Question

why is the knowledge of Earth's interior based on and indirect observation

Answer 1

Easily explained, that is because as you come futher down towards the core of the earth, the temerature is rising, and there is no way possible for anyone to survive that far down. Of course we have tools to help us out, but even those won't make it that far down.

I hope it made sense and I'm sure someone can explain it in a more accurate and proffesional way :)

Answer 2

Answer...

The knowledge off earth's interior based on indirect observation like :

1.) the behaviour of seismic waves or earthquake.

2.) the occurrence of volcanoes.

Hope it helps...