RIASSUNTO
Abstract
The aim of the Seismic Survey Environmental Assessment Tool (SSEAT) is to reduce the risks to the environment from seismic surveys. It facilitates environmental impact assessment and the management of environmental sensitivities whilst designing and undertaking a seismic survey. It is both a ‘check-list' guide and a sound propagation impact model. In the UK it guides the completion of an application to carryout a seismic survey, Petroleum Operations Notice (PON) 14A under the Offshore Petroleum Activities (Conservation of Habitats) Regulations 2001. SSEAT used properly ensures that the risks to the environment are understood simply and clearly by a survey team and enables the elimination of any that are adversely significant. The new tool allows BP to better understand the impact of seismic surveying and submit environmental information to the DTI, the UK Department of Trade and Industry, in a transparent and consistent manner. The Gausland model ‘received level = source level - transmission loss' is used to model the behaviour of a seismic pulse, and to later help estimate the zones of effects for sounds using presented scientific literature thresholds - Zones of: 1) Audibility; 2) Response; and, 3) Hearing Loss, Discomfort and Injury. This is achieved by taking the sound levels that are known to induce a response to mammals and/or fish and recording the distance from the seismic source that the sound level occurs from the plotted sound decay curve. Whilst designed for use in the UK it can be used anywhere in the world to assess the environmental impact of seismic surveying. If so it will set a performance benchmark. Its use has also highlighted areas where improvements could be made. The outcome is an abbreviated summary of an ‘environmental impact assessment' (EIA). It is similar in process to undertaking an Environmental Impact Assessment (EIA) under the UK Offshore Petroleum Production and Pipelines (Assessment of Environmental Effects) Regulations 1998, European Amended Environmental Assessment Directive 97/11/EC and World Bank EIA guidelines.
Introduction
Full scale seismic survey airguns (an array) generate noise pulses with very high peak levels, about 255 dB rel 1 µPa at 1 metre with a wave peak to peak time as long as 6 milliseconds (sound pressure levels are typically expressed with reference to standard pressure - usually one micropascal in water, and 20 micropascals in air and a standard distance - usually one metre) (Wardle et al., 1998). Smaller arrays often have source levels of about 210-246 dB rel 1 µPa-m0-p. The short duration of each pulse limits the total energy. Both high and low frequency energy is present in the pulses at considerable magnitude ranging from below 100 Hz to 22 kHz (Goold & Fish, 1998). It is the propagation of sound horizontally that is of interest as some acoustic energy is emitted into the wider marine environment (UKOOA, 1996). It is important to understand what impact these acoustic pressure waves might have on fish and marine mammals.
In 1994 the UK Joint Nature Conservation Committee (JNCC) produced guidelines for the UK Offshore Operators Association directed at minimising the acoustic disturbance from seismic surveying in areas where marine mammals may be present. These guidelines were updated in April 1998. The JNCC have also published a guidance note on implementing the 1998 guidelines together with guidance on using Marine Mammal Observers. The Scottish Executive's Fisheries Research Services (FRS) have also produced guidelines for minimising such disturbance during commercial fish breeding and spawning periods (Coull et al., 1998). A precautionary combination of methods is advised by experts and includes: acoustic monitoring; visual surveys; using recommended seismic windows set by the Department of Trade and Industry (DTI); and gradual air gun array start up (ramp-up) and shut-downs. The DTI regulate oil and gas companies in the UK using a system of Petroleum Operations Notices (PONs). Some of the notices are environmental. PON 14A is for example an application for consent and notification to carryout oil and gas surveys and shallow drilling offshore. An Environmental Assessment of seismic survey must be carried out as part of the application. The DTI produced statutory guidelines on this process in 2002.