動的スワス重合と深度可変広帯域通過フィルタによる水底下地層構造探査技術の開発―低周波広帯域音源を用いた有効性評価―

抄録

Recently, for shallow water civil engineering projects in rivers and harbors, and for offshore resource exploration on the deep-sea floor, it has become essential to develop a highly accurate structural investigation technology that can scan to depths of about tens meters under the bottom of the sea. In this paper, we present two new signal analysis methods for accurate sub-bottom profiling: dynamic swath signal stacking in an acoustic near-field, and the use of a depth-varied broad bandpass filter to clarify the stratal architecture under the sea floor. These methods are applicable to data obtained by measuring at a low altitude using a low-frequency broadband transducer. In the present research, we developed a prototype low-frequency broadband transducer and carried out a performance test for a 3-D exploration system in a very shallow area of the Hitachi-Tonegawa River in 2009. By applying the first proposed technique to the acquired data, we confirmed that these prototype systems and signal analysis methods are capable of clarifying the depositional-structure under the riverbed up to a depth of 10 m. Moreover, we explored just below the sea floor using an acoustic source fitted with JAMSTEC's Deep-Tow and Natsushima equipment as a single-channel sub-bottom profiler that was operated at low altitude in deep water in 2011. By applying the second proposed technique to the acquired data, we confirmed that our prototype exploration system and analysis methods are capable of clarifying stratal structures at depths of up to 60 m under the sea floor.