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 km², 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 |
