Surface-based Techniques

These methods are aimed at general searching for the presence of unknown buried utilities or for finding the location of known utilities for which impressed signal tracing approaches are not possible. Suitable techniques can identify the presence and approximate location and depth of buried utilities and other underground structures. A wide range of possible techniques are available and the most suitable techniques for a particular case are selected on the basis of the type and size of utility or structure being sought, the depth range for the investigation and the ground conditions at the site. The basic concept is to introduce a signal/wave into the ground from the ground surface. This signal can create reflections or refractions due to changes in underground conditions or due to the presence of buried objects. Signals returning to the ground surface are monitored and interpreted to indicate the presence, nature, location and depth of changes in the ground strata or the presence of buried objects or utilities. Simple metal detectors, for example, detect the changes in magnetic field that occur due to a buried metallic object. Cable and pipe locators, in addition to searching for evidence of EM fields already present due to the utility, can inject an EM signal into the ground to induce a current into a cable or metallic pipe which will in turn create its own EM field which is then detected at the surface. Another class of methods looks for the reflections of waves created by the change in transmission properties of the waves at the boundaries of objects buried in the ground or at changes in ground strata. The most common of these approaches is Ground Penetrating Radar (GPR) in which the presence and depth of buried objects can be determined from the signature of reflections as the survey device traverses the ground surface. A GPR system can identify non metallic utilities as well as not relying on prior knowledge of the utility’s existence. It is, however, limited in terms of the diameter of a utility that can be detected relative to its depth of burial and highly conductive ground conditions can render the method unsuitable even for normal depths of investigation. Acoustic/seismic signals also can provide reflection/refraction information as part of subsurface investigations. There are many other clues to the presence of buried utilities or objects that can be identified by a trained observer. These may include the presence on the surface of utility appurtenances, changes in ground vegetation cover due to changed drainage or thermal conditions adjacent to buried utilities, etc.