  Electrical and electromagnetic (EM) methods can be diagnostic geophysical
imaging tools for monitoring applications, such as carbon capture and storage
or hydraulic fracturing. In these settings, it is common that steel-cased wells
and other steel infrastructure are present. Grounded source methods, which use
electrodes to inject current into the earth are of interest for casing
integrity and monitoring applications. Electrostatic, or direct current (DC)
resistivity, experiments form the basis of our understanding of the physics of
grounded source experiments in terms of charges, currents, and electric fields.
Steel-cased wells are highly conductive and although their presence makes
numerical modelling more challenging, they can help targets of interest be
detected because they channel charges and currents to depth. Time-domain EM
experiments use a time-varying transmitter current. Understanding the EM
response requires that we consider both galvanic, or DC currents, as well as
image currents that are induced in the subsurface. As compared to DC
experiments, the physics of EM is more complex because of the multiple current
systems, as well as the need to consider magnetic permeability of steel-cased
wells. However, EM experiments have the advantage that they can provide a large
data set that is sensitive to a target of interest. Ultimately this will
increase the potential for being able to extract information about the target.
