Ground Penetrating Radar
One of the most versatile underground locating tools available is Ground-Penetrating Radar, or GPR. GPR uses high-frequency radar waves to create a detailed image of what lies under the earth. GPR is particularly good at helping to locate non-metallic subsurface utilities such as plastic, concrete and clay pipes, which conventional electronic locating equipment sometimes has a tough time with. Our newest addition to our GPR toolbox is the US Radar Ground Penetrating Radar. This GPR has very advanced 3-D software capabilities and the ability to profile long distances of ground, which in turns gives us the capability to provide our customers with data that represents large locate areas. Upon request we can also provide our customers with a detailed report along with pictures of our captured data from our GPR.
The above picture shows an underground storage tank being excavated and removed. The above scan shows how the still-buried tank appeared on our GPR.
GPR is also very handy for finding small, discrete objects that don't show up well against the background using other locating methods. It can locate pretty much anything you might need to find without tearing up your whole property. For instance, GPR is great for ledge profiling, and our instrument can even tell us how far down bedrock is: data which could save a contractor thousands of dollars in blasting costs.
GPR is also very effective at locating underground storage tanks. We were contacted recently to locate an abandoned 10,000 gallon storage tank in Portland. The reason the tank had never been removed was clear: it was sitting right in the middle of a bunch of buried utilities – including water, sewer and electrical lines – and no one was sure how they were all laid out.
Before the contractors could remove the tank, they had to perform test borings to look for any soil contamination which might be present if the tank was leaking. But, the drillers had to be sure that they wouldn't hit anything. Using GPR, Glen was able to give the drillers a number of clear places to do the test bores – and they never hit a thing. Glen was also able to precisely mark the locations of the tank and the utilities surrounding it.
The GPR scan image below shows the underground tank and two of the utilities near it. As you can see, those buried lines are only a few feet from the tank.
Ground-penetrating radar works better in some soil types than in others: GPR works best in gravelly or sandy soil, and less well in heavy clay materials. The efficiency of GPR is strongly affected by the target soil's conductivity (the soil's ability to carry a charge) and dielectric permittivity (the soil's capacity to hold a charge).
Since different soils have different electrical characteristics, the effectiveness and scan depth of GPR is highly dependent on the particular target site. Clay soils, along with subsurface brackish or salt water, greatly attenuate (absorb) the radar signal used by the GPR unit to "paint" a picture of the subterranean environment. So, in certain soils, the GPR's scan depth is limited. Especially salty or clayey soils can block the signal completely, although we don't often encounter that.
Assuming the soil composition of the target site is reasonably conducive to GPR operation, the other main factor influencing GPR's maximum scan depth and resolution is the type of antenna used. Generally, the lower the GPR antenna frequency, the deeper the GPR can scan, but the lower the resolution of the scan – so only larger objects will be detected. Conversely, a higher-frequency antenna will give a shallower scan depth, but at a much higher scan resolution, which allows smaller subsurface features to be detected. Naturally, we have a full range of antennas for our GPR units to suit any terrain and target. Our higher-frequency antennas are best for finding small buried objects, while our lower-frequency antennas are excellent for finding larger subterranean features, and ledge profiling.
The best conditions for ground-penetrating radar are sandy or gravelly soils, such as silica sands, fresh water, ice, and most solid rocks of low conductivity. Under such conditions, we can use a relatively shallow-scanning, high-frequency antenna, which will pick out the smallest detectable underground features while still providing all the scan depth we need. Using a lower-frequency antenna in compatible soils lets us find larger objects several dozen feet below the surface.
Under the right circumstances underground utilities can be located by GPR at depths exceeding ten feet, as seen in the screenshot below. This photo is of a large diameter metallic water line. Each hyperbolic reflection represents the pipe. We took multiple passes over the pipe at different intersects, and as you can see from the photo this line is increasing in depth as we keep making passes down the line. (Y-Axis=Depth, X-Axis=Surface Distance Travelled)