Low Frequency (0.1–6.0 cpd) Electromagnetic Study of Deep Mantle Electrical Conductivity Beneath the Hawaiian Islands

Summary Low frequency (0.1–6.0 cpd), naturally occurring electromagnetic fields on Oahu, Hawaii are used to compute E over B response functions that are complex functions of frequency containing information about the electrical conductivity structure at depth. Smoothly varying estimates of the response, unbiased by random electric field noise, are constructed with the aid of polynomials in frequency. Remarkably small standard deviations, for the smoothed response, between 1 and 3 per cent, result from using 22 months of hourly electric field data accurate to 1 μV m −1 . The island effect is found to be essentially frequency independent with zero phase shifts for frequencies less than 6 cpd. This enables us to estimate the mantle response that is consistent, at these frequencies, with a conductivity model dependent only on depth. A horizontally layered model is constructed by the inversion method of Schmucker and the layered profile is smoothed by the Backus‐Gilbert method that yields locally smoothed conductivity values varying from a near surface value of 0.08 mhom −1 to 0.8 mhom −1 at 700 km depth as in the Parker profile. In addition there appears to be a definite zone of high conductivity in the depth range 330–380 km with a locally smoothed conductivity value of 0.8±0.1 mhom −1 for a smoothing kernel of 30 km spread. The layer conductivity model and the island effect quite accurately reproduce all the observed complex response functions.