Anthropogenic noise prediction for light seismic surveys off the SW coast of Portugal

Marine seismic sources use extremely powerful sound pulses to penetrate the ocean bottom, which in consequence may be harmful for numerous marine species in the surrounding area. During the last decade the Portuguese Government has granted rights for oil and gas exploration in several offshore slots along the SW coast. In contrast to what happens in deep water seismic surveying, which is well studied and described, shallow water surveying effects, that usually rely on light seismic techniques, are under-investigated. Numerous cetaceans sightings along that coast denote a rich ecosystem, which may be subject to marine life harmful noise levels resulting from light seismic techniques and which may have acute, cumulative and chronic effects on marine organisms. The results suggest that there is an important contribution of the bathymetry transition from offshore to inshore for the propagation of the noise resulting from seismic sources. Indeed, it was observed that the continental platform works as a natural barrier precluding the sound resulting from offshore sources (located at more than 50km from the shore) to propagate towards land. It was also shown that sound level of sources located near the shore attenuate more rapidly than those offshore, even though high sound exposure levels may be reached due to shore proximity.