To perform surveying of a subterranean structure, electromagnetic (EM) wavefields traveling in opposite directions are determined. A relationship is defined among the EM wavefields, where the EM wavefields include a first set of EM wavefields in a first state in which a sea surface is present, and a second set of EM wavefields in a second, different state in which the sea surface is not present. At least one EM wavefield in the relationship is determined to perform removal of sea surface-related effects.