Gravity: Problems with traditional techniques

Since mass anomalies at depth will create predictable variations in the gravity field observed at or above the surface, models of density structure are created that mimic or reproduce the observations of gravity.  While these models are non-unique, they can provide constraints on heterogeneity at depth.
            This traditional technique may fail to constrain the thickness of cratonal lithosphere, however.  The chemical depletion of mantle lithosphere (e.g. high Mg:Fe ratio) reduces its density.  Conductive cooling of this chemically buoyant material, however, increases the density of the root.   The isopycnic hypothesis for the state of cratonal lithosphere holds that at all depths the cold, Mg-rich lithosphere is in gravitational equilibrium with warm, primitive asthenosphere.  Therefore, a cratonal keel does not represent a mass anomaly that would produce a signal in the gravity field. 
Recent work calls the exact balance between thermal and compositional effects at all depths into question [Djomani et al., 2001], but the density contrasts at any given depth are small and the overall column of lithosphere is still in or near isostatic equilibrium with asthenosphere.