Understanding the GPR radargram
With some examples
To an untrained eye the radargram produced after a GPR survey may seem like a contemporary black & white painting or a very low-quality UFO sighting. But with time and with enough staring, you get used to the GPR data imaging and you even start recognising certain patterns.
Here’s some GPR images and what they mean.
Square trench
This wave represents a double reflection that bounces off one side of the trench and the floor, which is then recorded. Two half hyperbolas can be seen in the centre of the trench.
A mistake that can be made here is to interpret the reflections as a result of several pipes located close to the corners of the trench. The reflections are simply from the multiple waves recorded.
Square trench with a pipe
The same square trench, but with a pipe in it has a complicated radar pattern. The combination of waves that bounce off the pipe and the walls of the trench can produce many faint hyperbolic reflections. The half hyperbolic tails from the wave can give a false impression that other materials or pipes may be buried when in fact all the complicated recorded reflections are caused by multiple waves bouncing inside the trench.
V-Trench
A V-trench has a very distinct reflection pattern. The wave in this structure can generate a rounded reflection at the apparent centre of the trench. Interpreters will often mistake this reflections as representing a pipe being located in the centre of the trench.
Another common interpretation mistake is the improper identification of direct reflection legs on each side of the trench. The observed reflections from the wave on each side of the trench centre are actually coming from the opposite sides of the trench, e.g. energy that is recorded from the right side of the trench is only detected when the antenna is on the left side of the trench, and vice versa.
The actually pattern that is recorded from a V-trench is very sensitive to the depth of the trench as well as the narrowness. For narrower V-trench the reflection may not be recorded or will not generate a pseudo hyperbolic reflection in the centre.
Semi-circular trench
A semi-circular trench extending to a depth of 1 m and with 1.6 m diameter can generate a reflection which is concave downward. This radar pattern is completely opposite of what one might expect.
For the circular trench buried at deeper depths, the concave pattern will almost look like a perfect hyperbola which can often be falsely interpreted as a pattern caused by a buried cylindrical object. The focusing of returned energy is dependent on the shape of the trench surface.
A specially designed elliptical trench can wholly reverse the above observations. The focusing or defocusing of GPR waves is very sensitive to small changes in the profile of the reflecting surface.
The learning point here is that the shapes recorded on raw radargrams may have the complete opposite structure in the ground.
Soils with contrasting velocity
Significant velocity changes between different materials can alter the shapes of radar structures from the “real” buried structures. In this example soil1 (light-grey) and soil2 (dark-grey) have a velocity contrast, with soil2 being much slower.
The total travel time for a ray to reflect off the bottom layer takes less time on the right side of the model than on the left side of the model. The reason being is that on the left side of the model soil1 is thinner and soil2 which is much slower in wave velocity is thicker — causing the overall travel time to the bottom layer to take much more time.
On the right side of the model, the faster material soil1 is thicker and there is less of the slower layer soil2 that the wave must travel through. The travel time here is thus much faster for the wave which reflects off the flat bottom layer. The net effect is to cause an upward warping of the flat layer on the right hand side of the model since overall faster materials exists here.
This effect is referred to as a “velocity pull-up” in seismology. Flat reflectors at depth may appear as not being recorded at equal times due to possible strong velocity contrasts and variable layer thicknesses of the highly contrasted soils.
Of course, there’s countless other use cases that can be observed when using a ground penetrating radar. This is why the post-processing and analysis of GPR data is just as important as the actual surveying in determining what lies underneath and what actions need to be taken.
Photos and explanations are taken from the book “Geotechnologies and the Environment”.
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