Key Words: Ground Penetrating Radar, Clandestine Burials, Geophysical Applications in Anthropology, Historic Cemeteries introduction and purpose
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| GPR Survey, an Overview The basic format of a GPR survey is fundamentally similar across the wide variety of situations it maybe used in. While post-processing, as discussed above, is an integral aspect of the GPR survey, it is meaningless without the successful detection of anomalies in the field in a manner that provides the best possible information on subsurface anomalies. For the purposes of presenting the process of a GPR survey, an example of a recent survey undertaken in part by the author will be used (Schultz and Gidusko, 2014). This survey was conducted at the Fountain of Youth Archaeological Park in St. Augustine, Florida (FOY). The park is operated as a tourist destination and sits alongside the Matanzas River, which periodically floods the park grounds. The site has been the location of extensive archaeological work and, in fact, this survey was initiated to attempt to locate previous excavation units from the middle of the last century. The location of past test units were sought in order to better map in previous work conducted at the site to an updated grid system that had been emplaced which was tied into GPS and GIS data. A GPR survey could thus inform future excavation at the site and save valuable field hours. Choice of antenna is largely affected by desired depth and resolution needed. These are factors of the soil type and condition as well as the type of feature being investigated. That is, a building foundation will require a different approach from an historic grave site one is large and requires relatively little resolution and the other is small and requires higher resolution. As this survey sought large (several meters long, but relatively shallow (>1 m) archaeological test units, there was little need for extreme depth or resolution. A 250 MHz antenna was decided on for the survey. Ground Penetrating Radar surveys should occur on a grid system that is either placed by the surveyors or works within an existing grid system put in place by a site’s principal investigator. This allows fora careful control of data collection. Within the grid system individual transects for the GPR survey can and should be adjusted dependent on subsurface resolution desired (Goodman et al., 2009). As the GPR unit pulses radar energy into the ground the wave is propagated in a roughly conical shape that spreads wider as it travels away from the antenna. Fora large subsurface feature, such as a building foundation, a wider spacing will provide reasonable results but would miss smaller-scaled anomalies. For this reason, it is best to keep data collection to shorter transect intervals in order to provide enough radar energy overlap. This provides a clearer depiction of subsurface anomalies, limits the possibility of missing these anomalies, and provides better data for D modeling software. For the survey here, intervals were set at .50 m. Grid lines should also beset in such away that the GPR is kept away from noise-inducing items such as metal demarcation nails or pin flags (Goodman et al., 2009; Annan, 2009). Before a survey can commence at a site it is also necessary to set the GPR’s velocity, which in newer GPR models is synonymous with depth. Velocity refers to the propagation of the radar energy from the antenna and is changed on a site to site basis according to several factors such as clay content of the soils, moisture in the soil, salinity in the moisture, and the relative dielectric permittivity of a known soil order. An excellent method of attuning the GPR unit to the velocity needed at a site is to either 1) collect data over a known subsurface feature that can be probed for precise depth or 2) bury a highly reflective, conductive material (rebar posts were used in this survey) at a set depth, then collect data over this area (Figure 4). This allows fora far more precise method of setting a GPR’s velocity at a given site than to apply preset relative dielectric permittivity settings. Download 0.69 Mb. Share with your friends:
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