Vertical Cable Seismic (VCS) Survey for Buried Hydrothermal Sulfide Deposit

Abstract

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.

Reverse Time Migration of Vertical Cable Seismic Data to Image Hydrate-Bearing Sediments With High Resolution

“…In addition, we establish the reference velocity model by accurate velocity analysis on common scatter point (CSP) gathers. In practice, constructing CSP gathers from common receiver gathers is an initial process of Kirchhoff pre-stack migration (Bancroft and Xu, 1999). By this processing, we can address large elevation changes and the travel times for the source and receiver ray paths can be computed to the actual locations.…”

Hydrate-Bearing Sediment Imaging of Ghost Reflection in Vertical Cable Seismic Data Using Seismic Interferometry

“…Derived from defence research, the VCS method was successfully applied to conduct offshore seismic survey, for highquality subsalt imaging [3,7], for low-angle fault scarp imag-ing and assessment of remaining exploration potential [8], for hydrothermal deposit exploration [5], for seafloor massive sulphide survey [9][10][11][12], and for natural gas hydrate exploration [4,[13][14][15]. All these applications reveal that the VCS technique is an effective approach to exploiting and developing the complex offshore oil and gas fields, which can provide superior seismic image compared with conventional marine seismic method.…”

The Sensitive Properties of Hydrate Reservoirs Based on Seismic Stereoscopic Detection Technology

“…In 2010, the successful development of an autonomous VC detection system with a detection depth of 100-2000 m further advanced the applicability of VC data processing technology. Then, a joint VC and submarine source was first applied in 2013 to conduct 3D seismic exploration within the Okinawa Trough to investigate vulcanization, thereby providing valuable seismic data for structural studies of mineral deposits (Asakawa et al, 2012;.…”