Identifying bottom hole rock properties while drilling

Acoustic signals generated by drilling a bore hole with a drill bit 340 are received at sensors 310, 320, 330 placed in the drill string. The acoustic signals received by the sensors are processed by a processor 350, e.g. using a Fourier transform, and a characteristic of the acoustic signals is used to identify lithology, rock strength, presence of oil, presence of gas, change in lithology, or the like.