It concerns me that there is so much confusion and misinformation about GPR.
Some equipment manufacturers would have you believe that GPR is straightforward. All you need is to buy their latest GPR system (usually an all in one system with fancy x-y-z technology) and you are virtually guaranteed GPR results. Little or no experience required.
No! For the most part, that is not the case. There is a lot more to GPR than the kit. It’s the surveyor that makes the difference.
I’ll give an example that will astound you later, but first, let’s look at things you should consider when you plan a GPR survey. Yes, you read that right. You need to plan the survey. There are four main stages:
- Survey objectives
- Survey method
- Equipment selection
- GPR survey methodology
- Data processing, interpretation and reporting
For a successful survey, you need to have a clear understanding and vision of what information is required. GPR is not a survey type, it is a survey method. It can be used for a wide variety of site survey investigations.
Establish what the client wants. Then go to the next stage to decide if GPR is the best method for the survey.
Is GPR the best survey method? GPR is not some kind of magic solution. It does not work for everything!
GPR detects changes in the sub-surface. It does this by looking for differences in the electrical properties (dielectric) between materials. For GPR to detect the target object, it must be sufficiently different from the surrounding material, otherwise, GPR may not be able to resolve the target.
Another consideration is the material type. GPR works best on materials with low electrical conductivity, such as concrete, asphalt and dry sandy soils. GPR has a limited penetration depth in conductive materials such as saturated ground or wet clay (it’s the dissolved mineral content that affects the GPR, not the water itself). GPR will not penetrate metal at all but it’s great at detecting it.
All GPR equipment is not the same! It is crucial to select the equipment best suited for the survey.
GPR antennas operate at different central frequencies. A high-frequency antenna (1.5GHz or higher) offers high resolution but limited depth penetration. A lower frequency has greater depth penetration but the tradeoff is that it will not resolve smaller targets. I will cover GPR equipment in more detail in a separate article.
You can find out more about the equipment we use on our web page Ground Penetrating Radar equipment.
GPR survey methodology
This involves equipment settings and distance and depth calibration of the GPR equipment. The importance of this should not be underestimated. An experienced GPR surveyor will know how to set up the equipment to ensure that the data collected is of the highest quality.
How scan lines are collected is crucial. For high survey detail, it is best to scan on a regular orthogonal grid. The spacing of scan lines should take into consideration the level of detail required and the size of the target objects. An experienced GPR surveyor will know how best to scan the area to ensure the objectives of the survey will be met.
Data processing, interpretation and reporting
GPR data can be interpreted on site in real time. This is fine for simple things such as locating reinforcement. For more complex surveys, it is far better to process the data and carry out the interpretation off-site, back at the office using specialist software. This produces far superior results.
The key is data interpretation. GPR detects changes, but it does not identify their exact nature. Certain detected features produce characteristic reflections in the data. This helps with their identification. Again, experience is key to accurate interpretation.
The day we were told that GPR was rubbish!
Here it is! As promised earlier in the article. We couldn’t believe what a client told us. We were requested to undertake a metal detection survey to locate embedded metal pins in a masonry façade. No problem; a combination of metal detection and GPR we told him. No, said the client. He had a full GPR survey carried out by a GPR surveyor using the latest kit and it was totally useless. The client insisted: No GPR, just metal detection.
After a little probing, we found out what had happened. The surveyor (I won’t use the term GPR surveyor) had used one of the latest all in one GPR systems, it even processed the data automatically (sorry, I won’t divulge the name of the company or the kit used). There was nothing wrong with the kit as such, except it had been set up to scan on a grid at fixed centres. Unfortunately, there was no correlation between the scan line positions and where the embedded pins were! Consequently, the survey failed to detect the pins.
We successfully demonstrated that positioning the scan lines correctly, could indeed detect the pins and differentiate between them and other larger structural steelwork, which was present. The client was happy, the reputation of GPR was saved and we got plenty of work!
Moral of the story
Always use an experienced GPR surveyor who understands the survey objectives, and who is capable of choosing the most suitable survey methodology and equipment for the survey.