We present evidence for approximate collocation of seismic and electrical transitions in the upper mantle. More than two years of very long period magnetotelluric \050MT\051 data were recorded at a lakebottom observatory in the central Canadian Shield. After processing to contend with non-stationary source effects, and removal of galvanic distortion, the underlying structure is 1D for periods of one hour to four days. The response was Sounding data to the MT curves. Minimum structure linearised inversion, nonlinear extremal inversion, and a new genetic algorithm for nonlinear hypothesis testing, reveal discrete jumps in conductivity at depths near the major upper mantle seismic discontinuities. The jumps occur over limited depth ranges
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