Ground Penetrating Radar is a key element in the utility surveyors toolbox. It also often the most misunderstood by clients. This may be down to the cost of a GPR survey and the occasional reluctance to spend the money getting a proper survey undertaken.
It is only one element in a box of tools that includes utility records searches, EM location and visual inspection among others.
The reason it is just one of the tools we use is partly due to ground penetrating radar limitations. A belief still exists within the industry that a GPR survey is an x-ray vision into the subsurface. That it can identify everything below our feet and is the key to solving the issues relating to service strikes.
Ground penetrating radar is a fantastic technology. If used correctly and in the correct conditions can make a huge difference. An underground utility survey can be a problem solving exercise, especially in locations with dense subsurface infrastructure. Many times we have had a particular problem solved by the GPR identifying a buried chamber or locating a pot ended cable shown on records.
These elements, after all, are why we have a full box of tools to work with. We use them all to build a picture of each location.
The x-ray vision myth exists because ground penetrating radar limitations are not widely understood by those outside the user base. In some rare instances, these limitations make the GPR element of a survey completely ineffective.
The following is a list of the ground penetrating radar limitations which are the principal drivers in geoscan survey success.
The ground penetrating radar equipment chosen for the survey
Not every piece of GPR equipment is suitable for the task at hand. Different frequencies on antenna are designed to perform various functions.
For example high frequency modules in the Ghz are designed to provide high resolution at a shallow depth and would be used for re-bar assessment or bridge deck analysis. At the other end of the scale the really low frequency antennas have tremendous penetration capability, but return a much lower resolution data. These are ideal for more geological work. The mid range frequencies at around 400Mhz – 750Mhz is where you would likely find the multi purpose GPR units used in fields such as utility mapping surveys.
The limitations of the GPR user
Depending on the survey parameters the user has a great effect on the ground penetrating radar limitations. If the survey is for utilities and the GPR element is a mark on site one, it is purely the skill and experience of the user identifying potential targets. Someone who is not trained, competent and experienced can often do more harm than good in this situation.
If the site area is one of highly dense infrastructure the human element can play a significant role in GPR survey success. If the data is being collected on site for later post processing this removes some of this risk. Post processing software can use filters to remove noise. Also, you are not relying on someone to make on the spot decisions. The data you are collecting provides a trail for cross checking and QA purposes.
Soil type & moisture content
Potentially the biggest geophysical factor in ground penetrating radar limitations is that of soil type. Highly conductive soils, such as clay, have high signal attenuation. This severely inhibits the effective penetration depth of a given GPR antenna frequency.
We are often asked, how deep can your GPR see? This depends on the antenna choice for a given application. A lower frequency antenna such as an 80Mhz will have a high potential penetration (40m). A high frequency one such a 750Mhz will be much lower (3.5m). The trade off is that lower frequency antennas have a lower resolution. In real terms, this means you will only identify larger targets at depth.
Any given frequency will have an effective depth rage it can work to. This, however, is based on ideal conditions, much like the mpg figures published by car manufacturers. The true penetration depth figure is determined by the site conditions, soil type in particular and often cannot be determined before the survey.
Another example of ground penetrating radar limitations due to soil type is that of signal scattering due features such as made up and rocky ground.
If a site is made up of a soil type containing lots of objects differing in size, such as a rock fill or crushed concrete material. The GPR signal scatters of these objects and in some instances renders it completely ineffective.
The other major factor in soil type is the presence of moisture. A saturated soil may completely remove reflections from GPR data. The problem here is that objects you would expect to see are not visible to the user.
Surface conditions
Condition and type of the surface on which survey is being undertaken has an effect on ground penetrating radar limitations.
If a surface is reinforced concrete with densely packed bars, the signal will suffer from dispersion. Rocky or made up ground provides the same result. This greatly reduces the ability of the user to identify potential targets. The same is true when the top surface is completely saturated due to a period of rain or if the top surface is made of a coarse gravel.
For a utility survey. It is these ground penetrating radar limitations, which make the full range of tools available to a surveyor so important. GPR is hugely important to us. Which is why we have invested in technology such as the IDS Stream C and have an in house geophysics team.
For clients, understanding ground penetrating radar limitations plays a huge role in the expectation of outputs from a survey. We are restrained by geophysical factors, but that does not detract overall from the quality of a survey. Good survey companies will ensure they use all of the tools in the box to meet clients expectations.
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