Acoustic logging while drilling tool to determine bed boundaries

A method for geosteering a bottomhole assembly, comprising the steps of:

• transmitting acoustic signals into the formation;
• receiving reflected acoustic signals;
• generating an electrical signal representative of the received signals;
• correlating the transmitted and received reflected acoustic signals;
• determining the time lag for the received reflected acoustic signals;
• determining the formation speed of sound;
• identifying bed boundaries based upon the time lag determination and the speed of sound determination;
• and
• steering the bottomhole assembly based upon the identification of bed boundaries.

A method for automatically steering a drill bit which is drilling through a downhole formation, comprising the steps of:

• transmitting an acoustic signal into the formation;
• receiving reflected acoustic signals;
• cross-correlating said received reflected signals with the transmitted acoustic signals;
• determining a time lag for a bed boundary based upon the time difference between transmitting the acoustic signal and receiving the reflected acoustic signal;
• changing the direction in which said drill bit drills based upon the time lag signal.

A system for a bottomhole drilling assembly, comprising:

• a transmitter assembly for transmitting acoustic signals into a formation;
• a receiver assembly for detecting reflected acoustic signals;
• a digital signal processor receiving signals indicative of the transmitted acoustic signals and the received acoustic signals, said digital signal processor cross-correlating the received signal to obtain a cross-correlated product;
• and
• a microprocessor connected to said digital signal processor, said microprocessor analyzing said cross-correlated product to determine time lag for the received signals.

An acoustic logging while drilling system for determining bed boundaries in the vicinity of a borehole, comprising:

• a section of tubing;
• an acoustic transmitter assembly located in said tubing section for transmitting an acoustic signal into the formation ahead of the drill bit, said transmitter assembly including a first transmitter located on one face of the tubing section, and a second transmitter assembly located on an opposite face of said tubing section for acoustic steering in the plane perpendicular to the tubing axis;
• an acoustic receiver array located in said tubing section for receiving reflections of the acoustic signal, said receiver assembly including a first receiver and a second receiver and said first and second receivers are separated a predetermined distance;
• and
• a processor for performing geometric corrections on the reflections received by the acoustic receiver array based upon a time delay that is selected to investigate a particular volume cell external to the borehole.