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In keeping with the requirements of the Texas Antiquities Code and the National Historic Preservation Act, most transportation construction projects require that an archaeological investigation be performed as part of the overall environmental assessment process. The investigation may simply involve a desk-top study of previously identified sites and surveys through various state data sources, involve a pedestrian survey that solely involves visual inspection of surface features or involve a more extensive subsurface investigation that includes systematic shovel tests and/or trenching. It is the latter where the application of remote sensing can be used to better define subsurface anomalies and reduce the level of effort required.
Geophysical remote sensing techniques are not new to archaeology. They have been used for more than thirty years in other countries. In the United States, geophysical techniques for archaeological prospecting are only now becoming more widespread.
There are several advantages in using geophysical techniques. First, the methods are non-intrusive and non-destructive. Copious amounts of data can be collected without the need for digging or trenching. Once the geophysical data is downloaded and processed, archaeologists can obtain a snapshot of what's beneath the soil and use the information to guide and focus ground-truthing strategies. This can reduce the effort required for subsurface activities and reduce investigation costs. Geophysical surveying is quick, reliable and less costly than traditional archaeological surveys. In addition, with excavation, the cultural features may be completely missed or the archaeological integrity of the site destroyed.
Different geophysical techniques measure different properties of the ground, which is why it is best to use a suite of such techniques.
Electrical Resistivity and Conductivity Electrical resistivity and conductivity are mathematical reciprocals of each other. The earth can be thought of as a huge conductor. If you induce part of the earth with electricity, some materials will conduct an electric current better than others. In order for the instrument to record or 'see' a rock wall, for instance, the material of the wall needs to be either higher or lower in resistance than the surrounding substrata material.
In conductivity surveys, the instrument records how well materials in the substrata conduct an electrical magnetic signal. A Geonics EM38 instrument transmits an alternating current in the ground, and this AC current creates a secondary magnetic field. The ratio between the fields is determined by what is beneath the instrument in the ground. Conductivity/resistivity instruments are often successful in detecting stone and brick buildings, prehistoric house depressions, ditches, midden deposits and gravesites.
Magnetometer/Gradiometer A magnetometer measures variations in the earth's magnetic field. The sensors on the fluxgate gradiometer are precisely arranged to pick up subtle changes in the earth's magnetic field. Changes in the localized magnetic field are then measured in relation to the background, and this is how it is determined if any cultural or manmade materials coexist in the upper layer or substrata of the earth.
Ground Penetrating Radar GPR is primarily dependent on the dielectric properties, shape and moisture content of the material. As the GPR antenna is pulled along the ground, a pulse is transmitted into the ground. The pulse of energy spreads and bounces back to the instrument's receiver that is housed in the antenna. GPR is good for detecting stone walls, burial and storage pits, prepared structure floors and historic graves, especially where there is little stratification visible in the radar profiles.
The use of geophysical remote sensing has been successfully used for transportation projects where landowners refuse digging on their property, when access for backhoe equipment is prohibited by land and vegetation features, or in the case of working in or near cemeteries (where the potential violation of unmarked gravesites may occur). While remote sensing will never completely replace traditional archaeological techniques and ground-truthing, geophysical surveying certainly can complement and improve the overall success of any survey requiring subsurface investigations.
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