Source field effects on transfer functions at mid-latitudes

Transfer function estimates are required to provide information regarding the lateral variation of conductivity structure within the Earth. Conventional techniques assume that the effect of external field characteristics on transfer function estimates can be removed by a least-squares procedure. This assumption is examined at three mid-latitude stations. It is found that, at all three stations, source field characteristics do affect the transfer function estimates. The source field effects increase with both latitude and period. The source field contribution is minimized by the determination of the transfer function from a weighted mean of estimates of transfer functions obtained from individual records. The weighting function is the vertical field partial coherence and it is, furthermore, necessary to provide a wide distribution of this parameter if a valid transfer function estimate is to be obtained.