Water-Borne Noise Levels from Airgun Arrays - Examples from Surprisingly Quiet Chicxulub Survey

In 2005 the US research vessel Maurice Ewing conducted an academic seismic survey of the Chicxulub impact crater in shallow water offshore Yucatan, Mexico. A combination of US legal requirements and Mexican government restrictions resulted in highly restricted time windows in which the work could be conducted. A casual observation by a diver in 20m-deep water just outside the predicted 180 dB (RMS) safety radius at 3.5 km that he could hardly hear the airguns led us to examine the data collected by the seismic streamer and by ocean bottom hydrophones. Calibrated plots of peak seismic arrival amplitude against offset show that in this environment, levels of deep, post-critically reflected and refracted arrivals outweigh those of water-borne arrivals at offsets greater than 1 km. Since the predicted 3.5 km radius was based on measurements of the same array in 60m-deep water we conclude that through a combination of very shallow water depth and geologic structure, seismic source energy was partitioned so as to reduce the intensity of water column reverberations. These results suggest that monitoring of hydrophone array data may provide a method of determining the safety radius in real time.