I have completed the delay modification on my SurfPi. The delay setting is normally done on the circuit board but I have replaced the preset trim pot with a potentiometer that is now mounted next to the threshold control on the housing. With this modification I am now able to tweak the performance wrt sensitivity and I am able to reduce "falsing" on normal soil, ie I can use the Pi on most types of soil without picking up false signals. Changing the delay could also give some crude form of discrimination. However, detection depth might be sacrificed with some settings.
Well done Barney. You seem to be quite knowkedgeable with electronics