Magnetic variations (2-30 cpd) on Hawaii Island and mantle electrical conductivity

Abstract

Naturally occurring magnetic variations (2-30 cpd) observed by an array of magnetometers on the island of Hawaii are used to compute estimates of the complex and frequency dependent Z over H response functions. These data contain information about the electrical conductivity structure at depth in the mantle but are also strongly affected by the conducting ocean. The lateral contrast in electrical conductivity between the island-mass and sea-water produces large amplitude spatial gradients in the observed Z variations. These spatial distortions are related to a local horizontal deflection pattern in electrical current which is induced over a large region in the conducting ocean and which is coupled to the deep conductivity by mutual induction. This mutual induction is accounted for in the analysis of the Z over H response to estimate the deep conductivity structure. The spatial distortions are studied by comparing the response estimates between different observation sites. For each site the response function defines a direction for H where the Z:H coherence is maximized. This axis of principle induction is found to be generally perpendicular to the coast at each station. When rotated into their local direction of principle induction the response functions for frequencies less than 30 cpd at each site are found to relate to a reference site by a real and frequency independent transfer parameter. This result is interpreted to indicate that the ocean caused field distortions are essentially static for the lower frequencies. Except for a constant multiplier at the reference site the spatial distortions are thus defined by the set of transfer parameters for the different stations. The unknown reference site parameter is determined simultaneously with the modeling of the observed response to the deep conductivity structure. General consistency is found between the estimated response and the theoretical response of the conductivity distribution determined by J. C. Larsen (1975) for Oahu.