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Warnings and Special Considerations
The following warnings and special considerations should be noted when their corresponding situations are encountered. These warnings and special considerations are generated in the results section of the Geophysical Decision Support System (GDSS) and are therefore most applicable to the GDSS.
Survey When Dry
A site needs to be surveyed when dry in the following situations:
- The surface contains intermittent wetlands.
- Monitoring seasonal variations subsurface water content / water flow.
Dry Surface Requires Significant Field Procedures
Significant field procedures are required to couple with a dry surface in the following situations:
- The site contains a bay or estuary.
- Minimal soil moisture content requires supplemental wetting of electrodes in electrical method surveys.
Some Methods Difficult or Impossible
Some methods become difficult* or impossible in the following scenarios: .
- The site is not generally accessible, including remote locations, site access / permission issues, dense vegetation.
- Dense vegetation can require additional costs/permission to clear survey areas.
- Asphalt and concrete surface material can require additional work for electrical methods.
- Topography can prevent logistical challenges; however, most common geophysical techniques can now incoporate topography into the processing results.
- Dense groupings of subsurface infrastructure such as urban & suburban utilities can contribute to significant interference.
- Presence of highly conductive surfacematerials can attenuate signal from electrical and electromagnetic methods resulting in invalid data.
- Pumps are present in supply or monitoring wells.
- Presence of significant above and below ground infrastructure can interfere with valid data acquisition and interpretation.
- The ground surface contains concrete with rebar or wire mesh.
- The ground surface contains thick vegetation(jungle, old growth forest, corn field).
- The ground surface contains a swamp or bog.
- Significant electrical or seismic noise is present.
- Local, grounded electrical systems such as electric fences and cathodic protection systems can prevent acquisition of valid subsurface electrical measurements.
- Proximity to roadways and railroads can present significant seismic noise sources preventing aquisition of valid seismic measurements.
*Some examples above represent difficult measurement situations; however, in many cases advancements in technology or exerienced professionals can effectively deal with the perceived difficulty, ultimately providing valid results.
The pages found under Surface Methods and Borehole Methods are substantially based on a report produced by the United States Department of Transportation:
Wightman, W. E., Jalinoos, F., Sirles, P., and Hanna, K. (2003). "Application of Geophysical Methods to Highway Related Problems." Federal Highway Administration, Central Federal Lands Highway Division, Lakewood, CO, Publication No. FHWA-IF-04-021, September 2003. http://www.cflhd.gov/resources/agm/
