The 2002 Ground Penetrating Radar Study:
Locating Unmarked Burials on [REDACTED],
Highgate/Swanton, Franklin County, Vermont
David Skinas, Jim Turenne
USDA Natural Resources Conservation Service
The 2002 Ground Penetrating Radar Study:
Locating Unmarked Burials on [REDACTED],
Highgate/Swanton, Franklin County, Vermont
David Skinas, Jim Turenne
USDA Natural Resources Conservation Service
Lands along [REDACTED] in the towns of Highgate and Swanton (Figure 1) contain many archaeological sites and burial grounds that passes a rich and extensive record of Native American occupations during at least the past 6,000 years. Over the last 29 years, and probably longer, the development of some homes on [REDACTED] Monument Road ? has disturbed hundreds of unmarked burials in several Abenaki cemeteries. The unintentional disturbance of yet another extensive cemetery in May of 2000 prompted town officials [REDACTED] Monument Road Landowners ? and the Abenakis to work together to develop a policy that would allow for the use on non-intrusive technologies to search for unmarked graves prior to any ground disturbance. During the week of August 26-30, 2002, the USDA Natural Resources Conservation Service, in partnership with the [REDACTED] Working Committee and the Vermont Land Trust, conducted a ground penetrating radar study of portions of six lots in the towns of Highgate and Swanton to determine it's potential for locating unmarked graves.
Figure 1: location of Study Area on the East Alburg and Highgate Center USGS quads.
FIELD SURVEY PROCEDURE:
At each site, a 1X1-meter grid of variable dimensions was established using a right-angle prism to locate corners and a meter tape for measuring lengths. Radar traverses were made along each grid-line and observation were placed at 1-meter intervals. Each traverse was identified by a GPR file number. A total of 109 transects were completed. Grid corner stakes were left at the site for a detailed survey of their coordinates, a differential GPS was also used to geo-reference the grid corners and test pits.
During the field survey, some of the radar files (GPR profiles) were reviewed to locate areas to ground-truth by the archaeologists. Areas which were interpreted as anomalies (hyperbolic-shaped patterns or point objects on the profile) or breaks in soil layers were a 1/4 inch (in) mesh screen for identification.
The radar unit is the Subsurface Interface Radar (SIR) System-2000, manufactured by Geophysical Survey Systems, Inc.1 Morey (1974), Doolittle (1987), and Daniels (1996) have discussed the use and operation of GPR. The SIR System-2000 consists of a digital control unit (DC-2) with keypad, VGA video screen ,and connector panel. A 12-volt battery powered the system. This unit is backpack portable and, with an antenna, requires two people to operate. A 500 MHz antenna was used in this study. The scanning time was a 50 nanoseconds (ns) time used at the Lavoie Site for transects 0 through 6.
The radar data (profiles) was downloaded from the SIR-2000 to a PC and copied onto a CD as unprocessed RADAN files. The RADAN (File###*dzt) files were converted into bitmap images using a conversion program. The bitmap files are labeled by the GPR file number of the transect, a spread-sheet to convert file number to grid coordinates is attached. The bitmap images can be opened using any image editing software such as MSPaint. Information on interpreting radar profiles can be found at: http://nesoil.com/gpr/profiles.htm.
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 layers) 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 ...
1. Manufacturer's names are provided for specific information; use does not constitute endorsement.
relationships among depth (D), two-way pulse travel time (T), and velocity of propagation (V) are described in the following equation (Morey, 1974):
V = 2D/T 
The velocity of propagation is principally affected by the dielectric permittivity (E) of the profiled material(s) according to the equation:
E = (C/V) 2 
Where C is the velocity of propagation in a vacuum (0.3 m/nanosecond). Velocity is expressed in meters per nanosecond (ns). A nanosecond is one billionth of a second. The amount and physical state of water (temperature dependent) have the greatest effect on the dielectric permittivity of a material.
