perrykolb,

Blind drill to auger/rotary refusal - then take a rock core. Your drillers will thank you. :)

Jeff

Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com

Jeff, I think perrykolb means like a seismic refraction test to determine distance from ground surface to top of rock.

Electrical resistivity testing may be a good way to go about it too.  It's usually cheaper and easier.

perrykolb,
A good deal depends on how much information you have in the area and how critical the knowledge is.
For shallow depths and good access, auger drilling is often the cheapest and best.  

Electrical Resistivity (my favorite) and seismic techniques need calibration, usually from drill holes or excavations. Even then, these are indirect methods. I have 2, very spectacular embarassments which I could expound upon. The electrical/seismic soundings are probably telling you most everything you need to know, but we are probably not smart enough or experienced enough to properly interpret the numbers or squiggles.

My 'rule of thumb' criteria is always a boring/coring and 1 boring/coring for every 4 electrical/seismic soundings.

Good advice emmgjld - many forget that there is a need for calibration!  Also, the method to use depends on the depth to the bedrock - rock high up will require very closely spaced phones; lower rock - you can spread them out.