Locating offshore hydrocarbon reservoirs has become more challenging with smaller, deeper, or shallow-water targets in complicated environments. Controlled-source electromagnetics (CSEM) is a geophysical method used to find reservoirs in marine settings. The diffusive nature of CSEM fields means that the signal from the target is only a small part of the total field. To reduce the impact of the complicated settings and to improve the detecting capabilities of CSEM, we have applied a synthetic aperture to CSEM data. The synthetic aperture virtually increased the length and width of the CSEM source by combining the responses from multiple individual sources. Applying a weight to each source steered or focused the synthetic aperture source array in the inline and crossline directions. We have developed an optimization method to find the optimal weights for synthetic aperture arrays that adapted to the information about the reservoirs in the CSEM data. To demonstrate the benefits of a weighted synthetic aperture, we have applied a 2D synthetic aperture array and a crossline-only synthetic aperture array to noisy, simulated electromagnetic fields. Both synthetic aperture arrays reduced the noise and increased the detectability of the modeled reservoirs when compared with the original CSEM response. The crossline-only synthetic aperture array also preserved the structural information within the measurements.