Some aspects of sensitivity analysis in one dimensional magnetotelluric modelling

Sensitivity analysis for a certain class of 1 D MT modelling is done to highlight a few points of principle. The analysis reveals that (1) skin depth at crossover point frequency in 1D MT response (explained in the text) roughly estimates the top of the second layer for a certain class of three layer earth models; (2) the MT signal can feel the presence of an inhomogeneity when it la beyond the skin depth level; (3) a linear relation exists between the depth and thickness of the middle layer for some three layered H and K type earth models for the same percentage anomaly; (4) sensitivity drastically falls when the resistivity contrast exceeds 100; (5) the relation between thickness of the middle layer for a three layered H type model and the frequency at which the apparent resistivity becomes minimum la also linear; (6) the relation between thickness of the second layer for both K and H type models and the percentage anomaly at minimum or maximum frequency (explained in the text) la nonlinear; (7) the ratio of the wavelength at the perturbation centroid frequency (explained in the text) and the target bed thickness varies within a few orders of magnitude; (8) sensitivity of MT response for resolving a resistive bed in between two conductive bed becomes low, and vanishes for a certain class of models; (9) one or two kilometers of conductive overburden significantly reduces the sensitivity of the MT response from well inside the upper mantle up to 300 km to 500 kin depth where the olivine structure of the mantle silicates changes to different phases of the spinel structures.