Visual and Passive Acoustic Marine Mammal Observations and High-Frequency Seismic Source Characteristics Recorded During a Seismic Survey

Abstract

In this paper, we present marine mammal observation statistics, high-frequency seismic source characteristics, and example denoising of marine mammal acoustical recordings using data collected during the mitigation and monitoring program for a 3-D seismic survey by EnCana Corporation, Calgary, AB, Canada, in the Northwest Atlantic during 2003. Marine mammals were observed both visually and acoustically. No marine mammal incidents or adverse reactions were observed during the survey. Acoustical observations were made by the Sea map Passive Acoustic Cetacean Monitoring System (SPACMS), consisting of two hydrophones placed 50 m apart, towed ahead of and to one side of the seismic source. Visual and acoustical detections were uncorrelated, indicating the complementary nature of the two observational techniques. Visual detections were more common per hour of effort than acoustical detections. Acoustical detection rates showed no significant day-night difference. Marine mammals appeared to have avoided very close ranges (100 m) from the seismic array during seismic acquisition, but the overall number of marine mammals in the observable radius (1-2 km) did not change significantly when the seismic source was ldquoonrdquo compared to ldquooff.rdquo Marine mammals were observed in larger groups and appeared to have become less vocal when the seismic source was active. It should be noted however, that the results from this data gathering effort may be affected by potential sources of bias (such as the combination of data from toothed and baleen whales). Signal processing of seismic source signatures indicated some high-frequency energy content consistent with expectations from earlier work. This analysis confirmed that most of the seismic energy was concentrated at lower frequencies (500 Hz). No low-frequency comparisons with near-field data could be made due to the geometry of the SPACMS recording hydrophones and seismic source, which resulted in the Lloyd's mirror effect obliterating low-frequency components in the SPACMS records. A wavelet-based denoising method was applied to improve the visibility of marine mammal vocalizations on a spectrogram display.