Physical properties and micro- and macro-structures of gassy sediments in the inner shelf of SE Korea

Abstract

In order to characterize physical properties and sediment structure (micro- and macro structures) for gassy sediments from the southeastern inner shelf of Korea, physical property measurements and sediment structure analysis for two sediment cores were carried out. One core (SG12-PC4) was collected from gassy sediments, and the other core (SG12-PC10) was obtained from gas-free sediments. The micro- and macro-structures for the gassy core were investigated using X-ray computed tomography (CT) scanning and soft X-radiography. In addition, scanning electron microscope (SEM) photomicrographs were used for understanding the clay fabric in the gassy sediments. Wet bulk density and porosity for both gassy sediments and gas-free sediments are characterized by a general trend caused by compaction with burial depth. On the other hand, the velocity shows different patterns in both burial depth and the correlation between velocity and other physical properties. Thus, the gas plays an important role in the control of velocity in the gassy sediments. This is clearly seen in the relationship between characteristic CT images and low velocity in the depth intervals characterized by gas and/or degassing cracks. In addition, shear strength measurements for gassy sediments have lower values than gas-free sediments, due to the weak bond between clay particles caused by gas and/or degassing. This suggests that original clay microstructure could be rearranged due to forces that gas exerts on clay particles. From CT images of both cross section and top-views, we may estimate possible conduits of gas migration within the gas-charged interval. Using SEM photomicrographs, the various traces (e.g., displacement of clay particles, oval-shape depressions) created by the escape of gas bubbles were identified on the images. Therefore, the presence of gas in marine sediments may control not only the variation of physical properties (e.g., velocity, shear strength) but also the arrangement of clay particles.