Filippo Biondi

That's the Doppler effect.

So you use this physical effect in order to grab the information of the superficial vibration that is present always and constantly on the surface of the Earth.

Because the surface of the Earth is always vibrating.

Let's say, because the Earth is interacting with the...

background of earthquake noise always present that's always present then you have i don't know in 100 kilometers far away from where you are observing there is a huge city the huge city is is is in life so they everything vibrates and that vibrations are always present on the surface of that

Right.

So it's just the surface vibration.

The sound is not penetrating.

There's not sound going from the sky.

The electromagnetic waves that you are transmitting, that is light, that is not penetrating.

No.

It's only interacting with the superficial vibration of the Earth.

So you can understand the density of specific structures going down as far as a thousand meters, potentially, just based on the surface vibration of the Earth.

It's like when you are observing the superficial waves of a swimming pool.

The swimming pool, you notice that there are superficial waves.

Think about the superficial waves on the earth.

So the earth is like a swimming pool.

And because there is an abrupt variation of the density of the material, so the earth, and then there is the atmosphere, the air.

And there is an abrupt variation of the density.

And that is called a cut of density.