Looking for Lost Graves
William J. Johnson, Rhea Engineers and Consultants, Gibsonia, PA, USA
Donald W. Johnson, Independent Geophysical Consultant, Ramsey, MN, USA
Marcella G. Johnson, Rhea Engineers and Consultants, Gibsonia, PA, USA
Tanya E. Raymond, University of Texas, College of Natural Sciences, Austin, TX, USA
Abstract
The identification of graves is an important issue at many historical cemeteries. Time and vandalism have often obliterated the trace of burials and the operator of the cemetery may not know where empty plots may still be available. Land development encroaching on an historical cemetery may also have to be careful about disturbing unmarked graves. In the extreme case where a cemetery needs to be relocated because of development, it is important to make sure that unmarked graves are not overlooked. As excavation is seldom a desirable solution to locating unmarked graves, methods of detecting burials from the ground surface can have an obvious benefit.
This paper is an update to a presentation made in 2003 at the Council for West Virginia Archaeology Spring Workshop where the three most commonly applied geophysical techniques (ground penetrating radar – GPR; magnetics; and resistivity/conductivity) were reviewed through case histories documenting the detection of graves. Over the past 10 or so years, the physics of detecting graves has not changed, but our ability to visualize the geophysical data has improved, in particular with respect to the GPR technique. Some new case histories demonstrate these changes.
Modern graves associated with concrete vaults are generally easy to identify on the basis of GPR reflections, but old graves where there is no vault or a casket has otherwise decomposed or was never present are much more challenging to map. In such cases, the main physical basis for grave detection is that grave shafts represent a disruption to the natural layering of the ground. Disruptions to soil layers can often be detected with GPR, although GPR usually best detects the base of a grave shaft. Graves are filled with a mix of the soil types excavated, so there is usually a physical contrast of the grave fill with natural soil. Graves are often manifested by magnetic lows because they disrupt the natural fabric of soil magnetization and are therefore delineated by magnetometry. Grave fill is not as dense as natural soil and can result in either resistivity highs or lows depending on site-specific conditions. The basic conclusion is that the detection of old graves is difficult, but usually achievable, especially when multiple techniques are applied. If a single technique is applied, and if field conditions permit, GPR is usually the most effective. Although individual graves can sometimes be identified with magnetics and resistivity, these techniques usually work best to give a geophysical framework to a cemetery, rather than identify individual burials.