional map of magnetometer anomalies based on the initial 1 meter test coverage. A relatively high resolution two-dimensional contour map of subsurface relations and disturbances for the southwestern segment of "Open Area South" (3% of total project area) was produced. Figure 2 plots the contours of the 1 meter transects for this measured segment spatially; the range of the magnetometer signals is from 46,894 to 53,754 nT. As shown, the most prominent anomalies are >3,000 nT. Significantly, the shape of loci with these higher readings are regular and often geometric (note trench-like configuration in southwest corner of Figure 2). These may conform to modern disturbances such as those registered by underground cable or pipe systems. It will be necessary to ground truth and/or check the distributions of existing utility corridors to verify the signals produced by modern disturbance. More irregular shapes for low-moderate nanotesla (nT) clusters would appear to be preferred candidates for buried archeological features. Potential locations for such investigation include the oblong shapes in the northeastern portion of the grid (co-ordinates 790-795 E/1210-1230 N), as well as the irregular clusters in the south (co-ordinates 780 E/1160 N). It is proposed that these locations be field tested. A complete map of the project area-including all five (5) discrete tracts-will constitute a baseline for the ground-truthing strategy.

Preliminary Recommendations

Initial applications of both GPR and Cesium Magnetometer surveys show that the latter is probably the most diagnostic and cost-effective method for the present SOW. The instruments for the magnetometer are both portable and easily interfaced with laptop computers. High resolution, broad coverage print outs are available and existing software produces detailed maps. GPR is of more limited utility because it produces isolated profiles along discrete spacings of the project area while the magnetometer offers comprehensive mapping. Perhaps even more significantly, the setting of the project area-at Lawson Airforce base-is not ideally suited for GPR investigation. Conyers & Goodman (1997: 51-52) note that GPR based survey objectives are often affected when antenna frequencies are highly variable "if the site is located near a military base or airport or near radio transmission towers."

Based on the initial results of the remote sensing study, we propose the following follow-up strategies under the parameters of the present SOW:

  • Use of magnetometer as the primary method for completing the survey of the five (5) tracts;
  • Use of GPR as necessary to verify signals obtained from the magnetometry study;
  • Expansion of the transect interval from 1 to 5 m.

The final product of the completed magnetometer survey will be a brief report summarizing results of the 5 m sampling strategies and a contour map of anomalies generated on this basis.

A second phase of investigation will begin by superimposing the map of anomalies with maps of utility corridors and land use modifications to account for disturbances of unequivocal modern origin. Once these anomalies are interpreted it is proposed that loci of "high signal" anomalies be further gridded and surveyed at 1 m intervals to finalize an investigative strategy for archeological ground-truthing. A scope and budget for the second phase will be submitted after the completion of the first phase of work.

References Cited

Conyers, L.B. & D. Goodman

            1997    Ground Penetrating Radar: An Introduction for Archaeologists. AltaMira Press. Walnut Creek, CA