A comparison of time and frequency domain geomagnetic sounding

A three-site geomagnetic data set from the Borders region of southern Scotland is used to examine the empirical relationships established by differential geomagnetic sounding experiments. A time domain analysis of such relationships reveals time domain transfer functions which appear to be almost independent of time. A frequency domain analysis is then used to illustrate both a significant phase rotation and a significant dependence on period of the equivalent frequency domain response functions. The formalism of conventional geomagnetic sounding is re-examined. It is found that difference fields are not required to establish the appropriate horizontal field relationships.