ELECTROMAGNETIC SURVEYS

What are Electromagnetic Surveys?

Two of the most effective and most frequently used geophysical methods for site assessment are Electromagnetic Surveys (EMS) and Ground Penetrating Radar (GPR). They are screening technologies that produce a rapid characterization of a site and can readily identify geophysical conditions, buried wastes and utilities, and potential migration routes.

Electromagnatic Surveys - as with GPR - should be carried out prior to more expensive traditional site assessment techniques such as drilling, since they can dictate the best location for borehole and monitoring well installation, as well as help in the remedial investigation to determine site remediation options.

The Equipment

There are a number of different Electromagnetic (EM) meters that can be used to quickly cover large areas with no surface contact necessary.

By recording responses to EM fields created by the instruments, we can identify changes in the subsurface such as voids, storage tanks, large pipes, ground contamination, mine workings, etc.

An EM31 will measure the conductivity of the ground to a depth of 5-6m, and will highlight a range of geological, archaeological and manmade features. All data are digitally logged and a GPS antenna can be added to accurately locate the survey area.

EM38's and RM15's apply similar principles with higher resolution and less depth penetration. Therefore they are widely used in archaeological surveys when investigating the top 1.5m of the sub-surface. Larger areas can be surveyed quickly; consequently necessary intrusive investigations can be planned, reducing on time and costs.

EM61 surveys detect the presence of all ferrous and non-ferrous metals to a depth of 4-6m. Again a large area can be surveyed quickly, in some cases the equipment is towed behind vehicles and tied in to GPS to increase speed. This technique is often used as part of a UXO investigation, quickly providing 'clear' areas for excavation/piling, etc.

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