Evaluation of rock boundaries and acoustic velocities using drill bit sound during vertical drilling

A method comprising:

• producing, by a well tool drilling a wellbore in a subterranean formation, an acoustic signal responsive to the drilling, wherein a first portion of the acoustic signal is reflected off a first subterranean formation boundary downhole of the well tool and a second portion of the acoustic signal is reflected off a second subterranean formation boundary downhole of the first subterranean formation boundary;
• pausing the drilling after producing the acoustic signal;
• receiving, after the pausing and at the well tool, the first portion of the acoustic signal and the second portion of the acoustic signal;
• and
• determining properties of the subterranean formation based on the first portion of the acoustic signal and the second portion of the acoustic signal.

A system comprising:

• a wellbore drilling tool configured to receive a plurality of acoustic signals produced in response to the wellbore drilling tool drilling a wellbore in a subterranean formation, the plurality of acoustic signals including a first reflected acoustic signal reflected off a first subterranean formation boundary downhole of the wellbore drilling tool and a second reflected acoustic signal reflected off a second subterranean formation boundary downhole of the first subterranean formation boundary responsive to a drilling sound produced during drilling the wellbore, at least a portion of the plurality of acoustic signals received after a pause in drilling the wellbore after the drilling sound is produced;
• one or more computer systems in communication with the wellbore drilling tool;
• and
• a computer-readable medium storing instructions executable by the one or more computer systems to perform operations comprising:
• determining properties of the subterranean formation based on the plurality of reflected acoustic signals.