It is possible, although other ways would be easier (for example using
lasers to measure the distance to the position markers).
GPS works by having various clocks tell the time to the receiver. DGPS
does the same, but with a transmitter in a KNOWN position instead of
the orbit. Use several fixed transmitters, and you don't need the
satelites any more.
But as I've written above, that would be a kludge to keep using the
DGPS receivers already installed - for geological purposes it would be
easier to use traditional surveying methods, it just wouldn't be as
comfortable because people would have to go on site to measure
distances. However, you don't really need PERMANENT measurement of the
position when the thing you're tracking is a continent moving a few
inches a year 
Juergen Nieveler
