Issues surrounding marine mammals and sound waves are technically complex, highly emotive, and not well understood, but it is a concern that must be addressed to ensure marine mammals are safeguarded. With this in mind, marine geophysical surveys implement mitigation measures. One important form of mitigation involves efforts to locate marine mammals within a prescribed area (frequently called the mitigation zone) surrounding air-gun arrays. These mitigation zones often extend to distances of 500 m from air-gun arrays. Independent marine mammal observers (MMO) visually monitor mitigation zones and operations are delayed or suspended when marine mammals are seen within the mitigation zone. Many marine mammals surface for no more than a few seconds at a time and can be difficult to see. Fog, precipitation, snow, and rough seas decrease the likelihood of visually detecting marine mammals, and visual monitoring is not as effective during hours of darkness. As a response to the need to better detect marine mammals in the vicinity of geophysical operations, passive acoustic monitoring (PAM) is sometimes utilized. PAM uses hydrophones to detect and localize vocalizations from marine mammals. PAM, like visual monitoring, may not detect all marine mammals, but it should improve detection rates. Unlike visual monitoring, it requires sophisticated hardware and software and its use for mitigation is not yet widely accepted.