The depth and size of a burial affect detection. Burials may range in depth from shallow (<50 cm) to very deep (>150 cm). Large, electrically contrasting features reflect more energy and are easier to detect than small, less contrasting features. Small, deeply buried features are more difficult to discern on radar records. Bones are generally too small to be distinguished with GPR (Killam, 1990; Bevan, 1991). In addition, bones are electrically similar to dry soil materials and rock fragments (Davis et al., 2000).
The shape and orientation of a subsurface anomaly may suggest its identity. Subsurface anomalies that are narrow and linear may suggest burials. Burials may be uniformly spaced or aligned in a particular direction. Bartel (1982) observed burials aligned with the orientation of the solar traverses. Multiple elongated anomalies occurring at a common depth suggest burials.
In soils such as the Au Gres and Windsor, burials are difficult to distinguish because of the presence of stratified or segmented soil layers and occurrence of rock fragments, tree roots, and animal burrows. Vaughan (1986) and Bevan (1991) found graves identification complicated by rock fragments, which introduced unwanted clutter and complicate the interpretation of radar records.
In the search for burials with GPR, success is never guaranteed. Even under ideal site and soil conditions, burials will be missed with the GPR. The usefulness of GPR for site assessment purposes depends on the amount of uncertainty or omission that is acceptable.
Calibration of GPR:
Ground-penetrating radar is a time scaled system. This system measures the time that it takes electromagnetic energy to travel from the antenna to an interface (e.g., bedrock, soil horizon, stratigraphic layer) and back. To convert the travel time into a depth scale, either the velocity of pulse propagation or the depth to a reflector must be known. The relationships among depth (D), two-way pulse travel time (T), and the velocity of propagation (V) are described in the following equation (Morey, 1974):
Figure 4. Time Sliced images of the [REDACTED] Site.
Figure 7 contains three time-slice images of the [REDACTED]. In Figure 7, all distance unites are expressed in meters. The origin is located in the lower left-hand corner (southwest corner of grid area) of each slice. The three horizontal "time-slices" represent depths of about 50, 100 cm, and 150 cm. These depths are based on an averaged signal propagation velocity of 0.10 m/ns through the soil. The width of each time-slice is about 25 cm.
The survey of the [REDACTED] provides radar records and 3D imagery from an area with known Native American artifacts and burials. The time-sliced images in Figure 7 are basically similar to the images seen in figures 3, 5, and 6 [REDACTED] has a long history of occupation and use. I has been repeatedly disturbed. In Figure 7, moderate to high amplitude reflectors do not increase monotonously with depth. However, patterns remain broad and patch suggesting undisturbed materials at greater soil depths. Three-dimensional time-slice analysis appears ill suited for the identification of Native American burial grounds in Vermont.
Figure 7. Time sliced images of the [REDACTED]
2. The interpretation of two-dimensional radar records and three-dimensional radar imagery led to the identification of several suspected locations within the Fournier and Spaulding sites. These locations were marked and will be examined by an archaeologist.
3. Although several suspected locations were identified, no burial was explicitly recognized.
4. The survey at the [REDACTED] provides useful information for the interpretation of similar sites in Vermont. [REDACTED] has long had a long history of known occupation and ...