Difficulties in the application of magnetic field gradient analysis to induction studies

Geomagnetic variation data recorded across an array of magnetometer stations in northern England have been used to compute an inductive response function. The function, if successfully determined, is appropriate for sounding vertical conductivity structures on a regional scale. The method employed is the horizontal spatial gradient method using station averaging and a linear approximation. It is found that, for the data under consideration, local and lateral anomalous fields exist in all three components at periods ranging from 10 to 10⁴ s. The presence of such anomalous fields invalidates the procedures used to estimate the regional field, whose characteristics are required to determine successfully the inductive response function.