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- Volume 21, Issue 10, 2003
First Break - Volume 21, Issue 10, 2003
Volume 21, Issue 10, 2003
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Tutorial: good practice in well ties
Authors R.E. White and R. SimmWell ties are a very important part of the interpreter’s trade. They provide a means of 1. correctly identifying horizons to pick, and 2. estimating the wavelet for inverting seismic data to impedance. Just as well ties are paramount in the calibration of a seismic interpretation, so too they are the cornerstone of using seismic amplitudes in impedance and AVO inversion, and ultimately of inferences fed into the risking process. Figure 1 shows an example where the geologically relevant amplitude information is highly focussed in a particular seismic loop (the blue loop in this case). The wavelet is ~30 degrees rotated from symmetry and its main loop is 24 ms deeper in time than time zero. This is nominally minimum phase data displayed with European polarity, i.e. with negative values corresponding to compressions and plotted as troughs. Given this convention, together with the fact that the red loop is closest to the checkshot time, some interpreters might expect that the red pick is the one to make. If you are after the amplitudes that map out the sedimentology, then it clearly isn’t. Once you know the wavelet, it is evident that the wavelet energy is concentrated in the blue loop and this loop is delayed relative to the checkshot time. Readers are referred to our previous tutorial article (Simm and White 2002) for a discussion of phase, timing, polarity and the interpreter’s wavelet and their impact on seismic interpretation. What is important in this example is that both the wavelet shape and timing were estimated without making assumptions about the wavelet (i.e. what the wavelet should look like) or the timing (i.e. which loop represents the top of the reservoir). The subject of this paper then is the process by which wavelets are estimated through a well tie procedure that results in quantitative measures of synthetic to seismic goodness-of-fit and likely wavelet accuracy. In the authors’ view this is a good practice approach, which should form at the very least the initial stages of a well tie study. Activities such as stretch and squeezing a synthetic to fit a seismic trace are unscientific and definitely not good practice! Regrettably, while ‘stretch and squeeze’ is a common feature of well tie software, q.c. features are at best rudimentary.
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Characterising landfills from the air
By D. BeamishThe British Geological Survey (BGS) continues to highlight the need for, and environmental relevance of, modern, multi-sensor airborne geophysical data in the UK. Here David Beamish, geophysicist with the BGS, describes the electromagnetic component of such survey data and how it may be used to characterise landfills from the air.
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Integrated geophysical surveys applied to karstic studies
Authors R. McDonald and R. DaviesRob McDonald, TerraDat (UK)* and Rob Davies, TerraDat (AUS) show how the use of a combination of non-invasive geophysical techniques can improve the reliability of near surface survey modelling
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Manual approaches to the removal of cultural noise from high-resolution aeromagnetic data acquired over hightly developed areas
By R.J. CussHigh-resolution airborne magnetic surveys in highly developed areas are prone to the problems of cultural noise or anthropogenic signal. This ‘noise’ may be defined as any part of the measured magnetic field that is derived from ‘non-geological’ sources that are not of interest to the study. In many surveys the ‘cultural’ component of the magnetic field can have significant amplitude and may mask features of geological interest, which can be of very low amplitude. The amount of cultural noise encountered tends to reflect the degree of human development in an area. Previous reports define cultural noise as singular, isolated anomalies (Muszala et al. 2001) or long linear magnetic features following pipelines (Gay 1986; Wilson 1997) or power-lines (Pearson 1996, Gharibi & Pedersen 2000). Several attempts have been made at automated removal of cultural noise, including successful application of wavelet analysis (Fedi et al. 2003). Cultural noise is highly troublesome in densely populated areas, such as large parts of the United Kingdom. Traditionally, magnetic surveys have been flown at increased altitudes in areas predicted to yield a high cultural noise content, or have not been flown at all. However, modern surveys are increasingly being conducted in urban and highly developed areas, as this is where water-resource, land-use and environmental problems are the greatest. This article aims to illustrate the level of cultural noise encountered during a high-resolution aeromagnetic survey of the English Midlands and the methods adopted to remove it. Frequency-based methods of filtering out cultural features were tried initially, but with limited success. The amplitude and wavelength characteristics of cultural noise and anomalies reflecting shallow geology were frequently similar. This spectral overlap presented a major obstacle to the numerical or automated removal of cultural noise and the failure of these existing methodologies resulted in our experimentation with manual approaches to deculturing. The two methods that were investigated both involve the identification of possible cultural sources of suspect anomalies on either in-flight video film or UK Ordnance Survey (OS) topography maps (at both 1:50 000 and 1:25 000 scales). Manual deculturing methods are traditionally viewed as slow, labour intensive and therefore costly. They can also introduce network levelling and gridding problems. The aims of our experimentation were to determine how effective the manual processes are at removing cultural noise and to produce a ‘clean’ baseline data set against which the products of quicker automatic and semi-automatic intelligent routines of deculturing can be compared. While little new insight is offered in this work, it is felt important to communicate the scale of the problem encountered and to show that by applying considerable care, it is possible to attain high-quality data.
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Envionmental applications of airborne radiometric surveys
Authors M. Lahti and S. KapotasAirborne gamma-ray measurements, primarily developed for uranium exploration (e.g. Bristow 1983), have many applications in environmental monitoring and geological mapping (e.g. IAEA 1991; Jaques et al.1997; Wilford et al. 1997). For example, they have been used to map radioactive fallout from nuclear accidents (e.g. Mellander 1989) and contaminant plumes from power plants (e.g. Rangelov et al. 1993) as well as monitoring the impact of uranium mining. Airborne measurements can be used for quick and effective mapping of large areas. Although the gamma-ray measurements record variations in the radioactivity of a relatively shallow surface layer (c. 0.3 m), the results are useful for both regional and targeted surveys. The results are normally presented as total gamma activity and as equivalent ground concentrations of uranium, thorium, potassium and other radionuclides (e.g. 137Cs) or as ratios (including ternary K, U, Th plots). Methods have been developed to separate natural and man-made radioactivity. This paper presents some examples of environmental applications of airborne gamma-ray surveys from Germany and the UK. In both cases, the results of multisensor (radiometric, electromagnetic and magnetic) airborne surveys are validated by ground measurements and sampling. The AERA project (Assessment of environmental risks by airborne geophysical techniques validated by geophysical field measurements) was an EC-funded project in south-east Germany, coordinated by the Geological Survey of Finland (GTK) (Gaál et al. 2001). A 1100 km2 site in the Zwickau area of Saxony was selected because of its extensive mining and industrial activities over several centuries. Uranium, black coal and nickel mining and smelting, military activities, modern heavy industry, and industrial and domestic waste have caused considerable environmental impacts. The HiRES-1 survey of central Britain was carried out to assess a range of environmental and resource applications of airborne survey data. The 14000 km2 area surveyed encompasses a wide range of rock and soil types. The area includes many urban centres and has a long history of extractive and manufacturing industry. There are regions with relatively high indoor radon levels and areas contaminated by fallout from nuclear weapons testing, the Chernobyl accident and discharges from the Sellafield nuclear fuel reprocessing plant. Small targets within the HiRES-1 area were flown as part of a collaborative GTK-BGS (British Geological Survey) project to investigate specific sites in more detail. These included landfills, colliery spoil heaps and gravel workings used for power station fly ash disposal. Although a multisensor approach was adopted for these projects, this paper concentrates on the environmental applications of airborne gamma-ray data.
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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)