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It concerns us 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 is required.
No! For the most part, that is not the case. There is a lot more to a GPR survey than the kit. It’s the surveyor that makes the difference.
I’ll give details of something a client told us, which amazed us later. But first, let’s look at things you should consider when you plan a GPR survey. There are five main stages:
- What are the survey objectives?
- What is the most suitable survey method? It may not be GPR!
- If it is GPR, what is the most suitable GPR equipment?
- GPR survey methodology.
- Data processing, interpretation and reporting.
For a successful survey, you must have a clear understanding and vision of the information you need to obtain. GPR is a survey method that can be used for various site investigations. You can find examples on our webpage, which lists our most popular GPR surveys.
Establish what the client wants. Then go to the next stage to decide on the most suitable survey method.
Is GPR the best survey method? GPR is not some 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 (the dissolved mineral content affects the GPR, not the water itself). GPR will not penetrate metal, 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 (1.5 GHz 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.
You can learn more about the equipment we use in our article about Ground Penetrating Radar equipment.
GPR survey methodology
Survey methodology involves equipment settings, including 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 the data collected is of the highest quality.
How GPR data is collected is crucial. For high survey detail, it is best to scan on a regular orthogonal grid with scan lines at close centres. The spacing of scan lines should take into consideration the level of detail required and the size of the target objects. Again, 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 the key to accurate interpretation.
The client told us that GPR was rubbish!
Here it is! As promised earlier in the article. This amazed us, and it’s 100% true.
We were asked 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 said. No, replied the client. He had a full GPR survey carried out by a contractor using the latest kit, and the GPR results were totally useless. The client insisted: No GPR, just metal detection.
After a little probing, we found out what had happened. The contractor who carried out the survey had hired one of the latest all-in-one GPR systems; it even processed the data on the fly. 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 a single pin.
We successfully demonstrated that by positioning the scan lines correctly, GPR could indeed detect the pins and differentiate between them and other larger structural steelwork, which was also present. The client was happy, the reputation of GPR was saved, and we got plenty of work!
Always use an experienced and reputable Company or surveyor that understands the survey objectives and has the experience and knowledge to choose the most suitable survey methodology and equipment for the survey.