A typical electromagnetic (EM) analog modeling apparatus consists of an electrolytic tank with embedded graphite blocks, representing conductivity anomalies. A plane wave magnetotelluric (MT) source is generated by alternating currents in a set of parallel horizontal overhead wires. A uniform horizontal magnetic field is produced over the surface of the electrolyte.A similar uniform field may also be generated by grounded semiinfinite vertical wires. Four such wires, two carrying current upward and two downward, when arranged at the corners of a rectangle of defined dimensions, generate a more uniform field than a corresponding pair of Helmholtz coils. If the size of the rectangle is large compared with a skin depth in the electrolyte, Cagniard's MT relationships are obeyed both on and beneath the electrolyte. The vertical current source has the advantage over the horizontal current source since it requires no ancillary external connections between the ends of the modeling tank to complete the induced current circuit.

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