Archaeo-Physics, LLC - geophysical survey and subsurface mapping

Case Study

Wyandotte County Cemetery

Kansas City, Kansas

The Wyandotte County Cemetery, in suburban Kansas City, was in use from the mid 19th century up to 1973. It is thought to contain hundreds of recorded burials, but only two graves were marked. The locations of other burials and the limits of the cemetery were not precisely known. A program of archaeological research was undertaken to define the limits of the cemetery integrating non-invasive geophysical techniques with conventional archaeological methods.

The geophysical investigation consisted of electrical resistance and magnetic gradiometer surveys of portions of the cemetery. Portions of the larger survey are presented here to illustrate the response of these techniques to historic graves (the geophysical investigation are discussed more broadly here).

The Geophysical investigation was followed by subsurface testing. Limited excavation largely confirmed initial interpretations based on survey results. Graves exposed during excavation showed both shroud and coffin burals. This variety of burial practices is reflected in a range of different geophysical expressions. In general, cemetery patterning was seen as north-south rows of anomalies. Even where individual graves were not clearly resolved, cultural patterning was distinct from undisturbed areas, which were generally very uniform.

Magnetic field gradient survey

Magnetic surveys are unsuited to many cemetery contexts because igneous rock (as monuments or occurring naturally) and ferrous metal are often ubiquitous. In this case, however, only two grave markers were known, and if present below the surface, these materials might be indicative of cemetery patterning. Possible targets included buried grave markers, steel caskets, reinforced concrete vaults, and former fences. The instrument can also detect more subtle anomalies caused by concrete or organically enriched, disturbed, or compacted soils, although these subtle anomalies are often obscured by the more intense fields associated with materials such as brick, metal, or igneous rock, and by post-depositional disturbance.

The Image below shows a high density of presumed graves with a variety of magnetic expressions:

(mouse over image to display interpretations)

Magnetic map showing unmarked burials

Comments:

  • Very strong anomalies (in the colored range of the magnetic scale) are caused by materials such as metal and igneous rock. Although these tend to obscure more subtle patterning, they are very likely significant in themselves, as they are largely confined within the cemetery boundaries. Possible sources include Granite grave markers, and metal ornaments and fixtures.
  • Cemetery patterning is offset several degrees from the survey grid, which is oriented to magnetic, rather than true north. Although patterning may be somewhat diffuse, the linearity and orientation of the patterning is unambiguous, and very unlike areas outside the limits of the cemetery.
  • The most obvious evidence for burials in dataset are magnetic highs with rectilinear patterning. The strength of these anomalies is moderate, generally 10nT or less, and they lack a visible negative component. This is consistent with a strongly magnetic source (such as ferrous metal, granite, or brick) at a relatively great depth. Their patterning, oriented with the cardinal directions, is consistent with Euro-American burial practices. Presumed sources are vaults or metal caskets. This type of anomaly may be interspersed with more subtle effects from disturbed soils, or obscured by stronger effects from near-surface phenomena (see below).
  • Subtler patterning is thought to be caused by disturbed soils. Disturbed soils may appear as either magnetic highs or lows, or may appear less homogeneous than undisturbed areas. In general, these effects are very weak, and may be indistinct, but their patterning may be visible in larger, magnetically quiet areas.
  • Strong bipolar anomalies (having positive and negative poles) are seen throughout the dataset. This type of anomaly is caused by materials with strong magnetic remanence (e.g., ferrous metal, granite, or brick) at or near the surface. Sources that would be of interest would include granite grave markers, the metal stems from plastic flowers, or corner irons marking cemetery plots. The patterning of metal debris or other anomaly sources not directly associated with graves may indicate areas of general activity, and may be indirect evidence of cemetery use. Road gravels can be seen as concentrations of very small and weak anomalies of this type.

(A portion of this of this dataset is interpreted in more detail in Case Study: Location of Human Burials: Magnetic Survey.)

Electrical resistance survey

Electrical resistance survey can be a very effective method for mapping historic cemeteries. Graves may be detectable because by the patterning of disturbed soils, or because of intrusive materials such as concrete vaults. In general, coffins or bodies were presumed to be too deep to be directly detectable, and the instrument was configured to best respond to near-surface phenomena.

mouse over image to display interpretations:

resistance map of historic burials

Comments:

  • The area showing cemetery patterning corresponds with a flat hilltop, which is the highest ground within the cemetery, and is thought to be the oldest part of the cemetery. A number of large trees were present in this area. Typically, trees tend to obscure subtle features, both because of the roots themselves and because of differential moisture uptake.
  • In spite of the trees, extensive patterning interpreted as graves (and verified by subsurface testing) can be seen throghout much of this area. Although individual graves may be indistinct, the patterning of rows of graves is evident.
  • A linear resistance anomaly is thought to be a former footpath, not visible on the surface, going from the driveway past the wooden cross.

Comparison of resistance and magnetic data

The use of multiple methods is generally complementary, and in this case, was indispensable for a confident interpretation of cemetery boundaries. With each of the instruments, response was better in some areas than in others. Fortunately, areas that were ambiguous in one dataset tended to have well-defined patterning in the other. The magnetic map below shows the area covered by the resistance map (above).

mouse over image to display interpretations:

Magnetic magnetic map showing burials, paths, and possible lightning-induced magnetization of a former fenceline

Comments:

  • The gravel footpath leading from the driveway and past the crossis visible as an alignment of weak bipolar anomalies.
  • A very strong linear magnetic anomaly may be the location of a former fence. Although bipolar anomalies of this strength are usually caused by masses of ferrous metal, none were found during subsurface testing in this vicinity. Another explanation may be remanent magnetization of soils caused by a lightning strike on a former fence. The north-south orientation, and its alignment with an extant gate supports the suppostion that this was the location of a fence, and thus a likely boundary of the cemetery.
  • The wooden cross appears to be newer than its stone and concrete pedestal. It is rather roughly made, and would seem to be an improvised replacement for an earlier original. We may suppose that an original cross may have been made of iron, by association with the ornamental gate of the cemetery. The location of the cross is associated with a surprisingly strong bipolar anomaly. It is possible that, with the posited fence, the pedestal may have been magnetised by a lightning impact that might have destroyed the original cross.
  • Although relatively weak and indistinct, some patterning indicating graves is present. In general, cemetery patterning in this area was much more apparent in the resistance data.

The investigation was commissioned by the Unified Government of Wyandotte County and Kansas City, Kansas. The Principal archaeological investigator was Chris Schoen of the Louis Berger Group, Inc., Marion Iowa, which contracted Archaeo-Physics, LLC to perform the geophysical survey.

Download high reslution images (*.PDF format)

Related content : Case Study: Location of Human Burials

Reference:

Jones, Geoffrey
2006 The Wyandotte County Cemetery: a case study in geophysical assessment of historic cemeteries. Proceedings of the 2006 Highway Geophysics - NDE conference, Saint Louis, Missouri, December 4-7 2006. (view online)

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