How to Find Buried Objects
(The text in this article has been extracted from a publication by the United States Environmental Protection Agency.)
Electromagnetic (EM) methods, also referred to as electromagnetic induction methods, are some of the most diverse and useful geophysical techniques.
Although they are commonly subject to cultural interferences, they can:
- Locate buried objects (metal and non-metal);
- Obtain geologic and hydrogeologic information; and
- On rare occasions, delineate residual and floating product.
The conductivity of a substance is its ability to transfer electricity by conduction.
Conduction means the transfer of energy by a medium without bulk movement of the medium itself.
In electrical conduction, energy is transferred by the movement of electrons or ions through substances (media); for example, soil, rocks, water, concrete…, anything.
EM methods here refer to the measurement of subsurface conductivities by low frequency electromagnetic induction.
A transmitter coil radiates an electromagnetic field, which induces electrical currents in the subsurface.
The electrical currents, in turn, induce a secondary electromagnetic field. The secondary field is then intercepted by a receiver coil.
The voltage measured in the receiver coil is related to the subsurface conductivity. These conductivity readings can then be related to subsurface conditions.
The conductivity of geologic materials is highly dependent upon the water content and the concentration of dissolved electrolytes.
Clays and silts typically exhibit higher conductivity values because they contain a relatively large number of ions.
Sands and gravels typically have fewer free ions in a saturated environment and, therefore, have lower conductivities.
Metal objects, such as steel underground storage tanks (USTs), display very high conductivity measurements, which provide an indication of their presence.
The EM receiver and transmitter coils can be configured in many different ways, depending on the objectives of the survey.
One common configuration for shallow environmental investigations utilizes transmitter and receiver coils that are attached to the ends of a rigid fiberglass rod at a fixed distance (i.e., fixed-coil separation). The equipment is then moved across the area of investigation.
This configuration is particularly suitable for detection of USTs and metal pipes.
The limitations of EM methods are primarily a result of the interferences, typically caused when this method is applied within 5 to 20 feet of power lines, buried metal objects (including rebar), radio transmitters, fences, vehicles, or buildings.
Its success depends upon subsurface conductivity contrasts: the difference in conductivity between an UST and surrounding natural or fill material is typically adequate for detection.