Something about non-destructive examination of burried steel ball valves.
Is the process fluid natural gas?
There is a possibility that you maybe able to get a detector that is very sensitive to one of you gas stream components. We had a demonstration of one type that was extremely sensitive to Methane.
I've used Helium with a portable He detector to check underground lines. The equipment I used was developed for the telephone company to detect leaks in buried telephone cables.
As mentioned above ultrasound would be a very good way to go.
Though I've never tried it I once proposed that we punch a hole through the cement and dirt covering a pipe to the detect a possible leak. This hole would enable us to get a ultrasound microphone in close proximity to the pipe.

MORE NEWS