The chart in the following link may help you it shows the temps at about 6 feet down
http://www.earthrivergeo.com/geothermal-hvac-loop-systems-information.php.
But should you go with a bore hole or more than one you can check the temps very easy with a laser temperature gun
Assuming a dry bore take readings at each 5 to 10 feet depths once the readings become the same as the previous you will have reached the depth of constant ground temperature to a certain degree of accuracy.
If you can have the bores made about 8" in diameter you could run 2 or possibly as many as 4 pairs of tubes but the efficiency of thermal transference will diminish with too many pairs in the same borehole.
Some recommend back filling with 1/4 to 3/8 pea gravel to near the top of the bore then finishing off with a grout that meets local code,where the bore is down to the water table to enhance thermal conductivity.
IMO' this can be both good and bad . Should the water table go down the gravel will offer very little thermal transference.
What little on the subject that was covered back when I was a college kid the most reasonable argument I remember was in the event a borehole penetrated an aquifer not just near surface ground water the bore should be filled with gravel only just a little above the lowest expected water level then several feet of sand then filled with Bentonite or thermally-enhanced grout
A bore where there is nothing but rock will require a thermally-enhanced grout for the entire depth of the bore.
But the driller or the installer should know what is required.
Also your county extension agent will or should have an archive of information.