Ultrasonics and Seismology Ultrasonics works because sound travels at different speeds through materials of different densities and rigidities. The same is true within the Earth: if there is a disturbance which sets the Earth jiggling at some location, the sound waves (or vibrations) will pass through the Earth itself and may be felt, and interpreted, some distance away. This is commonly used by geophysicists when they set off small explosions (sticks of dynamite buried in the ground) and use a network of nearby detectors to determine whether, say, the subterranean geology is likely to be housing an oil deposit. If we want to study the deep interior of the Earth, right down to the core, we will obviously need to give the Earth a pretty good jolt to start with, and then analyse the vibrations using an far-flung international network of detecting instruments. How
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