Boosting amperage to trigger a relay
The line is too long for Ethernet function.  Nominally, Cat5 twisted pair
is good for 200 meters, about 650 feet.

1500 feet of cable, means that your thin wire has a significant resistance.
A typical 12V relay takes 120 mA of coil current, means about 100 ohms of
coil resistance.
The 3000 feet (1500 out, 1500 back) of #24 wire in a cat5 pair has 77 ohms of
resistance.  So, delivering 12V to the relay means you have to apply
$$12V \times 177/100  = 21V $$ at the base, in order to get the
right power in the woods.  It varies, of course, depending on the relay.  

Something similar to PoE (power over Ethernet) can safely supply regulated 12V with a buck converter
to drive several such relays.   It takes a safe power supply (48V max, 350 mA max), at the 'home' base, driving one pair, into the woods where a buck
converter regulates it down to 12V.   You'd get 5 watts or
so, and the relay coil uses less than a third of that.

I'm guessing uses for the extra 12V power can be found...