Geophysical techniques are often useful for discovering unknown subsurface conditions. Most of these techniques are classified as non-invasive, requiring only minimal disturbance of surface cover. Recent projects involving successful application of engineering or environmental geophysics in Ohio and southern Michigan include:

• Seismic refraction mapping of depth to bedrock under a landfill. Results reduced the number of drilled holes needed to map an irregular concealed erosional contact.
• Seismic refraction locates refuse cells and cell walls in old, poorly documented solid waste landfill.
• Magnetometer survey locates concealed abandoned gas wells and cased water wells. Results confirmed by excavation. Lack of magnetic anomalies in remainder of area investigated provides physical evidence that all cased wells in the study area have been found and plugged.
• Magnetometer measurements at an abandoned landfill outline zones most likely to conceal buried steel drums.
• Electromagnetic conductivity and DC resistivity profiles map leachate escaping from a landfill. Results confirmed by drilling.
• Gravity, seismic refraction & reflection, and electrical resistivity delineate bedrock valleys concealed by glacial till. The integrated interpretation of geophysical and hydrogeological information suggests that water wells drilled into the axis of such valleys are more likely to encounter substantial ground water than wells drilled at random or wells drilled based on an incomplete knowledge of the valley location.

Techniques for geophysical detection and mapping of permeable bedrock fractures are being developed and tested. In addition to the surface techniques briefly outlined here, some subsurface conditions can be resolved by geophysical measurements involving boreholes.

Geophysical techniques do not eliminate the need to drill and sample. However, geophysical prospecting can reduce the overall costs and improve the overall quality of a site investigation by:

(1) targeting anomalous areas where sampling is most likely to encounter extreme values;

(2) providing physical evidence needed to justify interpolation of subsurface conditions between sample locations;

(3) providing physical evidence that areas not directly sampled are unlikely to conceal unexpected conditions;

(4) detecting locations where buried drums and their contents may pose a hazard to drilling crews or unprepared excavators.

Proper application of geophysical measurements can significantly reduce the number of borings needed to characterize a site while improving the confidence of interpretations based on direct borehole samples. Geophysical techniques become increasingly cost-effective as the area and depth to be investigated increase.

Certain routine geophysical methods sometimes can not be used due to cultural noise (electrical power lines or transformers, heavy vehicular traffic, buried pipes, pavement) or natural conditions. The experienced geophysicist knows how to recognize and minimize any influence due to such noise. Site visits are often required prior to finalizing plans for a geophysical prospecting project.

For additional information on environmental and engineering applications of geophysical methods, check out the EEGS.

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