Approximation Of Three - Dimensional Inhomogenjeities By Two - Dimensional Inhomogerelties In Magnetotelluhic Interpretation

The approximation of buried simple conductive three -dimensional (3 - D) inhomogeneities by two # dimensional (2 - D)inhomogeneities is investigated numerically using the Rayleigh-FfTmodeling technique. The technique is applicable to curvedinterfaces between regions of different conductivity.The effects of the increased aspect ratio of the 3 - Dmodels on the apparent resistivities, skew and tipper for theE - parallel and E - perpendicular modes are studied. Alsoinvestigated is the behaviour of these quantities when thecondition for electrodynamio similitude to exist between two modelsis satisfied. The results show that 3 - D E - perpendicular mode apparent resistivities and the 2 - D Transverse magnetic(TM) mode, values agree very closely.The 3 - D E - parallel mode apparent resistivities aresensitive to strike extent and frequency and approach the ? - DTransverse eleotric (TE) mode results as the aspect ratio isincreased. It is found that the ratio of the width of theinhomogeneity to the skin depth in the half-space is a veryimportant quantity in predicting the percentage difference betweenthe 2 - D and 3 - D values at a given aspect ratio. Very simpleguidelines for the above prediction are given for models havingconductivity contrast of 1 : 10 or its powers.The maximum skew values for the 3 - D models decreased byabout 60 percent for a change in aspect ratio from 2 to 4 and ..m remained constant when the aspect ratio is changed from 4 to 6. In general, as the aspect ratio is increased the tipperazimuths around the. middle of the inhomogeneities indicated thelong axes of the bodies.When the condition for similitude to exist betweentwo models is satisfied, it is found that the apparentresistivities are appropriately scaled while the skew values,tipper magnitudes and tipper azimuths are the same for bothmodels.|

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