N. Tada

1-D resistivity structures of the oceanic crust around the hydrothermal circulation system in the central Mariana Trough using Magnetometric Resistivity method

We present preliminary results of an electromagnetic survey over a region of hydrothermal circulation in the spreading axis of the central Mariana Trough. The Magnetometric Resistivity (MMR) method is used to determine the electrical resistivity structure of the oceanic crust around the hydrothermal system. 1-D electrical resistivity structure is calculated from the relation between the amplitude of magnetic field and source-receiver separation. The amplitudes of magnetic field variation of ocean bottom magnetometers (OBMs) placed on axis are larger than those of OBMs about 700 m away from the spreading axis, for source-receiver separations larger than 400 m. We estimated two resistivity structure models: one for the axial OBMs, and another for the off-axis OBMs. A region of lower resistivity between depths of 100–300 m is identified in the on-axis model. The hydrothermal source of the Alice Springs Field probably exists beneath the spreading axis, and the size of this source is smaller than 700 m. Lower resistivities at a depth of 50 m are identified in both the on-axis and the off-axis models, suggesting that the porosity in the uppermost oceanic crust is largest at this depth.