Near seafloor gas accumulation observed by T/B Hiyodori in the northern part of Tokyo Bay

Abstract

Recently, activity of seismic survey is extremely reducing in shallow waters where marine traffic is busy and fishing activity is high. For example, in the northern part of the Tokyo Bay, no seismic surveys have been conducted since in the late 2000s. In December 2017, T/B Hiyodori of Tokyo University of Marine Science and Technology （TUMSAT）made a seismic survey using an underwater speaker, which is a non-explosive source with a relatively low environmental impact on marine ecosystems. As a result, a gas layer was detected as a low-velocity zone about 7 m below the seafloor. The subsequent investigations revealed that the gas was composed mainly of methane and the gas layer was widely distributed in the northern part of the Tokyo Bay. In addition to the widely distributed near-seafloor methane gas, a local accumulation of gas was observed as a so-called fault trap about 17 m below the seafloor. Total volume of the gases is unknown at this moment. However, considering the high flammability of methane, potential risks from the gases might be assumed from a viewpoint of disaster mitigation. Further seismic surveys are strongly required in the area.