Indirect or Direct

DOE-HDBK-1092-2004

point without the grounding system being disturbed. Inductive couplers and clamps all apply a

signal to the utility in basically the same manner. The signal is induced onto the utility to be

located by an electromagnetic field created by the coupler and clamp. It is a requirement that

the utility have grounds in place at both ends of the section to be located. Missing bonds across

insulated sections of the utility will prevent this method from being used along the entire length

of the utility. Underground metal pipes and cables may not be joined with conductive materials,

thus making them short separated pieces of a broadcast antenna. There may be multiple

utilities bonded together (ie., cathodic protection and common grounding), making their

individual resolution difficult. These and other problems create the need for variety within the

electromagnetic method itself. Couplers and clamps should be positioned below the electrical

grounding point on CATV, electrical, and telephone cables. Systems that do not use earth

ground, such as railway signal cables, cannot be located with couplers or clamps. Gas meters

with insulated couplings should not be bonded. They should be direct-connected.

INDUCTIVE OR INDIRECT

A third method is the inductive or indirect mode. Here the transmitter is placed on the surface of

the ground above the known utility. The signal from the transmitter is induced onto the utility,

making the location possible. Inductive transmitter use should only be used when access points

for the utilities are not available. This method can lead to serious tracing errors, especially if

other utilities are buried/embedded in the same area.

DETECTOR FREQUENCY AND POWER CONSIDERATIONS

As an operator attempts to detect the location of underground/embedded utilities, the frequency

and power capabilities of the instrument being used must be understood. In many detectors, as

the frequency increases, the available power decreases.

Frequency selection can affect depth of penetration, distance of travel, resonance efficiency,

and resolution issues, as well as other operating characteristics. Most locator manufacturers

preset frequencies, and there will be an optimum frequency to use for a particular utility system.

These are some of the most frequently used :

a. ELF (Extremely Low Frequency) operates at below 300 Hz. It is typically used for

power cables energized and carrying a load, water pipes grounded to a power system

energized and carrying load, and deep, very conductive, long-length utilities.

b. VLF (Very Low Frequency) operates between 3kHz and 30 kHz. It is typically used for

very long, continuous conductors, and deep conductive long length utilities.

c. LF (Low Frequency) operates between 30kHz and 300kHz. It is typically used for

shallow (8 feet or less) conductors of medium length.

d. HF (High Frequency) operates between 3MHz and 30MHz. It is typically used for

cables; shallow short conductors.

Not all utilities are metallic or of sufficient length in the ground. Therefore, resonant

electromagnetic methods will not always be the answer. The current family of surface

geophysical methods available for utility and other near-surface structure or hazard

detection is extensive. Other electromagnetic techniques such as measuring eddy

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