How to Find Buried
Objects
Metal Detection
([The text in this article has been extracted from a publication by the United States Environmental Protection Agency.)
Metal detectors, also referred to as
pipeline and cable detectors, are widely used at UST sites for the
specific application of locating buried metal objects, both ferrous and
non-ferrous in a process called metal detection (MD). MD can be used at
UST sites to locate steel and composite (i.e., fiberglass-coated
steel) tanks; metal piping; and utilities.
There are two types of MD--frequency domain and time domain.
Frequency-domain metal detectors are typically used for
locating shallow metals (less than 2 feet) and for tracing piping and
cables at UST sites. Time-domain metal detection is useful for
investigations from 0 to 15 feet and for locating USTs or buried drums.
Both types provide good response to all metal objects.
Metal detectors operate by the same principles as EM
(Electromagnetic) methods, but they are adapted to the specific purpose
of locating metal objects. When the subsurface current is measured at a
specific level, the presence of metal is indicated with a meter reading,
with a sound, or with both.
Commercial metal detectors used for locating USTs also have
data recording capabilities although stakes or paint marks are typically
placed over targets as the survey proceeds.
The diagram below presents a schematic drawing of MD
operating principles. The depth of investigation with MD surveys is
dependent primarily on the surface area and the depth of the object. The
response of MD decreases dramatically with depth. As a target depth is
doubled, the response decreases by a factor of as much as 64 (the
response to small objects decreases more rapidly than the response to
large objects). However, metal detectors are very appropriate for UST
sites because they are capable of detecting metal utilities up to 3 feet
below ground surface (bgs), a 55-gallon metal drum up to 10 feet bgs,
or a 10,000-gallon steel tank up to 20 feet bgs.
MD is less sensitive to surface and subsurface interferences
than EM methods, but care must be taken to minimize noise from metal
fences, vehicles, buildings, and buried pipes. Rebar in concrete is
perhaps the most common problem for this method at UST sites. The
electrical conductivity of the soil does not cause significant
interferences for MD methods; however, mineralized soils and
iron-bearing minerals can provide significant natural interference with
surveys. |
