Geophysical
"Geophysical methods of subsurface exploration are an indirect means of gathering data pertaining to underground conditions. Using geophysical techniques involves taking measurements at the earth's surface or in boreholes to determine subsurface conditions. Geophysical methods may be employed during any stage of an investigation and include:
"Geophysical methods can be a useful and economic addition when used in conjunction with a test-boring investigation program. In contrast to borings, geophysical surveys are used to explore large areas rapidly and economically. They indicate average conditions along an alignment or in an area, rather than along the restricted vertical line at a single location as in a boring. Geophysical data can be used to help determine the best location of future drill holes as well as provide information to extrapolate foundation conditions between existing drill holes. These data are useful for detecting irregularities in bedrock surfaces and at interfaces between strata. The cost of performing geophysical surveys is often less than the cost of drilling; therefore, judicious use of both geophysical methods and drilling can produce the desired information at an overall lesser cost. Although geophysical field work is relatively inexpensive, interpretation of results is difficult and specialized. For test results to be usable and reliable, correlations must necessarily be made locally with exploration data from borings."[1]
"Geophysical methods are best suited to prospecting sites for dams, reservoirs, tunnels, highways, canals, and other structures. Also, they have been used to locate gravel deposits and sources of other construction materials whose properties differ significantly from adjacent soils. Downhole, uphole, and cross-hole seismic surveys are used extensively to determine dynamic properties of soil and rock at small strains."[1]
"All geophysical techniques are based on detection of differences between properties of geologic materials. If such differences do not exist, geophysical methods will not be useful. These differences range from acoustic velocities to contrasts in electric properties of materials. Seismic methods, both reflection and refraction, depend on the difference in compressional or shear-wave velocities through different materials. Electrical methods depend on contrasts in electrical resistivities. Differences in density of different materials allow magnetic surveying to be used. Differences in magnitude of naturally existing electric current within the earth can be detected by self-potential surveys."[1]
Best Practices Resources
National Engineering Handbook: Chapter 2 - Engineering Geologic Investigations, NRCS
Geophysical Exploration for Engineering and Environmental Investigations (EM 1110-1-1802), USACE
Citations:
Revision ID: 7061
Revision Date: 07/10/2023