Real-time evaluation of GPR profiles is quick and efficient, and a common method for projects with time constraints or where there are abundant obstacles in the survey area. The basic technique is to collect GPR profiles perpendicular to assumed burial orientation, and to identify potential burial-related targets and associated soil disturbances. Not all burials (especially older ones) will exhibit a hyperbolic target due to decay of wood coffins, but there should be evidence of the grave shaft as a result of excavation and filling of the grave. There are many environmental variables to deal with, therefore real-time cemetery surveying requires an in-depth knowledge of your GPR hardware, familiarity with local modern and historical burial practices, and a strong understanding of GPR theory and how cemetery targets appear on GPR profiles. In this post we will discuss the advantages and disadvantages of 2D data collection and offer advice on best practices. Real-time cemetery surveys can be challenging, especially for novice GPR operators, so we will start with a few key points on relevant GPR theory.
GPR Theory for CemeteriesThe performance of your GPR system depends on multiple environmental factors and local soil conditions. These include water content, soil conductivity, soil texture, and other factors. There are also hardware considerations, and chief among these is the GPR antenna frequency. Of equal importance is the burial container (or lack thereof) and whether it is constructed from wood, concrete, brick, or a modern synthetic material.
Water content is the single most important factor for GPR. Some water is required for GPR to penetrate the subsurface, but too much water can severely limit the depth of investigation. Water slows down the GPR velocity, increases dielectric, and can mix with soil chemistry to increase conductivity levels. In high-dielectric conditions hyperbolic targets are narrow and more difficult to see, reducing the chance of observing a burial target. Certain soil textures, like clay and silt, hold more water and may have inherently higher conductivity. High soil conductivity is like a lightning rod for GPR energy, whereby the GPR signal is dissipated into the ground and does not return to the antenna when reflected by a target or layer. This vastly reduces depth penetration, and in extreme cases may limit data collection to one or two feet below the surface.
Another soil-related factor is the presence of gravel, larger rocks, or boulders. Gravel can create “clutter” in the GPR profile, while cobbles and boulders create confusing hyperbolic targets that can look like coffins. You might also encounter animal burrows, and these too will generate hyperbolas. The key to differentiating actual burials from “false positives” is to look beyond the targets and to evaluate the entire GPR profile. Rocks, roots, and animal burrows may appear as hyperbolic targets, but they usually will not have a soil disturbance above them. Burial targets must have an associated soil disturbance, and this might appear as broken soil layers or an anomalous area above the target. Roots and animal burials may extend across the project area for tens of feet, but a human burial will not. Marking potential targets with paint or pin flags will help you visualize the length of targets and assist in ruling out some of them.
Your choice of GPR antenna is of critical importance. Higher frequency antennas, like 900 MHz and 2700 MHz, may exhibit impressive resolution but they cannot penetrate down to typical burial depths. Alternatively, lower frequencies greatly improve the depth of investigation but they sacrifice resolution in the process. As burials can be somewhat deeply buried, and they may not present large targets in profiles, we recommend a 400 MHz or 350MHz HyperStacking® antenna for most cemetery surveys. These antennas provide the ideal interplay between depth and resolution and will provide adequate depth penetration without generating unwanted soil clutter in your data.
Tip: Become familiar with local soil conditions, including soil composition and depth to bedrock/ledge, and ask to be present when someone is digging a new grave. If you cannot see a representative soil profile, check out the USDA NCRS soil map and other online soil science resources.
The various materials used for burial containers can degrade over time, especially if constructed from wood. Under the right circumstances wooden coffins may persist for a long time, though it most cases they will collapse and erode away fairly quickly. In these cases, you should understand that you will not be looking for the target; you will be looking for the hole, or soil disturbance, that the target is/was in. Brick and concrete vaults last much longer and should be easier to locate. However, these containers are larger than wooden coffins and the top of the container may only be 1-2ft below the surface. If you are only looking at the 4-6 foot depths, you might miss them! Lastly, you might encounter cemeteries where burials were previously exhumed. In this scenario the remains and container are removed, and all that is left is a filled-in excavation that is often larger than the original grave shaft.
Survey Area Considerations
Most of us cannot choose our project areas, which is why GPR operators rarely visit golf courses during work hours. As such, you might arrive at a project area and discover numerous issues that will slow down or complicate the survey. You should visit the project area prior to the survey and assess the ground conditions. If you are unable to visit the site in person, use Google Street View or Google Maps to check it out. Formal cemeteries are usually well-maintained, but older sections or those with burials of paupers or indentured persons may be overgrown with vegetation. A good rule of thumb is “if your lawnmower can’t go over it, the GPR can’t either”. Ask your client to clear brush, vines, and other problems; this will save you a lot of time and greatly improve the data quality.
Request maps of the cemetery and other relevant information. You might ask if there were ever any stones in the vicinity, or if historical records suggest the presence of burials. If you are surveying along a cemetery wall, ask when the wall was built. The wall could partially or completely cover burials that predate its construction, or there could be burials outside the formal cemetery boundaries.
Trees are ubiquitous in cemeteries, and though they add to the aesthetic their roots generate many “false positive” targets. Be cautious in areas with large and abundant trees and expect to find filled-in holes where trees fell in the past. Treefalls create soil disturbances that may look like grave shafts or mass burials.
Tip: Be wary of cemetery surveys after a large or prolonged rainstorm. If your boots “squish’ when you walk, or you leave deep muddy footprints, consider postponing the survey until the area dries out.
Cemetery targets are highly variable, so your system settings must be optimized to account for many different scenarios. This effort begins in the field, where you’ll want to configure an adequate depth range, perform a range gain/manual gain on “normal” background levels, and set a relatively accurate dielectric for depth calibration. Your goal should be to survey deep enough to image burials, optimize your gain settings to highlight lower amplitude areas and stratigraphic disturbances, and calibrate your depth scale for accurate assessment of target depth.
Burials are rarely “six feet under”, and depth to interment may vary wildly across the same cemetery. Coffin burials are usually four to six feet deep, but the top of a modern concrete burial vault or historical brick vault will likely be only one to two feet below the surface. Do not set the time range/depth to six feet; always overshoot the depth until the bottom 25% of the profile is attenuated. If your dielectric is inaccurate your depth scale will be as well, and this could lead to shallower-than-expected penetration. Deeper penetration will also reveal any potential stacked graves (more than one burial in a grave shaft) or will account for any historical or industrial fill emplaced over the cemetery. It is also important to look for marker beds or other relatively shallow stratigraphic indicators that will have to have been cut through to place an interment.
In addition, understanding dielectric changes and amplitude responses are critical for the GPR interpretation. Metal coffins and wooden coffins will all look different on the screen on your GPR system. The larger the dielectric change, the stronger the reflection and the brighter the target. The smaller the dielectric change, the weaker the reflection and the dimmer the target. The metal coffin will show up as a bright white to black to white target, while an an air-filled coffin will appear as a weak black to white to black target.
Tip: In 2D profiles, closely spaced graves can look like layers rather than individual targets. Furthermore, closely spaced burials, even with coffins, may not exhibit discrete grave shafts for each burial since all of the individual shafts could effectively coalesce into one large “trench” with no interior separations.
While 2D locating is fast and markable in the field, potential errors interpreting that data in the field could be minimized by collecting in 3D as well. Three-dimensional locating will maximize the data capabilities and achieve more certainty with post-processing the data. GSSI academy can help with RADAN and interpret data.
Credits// Author: GSSI