Archaeo-Physics, LLC - geophysical survey and subsurface mapping

Case Study

 

 

Cemetery survey with groud penetrating radar (GPR)
GPR in use on an historic cemetery

Location of Human Burials

Archaeo-Physics has conducted geophysical investigations of numerous cemeteries throughout the United States. The objective of these investigations was to locate both marked and unmarked burials within known cemetery boundaries, as well as locate and define boundaries where thay are not known. These investigations consisted primarily of ground penetrating radar (GPR) surveys, although electrical resistance and magnetic surveys are often effective where field conditions do not favor GPR.

Burials, whether historic or prehistoric, are often very difficult (sometimes impossible) to detect with geophysical methods. Ideally multiple geophysical methods are used in conjunction with careful mapping of surface features, historical research, and other data sources.

Ground Penetrating Radar

GPR has been successfully used to locate both marked and unmarked human burials in historic and pre-historic cemeteries and in forensic work. The location of graves with GPR can be accomplished in many ways. These include locating disturbed soil associated with the grave shaft, reflections associated with bones, coffins, grave goods, clothes and the detection of breaks in the natural soil stratigraphy. The location of disturbed soil associated with a grave is perhaps the most distinctive feature of a burial. The mixing of soil due to excavation causes changes in the porosity, leading to changes in the electrical and magnetic properties of the material, all of which create radar reflections differing from the surrounding subsoil. Burials can also be located by locating breaks in the natural soil stratigraphy. The bones, clothing, coffin, coffin hardware and grave goods are possible radar reflectors. A strong reflection may be caused by the skull due to the air void within. It has also been suggested that the decomposition of human remains leach calcium salts into the surrounding subsoil for many years. These salts change the electrical properties of the surrounding soil, making it visible to radar waves.

The chief disadvantage of the GPR is that its success is very dependent on specific site conditions, and is very difficult to predict. In general, sandy, homogeneous soils are ideal, while clays, silts, and rocky or heterogeneous soils greatly reduce the chances of success.

GPR profile plot showing unmarked grave
This figure shows a two-dimensional plot of an unmarked burial within a 19th century Native American cemetery. Discontinuities in the natural soil stratigraphy associated with the burial shaft are apparent at 0.5 to 1.5 meters below surface, while the strong reflections at approximately 1.8 meters below surface are thought to be associated with the actual burials.

Planview Presentation of GPR data

GPR data are traditionally examined as profile maps of individual transects. Time-slicing is a technique for constructing planview maps of an area isolating specific depth ranges. This not only makes interpretation of the data in the horizontal plane much more intuitive, but also allows us to isolate specific depths (or more properly, the two-way travel times of reflected waves) for examination. Data for time-slice analysis must be collected systematically at closely-spaced (generally 50cm) transect intervals.

GPR time slice of historic cemetery

A Time-sliced GPR map of a 19th century cemetery showing correlation between geophysical anomalies and existing grave markers (shown in yellow). Additional patterning (although less distinct) may be associated with unmarked graves. For a more detailed discussion of these data, see the Ellis Cemetery page.

 


Electrical resistance survey on an historic cemetery

Electrical Resistance Survey

Although resistance methods are more limited than GPR in their ability to detect low-contrast features at great depth, they may detect patterning cused by the disturbed soils within grave shafts. Resistance survey can be a valuable adjunct, even when conditions are favorable to GPR survey. Where site conditions (such as clay or silt soils) limit the effectiveness of GPR, resistance survey may be the principal survey method.

Graveshafts may appear as either high-resistance or low-resistance anomalies, and may appear as both within the same cemetery. Small-scale variance and anisotropy are also possible indicators of disturbed soils. Optimization of resistance methods for grave detection is a subject of ongoing research by Archaeo-Physics personnel.

Resistance / resistivity survey  of an historic cemetery

Electrical resistance survey results from an historic Euro-American cemetery. Although individual graves may be indistinct, rectilinear patterning indicates a high density of graves in north-south rows. Presumed graves fall generally within the green-blue range of the color scale. Interestingly, the few grave markers that occur within this area do not correlate well with the geophysical patterning. This suggests that many grave markers may be missing or displaced. The east-west linear anomaly appearing at approximately N26 is thought to be a utility line. For a more detailed discussion of these data, see Mapping unmarked graves at layman's cemetery.

Magnetic Survey

Magnetometers can be very effective tools for mapping cemeteries under certain conditions, but must be used judiciously. In many cases, igneous rock (as monuments or occurring naturally) and ferrous metal dominate the magnetic environment, obscuring more subtle patterning. In other cases these highly magnetic materials are buried below the surface and are indcators of cemetery patterning: Steel or iron in caskets, coffins or vaults, grave markers that have subsided and buried themselves, brick monuments, gravel paths and roads, even plastic flowers that have degraded, leaving their wire stems.

Where igneous rock, metal, and brick are not present, magnetometers can detect more subtle anomalies caused by concrete or organically enriched, disturbed, or compacted soils.

Magnetic map uf unmarked graves in an historic cemetery
Mouse over image to display interpretations

Key to interpretations:

  • Three subtle but distinct north-south rows of graves appear near the center of the map (indicated by red shading in the interpretative markings). Disturbed soils appear as rather weak lows.
  • Moderately strong magnetic highs within this region (outlined in black) may be caused by caskets or vaults containing highly magnetic materials.
  • Green shading indicates a jumbled bipolar (both positive and negative) response caused by road gravel.
  • Isolated strong bipolar anomalies among the graves may be granite grave markers (now buried), although these are not readily distinguishable from ferrous metal, which may also be associated with graves as ornaments, etc.
  • The eastern portion of the map is relatively uniform, and does not show indications of cemetery patterning, suggesting that no burials are present in this area.

In the example above, there are no extant grave markers or surficial indications of burials. The interpretations are supported by multiple data sources, including limited subsurface testing, resistance survey results, and topographic and landscape features.

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