Application of sensitivity patterns to inversion of magnetotelluric field data in Utah for selecting optimal input

Many different magnetotelluric response functions such as the impedance and tipper which are converted from measured electromagnetic fields to remove the effects of natural sources have been proposed and studied. To obtain good inversion results, it is important to know that the inversion results depend on the sensitivity patterns of the magnetotelluric response functions used as the input, indicating parts to which the magnetotelluric responses are sensitive. We introduce how to use the sensitivity patterns to determine optimal input magnetotelluric response functions for interpreting field data. For magnetotelluric field data, we use the data acquired at the Utah Frontier Observatory for Research in Geothermal Energy survey area. The target structure in the Utah Frontier Observatory for Research in Geothermal Energy survey area is associated with geothermal energy, and most of the sites are located to the left of the target structure. Examining the sensitivity patterns of the major magnetotelluric response functions, that is the components of the impedance tensor and tipper vector, for the target anomaly, the pattern of y-component of the tipper vector (Ty${T_y}$) covers the areas where most of the sites are distributed. This means information about the target structure is mainly contained in Ty${T_y}$. To investigate the impact of Ty${T_y}$ on inversion of the target anomalous body, we compare the inversion results for four cases that (1) the xy- and yx-components of the impedance tensor (Zxy${Z_{xy}}$ and Zyx${Z_{yx}}$) are used; (2) Zxy${Z_{xy}}$, Zyx${Z_{yx}}$ and Ty${T_y}$ are used; (3) Zxy${Z_{xy}}$, Zyx${Z_{yx}}$ and x-component of the tipper vector (Tx${T_x}$) are applied; and (4) the all components of the impedance tensor (Zxx${Z_{xx}}$, Zxy${Z_{xy}}$, Zyx${Z_{yx}}$ and Zyy${Z_{yy}}$) are used as the input. This comparison shows that Ty${T_y}$ selected as the optimal input from the sensitivity patterns for this case study contributes to retrieving the target anomaly. Through the case study for the Utah Frontier Observatory for Research in Geothermal Energy site, we show how to design a strategy of selecting optimal magnetotelluric response functions based on the sensitivity patterns to effectively image target structures. Our experiment supports that the sensitivity patterns can be used to increase the reliability of magnetotelluric inversion.