Seismic velocity decrement ratios for regions of partial melt in the lowermantle

For regions of partial melt in the lower mantle, both compressional and she ar wave velocities decrease monotonically with increasing melt volume fract ion. Those regions close to the core-mantle boundary thought to contain par tial melt have a velocity decrement ratio (relative change in shear velocit y over relative change of compressional velocity) of about 3. Arguments bas ed on effective medium theories have been given to show that such high valu es of this ratio are predicted for partial melt systems. The present work c onfirms the value of 3 near the core-mantle boundary. It is shown, furtherm ore, that this velocity decrement ratio can be estimated without detailed k nowledge of, or assumptions about, the microstructure of the partial melt s ystem by using Gassmann's equation of poroelasticity together with some rea sonable assumptions about the change (or lack of change) of the density and bulk modulus for lower mantle pressure and temperature conditions.