The velocity of propagation (E) was estimated at 6 for the sites, based on the soil and moisture conditions observed during the investigation. Incorporating the soil test pit log data with the representative radar profile can allow for better depth estimates. To determine the estimated depth to a layer or target on the radar profiles use the following equation :
Depth (feet) = (time to layer in nanoseconds)/4.89 :
Nine landowners in the Towns of Highgate and Swanton volunteered to have a ground penetrating radar study conducted on their property along [REDACTED]. One property was eliminated from the study because it was heavily wooded which prevented reliable scanning by the radar. Two landowners with property along the floodplain of the Missisquoi River were eliminated from the study because these land forms were formed in fairly recent times and are thus not considered old enough to contain unmarked Native American graves. The remaining six properties have land forms with the potential to contain unmarked burials (Figure 2). Four of these study plots are located in Highgate and two are in Swanton.
Figure 2. Location of the Six GPR Study Plots along Monument Road.
The [REDACTED] plot is located in Highgate on the northern side of [REDACTED] approximately meters southwest of an old oxbow of the Missisquoi River (Figure 3.) This GPR plot is also situated adjacent to the [REDACTED] over 80 human burials were unearthed in 1973. a 20x40 meter grid was established on the southeastern portion of the [REDACTED] property. Transects were run in an easterly to westerly direction with observation marks placed at 1 meter intervals. Radar Files 193 to 199 were run at a 60-nanosecond time setting, and files 200 to 212 were run at a 50-nanosecond time scale. At this setting the depth of profiling is estimated at 3.5 to 3.0 meters. The radar identified at least 20 anomalies on the [REDACTED] ranging in depth from 30-146 cm (12-57.5 in) below surface. Five of these anomalies were explored with shovel test pits (Appendix 1).
Figure 3. Distribution of the Five Test Pits Excavated at the [REDACTED] d.
The first test LV 1, was a 25 centimeter (cm) square pit dug to establish site stratigraphy and examine the radar hit revealed at Transect I-30. This initial test pit was excavated with a tile spade to quickly obtain a stratigraphic profile and identify the radar anomaly identified 30 meters west of the base line in Transect 1 at approximately 29 cm (11.5 in) ...
... below surface. The radar hit displayed at this location was interpreted as a geological intrusion of gravelly soil near the interface of the B2 and C1 horizons.
The initial 50 cm square test pit LV 2 was expanded into a larger 1 x 0.5 meter unit to determine the origin of a deep anomaly identified by the radar at approximately 79 cm (31 inches) below surface four meters west of the base line on Transect 2. The stratigraphy of this test unit was complex because of several root burns and filled in rodent burrows (Photo 1). The radar hit displayed at this location was interpreted as a geological intrusion of gravelly soil near the interface of the C1 and C2 horizons.
Photo 1. South Wall Profile of Test Pit LV 2.
The 50 cm test pit LV 3 was excavated at Transect 5, 19.5 meters west of the base line to identify the radar anomaly revealed at approximately 33 cm (13 inches) below surface. The radar hit displayed at this location was interpreted as a geologic deposit of compacted gravel that formed just below the interface of the B1/B2 horizons which is not a cultural feature.
Shovel test pit LV 4 was excavated 25 meters of the base line on Transect 5 to identify the anomaly identified at approximately 45 cm (17.7 in) below surface. The radar hit revealed at this location was a localized geologic feature of gravel and course sand that was deposited within the B2 horizon.
Shovel test pit LV 5 was excavated at Transect 8, 33.5 meters west of the base line to identified the pronounced anomaly identified at approximately 25 cm (9.8 in) below surface. A broken plow blade was identified at the depth indicated by the radar. This artifact is probably from the early to mid 20th century but could also be the result of 19th century agricultural activities.
The five test pits did not identify any unmarked burials. A broken plow blade was unearthed in test LV 5. The remaining radar anomalies were attributed to geologic processes that typically occur in this depositional environment. The radar also identified the location of the residential water and electric lines. No precontact Native American artifacts or cultural features were identified at the [REDACTED]. Not all of the radar anomalies were examined however. It is recommended that additional test pits should be excavated where deep radar hits were logged to determine the presence or absence of unmarked burials.
The [REDACTED] Plot is located in highgate in the so-called [REDACTED] about [REDACTED] east-northeast of an oxbow of the Missisquoi River (Figure 4). This GPR plot is situated on the other side of the oxbow bank from the Lavoie Plot. A small 9x16 meter grid was established in the backyard of the [REDACTED] residence. Transects were run in a southerly to northerly direction with observation marks made at 1 meter intervals. All transects were made at a 50 nanosecond time scale with an estimated profiling depth of 3.0 meters.
