Method and apparatus for enhanced acoustic mud pulse telemetry during underbalanced drilling

A borehole communication system for telemetry through a compressible drilling fluid comprising:

• a drilling fluid source configured to supply drilling fluid under pressure through a conduit towards a drill bit;
• a gas inlet in fluid communication with the conduit configured to supply gas into the drilling fluid thereby rendering the drilling fluid downstream of the inlet compressible;
• a pulser in the borehole configured to generate pressure pulses in the compressible drilling fluid corresponding to a predetermined pattern;
• a reflector positioned downstream from the gas inlet dimensioned so as to cause in response to an incident pressure wave travelling from the pulser towards the surface, a reflected pressure wave having the same pressure polarity as the incident pressure wave;
• and
• a pressure sensor positioned downstream of the reflector adapted to sense pressure in the compressible drilling fluid and generate electrical signals corresponding to the sensed pressure.