Acoustic Sensing with Azimuthally Distributed Transmitters and Receivers

A method comprising:

• transmitting an acoustic signal from a transmitter of a downhole tool positioned within a wellbore;
• and
• sensing an attribute of the acoustic signal with each of a plurality of receivers of the downhole tool, wherein the receivers include:
• a first receiver axially offset from and azimuthally aligned with the transmitter;
• a second receiver axially offset from and azimuthally aligned with the transmitter and the first receiver;
• a third receiver axially and azimuthally offset from the transmitter;
• and
• a fourth receiver axially and azimuthally offset from the transmitter, the first receiver, and the third receiver.

A method comprising:

• conveying a downhole tool within a wellbore, wherein the downhole tool comprises:
• a transmitter array comprising a plurality of transmitters azimuthally distributed around a longitudinal axis of the downhole tool at a first axial location of the downhole tool;
• a first receiver array comprising a plurality of first receivers azimuthally distributed around the longitudinal axis at a second axial location axially offset from the first axial location;
• and
• a second receiver array comprising a plurality of second receivers azimuthally distributed around the longitudinal axis at a third axial location axially offset from the first and second axial locations;
• transmitting a first acoustic signal from a first one of the transmitters;
• sensing an attribute of the first acoustic signal with at least first and second ones of the first receivers and at least first and second ones of the second receivers;
• evaluating a characteristic of a first region based on first signals generated by the at least first and second ones of the first receivers and the at least first and second ones of the second receivers, wherein the first signals are each indicative of the sensed attribute of the first acoustic signal;
• transmitting a second acoustic signal from a second one of the transmitters;
• sensing the attribute of the second acoustic signal with at least third and fourth ones of the first receivers and at least third and fourth ones of the second receivers;
• and
• evaluating a characteristic of a second region based on second signals generated by the at least third and fourth ones of the first receivers and the at least third and fourth ones of the second receivers, wherein the second signals are each indicative of the sensed attribute of the second acoustic signal;
• wherein the first and second regions are either:
• regions of cement substantially surrounding a casing within the wellbore;
• or
• regions of a subterranean formation into which the wellbore extends.