Most wells in oil fields are cased with metal. The feasibility of measuring low-frequency magnetic fields through such steel-cased wells is of great interest for monitoring fluid movement during production. We report on experimental and modeling results regarding the effect of steel casing on electromagnetic signals transmitted through them. This effect includes the attenuation and phase shift of a signal diffusing through the casing wall. Sample casings are studied under low field strength in the linear regime where the casing's electrical properties are independent of the applied signals. Frequency spectra are measured for transverse-electric and transverse-magnetic polarizations from 0.1 Hz to 1 kHz. Numerical models, developed to simulate the casing response, match the experimental data very well. Finally, studies of the effects of the earth's magnetic field and casing remanent magnetization show that these steady biases do not affect the ac signal detection.