Offshore Oil Exploration and Production
The activities associated with the oil exploration industry have historically constituted a major source of acoustic activity in shallow water (< 200 metres) and, in recent years, in deeper water as well. These activities range from oil and gas drilling and production operations to marine geophysical surveys and, in some geographic areas (e.g., the Gulf of Mexico) are of sufficient extent that they might be considered to be part of the background ambient noise level. Richardson et al  provides an extensive and comprehensive summary of the available data in this area and clearly shows that the strongest sources of sound are associated with seismic surveys. The extent to which our knowledge base is limited by the proprietary nature of oil exploration activities is unclear.
A paper that appeared prior to 1995, but which may be of interest, is that of Hall and Francine , who report on the measurements of underwater sound from a concrete island drilling structure located in the Camden Bay area of the Alaskan sector of the Beaufort Sea in late 1988. Noise was measured while drilling was taking place and at idle. They found that the low frequency limit of their instrumentation was probably affecting their results, in that a digital tape recorder with a low frequency limit of 0.2 Hz gave higher SPLs than a conventional tape recorder with a low frequency limit of 20 Hz. Their conclusion was that most of the sound produced by the drilling structure using a rotary turntable was below 20 Hz.
Some measurements from a towed airgun array used in a 2D survey off the west Wales coast during March 1996 are reported in Goold and Fish . Fourteen gun clusters, arranged along four lines of floats towed behind the survey ship, provided the sound source, and the authors took measurements using a two hydrophone array towed behind the guard ship, which steamed ahead of the survey ship. From the recordings which they made they derived plots for four source-to-receiver distances of the power spectral densities of the seismic and the background noise. The greatest difference between the source and background levels occurred at the low frequency end of the spectrum, and was still significant at the largest range used, viz. 8 km. The authors note that the background levels recorded were influenced by the noise of the guard ship itself and the flow over the hydrophones, and that if the true ambient level were used the source would have been even higher above background.
|Website produced by Subacoustech Ltd on behalf of Oil & Gas UK|