Disclosed are apparatus and methods for seismic exploration using pressure changes caused by sea-surface variations as a low-frequency seismic energy source. One embodiment relates to a method which obtains dual wave-fields measured below a sea surface. The measured dual wave-fields are decomposed into a down-going wave-field and an up-going wave-field at a selected observation level. Seismic images are then generated using the down-going and up-going wave-fields. Other embodiments, aspects, and features are also disclosed.