The volume concentration contained in the microcracks of the upper oceanic crust is examined in order to determine how porosity distribution changes as a function of depth. 58 cores recovered at increasing depths from the Deep Sea Drilling Project's Hole 504B were subjected to laboratory experiments in order to determine their compressional- and shear-wave velocities at a number of different uniform hydrostatic pressures. The inversion of the theory of Kuster and Toksoz (1974) computes the porosity contained in an entire spectrum of different size microcracks for each of the samples. It was determined that the increasing effects of pressure and alteration with depth effectively reduce the porosity contained in each size crack. A strong correlation between an abrupt increase in the degree of alteration and a large reduction in microcrack porosity concentrations was determined to occur at approximately 900 m below seafloor.