I would appreciate some advice on grounding for a long wire antenna.

I am just thinking about setting-up my new QTH mainly for 40m and 20m.
I would like to try a longwire (around 100m long) with an SG230 ATU,
largely because I have an opportunity to sling a long wire high and
clear. But how important is orientation of grounding elements? I could
bury wire under a small lawn (10m x 10m) but it would lie under just a
fraction of the feed end of the longwire. Is this workable ?
Alternatively (or additionally) I could run a long wire (again around
100m) in a straight line just beneath grass but at almost 90 degrees to
the antenna. I could also connect the far end of this long ground wire
to a big galvanised steel fence that runs for almost a mile, I guess
that would make a great counterpoise??

What do you guys recommend

Thanks