The ground beneath our feet in urban areas inevitably gets more and more congested over time with infrastructure comprising both underground structures and utilities. This, together with increasing urbanization and regeneration, aging infrastructure, climate change and flooding, are fueling a need for utility locating and better understanding of the subsurface conditions.
This is no easy task; utilities can, for example, consist of pipes, conduits, cables, tanks of differing material, size, and age. These utilities are buried at different depths in the ground which can be made up of widely varying natural and imported materials. The knowledge of both the location and depth can be lacking, due to obsolete, inaccurate, or non-existent subsurface maps. This lack of knowledge can lead to excavation or cutting in the wrong places, which in turn can cause serious damage, be dangerous, costly, and result in huge time delays.
So, knowing the locations of underground utilities can:
GPR (Ground Penetrating Radar) is an investigation method that uses electromagnetic pulses to produce an image of the subsurface, without digging or drilling.
GPR can be compared to a fish-finder (or echolocation device) on a boat. The technique is non-destructive and there is no need for any physical contact with the targeted utilities. A transmitter antenna sends a pulse into the ground, and that pulse reflects wherever there is a contrast in the dielectric properties of the medium it’s being sent into. Such a contrast can be caused by, for example, objects in the ground (such as utilities), different geological layers (such as bedrock), or voids. The reflected pulses are then collected by a receiver antenna, which will measure the time (which can be converted to depth) from when the pulse was transmitted to when a reflected signal was received. The receiver antenna will also record the amplitude of the reflected signals.
The transmitter and receiver are often put in one antenna box for easier handling in the field but can also be housed in separate units. Discrete objects, such as utilities, typically produce characteristic features, so called hyperbolas, in the resulting radargram.
The depth and resolution that can be achieved with GPR depends on the frequency of the transmitted electromagnetic pulse. A higher frequency results in higher data resolution, whilst a lower frequency results in better depth penetration. What frequency to use depends on the application and the required resolution and depth of your survey. For utility locating and mapping, frequencies of around 160-600MHz are usually suitable; see table below.
As seen in Table 1, the resolution changes with antenna frequency. This is also true for increasing depth, meaning that objects need to be larger, at depth, to be detectable.
|Antenna Frequency (MHz)||Suitable Target Size (m)||Approx. Depth* (m)|
*values seen in common soils and geology; significant improvements may be experienced in other media such as clean sands and frozen ground.
Frequency is not the only factor that determines the achievable depth and resolution; the ground through which the signal is sent, also determines the quality of the collected data. Resistive soils such as sand and gravel, as well as rocks, are usually suitable for GPR surveys, whilst less appropriate soils usually contain conductive materials, for example, clay. Soils saturated with water can also be problematic for GPR surveys.
GPR measurements are most often done by pushing the antennas in a cart along a predetermined line on the ground. During an investigation, the position of the GPR antenna can additionally be tracked by an external or internal GPS.
A 2D single-channel GPR system can only map a single subsurface profile with each pass. In order to better understand the size, shape and distribution of buried structures and objects it may be beneficial to build a 3D data volume where the GPR data can be used to create informative maps from various depths of the subsurface; there are two approaches to this.
The first is to build up a data block with a single-channel instrument collecting data at relatively coarse spacing (say 0.25m or 0.5m line intervals) in one or, ideally, two directions, orthogonal to each other.
The second approach, which will offer extremely high-resolution information of the subsurface, is to collect a “true 3D” data set, where the multiple parallel lines must be collected with extremely dense transect spacing. The ideal spacing is related to the frequency of the antenna and for a typical utility system this would be less than 0.1m. With a single-channel system, this approach would result in a time-consuming investigation with difficulties in positioning each individual line correctly. Instead, multi-channel solutions have been developed where an “array” of several transmitters and receivers provide the closely spaced data channels and precise positioning required for “true 3D” survey. It should be noted that these multi-channel array systems are distinct from instruments with multiple channels collecting fewer, more widely spaced profiles at once, or just multiple frequencies along the same line. As the true 3D systems create a data volume made up of extremely dense profiles, where line spacing and trace interval are similar, the survey swathes need only be collected in one direction.
Single channel 2D GPR is often sufficient for smaller utility locating and mapping projects, but when the investigation area gets larger, and the layout of the buried utilities gets more complicated, a multichannel solution can be a more efficient option.
When working with single-channel GPR, over good ground conditions, the identification of hyperbolas can often be done directly at the site, marking their position and/or depth on the ground surface. The location of the utility is set at the hyperbola’s highest point and all Guideline Geo single-channel GPR systems have a back-track function to simplify the work of directly marking out the detected utilities.