According to the manual this thing supposedly GB's all the way down to salt but I myself have never tried.
If you can get that thing balanced way down, 10 or under would be great, and turn the frequency up to 7 that might surprise you.
Wet black sand is your enemy, dry regular sand should not be a problem.
Here is some old advice using an F75 in this thread that seems to work from Tinfoil...
http://www.findmall.com/read.php?37,617795,618873#msg-618873
The flagship can manual GB down low but the F70 cannot, still, it is possible to fool it into balancing low if it won't go that far using the regular pumping method.
If you can find something around the ol' homestead that you can balance over, maybe a piece of iron that is not rusted, it might be possible to get into single digits fairly easily.
Then you can get into the wet and hopefully get deep.
This is the one environment I haven't had a chance to try my F70 in so far so curious on how all this stuff would actually work.
Confidence is high that you would do a bit better than that Compadre...maybe a lot better but the Compadre would still be fun but with less depth.