Impressed Signal Techniques

The methods grouped as impressed signal techniques all involve tracing the location/alignment of a buried utility by introducing a signal into the pipe or cable and then marking the strongest evidence of that signal on the ground surface in order to gauge its horizontal location. Examples falling into this category include:

• Tracing the electromagnetic field emanating from an electric cable
• Creating an electric current in a metallic pipeline and then tracing the electromagnetic field
• Introducing pressure waves into a fluid pipeline and then tracing the resulting ground vibrations (sonar).