The applicability of crosswell electromagnetic (EM) tomography is limited when the boreholes are steel cased. Steel casing is typically eight orders of magnitude more conductive than the formations around the borehole. Consequently, the high frequency signals which are needed to resolve the formation conductivity distribution in the interwell region cannot be radiated or received by antennas inside the casing. Introduction of small insulating gaps in the casing eliminates these problems by creating ports where signals can exit and enter the casing, making the casing an electric bipole antenna. The gapped casing can be operated in a shorted-port or open-port mode. In the shorted-port mode, the voltage is specified at a source port, and the current is measured at all the remaining shorted ports. In the open-port mode, the current is specified at a source port, and the voltage is measured at all the remaining open ports. For high-frequency shorted-port operation, the effective length of the casing bipole antenna is determined by the skin depth in the formations surrounding the casing. For high-frequency open-port operation, the effective length of the casing bipole antenna is determined by the casing gap interval, provided that it is less than the formation skin depth. The open-port mode is preferred.The interval between casing gaps is determined mainly by the well spacing. If the effective bipole length is proportional to the well spacing, the crosswell signal-to-noise ratio is approximately independent of the well spacing and the interwell resolution is approximately proportional to the well spacing at constant source input power. For large well spacings, gapped casing antennas outperform open-hole tools.Steel-cased wells completed or retrofitted with insulating gaps offer a stable, low-cost, permanent set of electrodes to implement long-term monitoring of petroleum reservoir fluid movements. Gapped casings offer a practical way to achieve high-frequency, wide-aperture EM tomographic datasets at large well spacings. Gapped casings can also be used for deep EM probing of the formations around a single well.