Exploration depth and resolution of controlled-source RMT with a long wire source: a sensitivity analysis

Abstract Controlled-source radiomagnetotellurics (CSRMT) is a widely used geophysical method for shallow exploration. In this study, we investigate the effective exploration depth and model resolution of electromagnetic field components for CSRMT with respect to resistivity, permittivity and magnetic permeability by analyzing the one-dimensional (1D) sensitivity matrix on the condition of fixed source-receiver distance. The study reveals that, sensitivity and effective exploration depth of Hy and Hz for three electrical parameters are poorer compared to Ex calculated on the same frequency. For resistivity, sensitivity and effective exploration depth of all field components at near surface can be refined when the displacement current is considered. Hy and Hz lack exploration ability in near-field zone and the effective exploration depth of Ex in near-field zone approaches a fixed value. For magnetic permeability, effective exploration depth of both Hy and Hz in near-field zone also tend towards fixed values. Under the condition of far-field zone, effective exploration depth increases with increasing formation resistivity or permittivity at the same frequency, while it decreases with increasing formation magnetic permeability. Analysis of model resolution indicates that, resistivity resolution at near surface decreases as the formation resistivity increases. The permittivity resolution initially rises and then declines with increasing depth. And permittivity resolution in various formations is significantly different at near surface, as an increase in depth, effect of displacement current weakens and resolution changes less significantly with variable permittivity. Effect of magnetic permeability changing on the resolution of magnetic field is more significant than that on electric field.