My company manufactures equipment specifically designed for detecting leaks in underground piping and storage tanks using helium (totally inert and non-toxic) as a trace gas. If helium is introduced to the system our instrument can detect the helium as it reaches the surface. Detecting leaks as small as 0.05 gallons per hour is not a problem. We have an office in Oxford and local personnel in Manchester. Let me know if I can be of assistance.
Just to let you know.
GPR is ideal for finding wet/dry soil boundaries for shallow leaks.
But 20m is a real challenge. I've penetrated 60m in clean sands
on Cape Cod, but typical soils will only let you go about 5-10m,
and clay soils won't even allow 2m.
You could do resistivity to map a long profile, or even EM measurements
depending on budget and time constraints.
Andrew's comment seems good, if nothing is yet installed.
What type of soil are you dealing with?