Figure 4. Location of LaFrance GPR Plot behind residence.
The radar identified four anomalies on the [REDACTED] ranged in depth from 49-107 cm (19-42 in) below surface. Because of time constraints no test pits were dug at this study area. Ground truthing the four anomalies identified on the [REDACTED] south-southeast of the burial site restored on the former [REDACTED] (Figure 5). This landform is relatively flat and is currently used as hay land. A 40x26 meter grid was initially established at this site, but because of time constraints only a 10x26 meter area was scanned. Transects were run in a westerly to easterly direction with observations placed at 1 meter intervals. All transects were made at a 50 nanosecond time scale with an estimated profiling depth of 3.0 meters.
Figure 5. Location of Test Pit LR 1 on the [REDACTED] Plot.
The radar identified nine anomalies on the [REDACTED] lot ranging in depth from 28-74 cm (11-29 in) below surface. One of these anomalies was excavated 21 meters east of the base line on Transect 3 to radar identify the origin of a radar anomaly identified at approximately 43 cm (17 inches) below surface. No precontact Native American artifacts or cultural features were identified in test pit LR 1. Historic period artifacts (spike, refuse bone, glass, nail and slag) were recovered from the lower plowzone in Layer 2. The 7 inch long iron spike recovered at the bottom of the buried plowzone produced the anomaly detected by the radar at approximately 43 cm below surface. It is typical to find historic artifacts mixed within the plowzone horizon.
No precontact Native American artifacts or cultural features were identified at the LaRocque Plot. It is recommended that the remainder of this 30x46m study area should be scanned with the radar, and that additional testing of identified anomalies need to be excavated to determine the presence or absence of unmarked burials.
The [REDACTED] GPR plot is located in Swanton along the edge of an oxbow of the Missisquoi River (Figure 6). The landform is relatively flat but slopes down somewhat towards the northwest and top of the oxbow bank. A 14x28 meter grid was established, transects were run in a S-N direction with observations
placed at 1 meter intervals. All transects were made a 50 nanosecond time scale with an estimated profiling depth of 3.0 meters.
Figure 6. Location of Test Pits DB 1 and DB 2 on the [REDACTED] GPR Plot.
The radar detected at least three prominent anomalies on the Dubois Plot that ranged in depths from 37-92 cm (14.5-36 in) below surface. Shovel test pit DB 1 was excavated 20 meters north of the base line on Transect 9 to identify the origin of an anomaly identified about 38 cm (15 inches) below surface. This test pit was dug approximately 7 meters south-southwest from the top of the oxbow bank. The several layers of fill observed in this test pit represent landscaping activities that built up and extended the back year north towards the oxbow. The radar hit represents a change in the soil matrix in Layer 3 that was a geologic intrusion.
Shovel test pit DB 2 was excavated 12.75 meters north of the base line on Transect 13 to identify the origin of an anomaly identified about 37 cm (14.5 inches) below surface. This test pit was placed 4.5 meters south-southwest of the oxbow bank. The anomaly identified at this location was a decayed tree root that measured approximately 20 cm in diameter. No precontact or historic period artifacts or culture features were identified in either shovel test pit. Additional shovel testing is recommended to determine the origin of the other pronounced radar anomaly scanned at the Dubois Plot.
The [REDACTED] GPR plot located in Highgate on the southwest of side of Monument Road along with the band of the Missisquoi River (Figure 7) and approximately [REDACTED] southeast of the [REDACTED] on the former [REDACTED]. This landform is relatively flat. A 18x30 meter grid was established, transects were run in a W-E direction with observations placed at 1 meter intervals. All transects were made at a 50 nanosecond time scale with an estimated profiling depth of 3.0 meters.
Figure 7. Location of Test Pits TP 1 and TP 2 on the [REDACTED] GPR Plot.
Approximately twenty-seven anomalies were revealed within the Thompson study area ranging in depth from 30.5-92 cm (12-36.2 in). Two shovel test pits designated TP 1 and TP 2 were excavated at this site.
