ncountering unmarked or improperly marked graves can create difficult and embarrassing problems for those managing cemeteries. While headstones and markers are normally positioned over burial sites, as time passes, these makers can be lost or mispositioned. For cemeteries that are hundreds of years old, unmarked graves are a common occurrence.

Problem

A grave is a relatively well-defined target, the size is typically 0.5 x 2 m (2 x 6 ft) and depth is normally less than 2 m (6 ft). However, locating unmarked graves can be difficult. There is often no surface expression of the burial location. The challenge is to explore the subsurface without disturbing the soil.

In this case study, a cemetery in Tuscaloosa, Alabama was unsure of the locations of graves in an older section of the property.

GPR Contribution to Solution

In soils that are favorable for GPR, sandy or silty soils, as was the situation in this case study, buried, intact coffins are often strong reflectors of GPR signals and appear as strong hyperbolas or localized, flat reflectors, depending on the shape of the coffin. Examples of GPR reflections from coffins are shown in Figure 1 and 2.

Figure 1: Typical GPR cross-section showing the varying depth and position of coffins.

Figure 2: 25-foot GPR cross-section showing different GPR reflections from two coffins; probably caused by the coffins differing in composition. The strong reflection from the coffin on the left may mean it has a high metal content, while the weaker reflection from the coffin on the right may mean that it is a wooden coffin.

The most practical way to sort out the Tuscaloosa Cemetery site was to carry out a detailed grid survey. The grid was 42 x 14 ft (12 x 4 m) with a line spacing of 2 ft (60 cm) and took about 15 minutes to collect.

Once the grid data was collected, it was processed in the field and displayed as a series of depth slices combined with a cross-section: as shown in Figure 3.

The collected GPR grid data can also be displayed in 3D (Figure 4)

As shown in Figures 3 and 4, graves are usually detected by a long and thin GPR response of the defined size in the top 2 meters (6.5 ft).

In soils with a high clay content, GPR signal penetration is limited and may not be deep enough to detect coffins directly. In those situations, it is necessary to look for the grave shaft that was dug to bury the coffin. The act of excavating the soil for a burial radically disturbs the natural soil structure and this disturbance is often detectable with GPR (Figure 5).

Figure 5: GPR cross-section showing a disturbed soil layer, an indicator of a potential hole from an unmarked grave.

Results & Benefits

This cemetery mapping with GPR case study demonstrates the value of GPR for confirming grave locations. Some key benefits are:

• GPR operation is simple and intuitive
• Users can be effective with only a few hours of training
• Line Scan mode displays real-time cross-sectional images of the subsurface that quickly provide areas of interest before conducting more-detailed grid surveys
• Grid Scan mode provides detailed area coverage, generating depth slice maps or 3D images that aid data interpretation
• Systematic search protocols are available as best practice guides

Click here to learn more about Noggin Ground Penetrating Radar used in this case study.

GPR responses vary greatly depending on the target being sought and the host material. GPR response variability can be challenging to new GPR users. When learning about GPR, the best practice is to review several similar GPR case studies to develop an understanding of variability. Check for other insightful information on the resources tab to learn more.

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