Variability of underwater radiated ship noise measured using two hydrophone arrays

The impacts of man-made underwater noise on the marine environment have received increased attention over recent years, primarily resulting from recognition of the increased pressures placed on the oceans by human activities. Underwater noise from shipping has long been acknowledged as a major contributor to overall ambient sea noise levels. Furthermore, recent evidence from long term measurements at a deep water location suggest that the increase in global merchant shipping has led to an increase in ambient noise levels in the oceans (although it should be noted that the data also show that this trend may be levelling off). To date, measurements of radiated noise from ships have been carried out primarily at fixed noise ranges. However, in recent years researchers have increasingly used other systems, most commonly portable measurement systems deployed from support vessels. The use of various bespoke or proprietary measurement systems and the various ways of reporting the data in the literature has prompted the recent publication of national and international standards covering measurement and data processing methodologies. These standards cover in detail many aspects of full scale ship radiated noise measurements including the measurement system specification, target vessel operating sequence and trials location requirements as well as the processing, analysis and reporting of the data. The objective of this paper is to present some analysis of measured radiated underwater noise data from a small vessel operating under realistic test conditions at sea in shallow water undertaken as part of the SONIC project. The measurements followed the requirements of the published standards as closely as possible although it was not possible to meet all of the criteria. Two multiple hydrophone arrays operated by two of the project partners, CETENA and the University of Southampton, were deployed from the same moored support vessel over a period of several days. The comparison of the data from these two systems provides an opportunity to investigate the typical variability and repeatability of radiated underwater noise from ships using mobile arrays.