3D Seismic Velocity Structure below Mounds and Pockmarks in the Deep Water Southwest of the Sado Island

Abstract

The METI（Ministry of Economics, Trade and Industry）Sado-oki Nansei 3D seismic survey was carried out in deep water southwest off Sado Island, Japan Sea. The survey area covered the Umitaka Spur, which features mounds and pockmarks. Other surveys sampled large masses of methane hydrate beneath the sea floor, discovered methane bubble plumes rising into the water, and investigated high resistivity anomalies below the sea floor. We applied 3D pre-stack time migration and continuous velocity analysis method to 3D seismic data, and investigated the geological and P-wave velocity structure below the sea floor of the Umitaka Spur and its surrounding area. This revealed high velocity anomalies, suggesting the methane hydrate occurrence below mounds and pockmarks. On the other hand, P-wave velocities below the sea floor in areas of the Umitaka Spur were lower than propagated in water layers. Therefore, it was suggested that concentrations of methane hydrates were limited below mounds and pockmarks, although some gas may be contained in low-velocity zones. The situation may be caused by the localization of methane supplies from deep layers.

Structural-stratigraphic control on the Umitaka Spur gas hydrates of Joetsu Basin in the eastern margin of Japan Sea

“…Extended studies on gas hydrate issues such as geochemistry of sediments (e.g., Freire et al, 2009), sea water chemistry at plume sites (Ishizaki, 2008), porewater chemistry Tomaru et al, 2007), methanerelated carbonates (Matsumoto et al, 2009a), seafloor topography (Matsumoto, 2005), and acoustic and seismic surveys (Aoyama and Matsumoto, 2009;Saeki et al, 2009) conducted in the last few years have provided much information about this gas hydrate area.…”

“…A 3-D seismic survey was conducted by the Ministry of Economy, Trade and Industry (METI) in 2003 (Saeki et al, 2009), and two exploratory wells were drilled in 2004 on the spur to confirm the petroleum system (Okui et al, 2008;Son et al, 2001).…”

Physical and thermal properties of mud-dominant sediment from the Joetsu Basin in the eastern margin of the Japan Sea

“…Mounds and pockmarks, 50-100 m in diameter, show a linear distribution pattern that is parallel to the axis on the top of the Umitaka Spur and the Joetsu Knoll Nakajima et al 2014). Two-and three-dimensional seismic data obtained from the Umitaka Spur identified seismic blanking that obscured reflectors beneath the mounds and pockmarks, suggesting subsurface gas migration from deep (Saeki et al 2009;Freire et al 2011). Active cold-seep sites with biological communities were discovered on several mounds at the summit areas of the Umitaka Spur and the Joetsu Knoll.…”

Influence of the carbon isotopic composition of methane and the proportion of methane-derived bicarbonate on the 13C/12C ratio of dissolved inorganic carbon at the sulfate–methane transition in the Joetsu Basin area, eastern margin of the Sea of Japan

“…The seafloor topography consists of undulating mounds (less thañ 200 m in diameter and 20 m in height), inactive pockmarks (~200e500 m in diameter and 10e20 m deep) distributed linearly, and a southern pinnacle (~20 m high). In total, five seismic blanking zones have been recognized under the two gas plume sites, the largest pockmark, and the southern pinnacle on the spur (Freire et al, 2011;Saeki et al, 2009). Three gas seep sites have been recognized at the Joetsu Knoll (Matsumoto et al, 2011) (Fig.…”

“…By comparing d 13 C DIC at the SMT and d 13 C of deep methane, we distinguish lower values of d 13 C at the SMT due to either the anaerobic oxidation of methane with lower d 13 C or to a higher proportion of methane-derived HCO 3 À at the SMT at a gas hydrate region in the Sea of Japan where seafloor gas hydrates and seismic blanking are distributed regionally (Freire et al, 2011;Hiruta et al, 2009;Matsumoto et al, 2011;Nakajima et al, 2014;Saeki et al, 2009;Snyder et al, 2007;Tomaru et al, 2007). In order to evaluate the data from piston core samples, the depth of no sulfate (DNS) was calculated from the SO 4 2À depth profile to estimate the SMT.…”