magnetotelluric and transient electromagnetic ... - Orkustofnun

Report 21 415 Wameyowhere μωf= 4π × 10 -7 (H/m);= 2πf;= Frequency (Hz),The high frequency in this case is such that the skin-depth is much less than the radius of the sphere.Figure 5c shows a crosssectioncutting an equatorialplane of the conductivesphere; at time t = 0 thecurrent is concentrated at thesurface layer of the sphere.However the amplitude of thecurrent commences todecrease due to ohmic lossesin the conductive spherematerial. The local magneticfield due to the current alsodecreases inducing electromagneticfield (throughFaraday’s Law) which causesnew current to flow as shownin Figure 5d. This processcontinues with the result thatthe current flow movesinward as time elapses. Thecurrents appear to bediffusing radially inwards asa result of the interaction withtheir magnetic fields in theconductive body. Thisperiod, when the currentdistribution is in motion, istermed as the ‘intermediatetimes’.During this period themagnetic field associatedwith the decreasing currentsdecays rapidly with time.ACEBDA stage is reached, however,where the relative spatialcurrent distribution becomesinvariant with time butdecreases at an equal rate,FIGURE 5: Target conductive sphere and current distributionwith the form indicated inFigure 5e. Near the sphere centre, the current density increases linearly with radial distance becomingrelatively uniformly distributed at one-half the radius and slightly decreasing towards the edge. Thisperiod is termed as the ‘late-times’. The inductance and resistance associated with each current ringhave stabilized and from this point onwards, both currents and their associated external magneticfields commence to decay exponentially with a time-constant given by (Geonics Ltd, 1980) as:2σμrτ = (11)2π