Relationships between porosity and shear modulus are applied to logs recorded in the Miocene Upper Terminal formation of the Wilmington Field, CA. to predict porosity from shear-wave velocity data recorded through casing using a dipole sonic logging tool. A relationship based on a modified Hashin-Shtrikman lower bound, appropriate for the unconsolidated nature of the formation, allows determination of porosity from the shear modulus. The results are similar to those obtained using standard log analysis in sands. Standard logs predict higher porosities in smectite-bearing wackes because of the inclusion in the standard analysis of bound water or micro-porosity in the clays which is not included in the porosity value derived from the shear-wave velocity.¿ 1997 American Geophysical Union