How Magnetic Locators Work

Magnetic Locators find underground objects with ferrous metal content, using the earth’s magnetic field. They do not emit a signal, but rather, they measure magnetic field distortion that surrounds a buried metal target.

Magnetic locating diagram with iron marker and illustration of the two magnetic fields from the locator that are bent by the iron marker.

Typical Locate: Magnetic Field Detection
Detecting the magnetic field of an iron marker

  • Magnetic field stronger at sensor A than B
  • Creates a signal strength that is larger than zero
  • Audio tone frequency increases as signal strength increases

Audio signal peaks directly over vertical target while the signal peaks over each end of a horizontal target.

Object Shape and Orientation Affects Findings
Signals from vertical and horizontal Targets

  • With locator directly over a vertical target, signal strength peaks
  • Audio signal, digital readout, and bar graph indications also peak over each end of a horizontal target

Magnetic Location strong audio signals

Strongly Magnetized Markers
Near-surface markers provide a weaker indication to either side

  • From point A to B: signals increase slightly and then decrease
  • Beyond B: signals increase rapidly, peak directly over marker, decrease at point C
  • From C to D: signals increase and decrease again

Magnetic Locating cast iron pipe weld joints

Cast Iron Pipe Weld Joints
Cast-iron pipes produce the strongest magnetic signals at joints

  • Sensitivity setting at maximum
  • Vertical hold 1 to 1.5 ft. above surface, walk without tilting or turning
  • Mark locations of maximum signal strength

Magnetic Locating a steel drum

Locating a Steel Drum
Orientation is key for accuracy

  • Signal pattern varies depending on vertical or horizontal orientation
  • Depth of target also affects signal pattern
  • 55 gallon drum can be located at depths from 5 to 12 feet