Test pit TP 1 qas excavated at Transect 7, 16.2 meters north of the base line to identify the origin of an anomaly identified at 55 cm (21.5 inches) below surface. This test pit is located in a residential yard approximately 17 meters from the top of the riverbank. The anomaly identified by the radar at 55 cm below surface was a geologic deposit that contained many iron concretions.
Shovel test pit TP 2 was excavated 14 meters east of the base line on Transect 16 to identify the origin of a radar anomaly identified approximately 42 cm (16.5 inches) below surface. This test pit is located about 19 meters from the top of the riverbank. One piece of window glass was observed in the plowzone layer (Ap) but was not collected. The anomaly identified by the radar was an intact tree root that measured about 18 cm in diameter.
No precontact or historic period artifacts or cultural features were identified in either shovel test pit. Remnants of the original gravel based Monument Road was identified from 6-10 meters east of the grid base line. The radar identified many anomalies in this study area, and additional deep shovel testing is recommended to determine the presence or absence of unmarked burials on the Thompson Plot.
The [REDACTED] GPR plot is located in Swanton along the Missisquoi River (Figure 8). The landform slopes up gently away from the river to the north-northeast. A 26x34 meter grid was established, transects were run in a zigzag pattern alternating between S-N and N-S directions with observations placed at 1 meter intervals. All transects were made at a 50 nanosecond time scale with an estimated profiling depths of 3.0 meters.
The radar located eleven anomalies on the [REDACTED]. One area was flagged for investigations. Shovel test pit WF 1 was excavated 27 meters north of the base line on Transect 29 to identify the origin of a pronounced anomaly scanned at 61 cm (24 inches) below surface. This test pit is located approximately 31 meters from the top of bank in a pasture. A rectangular slab of red sandstone was encountered at precisely 60 cm (24 inches) below surface as the radar indicated. The roughly 6x8x2 inch angular stone was lying flat at the interface of B and C horizons. There was no evidence of any soil staining (grave fill) above, below or adjacent to the rock, and no other stones were observed in the entire test pit. It is extremely odd to find such a large, angular stone at this depth within an alluvial soil where few stones are expected.
Figure 8. Location of Test Pit WF 1 on the [REDACTED] GPR Plot.
Figure 8. Location of Test Pit WF 1 on the [REDACTED] GPR Plot.
No precontact or historic period artifacts or cultural features were identified in shovel test pit WF 1. Because of the presence of other pronounced radar anomalies revealed at the Fournier Plot, it is recommended that additional excavation occur at these locations to determine the presence or absence of unmarked graves.
No human remains were identified during the study. However, a suspicious flat sandstone found at 2 foot depth was probably marking a grave. Although there was no evidence of human bone or grave fill it is highly unusual to have a large angular stone in such a depositional environmental. Evidence of the grave could have been completely leached away over time in the excessively well drained soils of the study plot.
Soil conditions at this site were very favorable (sandy alluvial and outwash soils with a few coarse fragments) for GPR techniques. The radar identified numerous "point objects" which are displayed as a hyperbolic pattern on radar profiles and layering or stratification within the soil. The stratification's identified by the radar were interpreted as the eolian ...
soil mantle/geologic deposit which were approximately 20 to 30 inches below the surface and geologic strata (layers of sand and gravel) below the eolian mantle.
Even with favorable site conditions the detection of buried cultural features with the GPR cannot be guaranteed. The detection of buried cultural features is affected by (i) the electromagnetic gradient existing between a cultural feature and the soil, (ii) the size, shape, and orientation of the buried cultural feature, and (iii) the presence of scattering bodies within the soil (Vickers et al., 1976).
During the investigation the radar identified numerous point objects on the profiles, but ground-truthing by the archaeologists did not yield any burials. Most of the features encountered in the test pits were more recent artifacts and dissimilar soil types. Based on the results of the field investigations, the radar was not successful in locating potential burials at this site. Preliminary Ground-penetrating radar data provided no information concerning the presence and distribution of additional burials at these sites. Another possible explanation is that there were no burials located where our grid-lines (transects) were placed.
