I am trying to find out what formula to use for the above issue. If I do a gravity measurement above a volcanic chamber, what do I have to do? I thought about adding the Bouger corrections, as it is a mountain I am measuring on, but I do not know the density of the mountain, only for the bedrock around the crater. How do I do this, then? I found something about "buried bodies", but I do not know if you can name a chamber (="hole") a buried body. Can someone help out?
As I recall you calculate the (repulsive) force from a body of negative matter the same size as the hole, then you add that to the Earth's gravity with no hole. The result is to reduce the Earth's gravity slightly.