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From the literature, it says that we can know what is beneath a site (say for prospecting for oil. metals, etc) from measuring the local "g" field/acceleration at various locations. Together with other information, we can know the mass distributions at the site below.

But according to gravity theory, the gravitational field and magnitude just above the surface of the Earth would be always pointing directly towards the center of mass of the earth. We can assume that this CM relative to a location is a constant in general if we take into account purely gravitational forces between an object and the earth.

Small g is a constant at a location depending on mass m of an object, mass M of the Earth and the distance r from the Earth's CM. It is a constant in general.