The use of group penetrating radar to identify unmarked burials is not always conclusive. Certain soils types and site conditions can confound the GPR signals, and if the contrast of a burial is negligible, as documented elsewhere on Monument Road, then these devices will not provide the conclusive results desired. In such situations and when house construction is imminent, an archaeologist and Abenaki representative should monitor the bulldozer as it carefully peels away the soil inches at a time. This last-ditch method may unfortunately damage some of the human remains as they are exposed but the remainder of the burial will be protected. Only then can we feel confident that the study area has been thoroughly surveyed and that no burials exist within the project limits. This last will only be used when development is imminent.
1. The radar data was interpreted in the field to locate areas to ground truth by the archaeologists. The file (radar profiles) were not post-processed using the RADAN program to see if the processed files showed more information. The RADAN program also allows for 3D grid modeling and time slicing of the gridded areas, which have been shown to be useful for archaeological surveys. Processing the 100 plus files in this manner would take an estimated week of work to complete and there are no signs the data would be any more successful at locating burials with any degree of confidence. Training on the RADAN program has not been made available to the radar operate at this time.
2. The use of other geophysical equipment (FM, magnetometers, etc.) or a different GPR system (higher-frequency antenna for example) may provide better data at this site. A GPR operator with more experience with archaeological applications may also be better suited for this study.
3. If a known burial is located, a traverse with the radar would be useful to discount the possibility that there simply were no burials were we profiled. The data from running the radar over a known burial and correlating the profile signatures would be helpful for future investigations.
4. Too many plots were scanned during the week long study. While the GPR scanning went smoothly and took less than a day to examine each plot, ground truthing the anomalies was more time consuming than initially thought with the result that not enough time was spent at each lot to thoroughly examine all of the suspicious anomalies recorded by the radar. Too much time was spent excavating the shallow radar hits. At depths below three feet (where most burials are expected) a much larger test unit is needed which greatly increases the excavation time and earth moved. Ground truthing deeper anomalies would have required that a much larger hole had to be excavated within well-kept residential yards, and would have taken more time than was available for the study.
4. [sic] The ideal way to adequately test the GPR technology along Monument Road is to investigate a parcel that will be developed or improved. Once the GPR study and ground truthing has been completed, the subsequent earth moving by heavy machinery would be monitored by the archaeologist to compare radar results with 'reality' to determine precisely what is located below ground. This investigation would tell us if the radar missed any unmarked graves because of a lack of contrast from the surrounding soil or it would show that there would be no umarked graves in the study area.
We have taken a large step towards locating unmarked burials along Monument Road by conducting the 2002 GPR pilot study. The depositional environment along Monument Road is complex and needs to be better understood before cultural features and burials can be identified with confidence. The information obtained during the week long study is extremely useful and begins to build a database that will help the next GPR operator and archaeologists to eliminate non-cultural anomalies and focus on the deeper radar hits where unmarked burials are more likely to exist.
Daniels, D.J. 1996. Surface-Penetrating Radar. The Institute of Electrical Engineers, London, United Kingdom. 300 p.
Doolittle, J. A. 1987 Using ground-penetrating radar to increase the quality and efficiency of soil surveys. 11-32 pp. In: Reybold, W. U. and G. W. Peterson (eds.) Soil Survey Techniques, Soil Science Society of America. Special Publication No. 20. 98 p.
Killam, E. W. 1990. The detection of human remains. Charles C. Thomas Publisher, Springfield, Illinois. p. 263.
Morey, R. M. 1974. Continuous subsurface profiling by impulse radar. p. 212-232. IN: Proceedings, ASCE Engineering Foundation Conference on Subsurface Exploration for Underground Excavations and heavy Construction, held at Henniker, New Hampshire. Aug. 11-16, 1974.
Vickers, R. L., Dolphin, and D. Johnson. 1976 Archaeological investigations at Chaco canyon using subsurface radar. pp. 81-101. In: Remote Sensing Experiments in Cultural Resource Studies, assembled by Thomas R. Lyons, Chaco Center, USDI-NPS and University of New Mexico.
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David Skinas, working for the USDA ... HAS had a "working relationship" with these Alleged and Re-Invented VT "Abenakis" for years! He has sat at the SAME TABLE with April St. Francis-Merrill of Swanton, Franklin County, Vermont for years!