Description: Most active marine electromagnetic surveys use an array of transmitting and receiving electric dipoles located on the sea-floor. There are many reports of successful detections of buried resistors with these arrays. Nevertheless, the concept has a significant drawback: Large offsets between the transmitter and receiver with respect to the target depth are needed. A logical alternative is the use of a vertical electric-dipole transmitter in a borehole deeper than the target, combined with receivers located at the seafloor. Because current must pass through the target, even data recorded at short offsets with respect to target depth should be sensitive to the target. We derived the equations for the electric and magnetic field components produced by the vertical electric dipole in a 1D layered medium. The equations are used to model the fields for resistive layers. An eigenparameter analysisis applied to the different field components, and the resolution is compared with the standard seafloor inline array. The analyses show that data for the downhole transmitter are more sensitive to the resistance between the source and the receiver than the sea-floor array. However, model geometry such as depth to the target is resolved less easily. The resolution of the model geometry is improved significantly when a long bipole transmitter is used, extended from below the target layer to a level above it. We carried out 3D modeling for the downhole transmitter through the use of a finite-difference algorithm. Results indicate that these data are very sensitive to lateral changes within the target layer. Differences between electric fields simulated at the seafloor for 3D models and a reference model show a correlation to lateral resistivity differences. This finding suggests that the array is suitable for mapping and monitoring resistivity anomalies outward from the borehole.