Acoustic data link and formation property sensor for downhole MWD system

A system is disclosed for transmitting and receiving acoustic data signals in a well containing a drill string. The system includes devices for transmitting acoustic signals through the drill string, drilling mud, and formation, and further includes methods for transmitting and interpreting the acoustic signal so as to maximize accuracy of the transmission. The methods of the present invention include correlating signals transmitted along different paths or paths of different lengths, using frequency shift keying transmission, using shear waves to transmit signals through downhole equipment and using compression waves to transmit signals through the mud. The signals further give information about the frequency dependence of formation speed of sound and formation acoustic attenuation. The method also give information for imaging the locations of reflective boundaries in the material surrounding the borehole. The system offers the advantage to the driller of receiving essentially real time information about properties of the formation surrounding the bit.