It isn’t often that you see a sign which effectively says “Beware! Geophysicists!”
A GPR is an interesting toy which I have never had a chance to play with. (Our students get to use one on the final-year geophysics field course but it is supplied by the French university with whom we share the trip.)
I’m intrigued as to what the survey is looking for; the sign is on Waverley bridge so I would expect that the survey would show the internal structure of the bridge, with a big empty space below it. What would the survey show that you could not see for yourself by standing on Platform 11 and looking up? (Or by looking at the engineering drawings, which they presumably have?) And is it bad enough that anyone on Platform 11, or on the Number 100 bus crossing the bridge, should get worried about it?