Seismic velocity decrement ratios for regions of partial melt in the lower mantle

For regions of partial melt in the lower mantle, both compressional and shear wave velocities decrease monotonically with increasing melt volume fraction. Those regions close to the core-mantle boundary thought to contain partial melt have a velocity decrement ratio (relative change in shear velocity over relative change of compressional velocity) of about 3. Arguments based on effective medium theories have been given to show that such high values of this ratio are predicted for partial melt systems. The present work confirms the value of 3 near the core-mantle boundary. It is shown, furthermore, that this velocity decrement ratio can be estimated without detailed knowledge of, or assumptions about, the microstructure of the partial melt system by using Gassmann's equation of poroelasticity together with some reasonable assumptions about the change (or lack of change) of the density and bulk modulus for lower mantle pressure and temperature conditions.