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- Volume 20, Issue 10, 2002
First Break - Volume 20, Issue 10, 2002
Volume 20, Issue 10, 2002
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Passive acoustic monitoring as an environmental protection tool in marine seismic operations
Authors C. Walker and D. HedgelandChris Walker and David Hedgeland, PGS Research Centre, London discuss measures being taken to mitigate the environmental impact of acoustic noise in marine seismic surveys and the uncertainty which still surrounds the issue of ‘acoustic pollution’. The oil industry has responded positively to concerns about the impact of exploration and production (E&P) activity on the environment by introducing a range of environmental protection measures that cover the full spectrum of oil sector operations, from exploration in frontier areas, through production in mature fields to abandonment of exhausted reservoirs. In recent years, however, one specific area of concern has arisen regarding the level of noise in the oceans world-wide – so-called ‘acoustic pollution.’ Whilst there are many natural sources of underwater noise (Fig. 1), the potential impact of man-made noise on the health and behaviour of marine mammals has been highlighted, particularly as activity has moved further and further offshore into areas which have previously been their exclusive preserve. With the adoption of the Agreement on the Conservation of Small Cetaceans of the Baltic and North Seas (ASCOBANS) treaty in Europe and the Marine Mammal Protection Act in the USA, it has become a regulatory requirement to minimise acoustic disturbance to these animals. Such agreements are enforced within each country by national guidelines or regulations. In the UK, the Joint Nature Conservation Committee (JNCC) advises the UK government on formulating guidelines in response to the European Union Habitats Directive and ASCOBANS. Similar organizations in the USA, Brazil and Australia are the Minerals Management Service (MMS), Instituto Brasileiro do Meio Ambiente e dos Recursos Naturais Renováveis (IBAMA) and Environment Australia respectively. In the USA, for example, the existing environment legislation is currently under review in order to take account of increased E&P activity in the deep water areas of the Gulf of Mexico.
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The role of environmental geophysics in the investigation of an acid tar lagoon, Llwyneinion, North Wales, UK
More LessConsultant John M. Reynolds1 offers a case study of how environmental geophysics can provide the tools for effective survey and analysis of pollution hazards. During the last decade or so, environmental geophysics* has really come of age. Two aspects have changed in particular: first, environmental geophysics is being used to investigate sites that 10 years ago would have been impossible or considered as ‘research’ sites. The technology for both acquiring and handling data, as well as increasingly sophisticated and easy to use software have enabled much more data (and with much finer spatial sampling) to be acquired with better quality and to be analysed more robustly. The downside of this ease of use is that more unqualified and inexperienced people are entering the ‘industry’ (note I did not say ‘profession’!) and there are more examples of bad surveys occurring. The second key factor is the regulatory and legislative climate that prevails; the UK is fast catching up with the USA in its increasing recourse to the law courts. Clients are not just looking for geological, environmental or engineering targets any more. Rather, there is a very rapidly developing use of environmental geophysics as a risk management tool.
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Structural geology using borehole wall imagery: case study of an OPTV log in flagstones, North Scotland
More LessAndrew W. B. Siddans, senior geologist, Robertson Geologging, describes the processing and interpretation of an optical televiewer log (OPTV) during a borehole drilled through flagstones in the North of Scotland for the UK Atomic Energy Authority as part of an ongoing geotechnical and environmental site investigation. The borehole-wall image involved in the project consists of rows of 720 pixels, colour-coded as 24-bit triplets. Each row of pixels represents 1 mm depth on the borehole axis and is oriented NESWN around the borehole wall. The nominal diameter of the borehole is 145 mm. Traces of various geological features are recorded on the imagery, among which bedding, fractures and calcite-veins can be identified (Fig. 1). Processing of such imagery aims to identify and label the significant geological features along with their orientation and depth on the borehole axis. Traditionally this is a time-consuming, interactive process. Methods of partially automating and generally easing the burden are discussed in the processing section below. Interpretation of the dips resulting from processing follows the two classic, broad fields of structural interpretation and fracture analysis. Structural interpretation aims to extract formation dip and identify geological structures such as unconformities, folds and faults, from the distribution and orientation of dips assigned to bedding. Fracture analysis aims to identify geometrical sets of fractures/veins, and then estimate variations in mean-dip and frequency within the sets and lines of intersection among the sets, with depth. Methods and results are described in the interpretation section of this article.
