A new seismic survey technology using underwater speaker detected a low-velocity zone near the seafloor: an implication of methane gas accumulation in Tokyo Bay

Abstract

Owing to the strict restrictions on the use of air guns in marine seismic surveys due to concerns about their potential impact on the marine ecosystem, there have been several cases where seismic surveys were not permitted. This tendency has been particularly significant in coastal waters where fishing activity is flourishing, which creates blank zones in seismic surveys. The authors, therefore, adopted underwater speakers as environment-friendly seismic sources that can be used under such restrictions. In December 2017, the applicability of underwater speakers as a seismic source was tested in a seismic reflection experiment in the northern part of Tokyo Bay. As a result, shallow subsurface structures were successfully imaged, and a low-velocity zone was detected 7–8 m below the seafloor. In this paper, the concept of environment-friendly seismic survey using underwater speakers is reported. In addition, the potential presence of a methane gas layer that was detected in the low-velocity zone is discussed. If the methane gas is widely distributed near the seafloor in the northern part of Tokyo Bay, a large amount of gas might be released into the water and then into the air when, for example, a large-scale earthquake occurs directly underneath the Tokyo Bay area. Given the high flammability of methane, the features and volume of its distribution must be precisely investigated from the perspective of earthquake occurrences in the metropolitan area.[Figure not available: see fulltext.].