Across the United States, there is a significant number of forgotten burials of unmarked and lost graves. Geophysical techniques, including ground penetrating radar (GPR), are required to non-destructively detect these burial sites in cemeteries and other locations.
Cemetery mapping utilizing GPR is becoming increasingly popular. Efficient and rapid two-dimensional (2D) real-time evaluation of GPR profiles is especially useful for projects with time constraints or when suspecting large amounts of obstacles in the survey area.
Lost Graves Stem from Cemetery Age and Population Growth
Historical cemeteries can date back centuries and missing, fallen or poorly placed headstones can complicate the presumed physical location of gravesites. Additionally, the original documentation may be unavailable or unreadable, which complicates the issue further.
GPR service providers are being employed on an increasing basis to generate up-to-date burial maps or clear areas for new burials. Moreover, as modern population growth means an increase in infrastructure and city sprawl, there are documented cases where contractors were granted permission to build over forgotten burial grounds.
In certain cases, a GPR investigation may be performed to detect a cemetery’s existence as well as the presence or absence of burials. GPR can also help identify whether the graves have been disturbed and other factors related to relocation recommendations.
Additionally, GPR may be used to assess overlooked graves during cemetery relocation as a result of urban expansion.
Image Credit: Geophysical Survey Systems Inc.
Three Keys to Successfully Using GPR Equipment
When choosing GPR equipment for cemetery mapping surveys, there are a number of best practices to consider. Users should know their GPR equipment and learn how best to use it in cemeteries.
The first key to success is identifying the most appropriate equipment to use and gaining familiarity with the chosen equipment. Read white papers or posters that detail specific GPR methods for cemeteries and become familiar with how burials appear in GPR datasets.
Furthermore, consider employing archaeologists or other experienced users for specialized training on using GPR equipment for cemetery mapping. For instance, GSSI offers GSSI Academy classes on cemetery mapping and equipment use.
Conduct Pre-Survey Research
Generally, the best experience is hands-on practice. Consider contacting a local cemetery and requesting permission to scan their grounds before conducting and offering cemetery surveys as a service.
Be present when a new grave is being prepared. Get to know the age of the cemetery and how it may have been maintained over time and then evaluate the existing marked graves in the cemetery to identify the direction and orientation of the graves.
Ideally, the GPR should be passed over burial sites perpendicular to their long axis (90 degrees). Before conducting any cemetery mapping project, get acquainted with local soil conditions, including the composition of soil and depth to bedrock/ledge.
If a representative soil profile cannot be readily accessed, check out the USDA NCRS soil map and other online soil science resources.
Consider the Variables
The quality of a GPR survey is significantly influenced by soil conditions, weather and the surrounding environment. Make a note of observances in your field notes and consider if other geophysical methods can be employed to complement the GPR survey.
Be particularly cautious if the ground is saturated with water. If water rises to the surface when walking over the site, consider returning once the water content in the soil has decreased. Look for large trees in the immediate area, signs of large rocks and animal burrows; these and other targets could produce false positive indicators of burials.
Peter Leach collecting 2D data with the GSSI UtilityScan system. Image Credit: Geophysical Survey Systems Inc.
2D Cemetery Mapping Using GPR
Evaluating real-time two-dimensional (2D) GPR profiles is both quick and efficient and a frequently used method for projects with limited time or where there are numerous obstacles in the survey area.
The fundamental technique necessitates collecting GPR profiles perpendicular to assumed burial orientation and determining possible burial-related targets and associated soil disturbances.
Not all burials (especially older ones) will display a hyperbolic target due to the decay of wooden coffins, but there should be sufficient evidence of the grave’s shaft as a result of excavation and filling of the grave. Water content is the principal factor affecting GPR performance.
Water impedes the GPR velocity, increases dielectric and can increase conductivity levels due to mixing with soil chemistry. In high-dielectric conditions, hyperbolic targets are narrow and more challenging to see, leading to a decreased chance of observing a burial target.
Some soil textures, like clay and silt, bear more water and may have intrinsically higher conductivity. High soil conductivity acts like a lightning rod for GPR energy, whereby the GPR signal is absorbed into the ground and does not return to the antenna when reflected by a target or layer.
This significantly reduces depth penetration, and in dire cases, may significantly impede data collection to just one or two feet below the surface. Another factor related to soil is the presence of gravel, larger rocks or boulders.
Gravel can generate clutter in the GPR profile, while cobbles and boulders produce perplexing hyperbolic targets. The disturbance in the ground from an animal burrow can also create hyperbolas that would puzzle a user when attempting to locate burials.
The critical factor to consider when differentiating actual burials from false positives is to look past the targets and assess the entire GPR profile. Rocks, roots and animal burrows may show up as hyperbolic targets, but they generally do not have a soil disturbance above them.
