What can you see?
Davide Campo
MInstNDT | Technical Manager (Non-destructive testing) | GPR Specialist | Consultant
How many times did we hear this sentence associated to a radargram? I found entertaining LinkedIn posts where the author asks to guess what the radargram shows, it is a good challenge where I can spot something familiar, where I can use the knowledge/experience I have to try to understand the context from the data and where I can learn something new (main thing).
But how do we interpret GPR data in the real word?
GPR is not an instrument, it is a geophysical methodology (no, it is not a utility locator or a rebar finder) and, as all the geophysics, it needs a context to have a meaning. At the end of the day, any geophysical output is a bunch of numbers converted in a more or less colourful image. Numbers wouldn’t make much sense without background information.
What is the context?
Very simple:
That is the minimum for a context. It is not always necessary to know everything (and sometimes all the above is not enough) and it is always better to know as much as possible.
So to come back to the radargram above (here a better a view: https://www.dhirubhai.net/posts/davide--campo_gpr-activity-6823372073100959744-B3sM), what can we see? Some features may be familiar, it depends on the analyst’s experience and knowledge.
On the left hand side, there are 3-4 hyperbolas at different depths, maybe top and bottom of a pipe with some multiples.
On the right hand side there are two more hyperbolas, other two services.
Between 3 ?and 4 m, those little hyperbolas resemble reinforcement: a potential structural element buried into the ground.
Ground? We have no background information. Where was the scan taken? We assumed it was on the ground.
It is actually a numerical simulation of GPR data: the hyperbolas on the left are generated by a triangular shaped feature with relative permittivity of 19 (did we say pipe? it is not even circular), the two hyperbolas on the right are actually the edges of a limited interface, the small hyperbolas resembling reinforcing bar are circular voids: no metal, no bars.
Background information is not an option: to interpret data without it is a risk and for sure is wrong. Your experience helps but cannot replace background information.
Managing Director at GB Geotechnics UK (GBG)
2 年I was once contacted by a lawyer in the US asking if I could interpret the data from a GPR operator who had incorrectly interpreted data leading to major structural damage (which would have been avoided if they had just used a metal detector, but I digress), and they refused to accept that data out of context is largely meaningless. I could have a guess from their very vague description what the data was, I could even give context to the condition of and arrangement of the materials, but without even knowing the relocation of the scans, they were asking a very impossible thing!
President, Bigman Geophysical | Founder, LearnGPR | Utility Locating, Concrete Scanning, Archaeology, Civil Engineering
3 年Great perspective Davide! I agree. I generally wont comment on those types of posts. Even worse, we have had tgis happen on sites. Asked to locate something, we requested information. We were told that we couldnt have it since they wanted independent data sets and wouldnt share information
Auscultation et diagnostic béton - Détection des réseaux souterrains
3 年Rebar in concrete, perhaps protection of underground networks?
Regional Business Manager, Detection at Leica Geosystems part of Hexagon
3 年Hi Davide, is there any simulated model drawing you can share here so we can compare side by side?