Cemeteries and Geophysical Survey

Cemeteries are absolutely some of the most challenging conditions within which to conduct geophysical surveys. No one geophysical surveying instrument can detect all graves 100% of the time. Furthermore, graves may still be present even if no obvious signs of them are detected during the geophysical surveys. That said, and assuming the ground/site conditions allow for surveys to be done, multi-instrument geophysical surveys are the best approach for non-destructively identifying the presence of unmarked graves in cemeteries of any age. Three instruments that we have used in the past have proven useful for detecting graves in a variety of settings:

Magnetometer

Magnetometers are very sensitive to iron-based materials, including iron objects and iron in the soil. In cemeteries the primary thing of interest that magnetometers can detect is the disturbed soil in grave shafts. It will not detect buried human remains or coffins unless the coffins are buried in a brick or reinforced concrete vault or the coffins are made from cast iron or some other magnetic material (that does not include copper or brass). They will not detect the hardware on coffins because in most cases it is made of brass or is buried too deeply to be detected. In older cemeteries, where graves have subsided as the wooden caskets disintegrate and collapse, magnetometers are good at detecting the soil brought in to fill in the depressions over the collapsed coffins. One major downside to the use of magnetometers in cemeteries is that they are very sensitive to iron objects, and in cemeteries there are many sources for iron objects that can, in essence, blind the magnetometer. Such iron sources include cast iron fences that surround the cemetery or individual family plots, headstones with iron components (such as pins that attach the headstone to a base stone), iron plot markers, and iron-based grave furniture (like war veteran stars, plastic flowers and wreaths that have thin steel wires, etc.). Magnetometers can be used to survey over gravel or paved drives and can be performed in the winter on frozen ground.

 

 

Electrical Resistance Meter

Like magnetometers, electrical resistance meters are good at detecting disturbed soils, which, in cemetery settings, include the soils in grave shafts. Resistance meters send out a little electrical current into the ground and then measure how easily it travels from one place to another. Areas of more or less soil moisture are essentially what this instrument measures. Many aspects of a grave can alter soil moisture. Because graves are areas of disturbed soil, moisture may be able to more easily penetrate, and accumulate in, a grave. Such graves will have a lower resistance. Graves with burial vaults, be they brick or concrete, will have a much higher resistance since the vaults are a barrier to moisture. Resistance meters cannot detect iron objects within or on top of the soil, so they are not hindered by iron fences or iron objects around the graves. However, soil moisture conditions are very important to acquiring useful resistance data. Resistance surveys cannot be done in the winter or when the ground is even partially frozen. They can also be ineffective when the ground is very wet, as in late winter/early spring, or very dry, as in late summer. That said, ultimately the results of a resistance survey are dependant on the soil types present in the survey area, the kinds of things that one wants to find (like graves), and the soil moisture conditions at the time of the survey. These factors can work together in many different ways to make resistance surveys better or worse for finding unmarked graves. Resistance data cannot be collected over gravel or paved drives.

 

Ground Penetrating Radar

Ground penetrating radar (GPR) surveys work by sending electromagnetic waves (radio waves) into the ground and then detecting them as they bounce off of things below ground and come back to the surface. The strongest radar reflections occur when there are buried layers, or objects, below ground that are very different from their surroundings. GPR is very good at detecting buried layers of different materials. In cemeteries, GPR tends to detect the sides and bottom of a grave shaft, rather than the grave shaft fill. If the coffin is still intact and contains air, the air will create a strong radar reflection. Metal coffins, of any type, and burial vaults will also create strong radar reflections. Thus, at first glance GPR seems the most ideal of the instruments for locating unmarked graves. However, it has its limitations. Soils that are very wet and clayey tend to absorb the radar waves such that very few bounce back, making it hard to detect anything below ground. Soils with lots of gravel have the opposite effect—they produce so many reflections that the GPR results can be too cluttered with reflections to be of use. And, of all the instruments, GPR has the most particular requirements for surface conditions—it does not work well in tall grass or weeds or on very bumpy terrain. The GPR must be in direct contact with ground, while the other instruments do not have such requirements. Fortunately, most cemetery settings have ideal ground surfaces for GPR surveys—mowed grass. GPR can be used in the winter on frozen ground and it can be used over gravel and paved drives. The great advantage of the GPR over the other instruments is that it produces 3-dimensional data blocks--that is, GPR data can be used to examine the ground at different depths. The example included here is just one slice, at about 56-76 cm below surface, from the radar data collected at this cemetery.

Looking for the remains of the missing Goettge Patrol (WWII), Guadalcanal, Solomon Islands.

 

  

While there are other geophysical instruments that could be used to locate unmarked graves in cemeteries, the three mentioned here are commonly used and have located unmarked graves all over the world. All three instruments are fairly quick to use and maps of the survey results can be made in minutes. Nevertheless, effective surveys require knowledgeable processing of the data and interpretation of the survey results. And, in the end, it is important to remember that the instruments detect differences in various electromagnetic properties of the soil and objects within it, not graves. The determination of whether or not graves have been detected is an interpretation and the only way to verify an interpretation of geophysical survey data is to core or dig up the things detected. However, the use of multiple geophysical instruments is the next best means, after excavation, for identifying the presence of unmarked graves. One instrument’s survey results can serve to support or refute the results of another, which helps improve the reliability of our interpretations.

In addition to collecting the geophysical data, it is also important to make an accurate map of the cemetery, at least in the area being geophysically surveyed. We use a laser transit/total station for making our maps as it is about the only way to make fast, and highly reliable maps. Only survey-grade GPS systems are accurate enough to make adequate maps of cemeteries for use with geophysical survey data.