Bioassay-guided fractionation of a methanol extract of the brown alga, Ishige sinicola, led to the isolation of five algicidal compounds. Their structures were determined to be α-monoglycerides of eicosa-5Z,8Z,11Z,14Z-tetraenoic (arachidonic) acid, octadeca-6Z,9Z,12Z,15Z-tetraenoic acid, linoleic acid and oleic acid, and 1-O-palmitoyl-3-O-(6-sulfo-α-D-quinovopyranosyl)-sn-glycerol on the basis of spectroscopic data and a comparison with the data in the literature. These glycerolipids showed moderate-to-high cell lysis activity against the red tide microalgal species, Heterosigma akashiwo, Karenia mikimotoi and Alexandrium catenella, at a concentration of 20 µg/mL.
Gas hydrates are located in the permafrost and in deepwater shallow sediments, where low temperature and high pressure satisfy the stability conditions of methane clathrates to remain as solid compounds. Hydrates are found in a form of fine-layered or altered-layered structure with hiatuses and necessitate high-resolution surveys, which may not be achieved by conventional marine acquisition using towed streamers. We have developed a recent case study in which the vertical cable seismic (VCS) method has been used for high-resolution subseafloor imaging using a set of buoyed vertical-arrayed receivers that are anchored to the seafloor. The observation close to the target in the deepwater environment provides a higher signal-to-noise ratio and higher resolution. The primary reflections, however, could not achieve reliable depth images in the data processing due to their limited subsurface coverage. We used a reverse time migration (RTM) implementation of mirror imaging to extend the spatial subsurface coverage by using receiver ghost reflections. Because conventional velocity analysis methods are not applicable to the VCS survey due to the asymmetrical reflection path between the source and receiver, we implemented seismic interferometry and generated virtual surface seismic data from VCS data for velocity analysis. To preserve the resolution, amplitudes, and phase characteristics, we applied mirror RTM on the ghost reflections in the original VCS data rather than imaging the virtual data. The introduced case study using a VCS survey for identifying the methane hydrate system of the Umitaka Spur in the Sea of Japan led to high-resolution images, which suggest that a large gas chimney exists beneath a pockmark and is responsible for transferring methane gas from a deep hydrocarbon source to the shallow sediments. A bottom-simulating reflector as the base of the gas hydrate stability zone was also imaged.
In the Seafloor Massive Sulfide (SMS) exploration, the buried sulfide deposits without hydrothermal activity are important for mining development. We had proposed Vertical Cable Seismic (VCS) for exploration of SMS and found out it could reveal the buried deposit through surveys. The conventional VCS data processing gave us the large-scale 3D structure. We re-examined the data processing to focus on the very shallow part so that the detailed structure of the buried sulfide deposit could be recognized. We adopted the Three-dimensional Pre-Stack Depth Migration (3D-PSDM) method that did not assume a horizontal structure since the hydrothermal area has a complex structure. In order to focus on the shallow part, we used a high-density gridded velocity model to achieve high accurate travel time calculation. Grid spacing is about one-tenth shorter than that of normal oil/gas exploration case. Velocity model is not complex but a two-layer model consisted of the seawater and sub-seafloor. The water velocity is adjusted to match the migrated ocean bottom reflection to ship-born multi-beam bathymetry.The first VCS survey with GI-gun in 2011 and the second one with high-voltage sparker in 2013 in Izena Hole, Okinawa Trough, gave the high-resolution 3D PSDM volumes of 500m x 200m x 200m and 400m x 200m x 200m respectively. These results successfully delineated the top interfaces of the buried sulfide ore deposit that are consistent with the geological model provided by Japan Oil, Gas and Metals National Corporation (JOGMEC) using the core data of their intensive drilling. In addition, the existence of the high velocity zone was suggested beneath the seafloor by the estimation of velocity structure by Common Reflection Point (CRP) gather analysis. We consider the high velocity anomaly implies the existence of buried sulfide ore deposit.These results suggest that VCS can reveal the distribution of buried sulfide ore deposits. We are developing a detailed velocity analysis method to reveal the velocity anomaly caused by the buried deposits. Furthermore, it is important to estimate other geophysical properties using EM, magnetic and gravity surveys. To confirm the distribution of the buried deposits represented by the VCS data, it is necessary to compare the VCS results and drilling cores by JOGMEC directly.
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