An Acoustic Survey of Environmental Noise in the Meldorfer Bucht

 

 

by J. Nedwell (Subacoustech Ltd.), A. Turnpenny (Fawley Aquatic Research Laboratories Ltd.) and C. Snow (Subacoustech Ltd.). Subacoustech Ltd. report reference 239R0102.

RWE-DEA are gratefully acknowledged for the commissioning of this work and giving permission for the appearance of information on this page.

Summary

A seismic survey was conducted on behalf of RWE-DEA in the Wadden Sea, along the western and northern coasts of Denmark, Germany and the Netherlands, in October 1996. Environmental noise monitoring of underwater noise was conducted by Subacoustech from a hydrophone cable laying vessel as the survey was in progress.

The monitoring was intended to provide information about the consequential impact by airgun activity on local marine life.

The Wadden Sea is a shallow coastal sea along the western and northern coasts of Denmark, Germany and the Netherlands. Towards the North Sea it is bounded by 17 large, inhabited barrier islands as well as numerous small, uninhabited sand banks and tiny islands. The boundary towards the mainland is characterised by the presence of salt marshes, sea walls and some pleistocene cliffs. Three large rivers – Ems, Weser and Elbe – as well as a number of smaller rivers have estuaries opening into the Wadden Sea.

The tidal range within the Wadden Sea and the adjacent estuaries varies between 1 and 4 metres. During ebb vast areas of tidal flat emerge and normally about two thirds of the area is exposed during low tide. The barrier islands included, the Wadden Sea area occupies about 10,000 kmĀ², thus being the largest esturine area in Europe.

The Meldorfer Bucht seismic survey involved firing an airgun array in very shallow water (typically of one or two metres depth). Concerns were expressed over possible environmental effects on wildlife in the vicinity of the survey, as the shallow waters may harbour a high density and rich variety of species. As the Meldorfer Bucht area lies within the Wadden Sea National Park, and little information was available concerning airgun sound propagation in shallow waters, it was considered important to investigate and monitor the sound pressure levels in the water surrounding the airgun, and to interpret the findings in the light of any possible environmental effects.

Environmental monitoring was conducted by Subacoustech as the survey was underway, and recordings were made over a wide variety of conditions and ranges.

The data were assessed in terms of their peak pressure levels. The results from this analysis have been summarised in figure 1, which presents peak pressure in dB as a function of distance from the source. The figure also illustrates some specific levels of interest, ranging from ambient noise levels through to the level at which injuries to fish may be expected to occur.

It was concluded that on a peak pressure basis and by taking the lowest estimate of avoidance threshold for cetaceans, an avoidance reaction may occur upto 3 km from the airgun array. This is where the level of noise from the array has reduced to that of background noise. For flatfish and swimbladdered fish, again taking the lowest avoidance threshold, the range of disturbance would be less than 1.1 km from the airgun array.

A summary of peak pressure measurements as a function of range

Survey Details

Customer RWE-Dea
Survey area Wadden Sea
Water depth 1 – 4 metres
Total airgun volume 480 cubic inches, Bolt airgun array
Number of airguns 15 per array
Survey date October 1996

Monitoring details

Number of recordings 564
Frequency range 20 – 20000 Hz
Dynamic range 96 dB for recording, 72 dB for analysis
Recording range 65 m to 4500 m
Recording depth 1 metre