Electrically conductive fluid injected during the operation of hydraulic fracturing can be mapped by anomalous electromagnetic (EM) fields under the excitation of controlled EM sources on the surface. Steel casings, existing in the area of fracturing operation and sources of strong EM responses, need to be considered during survey design and data interpretation. We develop a novel numerical approach that efficiently simulates the effect of steel casings by assigning lumped conductive properties to mesh edges, so finely discretized cells for casings are avoided. In our numerical study, a multistage fracturing along a horizontal well is considered, and the operation is monitored by two survey configurations: long-offset excitation and near-well excitation. Time-lapse analysis shows that the steel casings always have nonnegligible effect in EM field data, even for the long-offset excitation with a distant EM source; the near-well excitation, compared with other conventional configurations, has stronger EM responses and can maintain a high level of sensitivity throughout the entire fracturing procedure. Our results help the design of a controlled source EM survey for a more effective fracturing monitoring campaign.