Probably the question I am asked the most regarding the effectiveness of GPR is how deep can ground penetrating radar see? Of course this is both a simple question and one which is perfectly reasonable for someone employing the services of a GPR practitioner to ask.
Unfortunately the answer can get rather complicated. There are a range limitations of ground penetrating radar and factors which affect the effective penetration depth of GPR. We start with what the theoretical maximum depth range is and then start to add in the variables which reduce that depth.
Here we are going to take a look into the various factors relating to antenna frequency and the environmental conditions which reduce the effective depth of GPR.
What are the factors that affect how deep a GPR can see?
The depth that a Ground Penetrating Radar (GPR) can penetrate is affected by several factors, including:
- Frequency of the radar signal: Higher frequency signals can provide higher resolution images but are limited in their penetration depth, while lower frequency signals can penetrate deeper but have lower resolution.
- Subsurface material conductivity: The electrical conductivity of the subsurface materials affects the penetration depth of the radar signals. High conductivity materials, such as clay soils can cause the signals to be absorbed, reducing the penetration depth.
- Soil moisture content: Moist soil can increase the conductivity of the subsurface materials and reduce the penetration depth of the radar signals.
- Ground make up of the subsurface: The type of subsurface being imaged can also affect the penetration depth of the radar signals. Differing particle sizes cause scattering of the signal.
How does ground penetrating radar frequency affect penetration depth?
Not all GPR systems are the same. There are different frequency antennas which are designed to perform a specific task. The resolution of the data and the effective penetration depth are governed by the frequency of the antenna being used.
GPR antenna frequencies are a subject in themselves. The quoted frequency is actually the central frequency with the equipment operating in a range on either side.
For example a high frequency system with a frequency in the Ghz range would give high resolution at a shallow depth for tasks such as concrete inspection. Very low frequency antenna systems will give a low resolution but have a very high penetration depth and would be used for applications such as geology.
The following table indicates the common frequencies of GPR systems, their designed penetration depth and common applications for those systems.
|Frequency (MHz)||Penetration Depth (m)||Potential Applications|
|1600||0.5||Concrete and reinforcement inspection|
|900||1||Concrete inspection and void detection|
|400||4||Utility surveying, archaeological prospecting and void detection|
|250||6||Utility surveying, archaeological prospecting, void detection and geotechnical|
|100||20||Geotechnical and mining|
|<80||Up to 50||Geotechnical|
How does soil type affect the depth GPR can see?
The type of soil you are dealing with will have a fundamental impact on the effective penetration depth achievable with GPR. Tee penetration depths listed in the table above are based on ideal conditions, which we rarely encounter on site. Really good penetration depths can be achieved in dry sandy soils or dry materials such as limestone or granite.
GPR is greatly affected by highly conductive soils. In soils, such as clay, the electrical conductivity is high. This means that the GPR signals are absorbed and dispersed by the conductivity of the soil, reducing the penetration depth. Essentially they are attenuated, this results in the shallower penetration depth we see in these situations.
Adding other factors to a conductive soil, such as the presence of moisture, can make a site unsuitable for an effective GPR survey as the penetration depth will be insufficient for meaningful data to be collected.
It should also be noted that the conditions can vary across a large site, where in one area you may achieve better results than another.
How does soil moisture content affect the penetration depth of GPR?
If highly conductive soils such as clay affect penetration depth, adding moisture to the equation makes huge differences to the effectiveness of GPR. Adding water to soil increases the conductivity of the soil, therefore amplifying the attenuation effect that reduces penetration.
This is significantly noticeable where the surface of a soil is heavily saturated. For example, if you are undertaking a GPR survey in a field after a period of heavy rainfall, the moisture content of the first few cm of the soil will have a dramatic effect on penetration depth.
Another example of this is in san. A dry sandy soil is perfect ground conditions for the effectiveness of GPR. If that same sand becomes saturated in any way, the quality of the data will be greatly reduced.
As is the case with clayey soils, water content may make an area unsuitable for a GPR survey, or at the very least, provide very limited information.
How subsurface scattering affects GPR penetration depth
Unless you are dealing with uniform soils, The material of the ground being surveyed can cause a scattering effect of a GPR signal. This essentially means the signal is losing energy and being sent in random directions, meaning it is not traveling as far underground as it otherwise would do.
A degree of scattering is inevitable in most situations, because uniform soils are not necessarily always present, especially close to the surface.
This phenomenon is most commonly a greater factor of areas of made up ground and brownfield sites, where the fill materials used often have a range of different sized particles. Recycled crushed concrete being a good example.
The energy loss of the signal can reduce the penetration to an amount that makes the site unsuitable for the use of GPR in the most extreme circumstances.
How does target size affect penetration depth?
Although the size of the objects you are looking for does not directly affect the penetration of the signals, the antenna frequency selected will determine the size of target you are likely to see, therefore reducing the effective penetration based on your desired target.
For example, a low frequency antenna of around 100 Mhz, may have a potential penetration of around 5 meters, but it will only be able to identify a target of approximately 0.5m in size at that depth. Anything smaller will not be visible due to the low resolution of a low frequency antenna.
If you were mapping a site to identify a target of around 0.2m in size, you would need a 500 Mhz antenna, but your effective penetration would be reduced to around 2m.
How deep can GPR actually see?
From all of the above information you will have discovered that there is a penetration depth achievable based on ideal conditions for any given ground penetrating radar antenna. To get to what the actual depth is in reality, all of the factors above reduce it from that ideal number.
In the USA, a map has been produced that provides a broad outline of the suitability for different areas for radar surveys based on the soil conditions.
The table below offers some guidance on what you can expect to see from different antennas in real world conditions. This is published information from Sensors and Software, a Canadian GPR manufacturer.
Remember that these are a guide as to how deep GPR can see. The actual penetration depth achieved will vary greatly based on local conditions. If you are looking into the cost of a GPR survey, this should be a factor you consider.
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