Semblance processing for an acoustic measurement-while-drilling system for imaging of formation boundaries

A method of determining the position of a bed boundary during the drilling of a wellbore, comprising:

• (a) using a transmitter on a downhole tool to transmit acoustic signals at at least one downhole location;
• (b) using a plurality of spaced apart receivers on the downhole tool to receive acoustic signals, said received signals including reflected signals from the bed boundary and unwanted signals traveling from the transmitter to the plurality of receivers;
• (c) using a processor for prefiltering the received signals based on a threshold determination and selectively attenuating the unwanted signals, giving a plurality of prefiltered signals;
• and
• (d) using the processor for determining a relative position and orientation of the bed boundary with respect to the downhole tool from the plurality of prefiltered signals.

A method of determining the position of a bed boundary during the drilling of a wellbore, comprising:

• (a) using a transmitter on a downhole tool to transmit acoustic signals at a plurality of downhole locations of the tool;
• (b) using a plurality of spaced apart receivers on the downhole tool to receive acoustic signals reflected by the bed boundary at each of said plurality of spaced apart receivers at each of said plurality of downhole locations;
• and
• (c) using a processor at each of said plurality of downhole locations to:
• (i) prefiltering the received signals to attenuate an unwanted portion of the acoustic signals traveling from the transmitter to the receiver;
• (ii) determining semblances of the signals received by the plurality of receivers in a slowness-minimum offset time domain,
• (ii) filtering the determined semblances to give a set of filtered semblances,
• (iii) determining from said set of filtered semblances and a velocity of the acoustic signals, a relative position and orientation of the bed boundary with respect to the tool, and
• (d) determining an absolute position and depth of the boundary from said relative positions and orientations and from supplemental information.