Abstract
Vertical magnetic transfer functions (tippers) estimated at island observatories can constrain the one-dimensional (1-D) conductivity distribution of the oceanic lithosphere and upper mantle. This is feasible due to the bathymetry-dependent ocean induction effect (OIE), which originates from lateral conductivity contrasts between ocean and land and leads to non-zero tippers even for 1-D conductivity distributions below the ocean. Proper analysis of island tippers requires accurate three-dimensional (3-D) modeling of the OIE, for which so far was performed assuming constant sea water electric conductivity with depth. In this study, we explore—using rigorous 3-D electromagnetic modeling—to what extent realistic, depth-dependent, oceanic conductivity affects island tippers. The modeling is performed for 11 island observatories around the world in the period range 10^{-1} to 10^{4} s. We also investigate the effect of seasonal variations of the oceanic conductivity and to which extent this could explain the observed systematic seasonal variation of tippers. Our model studies suggest that for most of the considered island observatories the effect from depth-varying oceanic conductivity is tangible and exceeds the error floor of 0.025, which usually is assigned to tippers during their inversion. The effect varies significantly with location, depending on regional bathymetry. Contrarily, the effects from seasonally varying oceanic conductivity were found to be too small to be worth consideration.
Highlights
One of the geophysical methods to probe the physical parameters of the Earth’s mantle is GeomagneticDepth Sounding (GDS; Banks 1969; Weidelt 1972)
Red and orange curves correspond to tippers calculated in the models with depth-varying and depth-constant oceanic electrical conductivity, respectively
The analysis is based on 3-D EM modeling, which was carried out for 11 island observatories located in the Pacific, Atlantic, and Indian Oceans
Summary
One of the geophysical methods to probe the physical parameters of the Earth’s mantle is GeomagneticDepth Sounding (GDS; Banks 1969; Weidelt 1972). Long-period (> 3 h) variations are routinely used in GDS to constrain electrical conductivity of the Earth’s mantle (approximately from 400 km down to 1500 km) either in terms of local onedimensional (1-D; Olsen 1998; Utada et al 2003; Munch et al 2018; Chen et al 2020; Zhang et al 2020, among others) or three-dimensional (3-D; Kelbert et al 2009; Semenov and Kuvshinov 2012; Koyama et al 2014; Sun et al 2015; Li et al 2020, among others) conductivity distributions. (2021) 73:3 transfer functions (tippers) estimated from short-period (< 3 h) variations at island observatories can be used to constrain conductivity distribution beneath oceans, where our knowledge is still very limited. The OIE leads to non-zero tippers even for 1-D conductivity distribution beneath the ocean
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