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Methods: Utility Location / Borehole Investigation

    Utility location methods are commonly employed for the following types of site investigations:

    The utility location method is a surface method that uses electromagnetic induction to locate and delineate long, linear metallic or conductive pipes and cables. Spectrum takes pride in our utility location expertise: with 25 years of experience our reputation was, and continues to be, built on it. The utility location method is used in an effort to identify the surface trace of detectable underground utilities and abandoned piping for a variety of environmental and engineering investigations. These utilities include but are not limited to: electric, water, telephone, natural gas, storm drain, sanitary sewer, fuel piping and compressed air.

    Utility locators may be operated in either active or passive mode. Passive locating is possible when electrically conductive conduits are energized by ambient radio frequencies (RF) that are often produced by 50/60 cycle electrical, radio, audio, television, and communication transmissions. A receiver tuned to these frequencies can be used to locate the re-radiated signal emitted by the conductor (i.e., conduit). Active locating is initiated by connecting to an exposed conduit and propagating an EM signal at a known frequency (8 and 33 kHz for example) along the conduit. A receiver, tuned to these frequencies, is then used to locate the signal maxima (or surface trace) of the applied signal. Spectrum uses a Radio Detection 7000 transmitter w/ matched receiver, Fisher TW-6 M-scope shallow focus metal detector (M-Scope), and Dynatel 500A transmitter w/ matched receiver, among other instruments, for utility location. Once located, utilities are marked on the ground with surveyor’s chalk using the American Public Works Association color code found below.

    Table 1: APWA Color Code




    Storm Drain/Sanitary Sewer


    Natural Gas




    Unknown Conduit



    Utility locators are specifically designed to accurately locate and delineate metallic underground pipes and utilities. These locators are designed to detect the magnetic field resulting from an electric current flow on a line. During the use of a locator, the transmitter emits a radio-frequency source signal that, by Faraday’s Law of Induction, induces a secondary electromagnetic field in nearby utilities. A receiver unit measures the signal strength of this secondary magnetic field and emits an audible response to allow the precise location and tracing of the pipe, cable, or other conductor in which the signal is induced. If the utility is accessible, the source signal can be directly connected to it, which makes the secondary field much larger and more readily measurable. Where no direct connection is possible, utility locators can be used to inductively trace the pipe or cable.

    The M-Scope has a transmitter and a receiver at the ends of a short boom; this instrument also operates on the principles electromagnetic induction. The M-Scope provides an audible response when shallow subsurface metallic objects are encountered. In addition to locating utilities, the M-Scope is useful for identifying buried vault lids, cleanouts, UST fill ports and storm grates.