Environmental Enlightenment #72

By Ami Adini - Reissued June 25, 2010

This is a SHORT, LIGHT and SIMPLE newsletter. Its purpose is to rekindle in the initiated terminology they have once learned, and enlighten the uninitiated on terms they may have heard but never known the meaning of.

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.

You can find past issues of our  "Environmental Enlightenment" at amiadini.com Wealth of information about environmental site assessments in the real estate transactions and issues concerning assessment and cleanup of contamination in the subsurface soil and groundwater.

Call me if you have any questions. There are no obligations.

Ami Adini
Ami Adini & Associates, Inc.
Environmental Consultants
Underground Storage Tank Experts
323-913-4073; 323-667-2336 fax
mail@amiadini.com
www.amiadini.com

Ami Adini is a mechanical engineer, California Registered Environmental Assessor, Level II, and president of AMI ADINI & ASSOCIATES, INC. (AA&A), an environmental consulting firm specializing in all phases of environmental site assessments, rehabilitation of contaminated sites and upgrading of underground storage tank facilities. AA&A supplies practical solutions to environmental concerns using the highest standards of ethics and integrity while providing its clients with maximum return on their investments.