by J. R. Nedwell (Subacoustech Ltd.), K. Needham (Subacoustech Ltd), A.W. H. Turnpenny (Fawley Aquatic Research Laboratories Ltd.) and R. M. H. Seaby (Fawley Aquatic Research Laboratories Ltd.)
BP are gratefully acknowledged for the commissioning of this work and giving permission for the appearance of information on this page.
In 1994 BP conducted a number of seismic surveys in Poole Bay on the south coast of England. Poole Bay is an estuarine area comprising a shallow bay surrounded by shallow creeks and islands. Within the bay, Poole Harbour, one of the largest harbours around the UK coast, has shallow water depths of about 1 to 5 metres. Generally the water in the bay is between 5 and 20 metres deep, with a gradually shelving sandy bottom. The harbour contains several important wildlife areas, including Brownsea Island, an important nature reserve.
A literature review for this work revealed the general belief that the propagation loss that results in shallow water is rapid and occurs at a rate of 40 log(R), where R is the range from the airgun. A simple shallow water propagation model was formed, which confirmed that under perfect conditions with an undisturbed water surface a propagation loss of 40log(R) would result, but under typical climatic conditions wave action on the surface could lead to a much lower propagation loss of about 23log(R).
This result was significant because the area effected by a seismic survey is very dependent on the transmission loss.
The measurements confirmed this view. The first two figures present two sets of measurements in different locations. A single airgun was operated from a fixed position in Poole Bay at two different pressures, 1500 PSI and 2000 PSI. Measurements of the waterborne noise with the airgun operating at these two pressures were made along two straight line transects from the airgun. In all cases propagation losses between 20 and 25 log(R) were noted.
Measurements were taken from a variety of locations inside Poole Harbour. Results from this are shown in the figure left. Propagation loss here was found to be between 21 to 26 log(R).
Unfortunately, at the time that these measurements were taken the dBht method of analysis had not been developed, so the data has not yet been fully interpreted in terms of its potential for environmental effects. It is, however, interesting to note that the low transmission loss is at variance with the general view that high losses occur in shallow water.