Evolution of Porosity and Seismic Properties of Shallow Oceanic Crust

摘要

This study sought to use seismic measurements to infer porosities in order togain an understanding of crustal aging. Techniques used were development of the theory linking porosity to seismic velocity, microscope investigation of seafloor lavas, measurement of ultrasonic velocities in the laboratory, comparisons of theory with field measurements of seismic velocity, and field investigations of seafloor extrusives exposed on land. The project demonstrated that seismic velocities in the seafloor are strongly dependent on pore shape. Those velocities, and the nature of their increases with age and depth, were shown to be completely consistent with the concept of progressive sealing of cracks by alteration products. The project also showed how seismic measurements can be interpreted in terms of overall porosity and the distribution of that porosity over different pore shapes. Together with other ONR-supported crustal aging studies, the project confirmed the idea that the evolution of the oceanic crust is governed by its circulating fluids, making permeability, and hence porosity, key factors in controlling the state of the crust.