[S-EM15] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM15-P03] Appropriate configuration of resistivity models for marine magnetotelluric investigations: A case study on the Northwest Pacific plate

*Miku Sakurai¹, Tada-nori Goto¹ (1.University of Hyogo)

Keywords:the Pacific Plate, magnetotelluric, ocean-bottom electromagnetometers, skin depth

Marine magnetotelluric (MT) surveys are widely used for resource surveys and active fault surveys of the ocean crust. In this study, we measured electromagnetic data off the coast of Sanriku for one year and attempted to estimate the underground resistivity structure using the MT method. Water in rocks is involved in the occurrence of plate boundary earthquakes, and friction is influenced by the moisture from the subducting plate. Fujie et al. (2018) reported that the seismic wave velocity of the uppermost mantle of the Pacific plate decreases and the water content increases before subduction. MT surveys should reveal the influence of deep water. However, in order to obtain an accurate resistivity structure from the observation data, a certain level of accuracy is required for the calculation of the MT response using a numerical model. Currently, analysts must build an appropriate resistivity model through trial and error. Zmax must be large enough so that below Zmax either the secondary or total Ex or Hx field is negligibly small (Rodi, 1976), although no clear threshold exists. Here, we performed a one-dimensional forward analysis focusing on the skin depth and the bottom of the model (Zmax) using a one-dimensional aligned structure and a two-layer structure.
Therefore, in this study, we calculated how many times the bottom of the model (Zmax) is larger than skin depth. As a result, we found that the bottom surface of the model needs to be 80 times or more than the skin depth in order to obtain a calculation accuracy of the phase equivalent to the observation error.