Thanks for that. I suspect that your advice is correct and am now considering the rental of a ground probing radar unit, in particular the Sir-3000 with a ground-coupled attenna. This would allow me to walk the fields on a free-run (with carrying harness), then generate 3D images on my laptop inside a day.
Penetration depends on the attenna. The GSSI 3207 Ground-Coupled Antenna has a center frequency of 100 MHz and a depth of penetration of 20m, which is more than I need.
Some links:
product file
Use in rescue
Ground Penetrating Radar Antennas
Thanks again.