Abstract: A transmitter, to be used in an airborne or ground electromagnetic survey system for detecting conductive bodies in an area, said system being adapted to generate a frequency modulated signal and radiating it to the said area, the said transmitter being provided with a power supply means being adapted to be connected by switch means to at least two circuits, each circuit comprising a radiating means, said radiating means being adapted to be connected to the power supply means, in such a way that assembled radiation means are present, each circuit comprising a radiating means being connected by its own switch to the said power supply means.

Abstract: A method and apparatus for reducing ground current interference with electrical signals representative of seismic signals, said electrical signals passing from at least one geophone to a recording system via an electric circuit including a balancing circuit coupled to ground, whereby a bridge circuit is formed which includes the geophone, the electric circuit and at least part of the recording system, and wherein the balancing circuit comprises two pairs of arms, each pair having at least one arm with variable impedance. Automatic adjustment of the variable impedance occurs based upon electrical information received by a detecting circuit for determining the variation in unbalance of the bridge circuit. Impedance adjustments may be made stepwise or continuously in alternative embodiments of the invention.