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By fusing data obtained from finely spaced continental-scale, magnetotelluric (MT) measurements used for geophysical imaging of the electrical conductivity variations of the Earth's crust and mantle, real-time data of the geomagnetic field variations at a sparse network of fixed geomagnetic observatory and variometer stations, power transmission system sensor data such as neutral ground return current, synchrophasor and other sensor data, information on power grid topology and state, and by applying algorithms we have developed to project the real-time stream of magnetic observatory and variometer data through the frequency-dependent tensor impedances derived from the MT data at each temporary station, we calculate the anomalous voltages on power transmission substations induced by geomagnetically induced currents. Our solution accounts for the first-order impacts on the induced voltages that are due to the effects of 3-D variations in ground (Earth's crust and upper mantle) electrical conductivity structure. These effects when convolved with the path integral of the induced vector ground electric field along the transmission lines are the dominant term in determining the intensity of the geomagnetically induced currents in the system; considerably more so than geomagnetic latitude scaling effects. We discuss integration of real-time geophysical estimates of geomagnetic disturbance induced substation voltages with DC and AC power flow simulations on increasingly realistic models of the topology and state of regional power grids. We are integrating our workflow with the open source PowerModelsGMD.jl power flow simulator developed at Los Alamos National Laboratory, and describe simulations of real-time assessments of stress on critical assets of the power grid including reactive power loss, phase deviations, transformer heating and other metrics of transmission system stress. Such efforts to provide a real-time assessment of risk to critical assets can also inform statistical assessments of system vulnerabilities to "100-yr" or "Carrington" level geomagnetic disturbances. We will discuss how such efforts are informing the development of updated standards that may impact the future regulatory environment that governs efforts to maintain a resilient power transmission system.