Ultrasonic sensors are commonly used for non-destructive examination of metals and welds in the structural and mechanical engineering world (looking for defects). There is a vast, rich literature available on the subject.
Something similar called "pulse echo" testing is done to look for defects in cast-in-drilled-hole (CIDH) or drilled shaft concrete piles...tap on the top and look at the return signal.
I googled "sonic diagnosis of skeletal defects" and found an interesting paper about 40 years old on the use of ultrasonic sensors for examining bones (through soft tissue!); it mentioned which sensor they used, and it had an extensive bibliography. So no doubt there is a ton of information available about that application, too.
Suggest you hone your google skills.