An in-depth lab analysis of soil samples was necessary to gather additional information to explain statistical correlations and qualitative analyses.
To do this, soil samples were collected at each site using an auger at 20, 40, and 60 cm depths. The samples were tested at the United States Geological Survey (USGS) in Denver, Colorado for their physical and chemical properties. Tests determined the samples' relative dielectric permittivity (RDP) and how those values changed with differing water content. This was done by passing a range of electromagnetic frequencies through the material and then measuring its response. Tests were performed after the samples were artificially dried, and then after they were given a specific amount of water which was allowed to be absorbed for varying amounts of time.
An RDP value is a way to measure radar velocity in that soil. Therefore, an analysis of the changes in RDP values of different materials can be used to determine whether buried features will produce higher or lower amplitude reflections at their interfaces (See also, GPR Method and Theory).
Results:
For all samples at both the CATS and Hammer sites, the RDP value was between 3-5 when dry, regardless of whether it is composed of clay, silt, or sand. When wet, the RDP values changed significantly. It appeared that the addition of water (and the amount) was the determining factor in the RDP value. The amount of time the water had to filter was far less important than the amount of water put in the sample. Therefore, it is the way materials in the ground hold and distribute water that matters most in producing radar reflections.
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This conclusion explains the results at the CATS and Hammer sites perfectly, as very different reflection intensities and the distribution of reflective surfaces changed drastically during different moisture regimes.
For further information, download Conyers 2004
Clay Conductivity and Mineralogy
CATS is a very clay-rich site. Therefore the conductivity of the site's clays was measured by studying its cation exchange capacity.
The higher the conductivity of a clay, the more rapidly the electrical component of an electromagnetic wave will be lost through attenuation. The more the radar energy attenuates, the shallower the depth of penetration will be. This in turn limits the depth of the investigation. No matter what the frequency of the antenna being used, highly conductive ground will limit depths of penetration.
X-ray diffraction of the samples showed that the clay at CATS was not a mineralogic clay, but rather a clastic clay. The soil was composed of quartz that was of clay size, but did not contain significant clay minerals derived from pedogenic processes.
The same tests were performed at the Hammer site, which is composed of dry sandy soil.
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This
is an x-ray diffraction test of three soils samples from the CATS site.
There is little mineralogic clay, but a high percentage of quartz in the
soil.
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This
is an x-ray diffraction analysis of the samples from Hammer site. Even
though soils in this area is described sandy, they have some illite and
kaolonite clays.
Conclusions of Lab Analyses:
1. Soil conditions are often complex, and it can be very difficult to predict the ground conditions at a site.
2. Dry ground conditions allow some features with certain physical and chemical properties to be reflected, while wet ground conditions allow other features to be reflected.
3. Moisture can greatly enhance the imaging of some features, while detracting from others.
