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 making GPR data challenging to interpret.
Challenge #2: Weak Targets
A weak response from a deeply buried utility (boxed in the above cross–section) can often be missed if only strong responses are noted.
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. 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
User marks interpretations on the screen while collecting data. (Pink – deep response, Blue – medium, Yellow – shallow)
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 leaviing 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.
MapView showing linear utilites (blue and pink) and point targets (yellow).
At any time during data collection, selecting MapView will show the entire area surveyed with all the interpretations clearly marked on the screen. Since buried utilities are normally ling linear features, examining MapView for sequences of linear dots is a powerful methods for differentiating utilities from localized soil features. In the example above in Figure 3, the operator decided to mark the deepest response blue, the middle depth target response as pink, and shallow targets as yellow. When displayed in MapView, the blue and the pink interpretations clearly form lines suggesting that these are indeed the sought after utilities(Figure 4). The yellow interpretations appear to be single point targets.
Produces a 2D map image of GPR data at varying depths based on signal amplitude. When locating in an area with many utilities, depth slicing is a powerful way of looking for linear subsurface objects likely to be utilities. Sensors & Software GPR systems guide the user through setup, grid data collection, data processing and display depth slices right in the field (Figure 5). If your GPR has an accurate GPS (< 1m) attached and you are working in an open area with lots of sky visible, you can also generate a depth slice from line collection using the GPS for positioning the GPR data.
Collecting a grid allows data to be processed into depth slices to reveal utilities at different depths. This helps in mapping utilities at complex sites.
Cover the area by walking back and forth with the GPR system to collect closely spaced lines. The data is then processed into depth slices using the new SliceView-Lines capability of the EKKO_Project V5 PC software (Figure 6, 7). Depth slicing aids significantly with Challenge #1, as it helps to visualize the targets and highlight linear features like utilities.
Collecting GPR data in a pseudo-grid, using an accurate GPS for positioning
Depth slices that reveal utilities at different depths
Interpretations overlaid on Depth Slices
Depth slicing is complimentary to MapViews of user selected targets. MapView allows the operator to use experience and knowledge to highlight even weak responses that may not be strongly presented in a depth slice. Combining both approaches is definitely beneficial in complex areas. In the situation where target responses in the GPR cross section are visible to the eye but too weak to appear in a depth slice (such as depicted in Figure 8, red dot), it is possible to map the weak hyperbolas by adding user selected point interpretations to the weak responsess directly in the field or in EKKO_Project PC software. There are different ways to choose what color of interpretation to put on a hyperbola. For example a user could choose to color an interpretation based on the utility marking color code, for example using blue to mark a target believed to be potable water. Another option would be to mark based on depth of the utility; shallow green, medium depth red, deep blue (figure 9). One could also mark based on how strong the hyperbolic response is. The options are endless; however, the user must choose a convention for the project and stick with it for this method to be effective.
Add point interpretations to the GPR lines. Once displayed in MapView, linear utility features are revealed
Add point interpretations to interesting features like ringing metal responses (green dot)
After adding the point interpretations, display MapView in the EKKO_Project PC software and look at the patterns the interpretations make (Figures 10 and 11). Some point interpretations will line up, indicating a linear object, probably a utility. Some interpretations will not line up with any others; these are from point targets and are usually not of interest to utility locators. Adding interpretations and superimposing them on your depth slices is the best way to confirm that the strong linear features that appear in the depth slices really are a result of strong hyperbolas (Figure 10) and, more importantly, to detect utilities from weak hyperbolas that do not appear clearly depth slices (Figure 11). Depth slices and interpretations are two key methods to help unravel complex sites and reveal weak responses from utilities that are often missed. Safety and damage prevention is of utmost importance to everyone; use all the weapons in your arsenal to resolve the challenging utility locating problems you face.
Depth slice showing the strong linear response from a utility (top) is confirmed after adding interpretations (bottom)
Depth slice (top) does not show any obvious linear utilities but after adding interpretations to all hyperbolas, including very weak ones, the interpretations reveal a utility that could easily have been missed (bottom)