Towards standardization of the measurement of underwater noise radiated from marine pile-driving

Pile-driving of marine foundations can radiate substantial levels of low-frequency impulsive noise into the water column, which given the right acoustic propagation environment conditions can propagate over large distances. Concern over the potential for impact on marine fauna often results in a regulatory requirement to measure the radiated noise level over distances which may extend tens of kilometres. Furthermore, if the transmission loss is to be established or validated it would be necessary to measure as a function of range at a number of positions. Because of the variation in hammer energy during the pile-driving activity, particularly if a soft-start is employed, it is also desirable to perform the measurement independent of distance for the duration of the pile-driving operation. A number of others factors will also influence the noise radiated into the water column, including the water depth (exposing a different amount of the surface area of the pile), the seabed properties, the penetration depth into the seabed by the pile, the pile dimensions, and the hammer energy. Importantly, some of these factors may change during pile-driving of an individual pile, for example tidal variation and sediment penetration, and this means that the acoustic output is likely to change during the driving of a specific pile. A method for the measurement of this underwater noise, that is generally in relatively shallow water, which considers both the spatial and temporal variation of the sound field is described. The method includes a combination of fixed autonomous recorders and vessel based hydrophone deployments. The paper describes the activities within the International Organization for Standardization (ISO), Technical Committee 43, Sub-Committee 3, Working Group 3 to produce an international standards document to define the appropriate measurement methodology, which should be followed when measuring underwater noise radiated from marine pile-driving.