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Airborne EM applied to environmental geoscience in the UK
By D. BeamishThe British Geological Survey (BGS) has been highlighting the need for modern, multi-sensor airborne geophysical data in the UK. Here David Beamish, geophysicist with the BGS, describes the first trial airborne electromagnetic data acquired and its relevance to environmental geoscience. The lack of modern, multi-sensor (magnetic, radiometric and electromagnetic) data represents one of the most serious gaps in the geoscience knowledge base of the UK, and a national, high resolution airborne survey has been a stated corporate objective for many years. In 1999, the fixed-wing, frequency domain, airborne EM (AEM) system developed and operated by the Geological Survey of Finland was used in a series of trials to acquire detailed EM data sets in addition to magnetic gradiometer (wing-tip) and radiometric information. The purpose of the trials was, in part, to assess the case for the inclusion of AEM in future strategic airborne geophysical surveying. The limited data acquired (3324 line km in 5 days’ flying) constitute the first high resolution AEM survey information to address specific environmental issues in the UK. It was anticipated that the AEM data would provide pathfinder information for the general assessment of land quality issues such as planning and pollution control and water supply/resource protection.
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Faeroe sub-basalt seismic imaging: a new iterative time processing approach
The main problem for seismic imaging in the Faeroe Basin is the presence of a widespread basalt body of variable thickness (up to several thousand metres) overlying the possible sedimentary basin that is the exploration target. Conventional seismic reflection methods (based on P-wave reflections) have difficulty in obtaining reliable images of the sub-basalt units due to the very high absorption, scattering and reflection capacity of this volcanic formation for P-wave energy. To overcome this problem, different approaches have been suggested. One of them is the use of converted waves to image beneath the basalt (Joppen & White 1990; Li & MacBeth 1996; Masotti et al. 1996). Converted waves generated at the interface between formations with strong velocity contrast may contain more energy than the standard P transmitted waves, especially at long offsets. As a basic requirement, data sets suitable for such an approach need ultra-long offsets, typically in the range of 10–20 km. Nevertheless, recently, Silva & Corcoran (2002) demonstrated the use of an iterative pre-stack depth migration approach employing conventional seismic data (< 4 km offset) for imaging beneath the basalt. The data set used in our analysis (courtesy of Veritas DGC) has been acquired with a single vessel towing a 12 km offset cable. Here, the major problems which are related to dual-vessel acquisition (gaps, noise, different feathering, etc.) are eliminated. The total length of the line was approximately 93 km (Fig. 1). Figure 2 shows the time migrated PP section processed in a standard manner using the first 6 km of offsets. A tentative interpretation has been made, but as shown in Fig. 2, no coherent energy is visible beneath the top of the basalt. The sub-basalt sequence has been interpreted using knowledge of the regional settings. The scope of this paper is to present the ENI-Agip approach to processing such challenging long offset seismic data sets.
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Volumes & issues
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Volume 42 (2024)
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Volume 41 (2023)
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Volume 40 (2022)
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Volume 39 (2021)
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Volume 38 (2020)
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Volume 37 (2019)
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Volume 36 (2018)
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Volume 35 (2017)
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Volume 34 (2016)
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Volume 33 (2015)
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Volume 32 (2014)
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Volume 31 (2013)
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Volume 30 (2012)
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Volume 29 (2011)
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Volume 28 (2010)
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Volume 27 (2009)
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Volume 26 (2008)
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Volume 25 (2007)
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Volume 24 (2006)
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Volume 23 (2005)
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Volume 22 (2004)
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Volume 21 (2003)
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Volume 20 (2002)
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Volume 19 (2001)
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Volume 18 (2000)
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Volume 17 (1999)
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Volume 16 (1998)
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Volume 15 (1997)
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Volume 14 (1996)
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Volume 13 (1995)
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Volume 12 (1994)
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Volume 11 (1993)
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Volume 10 (1992)
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Volume 9 (1991)
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Volume 8 (1990)
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Volume 7 (1989)
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Volume 6 (1988)
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Volume 5 (1987)
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Volume 4 (1986)
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Volume 3 (1985)
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Volume 2 (1984)
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Volume 1 (1983)