There are many different scientific instruments to study the surface and interior of the earth like a bathymeter which is used to measure contours of deep oceans and a clinometer which is used to measure slopes and elevations.
Seismic waves can also be used to find interior of earth. Three types of wave travel away from the focus of seismic event these waves travel at different speeds. In addition of surface waves, the waves that are proving critical to providing evidence of the interior of the earth are 'p' waves(p for primary) and s for 's' waves(s for secondary). p waves are faster than the s waves so p waves can travel through solid rocks and fluids and their speed will vary depending on the density of rock through which they travel, increased speed increase density, thus providing information 
about the density.
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Scientists get information about the earths interior by tracking seismic waves, and analyzing rock and minerals samples and scientist also study seismic activity.

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