Visualization and Quantification of Three-Dimensional Porosity Variation and Anisotropy in Altered Igneous Rocks from Deep-Sea Drilling

High-resolution X-ray computed tomography (CT) maps the X-ray attenuation of solid objects in three dimensions. We have investigated its utility in analyzing ocean floor samples of diverse mineralogy, to identify structural relationships and compositional variations, and to characterize porosity and permeability. We examined half-round and minicore samples from the Manus Basin located in the Bismarck Sea that were obtained from Ocean Drilling Leg 193. The samples are mainly felsic volcanic rocks of rhyodacite composition with varying degrees sulfide minerals and alteration. Porosity analysis consist of CT scanning the cores first dry and then fully infiltrated with water. In-plane resolution for 25 mm minicore samples was 0.047 mm and between-slice resolution was 0.056 mm, although better resolution is possible with the equipment. The two data sets were then overlaid using a three-dimensional alignment algorithm and subtracted to determine the location of full and partial volumes of water in the sample. These images reveal interesting details of porosity structure, such as the fact that sulfide veins appear to have very low porosity and may be barriers to flow. Vesicle-filling materials also tend to have low porosity, but because vesicles are dispersed they are expected to have a less severe effect on permeability. Quantitative analysis of the differenced images by ascribing porosity values to grayscales results in porosity estimates that are very close to those obtained by weighing dry and infiltrated samples. The data can be used to quantify porosity homogeneity using a CDF curve of percent porosity on the 50-æm scale versus percent sample volume. These data can also be used to quantify anisotropy in porosity structure using a number of methods. These approaches promise to improve understanding of the link between porosity and permeability in complex lithologies, and in particular provide information on three-dimensional bulk flow properties that are difficult to obtain otherwise.