A New Procedure for Quick Estimation of Porosity and Density of
Hydrate-bearing Sediments by Using Medical X-ray CT Scanner
Abstract
Gas hydrates have been reported to exist in marine sediments from
various parts of the world ocean. The hydrates start decomposing soon
after recovery of the sediments through coring operations due to changes
in ambient pressure and temperature. This decomposition leads to changes
in sedimentary structures, and thus complicates physical property
related measurements of the sediments by conventional methods. In this
study, we used a medical X-ray CT scanner to quickly scan the recovered
cores, and then used raw data from the CT, and thus avoided image
processing steps, to estimate porosity and density of the sediments. The
raw data were in terms of CT numbers, which were obtained by drawing a
circular region of interest (ROI) to cover most of the sediments visible
in a cross section XCT image of the sediments. The data were weighted
for relative contribution of liquid and solid in sediments before
estimating porosity. On the other hand, density was estimated by using
an average CT number that was automatically calculated by the Osirix
software used for drawing the ROI on an XCT image, and by using a
calibration equation based on a set of standards. Although some
uncertainty in estimation of relative volumes of solid, liquid and gas
could not be avoided, the results obtained by this new procedure were in
good agreement with those obtained by conventional methods. Since
porosity and density estimates by the new procedure can be made in a
matter of minutes after core recovery, it can guide progress of coring
operation and further processing of hydrate-bearing sediments.