When oil companies hunt for ‘black gold’, what they are really looking for is underground rock formations hinting at the presence of oil. One common method is to fire sound waves (lower frequencies than ultrasound) into the ground.
When these waves encounter the interface between two layers of rock, they either keep going deeper or bounce back like echoes depending on the type of rock.
Microphones record the resulting pattern, allowing geophysicists to build up a picture of the rocks underground. Ultrasound doesn’t penetrate the rock deeply enough to do this, but it is often used to image borehole walls later on.
Answered by Alexandra Cheung