抄録
Marine electromagnetic (EM) sounding methods were developed originally for imaging the deep mantle, partial melt below mid-oceanic ridges, active faults in the crust around subduction zones, etc. Recently, marine EM sounding has been applied extensively for oil, gas, and methane hydrate exploration. In this paper, we introduce various marine EM sounding methods and case studies. Natural fluctuations of EM fields from the ionosphere can be observed on the seafloor using ocean-bottom electromagnetometers (OBEMs), and the marine magnetotelluric sounding method can be applied for imaging the deep structure (> several km below the seafloor). However, the natural signal is less powerful in the higher frequency band and provides less resolution for near-seafloor structures. A high-frequency controlled electromagnetic source on/near/below the seafloor is necessary for evaluating the shallow structure. Marine controlled-source EM (CSEM) sounding using a horizontal electric dipole, towed near the seafloor and sending an electric signal, and OBEMs as receivers settled on the seafloor is widely used for oil and gas exploration, and occasionally for methane hydrate explorations. A vertical electric dipole is also used for magnetometric resistivity (MMR) sounding when exploring hydrothermal systems around ridges. On the other hand, both sources and receivers attached to a cable can be towed near the seafloor for a continuous survey. Towed-CSEM sounding with a time-domain analysis is used especially for methane hydrate exploration. Another type of towed-CSEM with magnetic signal obtains porosity information on near-seafloor sediments. A deep-towed marine DC resistivity survey has been applied near the seafloor, and successfully imaged shallow methane hydrate distributions in sedimentary layers. Thus, various marine EM soundings can illuminate sub-seafloor methane hydrate, and are useful for discussing the accumulation/dissolution process of methane hydrate and the potential of methane hydrate as a new energy resource.
