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Applied Geology and Geochemistry

Geophysical Methods

Applied geophysics has steadily increased in environmental investigations due to one significant development - increased computing power. Field-grade geophysical instruments are equipped with digital signal processing and recording capabilities once restricted to large corporate computing centers. This improved computational strength has provided investigators with near real-time results that, in turn, drive improvements in instrument sensors and processing algorithms. The most useful methods amenable to these enhancements are:

  • Ground-penetrating radar
  • Seismic reflection and refraction
  • Electromagnetic induction
  • Electrical resistivity
  • Gravity
  • Magnetics

Most ground-penetrating radar surveys involve locating buried utility lines or abandoned underground storage tanks. Advances in the last decade have lead to ground-penetrating radar surveys at archeological sites, moisture content surveys, and subsurface facies characterization. Current research is directed at using ground-penetrating radar for plume migration detection, deep sedimentological assessment, and three-dimensional surveys.

Seismic reflection and refraction surveys historically formed the foundation of geophysical studies in the petroleum industry. Seismic surveys have also been applied to study the Earth's crustal development throughout the world. Near-surface seismic techniques have advanced in tandem with deep borehole surveys performed in the oil industry. Although the surveys do not map the areal extent of those in petroleum fields, they do provide the same level of detail offered in the larger surveys. Recent advances in geophone construction, high-speed microcomputers, and processing algorithms have moved seismic techniques into the fields of subsurface moisture mapping, aquifer boundary studies, and three-dimensional vadose characterization.

An electromagnetic induction system works through the application of one electromagnetic field to the subsurface and monitoring the induced electromagnetic field. A map of subsurface conductivity can be produced from the data obtained in an electromagnetic induction survey. The map shows areas of disturbed soil, buried metallic objects, and changes in soil conductivity that may be related to disposal of highly conductive substances.

Resistivity surveys also have a large pedigree in the mineral resources industry. As with seismic techniques, advances in computing power have provided investigators with a new tool for monitoring moisture travel in the vadose in real-time. The latest deployment of high-resolution resistivity, a modified resistivity network with advanced analytical techniques, is revolutionizing soil-moisture monitoring in highly contaminated zones by moving beyond dry well logging. The data obtained from high-resolution resistivity surveys indicate both the time of release and the direction of a spreading plume. High-resolution resistivity can also be applied to existing contaminant plume by mapping the charge distribution of a current-fed plume. The resulting map shows areal distribution of the charged body in plan view. Similar cutting edge applications of DC resistivity include mise'a la masse (excitation of mass) and Residual Potential Mapping (RPM), both of which have been successfully applied to contamination studies by HGI.

Gravity surveys are used to determine whether ore bodies are present at depth and the distribution of gravity anomalies, normally associated with geologic structures. The survey is normally performed in conjunction with other geophysical techniques to map geologic features over large areas. Density differences between high-density materials and the overburden can be distinguished without resorting to a two-method survey. Adding seismic or electrical resistivity can enhance the information derived from either method alone and is often used to characterize large alluvial basins, the location of many municipal water supplies.

Another technique used primarily for utility location relies on the differences in magnetic properties of features in the subsurface. Magnetometers have also benefited from embedded computing devices, allowing for large-scale logging relying one field device. The information is usually combined with data from other survey methods including electromagnetic induction or ground-penetrating radar surveys.

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