An experienced drill operator is able to make a log of a drill hole defining physical properties of the earth such as hardness or structure as they vary with depth. This log is based on intuition or how the drill feels'. Rock characterisation using measure while drilling (MWD) parameters aims at refining this intuition into a package that accurately defines the properties of the subsurface as the drill bit propagates. Subsurface investigation using MWD parameters aims to combine physical drill parameters: Pull down force, rotary speed, penetration rate, torque and vibration in such a way to yield real time rock properties. MWD is investigation in its most primitive form, put simply MWD is merely a physical probe feeling' what the rock mass is at depth.