[S-EM16] Electromagnetic Induction in the Earth and Planetary Interiors, and Tectono-Electromagnetism

[SEM16-05] A novel static shift correction method based on evaluation of statistical properties of MT responses

*Nagi Yamashita¹, Tada-nori Goto¹ (1.University of Hyogo)

Keywords:Magnetotelluric, Geostatistics, Variogram, static shift

In the geothermal energy usages, detailed understanding of spatial distributions such as fractures and reservoirs are required. The magnetotellurics (MT) is one of the geophysical methods used to estimate the underground structure around geothermal areas. In the MT method, the resistivity structure is estimated based on the measured apparent resistivity and phase (MT responses) for each frequency. The severe problem on the MT survey is the influence by small-scale inhomogeneities near the surface (so called "static distortion"). The correction methods of static distortion have been proposed by many authors. They are mainly based on parameter adjustments in the inversion procedures, rely other geophysical information not affected by the near-surface heterogeneities, or use some reference structures widely buried in the target area. However, few researchers mentioned the spatial characteristics of MT responses affected/not affected by static distortion, whose characteristics will be greatly useful for the correction of static distortions.
In this study, we discuss on the characteristics of the spatial distribution of MT responses on a simple three-dimensional (3D) resistivity structure, and also those affected by static shift. We investigated the spatial correlation of MT responses using a variogram, developed in the geostatistics. The degree of influence of static shift on MT response was also evaluated by the same method. As a result, we found a spatial range in which the apparent resistivity has a correlation. The range was greatly reduced when a static shift was given. In addition, we found that the range of the apparent resistivity is similar or wider than one of the phase. Based on this relationship, we developed a new method to correct the effect of static shift by appropriate spatial smoothing of the apparent resistivity. Our method was tested using 2D/3D numerical simulations. It is concluded that the accuracy of underground resistivity structure analysis was improved.