NOTE:

Two-dimensional gravity modeling of subsurface mass distributions. Bodies
of anomalous mass are polygonal in cross section, maintaining their shapes
in infinity in direction in and out of the page.

a) Relative to surrounding material, a body with excess mass
results in a positive contribution to the gravity anomaly
profile (delta g).
b) A negative contribution results from a body with a deficiency
of mass.
c) The gravity anomaly for the sum of the contributions shown in
(a) and (b).

A crustal root, having a mass deficiency due to its low density, would
show a negative contribution of the gravity anomaly profile.