Burial targets should exhibit an associated soil disturbance, and this usually appears as an anomalous area above the target or as broken soil layers. Roots and animal burials may straddle wide sections of the project area (tens of feet), but a human burial site will not.
Using paint or pin flags to mark potential targets will help visualize the length of targets and assist in excluding some of them. The choice of GPR antenna is of utmost importance to the success of 2D cemetery mapping. Higher frequency antennas, like 900 MHz and 2700 MHz, may display impressive resolution but cannot reach typical burial depths.
Alternatively, lower frequencies antennas significantly improve the depth of investigation, but they give up on resolution in the process. This is because burial sites are generally buried deep underground, and they may not exhibit large targets in profiles.
People who are performing a cemetery survey should employ a 400 MHz or 350MHz HyperStacking® antenna to gain optimal results. These antennas offer the ideal balance between depth and resolution and will supply users with sufficient depth penetration without producing unwanted soil clutter in your data.
Another key factor is burial container material. Some wooden coffins may remain for a long time, but in the majority of cases, they will quickly collapse and decay. In these cases, cemetery mappers will not be searching for the object as a target, but rather the void, or soil disturbance, that the target is/was in.
Brick and concrete vaults should be easier to locate as they stand the test of time. However, these containers are much larger than wooden coffins and the top of the container may only be 1-2 feet below the surface; they can be missed easily if the survey is concentrated on depths of 4-6 feet.
Finally, one might work in cemeteries where burials were previously exhumed. In this scenario, the remains and container are withdrawn, and all that is left behind is an excavation that has been filled in and is usually larger than the original grave shaft.
Peter Leach conducts a 2D data collection in a cemetery in Massachusetts with the UtilityScan Pro with the 350 HS antenna. Image Credit: Geophysical Survey Systems Inc.
Advantages and Disadvantages of 2D Data Collecting
The advantages of 2D data collection in a cemetery include the speed of data collection, on-the-spot marking, and simple data interpretation.
Speed of Data Collection
Since 2D data collection does not necessitate setting up a grid, most of the time is usually spent prepping the area is cut down or eliminated.
When collecting a 2D file, one can prep out the area easily with a few passes around the location to look for the prime location to conduct the survey.
On the Spot Marking
Spray paint and flags can be employed as on-the-spot markers to simply identify a possible grave location in the cemetery. When marking the target on the ground, also make a note of it in a notebook, along with notations of possible graves and other obstructions. Additionally, take photographs of the landscape at the time of marking.
Viewing 2D data on the screen and analyzing the results on the spot is relatively straightforward. Unlike 3D data, 2D data collection does not necessitate post-processing to observe the data.
Use a notebook to mark possible targets, obstructions, and possible graves. Image Credit: Geophysical Survey Systems Inc.
Disadvantages of 2D data acquisition for cemetery mapping include interference that impedes real-time interpretation, difficulty in distinguishing targets and the risk of incomplete coverage.
Interference GPR data from cemeteries can be influenced by external noise, soil-related issues and other unavoidable data problems that hinder real-time interpretation.
Continuous EM noise will produce horizontal noise bands that span soil disturbances and create an impression of continuous soil layers; proximity to the transmission source will further compound this issue.
Sporadic EM noise can produce a snowy or static overprint and cloud real data. Soil conditions generate their own noise signatures, and typically these are aggravated by certain clay varieties and general water content.
Salt, nitrates, calcium carbonate and other chemical components will enhance conductivity and limit penetration depth while greatly reducing interpretive potential.
Another key concern is the equifinality of GPR reflections, as targets and layers from totally different origins can be indistinguishable on GPR profiles.
For instance, animal burrows, coffins, rocks, roots and other point sources all produce hyperbolic targets. Tree removal, clandestine burials, pet burial, utility trenches and other disturbances all have similar characteristics and real-time interpretation of solitary GPR profiles can be a challenge.
To avoid this data interpretation issue, keep an eye out for disturbances in the ground and the hyperbolic targets on the entire 2D GPR scan rather than individual hyperbolic targets.
When conducting 2D data collection, it is relatively easy to miss areas and not realize it since you do not have a grid that compels you to accumulate data in straight lines and at regular intervals.
Consider Expanding into 3D Locating
While 2D locating is rapid and remarkable for use in the field, potential errors interpreting that data could be reduced by also employing three-dimensional (3D) data collection. 3D locating will optimize the data capabilities and deliver more certainty during post-processing of the data.
This information has been sourced, reviewed and adapted from materials provided by Geophysical Survey Systems Inc.
For more information on this source, please visit Geophysical Survey Systems Inc.