In less than 100 seconds, William Symes of Rice University in the US explains how geophysicists use sound to infer structures within the Earth. The basic idea is to fire sound waves into the Earth (or track natural seismic signals) and then measure the distribution of reflected waves at the Earth's surface and the time it took for them to return. This information can help geophysicists to identify features of interest within the planetary interior.

Of course, it is not quite as simple as that because sound varies in its speed within the Earth depending on the physical properties of the material through which it is passing. Symes explains that geophysicist have teamed up with mathematicians to develop computational models to help them to interpret their echoes.

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