Example $18\mathrm{.}6$: A seismic refraction study of an area has given the following data:

$($a$)$ Plot the time travel data and determine the seismic velocity for the surface layer

and underlying layer.

$($b$)$ Determine the thickness of the upper layer.

$($c$)$ Using the seismic velocity information, give the probable earth materials in the two

layers.