No abstract

The problem of global conductivity sounding has been re-posed in order to investigate the possible existence of large-scale lateral heterogeneities in mid-mantle conductivity. A response function (Z/H> is robustly estimated and scaled into an equivalent scalar magnetotelluric impedance by assuming the geomagnetic field spatial variations are adequately described by a Py spherical harmonic. Data found to be inconsistent with this representation in either the time, frequency or spatial domain are excluded from the analysis. Making use of this relationship, robust statistical techniques and careful data handling procedures have been applied to a new global magnetic database containing well in excess of lo6 daily mean values (1358 observatory-years spanning the period 1883-1980). A set of spatially distributed scalar frequency domain impedance functions for a variety of tectonic provinces result from this analysis. Response functions consistent with both the Py assumption and local I-D earth structure were found for 22 of the 79 observatories examined. The set of acceptable response functions is presented, along with a summary of data quality for the observatories represented in the database.

S U M M A R YRobust estimates of the magnetotelluric (MT) transfer function are found using an iterative reweighted method on time series data corrected for outliers and gaps. The MT transfer function, composed of several analytic functions smoothly varying in frequency, is used to represent the frequency-domain relationship between electric and magnetic time series. The smoothly varying transfer function facilitates identification and removal of electric and magnetic outliers (spikes), construction of the frequencyand time-domain weights used for obtaining robust smooth and band-averaged estimates, and separation of the time series into MT and correlated noise signals if a remote site exists that is free of the correlated noise. Errors in the transfer function are calculated using jackknife estimates of the solution covariance. The method is tested on: time series from a relatively clean MT site in central California; a test time series based on Tucson magnetic time series plus synthetic noise for a given transfer function; and time series from the Larderello geothermal region in central Italy where there are strong signals from d.c. electrified railways.

Low frequency (0.1-6-0 cpd), naturally occurring electromagnetic .,:ids 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 pV m-I.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 0.08mhom-' to 0*8mhorn-' at 700km 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 mho m-' 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. on the electrical conductivity as determined from the ionospheric generated signals.The oceanic generated signals will be treated in a separate paper.A major obstacle in the interpretation of island fields is the complication caused by the island conductivity structure. Fields measured at sea-floor sites distant from topographic features such as ridges, islands or continents are less complicated by topography effects. However, islands are limited in size and have known offshore