For many years, the myth that GPR does not work when the ground is wet has persisted. In the earliest days of GPR, one of the first things discovered was that the radio wave propagation velocity in the ground was strongly controlled by the volumetric water content. This concept is embedded in the classic Topp equation that relates water content to dielectric permittivity and radio wave velocity. Until 1980, the relationship between water and GPR was generally understood and there was no confusion.
Somewhere in the 1980s, a badly designed GPR instrument was offered for commercial use, primarily in Europe. When the unit failed to work properly, the vendor said that the equipment failed because GPR could not be used when ‘the ground was wet’. The goal was to convey the impression it was not an instrument problem but a physical fact of life! This totally bogus statement was repeated and has gradually propagated around the world. This myth persists despite the plethora of superb GPR data sets collected on lakes and rivers and in water-saturated sediments.
The basic facts are as follows:
- Water is important and ubiquitous in most terrestrial environments
- Water has a high dielectric permittivity which can have a large impact on electrical properties of the ground
- The presence of water can strongly affect the GPR wave velocity, slowing propagation
- The polar nature of the water molecule makes it a solvent for ionic materials; dissolved ions make water conductive and thus attenuate GPR signals
In summary, water is an important factor in GPR use and data analysis. Its presence does not, by itself, cause GPR not to function or be ineffective. In fact, as stated previously, some of the most spectacular GPR data ever collected have been acquired in water saturated sands and gravels.
The critical pros and cons about water and GPR are follows:
- An increase in dielectric permittivity associated with water saturation decreases GPR wave velocity, resulting in greater depth resolution
- As the ground dielectric permittivity increases, more energy is coupled into the ground and downward focusing increases
- On the downside, if the water is saline, containing a substantial amount of dissolved solids, GPR signals can be strongly attenuated
- If water is distributed irregularly with high and low concentration pockets in the ground material or on the surface (puddles are an example), these zones generate high gradients in electromagnetic impedance which become localized scatters of electromagnetic signals. These scattered signals can generate ‘clutter’ that may mask desired target responses.
All in all, water can actually improve GPR performance in some conditions. The key is to understand the effect that water can have and be able to take that into account in data evaluation.
The GPR community must become more factual about the role of water in GPR. We all have to play our part in stopping the dissemination of the water myth!! No matter how many times I have explained that the presence of water is not a bad thing, the ‘water is bad myth’ is accepted as fact by many users.
While I hope we can lay this myth to rest, I suspect it will still persist in 100 years time. Alas!!!!