Bathymetric refraction effects associated with marine pile driving

Abstract

Measurements of underwater noise from vibratory pile driving from a marine construction site in Puget Sound, Washington, are studied using line array-based measurements made at range 16 m from the pile source and single hydrophones at range 417-m on one transect, and at ranges 207-m and 436-m on another transect running approximately parallel to a sloping shoreline. Using adiabatic modes, the field from an incoherent line source model is propagated along these two transects. This approach works well except for the 436-m measurements located on the along-shore transect, where the observed levels for frequencies <300 Hz are significantly lower than predicted. These observations are interpreted in the context of shadow zones associated with bathymetric refraction for a downward sloping beach [Deane and Buckingham, J. Acoust. Soc. Am. 93, 1319 (1993)]. The modal amplitude due to horizontal refraction is evaluated with the parabolic wave equation, written in terms of the depth dependent phase velocity of each mode [Ballard, Proc. Meetings Acoust.ics 15, 070001 (2012)]. The results are significant as sloping beaches form many of the boundaries to waters where pile driving occurs, and where protective environment underwater noise monitoring is necessary, such as in Puget Sound.