It is common for all of us to see technicians along the side of roads with hand-held utility locating equipment. Typically they will mark their findings with spray-paint and/or pin-flags. What, you may ask, are they doing exactly? And the answer is they are locating and marking subsurface utilities (think electricity, telephone, natural gas) so that workers and machines don’t have unpleasant encounters during digging. Damaging a utility with heavy equipment can ruin your day, not to mention messing up the utility service and probably costing your employer a lot of money.
The equipment used is typically an electromagnetic (EM) device with either audible or visual, or both, feedback that lets the operator know where a subsurface pipe/utility is located. The instrument can give a fairly accurate measurement of depth as well. Most of this equipment operates on the principal of either electromagnetic induction, or in some cases conduction. When an electronic wave is transmitted into the ground, the signal travels through the material, and is modified or distorted by interactions with metal objects. This modification is what the instrument reads, and this is how subsurface metal utilities are identified. Typically, the instrument incorporates a transmitter that will output an electronic signal at a specific frequency, and then a receiver “wand” is set to receive the same frequency to track a particular utility. The transmitter can be directly connected to exposed metal/wire at a junction box or outlet if it is accessible, or a ring clamp can be attached around a pipe casing to send out the signal. I have traced such a signal as far as 2,000 feet. In cases where these methods are not possible, the transmitter itself can induce a current into the ground that can travel along a buried metal conduit.
There are some other methods to non-invasively detect objects under the ground surface. Ground Penetrating Radar (GPR) operates on a system similar to aviation radar, where a signal is broadcast and then reflected back to a receiver. Real fast!! GPR can be useful in detecting utility lines that cannot be easily traced with an EM device, and locating isolated objects such as underground storage tanks. Some non-metallic utility lines such as PVC water lines or septic leach field lines can also be identified by GPR, dependent on their diameter, depth, and the soil conditions.
Sonic waves can also be used as a detection method. This is particularly useful for locating water or air leaks, where a sound wave is emitted from the leak itself.