Ground Penetrating Radar Survey of a Portion of the Riverside Cemetery, Hopkinsville, Kentucky

Transcription

1 Ground Penetrating Radar Survey of a Portion of the Riverside Cemetery, Hopkinsville, Kentucky October 2012 Report prepared by Anthony L. Ortmann, Ph.D. Assistant Professor Department of Geosciences Murray State University aortmann@murraystate.edu Report prepared for Dr. William Meacham, Center of Asian Studies, University of Hong Kong

2 On October 20 th, 2012 a small group of Murray State University (MSU) graduate and undergraduate students conducted a ground penetrating radar (GPR) survey of a portion of the Riverside Cemetery in Hopkinsville, Kentucky. The survey was supervised by Dr. Anthony Ortmann and was undertaken in collaboration with the Kentucky Veterans Affairs Department, the Kentucky Archaeological Survey, and Dr. William Meacham of the University of Hong Kong. This survey was undertaken as a service learning project to provide students with experience in geophysical surveying methods and to provide a public service to the western Kentucky regional community. The purpose of the ground penetrating radar survey was to identify the locations of potential unmarked graves. During the winter of , nearly 300 Confederate soldiers from Camp Alcorn were interred in Riverside Cemetery. None of these graves are currently marked with headstones or other designating features, however, over 200 are believed to still be buried in the cemetery. Unfortunately, historical records have provided only vague information about the locations of these graves. Archival research suggests that the southeastern portion of the modern cemetery (Figure 1) has the highest probability of containing these soldiers graves. The graves are believed to be distributed in a series of rows containing between 3 and 32 graves per row. On October 17 th and 18 th, 2012 members of the Kentucky Archaeological Survey (KAS) conducted a ground penetrating radar survey to assess subsurface features and identify possible unmarked graves. The KAS survey consisted of three non-contiguous survey grids (Figure 2). Two of the KAS survey grids (Grids 1 and 2) measured 44 m (north-south) by 22 m (east-west). The third grid (Grid 3) measured 30 m (north-south) by 44 m (east-west). The MSU survey was undertaken to supplement the earlier KAS survey using different instrumentation. The KAS survey was conducted with a Mala GPR system. The subsequent MSU survey utilized a Geophysical Survey Systems, Inc. (GSSI) SIR-3000 GPR unit equipped with a 400 MHz antenna. Methods The MSU survey consisted of two non-contiguous survey grids (Figure 2). The East grid measured 20 m (north-south) by 20 m (east-west) and was isomorphic with the northeastern portion of the KAS s survey grid 3. The West grid also measured 20 m (north-south) by 20 m (east-west). The West grid was centered at the northeastern corner of the KAS s survey grid 1, so that only one quadrant of the West grid overlapped with the earlier KAS survey. Data were collected from each of the MSU survey grids in both the north-south and east-west directions to provide better resolution and enhance the shape of subsurface features. Each grid was surveyed along transects spaced at 50 cm intervals to ensure adequate coverage for data acquisition (see Table 1 for a complete description of all data collection parameters). While GPR is capable of producing high resolution images of subsurface features, the technology is limited by several factors related to the natural environment as well as the specific data collection parameters utilized during survey. Soils in the surveyed portion of Riverside Cemetery belong to the Elk soil series which consists primarily of well-drained silt loams and silty clay loams formed from mixed alluvium (USDA Natural Resources Conservation Service soil surveys At the time of the MSU survey, the soils in the survey area 2

