A one-dimensional model of solid-earth electrical resistivity beneath Florida

Abstract

An estimated one-dimensional layered model of electrical resistivity beneath Florida was developed from published geological and geophysical information. The resistivity of each layer is represented by plausible upper and lower bounds as well as a geometric mean resistivity. Corresponding impedance transfer functions, Schmucker-Weidelt transfer functions, apparent resistivity, and phase responses are calculated for inducing geomagnetic frequencies ranging from 10−5 to 100 hertz. The resulting one-dimensional model and response functions can be used to make general estimates of timevarying electric fields associated with geomagnetic storms such as might represent induction hazards for electric-power grid operation. The plausible upperand lower-bound resistivity structures show the uncertainty, giving a wide range of plausible time-varying electric fields. Introduction Geoelectric fields, induced in the Earth’s conducting interior during magnetic storms, represent a natural hazard for the operation of electric-power grids (for example, Bolduc, 2002; Boteler, 2003). In response to the possible vulnerability of U.S. power grids, in May 2013, the Federal Energy Regulatory Commission of the Department of Energy issued FERC Order No. 779 directing the North American Electric Reliability Corporation (NERC-DOE, 2010) to develop power-grid standards to help mitigate the deleterious impact of storm-induced geoelectric fields. In support of these developments, the Geomagnetism Program of the U.S. Geological Survey (USGS) has assembled a simplified model of electrical resistivity beneath Florida, a part of the United States that was not specifically covered by an earlier effort undertaken by Fernberg (2012). This resistivity model for Florida contributes to a larger USGS and interagency U.S. Government project for evaluating induction hazards associated with geomagnetic storms (Love and others, 2014). Method Summary To develop a one-dimensional (1D), depth-dependent model of electrical resistivity for Florida, we first assemble a layered-earth structure sequence for the lithosphere (crust and upper mantle) beneath the tectonically distinct northern and southern parts of Florida. For a given rock type, electrical resistivity can differ by orders of magnitude depending on mineralogy, water content, chemistry, and degree of weathering. Therefore, for each of the northern and southern Florida layered-earth sequences, we assigned a range of resistivity values based on rock type and published results from geological and geophysical surveys and from direct laboratory measurements of standard rock types for each layer. A 1D model, applicable to all of Florida, has a set of layers and resistivity values that encompass the