Key Words: Ground Penetrating Radar, Clandestine Burials, Geophysical Applications in Anthropology, Historic Cemeteries introduction and purpose

Soil and GPR

Soils and the different properties of soils are important to understand in order to benefit from a GPR survey. Soils encompass the mineral, organic, gaseous, and liquid material that encompasses most surface land on earth. The type and distribution of soils is dependent on a number of geological and environmental factors which changeover time. Soils are characterized by their chemical composition as well as their horizons, or layers. Horizons are formed by the accumulation, transformation, or loss of material at a given location. Soil horizons, or layers of soil, do not have homogenous electrical or chemical properties with layers above or below. These differing properties register as differing amplitudes by the antenna’s receiver during a
GPR survey. It is this function of GPR that allows for the location of anomalies given a site with a known soil profile, the GPR registers that which is outside the norm for the profile (Soil Survey Staff, 1999; Doolittle and Butnor, 2009; Soil Survey Staff, 2010). One of the greatest hindrances to the feasibility of a GPR survey is the type and condition of soil at the site (Doolittle and Collins, 1995; Conyers, 2004; Schultz et al., 2006; Schultz et al,
2008; Conyers, 2012). The electromagnetic pulses from GPR antennas are susceptible to signal attenuation in certain soil and weather conditions. As anomalies are the representation of electrical or chemical differences detected in the soil matrix, signal attenuation due to soil characteristics renders GPR surveying difficult or even unviable at certain sites and at certain times of the year. As such, a basic knowledge of soil characteristics and limiting factors should betaken into account prior to a GPR field survey. In the United States this can usually be ascertained through an examination of soil taxonomy information provided by the USDA (Soil Survey Staff, 2010) or state and local soil surveys which are largely generated and refined utilizing GPR (Doolittle and Collins, 1995).

In soils with high conductivities the dissipation of radar energy will occur quickly and provide loss to both resolution and depth. Doolittle and Collins (1995) note four principle factors that influence the conductivity of soils 1) porosity and water saturation 2) amount and type of salts in solution 3) amount and type of clay and 4) scattering (Table 4).
Table 4. Factors that influence the conductivity of soils (Doolittle and Collins, 1995). Factor Effect Porosity and degree of water saturation Generally, soils have lower signal attenuation when dry than when wet
2. Amount and types of salt in solution Higher levels dependent on presence of clay minerals, pH of soil solution, and degree of water saturation
3. Amount and type of clay In clayey soils ions absorbed on clay particles undergo exchange reactions with ions in the soil solution and contribute to electrical conductivity of the soil.
4. Scattering A subsurface plane sloping away from the antenna or having a convex upward surface will dissipate radar energy return. While these factors may inhibit GPR surveys they do not necessarily disallow them all together. The use of GPR in an archaeological context is often used to map larger areas a portion of a site versus an individual test unit, for example. In the published literature these GPR surveys are often focused on locating and mapping the foundations of structures (Bevan, 2007;
Conyers, 2010; Conyers and Leckebusch, 2010; Pettinelli et al., 2012) or large, consistently used burial sites (Chilton, 2007; Wardlaw, 2009; Doolittle and Bellantoni, 2010). Both of these situations provide ample opportunity for testing and retesting utilizing different antenna frequencies or allowing fora change in weather. Soil conditions and weather may provide a more daunting situation for forensic GPR surveys, however, as these surveys are often under time and location constraints (Schultz, 2007; Schultz and Dupras, 2008; Schultz, 2012).

Soils are classified in the United States according to the United States Department of Agriculture (USDA. The
USDA’s Soil Taxonomy (1999) provides information on the twelve recognized soil orders found in the United States Alfisols, Andisols, Aridisols, Entisols,
Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols. This soil information provides some of the most basic data necessary to determine the efficacy of a GPR survey at a given location. In 1954 (Fine) an early map outlining the relative soil conductivity of the United States was produced. This map provided general information based on state soil surveys, but had issues with its scale (the scale was far too small to effectively differentiate GPR applicability at the local level) and its sample size was far too few to be truly effective (Doolittle,
2012). More recently, a more precise GPR soil suitability map was produced by the USDA with the aid of state soil surveys and afar more defined delineation of soil orders at the local level Doolittle and Butnor, 2009; Doolittle, 2012). Soil in Florida (Figure 3) falls into seven of the twelve soil orders as outlined by the
USDA’s Soil Taxonomy (1999). The soil order is the highest hierarchical level for soil definition. Collins (1985) states that soil orders, are distinguished in relation to the five soil- forming factors (1) climate and (2) living organisms acting on (3) parent materials overtime as conditioned by (5) relief Soil orders are broken down into more defined sub-units dependent on formation processes, presence or absence of minerals, and even utilization by humans. For the purposes of this paper, however, it suffices to contain the discussion to the seven soil orders found in Florida, their overarching characteristics, and what effects these characteristics have on the potential use of GPR in afield survey.

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