Question

What tool have scientists used to help make inferences about the interior structure of earth?

Answer 1

Primarily sound waves.

Explanation:

Material samples from deep wells and volcanic eruptions provide some physical clues for the mantle. For the deep interior the primary method has been sonic waves – some recorded from natural events like earthquakes, and others generated intentionally at different points.

The different rates of sound transmission in different materials (including reflections) can be used to “map” different regions of the planet interior by mass, material properties and temperatures.
Also see:
http://pubs.usgs.gov/gip/interior/