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Measurement of Sound During a 3D Seismic Survey in D Blocks 14/14a of the North Sea

 

 

 

 

 

by J. R. Nedwell (Subacoustech Ltd.), K. Needham (Subacoustech Ltd.), A. W. H. Turnpenny (Fawley Aquatic Research Laboratories Ltd.) and D. Thompson (Sea Mammal Research Unit, University of St. Andrews).

Texaco Britain Limited are gratefully acknowledged for the commissioning of this work and for giving permission for the appearance of information on this page. Thanks go to Gary Hampson for many helpful discussions during the period of this project.

Introduction

A 3D survey was conducted on behalf of Texaco Britain Limited in zones 14/14a of the North Sea, in July and August of 1998. Environmental noise monitoring was conducted by Subacoustech from the survey guard vessel on an opportunity basis as the survey was in progress. A large number of good quality measurements were taken at depths of 5, 10 and 20 metres of the broadband sound resulting from the discharge of a seismic airgun array. The frequency range of recordings was 0 to 95 kHz. This wide bandwidth spanned all of the frequency ranges of hearing of the indigenous species of interest in and around the survey area. The measurements included a “soft start” procedure.

Survey Details

Customer Texaco Britain Ltd
Survey Area North sea zones 14/14a
Water Depth ~ 100 metres
Total Airgun Volume 3335 cubic inch, tuned clustered Bolt airgun array
Number of Airguns 36 in 2 arrays, operating in flip-flop mode
Number of Streamers 6, each 4000 metres long
Surveyor Veritas DGC
Survey boat SRV Veritas Viking
Survey date July – August 1998

Monitoring Details

Total number of recordings 1586
Number of analysed recordings 596
Frequency range 0 – 95 kHz
Dynamic range 72 dB
Recording range 1400 metres to 12000 metres
Recording depth 5, 10 & 20 metres

The positions in which measurements were made relative to the direction of travel of the airgun array are shown in the diagram below. Measurements were made at 5 m, 10 m and 20 m water depth.

 

Unweighted Results

The airgun array was found to be directive at low frequencies, with the levels typically higher than the general trend by about 10 dB within an arc about ±10° from the perpendicular to the array. The results were modelled reasonably well by a simple N log (R) curve. The effective Source Level was found to be rather higher than expected, possibly due to local acoustical effects near to the array.

There was a large degree of scattering of the peak levels recorded which was thought to be caused by spatial or temporal inhomogeneities of the sea.

Weighted Results

The data were analysed using the dBht approach developed by Subacoustech. The figure above shows a typical pressure time history during an airgun array release; to the right is the same pressure time history, but weighted to allow for the hearing ability of the harbour porpoise. The results were weighted in the frequency domain using the dBht(Species) approach, to allow the effect, if any, on individual species to be evaluated. When the weighted peak sound pressure levels were investigated it was found that the value differed very markedly between species, showing that the peak pressure level as measured from an unweighted sample of a seismic signal has no general biological validity.

For the three fish species presented, the Transmission Losses were found to be similar, but the effective Source Levels were very dependent on the species, varying from 201 dBht(Ictalurus nebulosus) for the catfish to 166 dBht(Limanda limanda) re. 1 µPa @ 1 metre for the dab at 5 metres depth. This agrees with the marked differences in hearing sensitivity that are found amongst different species.

For the mammals investigated, the Transmission Loss was again similar for the three species, because of their hearing ability being similar in frequency terms. The effective Source Levels are somewhat different, but the effective levels were highest for the killer whale, which has the most sensitive hearing.

Soft Start Procedure

In terms of the unweighted peak sound pressure levels measured at a range of 2500 metres the soft start procedure achieved its objective of gradually raising the sound pressure level during the start of the firing of the array. This was also found to be true of the results in dBht(Gadus morhua) for cod, although there was rather more scatter in the results at the higher volumes than was noted for the unweighted case.

There was a very significant scatter in the results for marine mammals such that the changes in level which result from the soft start procedure were dominated by the random variability in level.

It could not be determined from these results whether this arose from variability in the propagation (which for marine mammals is dominated by the high frequency behaviour) or from variability in the characteristics of the airgun array.