Well, I'm not a pipeline surveyor, far from it, but what I understand is that they're not surveying the pipeline per se. For starters because it's normally buried underground, and an actual leak would be detected via sensors in the pipe itself, or via an inflow/outflow discrepancy, pressure drop or something. Instead, what they are actually surveying from the air is activity on the surface which may have an impact on the pipe. Unannounced/uncoordinated digging/site development for instance.
I assume this is done at normal cruise speeds, with the ability to hover and take photographs as required.