ASCE 38-02 has an extensive listing of techniques for electronically detecting underground utilities which is part of the process for certifying utility data as “Quality Level B”. The methods described are as follows:
Electromagnetic Pipe and Cable Locators Terrain Conductivity Resistivity Methods Metal Detectors Ground Penetrating Radar Optical Methods Infrared (Thermal) Methods X-Ray Methods (Penetrating Radiation) Total Field Measurements Gradiometric Measurements Acoustic Emission Micro gravitational Techniques Isotopic (Radiometric) Techniques Chemical Techniques Borehole Geophysics Geophysical Diffraction Tomography
While all the above methods are valid, many have limitations based on cost, site conditions, limited application and current state of development. Based on a survey of Subsurface Utility Engineering Professionals it appears that at least 70% of Quality Level B data is generated using Electromagnetic Pipe and Cable Locators with other methods being used to either verify the information or for situations where Electromagnetic Pipe and Cable Locators yield poor or no results. For this reason only a description of Electromagnetic Pipe and Cable Locators is provided.
The following is not a comprehensive discussion but rather a brief overview.
The Electromagnetic and Radiofrequency line locators operate by locating either a background signal (an ambient energy field) (see Fig 1.1) or by locating a signal introduced into the utility line using a transmitter (an applied energy field). (See Figs 1.2 & 1.3)
The signal carried by the utility line can be located horizontally on the surface using a receiver. The receiver is moved across the estimated location of the utility line. The location of the highest signal strength is marked as the approximate horizontal location of the utility.
The receiver is then rotated until the highest signal strength is achieved. This will give the approximate orientation of the utility. (See Figure 1.4) The approximate vertical locations can be estimated using the above techniques. In some situations, however, errors can be introduced that render the vertical location no better than good judgment would predict.
In addition, errors can occur in situations when lines are running one on top of the other. What results is a combined field, which would indicate a single utility with greater depth than the actual depth. See Figure 1.5 Conversely, the presence of a conductor above the utility being traced could lead to a shallower depth than actual. Situations such as these make the determination of vertical location with these techniques unreliable.
The best way to determine the vertical location is to expose the utility using minimally intrusive methods. [click image to zoom]