Case Study
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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.
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.
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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.
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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. |
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Electrical resistance survey on an historic
cemetery
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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.
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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.
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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