CONSTRAINTS ON THE LARGE-SCALE POROSITY AND PERMEABILITY STRUCTURE OF YOUNG OCEANIC-CRUST FROM VELOCITY AND RESISTIVITY DATA

Geophysical measurements of a single physical property provide weak constraints on the large-scale porosity structure of the sea-floor. This limitation arises from fundamental principles which may be overcome by examing two independent pieces of information. As an example, electrical resistivity values and seismic compressional velocities from the East Pacific Rise at 13 degrees N are simultaneously compared to various combinations of empirical expressions. The best agreement between theory and the data sets is found using either the connected Hashin-Shtrikman bound or, equivalently, Archie's law with a low value of exponent to describe the resistivities, and Wyllie's time-average equation for the seismic velocities. The combination implicitly requires a connected fluid distribution and, hence, a highly permeable crust throughout the uppermost 1 km of sea-floor. Porosities in the surface regions remain poorly constrained. Assuming a fluid temperature below 15 degrees C predicts porosities of at least 20 per cent in the uppermost 50 m, decreasing to less than 1 per cent at depths around 1 km. Permeabilities of around 5 x 10(-12) m(2) are inferred for the surface regions decreasing to 3 x 10(-17) m(2) at depth. The method described should prove useful in constraining the structure in older sea-floor as more resistivity profiles are gathered along with seismic data.