The difference is that sound waves can travel through the earth and reflect back again, while light waves can't. Another phenomenon you've probably experienced with light, other than reflection , is its refraction or bending. The appearance of bending is a result of the refraction of light waves. The light waves slow down and bend when they hit the water at an angle, causing a distortion of the subsurface image. This bending of the light is proportional to the velocity change the light experiences going though the boundary separating the two materials.
Sound waves behave in a similar fashion.
Unfortunately, the clocks slow down as they descend into a colder environment at the bottom of the ocean. The position, determined by GPS to within a few meters, and the clock's timing are of critical importance. Without the specified accuracy the velocity of the sound traveling from the guns through earth's crustal structure could not be determined. These velocities allow the geophysicist to draw a picture of the earth's structure. The recording sensors consist of three geophones sensing ground motion, and, one hydrphone, sensing water pressure.