9:05 AM - 9:25 AM
[HTT17-02] Accumulation pattern of near-seafloor methane gas revealed by a shallow seismic survey in the northern Tokyo Bay
Keywords:shallow seismic survey, underwater speaker, environment-friendly, Tokyo bay, gas
Recently, activity of seismic survey is extremely reducing in shallow waters where marine traffic is busy and fishing activity is high. Accordingly, the shallow waters are becoming brank zones in seismic survey. For example, in the northern part of the Tokyo Bay, no seismic surveys have been conducted since in the late 2000s. Therefore, we have been developing a new seismic data acquisition system using an underwater speaker (UWS) as a seismic source available even in the extremely shallow waters. The UWS shoots a non-pulsive source wave with significantly smaller sound pressure level than explosive sources such as airgun guns, mitigating environmental impacts to marine ecosystems. This mitigation effect is similar with that of a vibroseis survey on land.
In December 2017, T/B Hiyodori of Tokyo University of Marine Science and Technology conducted a seismic survey using the USWS off the Urayasu in the inner part of the Tokyo Bay. As a result, a low-velocity zone was detected about 7 m below the seafloor, whose stacking velocity was estimated 1100 m/s from velocity analysis, suggesting the existence of gas. 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 possible local gas pool was found about 17-20 m below the seafloor, from its low-velocity and reverse reflection polarity. The local gas pool is laterally bounded by a fault. Interestingly, no low-velocity anomalies can be detected in the other side of the fault. The gas appears to be sealed laterally by the fault, like a so-called fault trap. If the fault is activated by an earthquake in future, the gas will be released from the seafloor into the water. Total volume of the gases is unknown at this moment. However, considering the high flammability of methane, potential risks from the methane gases might be assumed from a viewpoint of disasters on marine traffic and offshore structures. Further seismic surveys would unveil potential risks as well as potential resources in the shallow waters.
In December 2017, T/B Hiyodori of Tokyo University of Marine Science and Technology conducted a seismic survey using the USWS off the Urayasu in the inner part of the Tokyo Bay. As a result, a low-velocity zone was detected about 7 m below the seafloor, whose stacking velocity was estimated 1100 m/s from velocity analysis, suggesting the existence of gas. 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 possible local gas pool was found about 17-20 m below the seafloor, from its low-velocity and reverse reflection polarity. The local gas pool is laterally bounded by a fault. Interestingly, no low-velocity anomalies can be detected in the other side of the fault. The gas appears to be sealed laterally by the fault, like a so-called fault trap. If the fault is activated by an earthquake in future, the gas will be released from the seafloor into the water. Total volume of the gases is unknown at this moment. However, considering the high flammability of methane, potential risks from the methane gases might be assumed from a viewpoint of disasters on marine traffic and offshore structures. Further seismic surveys would unveil potential risks as well as potential resources in the shallow waters.