GPR is now a complementary tool to EM when locating buried utilities. Unlike active EM methods where an individual utility is excited, GPR sees any buried feature that differs from its surroundings which can result in many targets being observed. This article focuses on getting the most out of your utility data in two common but challenging situations:

Challenge #1: Too Many Targets

When there are many buried targets present, sorting out the identity of responses from at different depths, oriented in different directions, and separating utilities from other targets (such as rocks, tree roots, etc) – can be challenging. Figure 1 shows an example of a complex multi-target GPR cross section.

Multiple targets create numerous hyperbolic responses makingGPR data challenging to interpret.

Figure 1:
Multiple targets create numerous hyperbolic responses making GPR data challenging to interpret.

Challenge #2: Weak Targets

Targets of differing composition, at varying depths in differing soil conditions, result in highly variable varied GPR reflection responses amplitudes. As a result, the sought after buried utility may not be the strongest most prominent response in the GPR cross section. Non-metallic pipes and conduits utilities often produce weak responses since their composition represents a small contrast to the host material properties. In many instances a desired targets may be overlooked if the response is weak as illustrated in Figure 2.

Weak hyperbolas from deep hyperbolas are often missed by utility-locators.
Figure 2:
A weak response from a deeply buried utility (boxed in the above cross–section) can often be missed if only strong responses are noted.

Let’s look at some strategies to help to get the most from your data in these situations or a combination of these situations.

Interpretations & MapView

When locating utilities, existing plan map records often provide initial guidance as to what to expect on site. Further, standard practice requires creation of site sketch maps of observed features and targets prior to leaving the locate area. MapView is an integrated display feature that enables GPR data to be viewed in a similar manner on site. During data collection, the operator simply adds color-coded dots (interpretations) on all responses (normally each hyperbola) by touching the screen at the top of the hyperbola (Figure 3). Little discrimination is needed in this first step, every potential target can be marked. Attention should be given to the pattern, direction and spacing of the GPR data collection path to ensure that the area is properly covered.

Subsurface Reflections by Peter Annan