How to Find Buried Objects
Magnetic Methods
[The text in this article has been extracted from a publication by the United States Environmental Protection Agency.]
This is the last of a series
of articles on using the science of geophysics to detect buried objects.
Magnetometers are useful at underground storage tank (UST)
sites for locating tanks and piping made of ferrous materials. Although
highly sensitive magnetometers have been developed that can detect the
void space within large buried objects of any material (e.g.,
fiberglass tanks), this technology is not often used in UST
investigations
because many cultural interferences present at UST sites will mask the
effect.
Magnetometers that are commonly used at UST sites work by
measuring the earth's total magnetic field at a particular location.
Buried ferrous materials distort the magnetic field, creating a magnetic
anomaly. There are two methods
for measuring these anomalies--the total field method and the gradient method.
The total field method utilizes one magnetic sensing device to record the value of
the magnetic field at a specific location. The gradient method uses two sensors,
one above the other. The difference in readings between the two sensors provides
gradient information which helps to minimize lateral interferences. Total field
magnetic methods are often used at sites with few cultural features. Gradiometer
methods can be used in culturally complex areas. As a result, gradiometers are
more applicable for UST sites.
Magnetometers may be useful for reconnaissance surveys of
UST
sites because they are very fast and relatively inexpensive.
Potential cultural interferences include steel fences,
vehicles, buildings, iron debris, natural soil minerals, and underground
utilities. Gradiometer methods are useful for
minimizing these interferences.
Power lines are an additional source of
interference that can be neutralized with the use of very sophisticated
equipment that synchronizes readings with the oscillating electrical
current.
Some magnetometers are very simple and do
not have a data recording or processing ability. They indicate the
presence of iron with a sound or meter and can be used as a rapid
screening tool.
Magnetometers that record
data can, with the aid of data processing software, be used to estimate
the size and depth of ferrous targets.
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