3 were partially saturated. As a result, the GPR survey yielded poor data resolution below approximately 90 cm depth (Figure 3). The effect of these water saturated sediments on the GPR data does not preclude the ability to identify subsurface features (i.e., unmarked graves), but does make it more difficult to discern the size and shape of these features. This poor data resolution can be compensated for, to some extent, by applying various filters to the data and adjusting the amount of contrast between buried targets and their surrounding sediments. In order to enhance the appearance of subsurface features, each data set was processed using GSSI s Radan version 6.5 GPR software. Results The results of the MSU survey of Riverside Cemetery are inconclusive. The survey revealed a few subsurface anomalies, particularly in the West grid, but nothing definitively identifiable, in terms of size or shape, as a burial or grave shaft. In addition, none of the anomalies identified in the survey data displayed any patterning in their arrangement as we would expect from rows of graves. The East grid revealed virtually no subsurface anomalies aside from a series of high amplitude reflections that are presumably tree roots associated with a standing pine tree located within the survey grid at approximately 4.0 m northing and 11.5 m easting (Figure 4a-e). These tree roots do not appear to extend beyond approximately 50 cm below ground surface. Another series of high amplitude reflections were identified almost due north of the pine tree at approximately 16 m northing and 11 m easting. These reflections are only visible to a depth of approximately 40 cm below ground surface. These latter targets do not appear to correspond to any surface features, but the similarity to the tree roots suggests they may represent the remnants of a tree that has been removed from the site. No other features were identified in the data from the East grid. Data from the West survey grid revealed similarly few subsurface anomalies. A few high amplitude reflections are visible along the southern edge of the survey grid at approximately 11 m easting (Figure 5a-e). These features are presumably tree roots associated with a pine tree situated less than 20 cm outside of the West grid. Another series of high amplitude reflections are visible along the northern edge of the survey grid at approximately 11 m easting. Although there are no trees in this area, these anomalies are similar in size and amplitude to the tree roots along the southern edge of the survey grid. One stone resting on the ground surface is also visible in the uppermost time slices. This feature is located at approximately 8 m northing and 18.5 m easting. A few additional high amplitude and low amplitude radar impulses are visible within the West survey grid, however, none of these targets appears to be the correct size or shape for burials. In addition, none of these anomalies displays the kind of patterned layout that would be expected from the distribution of graves arranged in rows. Some of the high amplitude reflections appear to be small pieces of metal and possibly other historic debris. Some of the low amplitude reflections are probably small stones or even the byproduct of moisture in the sediments. Conclusion 3

4 No obvious burials are visible in either the plan view time-slices nor in the profile view traces collected from the East and West survey grids. If any graves are present in either of these survey areas, they are either being masked by poor survey conditions (i.e., ground water retention) or they have been altered by natural, historical processes (i.e., decomposition and homogenization of sediments) such that they are not readily identifiable in the GPR survey data. Alternatively, any unmarked graves may be located in a portion of the cemetery that was not tested with the GPR survey. Further research is warranted to determine the locations of the unmarked graves in Riverside Cemetery. 4

5 Figure 1. Modern boundary of Riverside Cemetery and probable location of historic, unmarked graves. 5

6 Figure 2. Approximate locations of KAS and MSU survey grids in southeastern portion of Riverside Cemetery. 6

7 Figure 3. Portion of a survey transect from East survey grid showing the zone of saturated sediments encountered at approximately 90 cm below ground surface. 7

8 Figure 4a. Plan view (25 cm time slice) of East survey grid at 0 cm below ground surface. 8

9 Figure 4b. Plan view (25 cm time slice) of East survey grid at 50 cm below ground surface. 9

10 Figure 4c. Plan view (25 cm time slice) of East survey grid at 100 cm below ground surface. 10

11 Figure 4d. Plan view (25 cm time slice) of East survey grid at 150 cm below ground surface. 11

12 Figure 4e. Plan view (25 cm time slice) of East survey grid at 200 cm below ground surface. 12

13 Figure 5a. Plan view (25 cm time slice) of West survey grid at 0 cm below ground surface. 13

14 Figure 5b. Plan view (25 cm time slice) of West survey grid at 50 cm below ground surface. 14

15 Figure 5c. Plan view (25 cm time slice) of West survey grid at 100 cm below ground surface. 15

16 Figure 5d. Plan view (25 cm time slice) of West survey grid at 150 cm below ground surface. 16

17 Figure 5e. Plan view (25 cm time slice) of West survey grid at 200 cm below ground surface. 17

18 Table 1. Setup parameters for GPR survey conducted with GSSI SIR-3000 GPR unit. Antenna 400 MHz Dielectric 14 Range 70 ns Scan Rate 50 scans per meter Samples per Scan 512 Resolution 16 bits Range Gain 3 Vertical High Pass IIR Filter at 100 MHz Filters and Vertical Low Pass IIR Filter at 800 MHz 18