Development of vertical cable seismic (VCS) system for seafloor massive sulfide (SMS)

The Vertical Cable Seismic (VCS) is one of the reflection seismic methods. It uses hydrophone arrays vertically moored from the seafloor to record acoustic waves generated by sea surface, deep-towed or ocean bottom sources. This method is useful to delineate detailed structures in a spatially-limited area below seabed in the deep sea where conventional reflection methods are not effective. Since 2009, we have been successfully developing an integrated VSC system with financial support from the Japanese Government in a technology development program for the exploration of hydrothermal ore deposits such as Seafloor Massive Sulfide (SMS). Since 2010, we have made several surveys (both 2D and 3D) using the newly developed VCS system to demonstrate the effectiveness of this system in the areas where ore deposits are thought to be accumulated near hydrothermal vents around Japan in the western Pacific Ocean. Through these experiences, we recognize the importance of accuracy of the positioning of sources and receivers. The errors in the positioning sometimes deteriorate the quality in data processing. To overcome this problem, we have developed an inversion method to determine the most possible coordinates using the seismic wave traveltimes from the sources to the receivers. In addition we have developed data processing techniques to properly image the detailed subsurface structure. The 3D data volume of Prestack Depth Migration (PSDM) which was acquired in the Izena Cauldron in August 2013, successfully delineate the sub-seabed structures that suggest the existence of a body of hydrothermal ore deposits about 30m beneath the seabed.