Most line locating equipment operate through principles of electromagnetics (EM), designed to detect underground utilities constructed of electrically conductive materials.  An active signal is applied to the underground utility by means of a radio frequency (RF) transmitter and then traced with a receiver.  With direct coupling, an RF signal is applied to a cable or pipe where there is access to a contact point.  With no access to the utility, the indirect mode is used.  A transmitter is placed or walked along the ground surface above the conductor and the signal is induced through earth onto the pipe or cable.  For non-conductive pipes and conduits, a plumber's snake can be inserted into the line and a signal induced along its length.  If electromagnetic methods will not detect a particular utility, ground penetrating radar (GPR) can be employed to image the subsurface feature. 

Field Procedures

NAEVA Geophysics' approach to utility location begins with a highly experienced and motivated crew equipped with a variety of state-of-the-art utility location instruments.  Our utility locators, without exception, are all geophysicists and geologists with a strong understanding of geophysical principles and how they are effectively applied to utility location.  Utility location is an exacting science.  When a complete and accurate utility mark-out is desired, NAEVA Geophysics provides geophysical consultants, not service technicians.

Equipment

To conduct a thorough and accurate subsurface utility mark-out, a variety of equipment types should be employed.  No single instrument can detect all types of underground utilities.  NAEVA Geophysics uses four different utility location instruments and GPR to provide a complete on-site utility mark-out.

1)  Radiodetection RD-600 EM instrument 
2)  Fisher TW-6 pipe and cable locator
3)  Metrotech 50/60 EM receiver
4)  Dynatel 500A EM transmitter and receiver 
5)  GSSI-Sir-3 GPR

Applications

• Electric
• Natural gas
• Telephone
• Fuel
• Water
• Sewer
  

Providing mark-outs where subsurface utilities are an issue:

• Generate or update subsurface utilty maps for future planned development
• Clear drilling location for subsurface interferences
• Identify utilities for avoidance during excavation
• Map utility pathways that may be related to contaminant migration
• Accurately mark-out utilities scheduled for repair or replacement
  

Data Processing and Presentation

Utility location equipment is designed for direct detection and recognition of targets, real-time in the field.  The surface traces of underground utilities are marked directly o n the ground as they are identified, using the color code established by the American Public Works Association.  Field sketches are made and later digitized using our CAD facilities, showing all detected underground utilities as they relate to permanent above ground facilities.
 
 

A utilities mark-out prior to excavating or drilling should be the rule, not the exception.

Information provided by: http://www.naevageophysics.com