Advances in Vertical Cable Seismic (VCS) for Seafloor Massive Sulfide exploration
Abstract:In 2014, the Japanese government started the Cross-ministerial Strategic Innovation Promotion Program (SIP), which includes ‘Next-generation Ocean Resource Exploration Techniques’ as an area of interest. J-MARES aims to establish “Multi-stage and integrated approach for SMSs exploration” using effectual geophysical exploration method and tools.
JGI proposed the Vertical Cable Seismic (VCS) technique which is a reflection seismic method that uses hydrophone arrays vertically moored from the seafloor. It is useful to delineate detailed structures in a spatially-limited efficiently. We have developed autonomous VCS systems and carried out several VCS surveys in actual hydrothermal area in Okinawa Trough. These results successfully delineated sub-seabed structures that suggest the existence of buried SMS deposits. Based on the successful results of these surveys, we are continuing to polish up the VCS system with data processing methods.
To obtain more detailed structure, we have manufactured four new-type of VCS with 16 hydrophones. Then we carry out the VCS survey using deep-tow high frequency source. The key points are (1) a high-frequency source close to the target, (2)efficiency of the surveys and (3)wide-angle reflections to detect of bottom interface of sulfide ore body. The most crucial technical issue is the positioning the deep-tow source.
As for the data processing, we have applied Prestack Depth Migration to obtain the subsurface structure but the velocity cannot be estimated adequately. We adopt CSP-EOM processing to VCS data. It is based on scattering phenomena which is useful for the scattering dominant area such as SMS area. This method gives us the velocity estimation of the SMS.
We consider VCS has high-potential for SMS exploration. The system will continue to be improved as part of the SIP project, along with other geophysical exploration techniques such as EM, magnetic and gravity.