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- Volume 6, Issue 3, 1988
First Break - Volume 6, Issue 3, 1988
Volume 6, Issue 3, 1988
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A microcomputer program for transformation of two-dimensional magnetic anomalies
By A. AinaThe BASIC computer program described here performs reduction to the equator and orthogonal reduction as used in the interpretation of magnetic survey data, whereby a total field magnetic intensity profile due to a two-dimensional (2D) body with known dip and remanence directions observed at any magnetic latitude is converted to one that would have been observed at the equator or at a magnetic latitude of 45°, respectively, had the body been situated at these locations. Since field data are necessarily of short length, the program provides the option of applying windows of the Hanning or Hamming types to smooth the spectrum of the data, therby minimizing the instability of the edges of the output anomaly observed for short lengths of data. The principal use of the techniques is to determine the position of the sources of magnetic anomalies on the ground surface, thereby facilitating the selection and siting of exploratory drill holes. The calculation of apparent susceptibility distribution via reduction to the pole or equator is also possible (Gunn 1975; Silva and Hohman 1984). An advantage of the methods is that they are independent of the body geometry of the magnetic sources. Other techniques are either based on models which idealize particular geological situations (e.g. Parasnis 1986, p. 44) or utilize some involved geometric construction procedures. The program is based on convolution in the space domain. It is illustrated on field data from Botswana.
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Near-surface sand effects
By V. FriedmannThe Rub' Al-Khali desert, located in the southeastern corner of Saudi Arabia, is an area of gravel plains and salt flats overlain by mountainous sand dunes that are typically 90 m high and commonly reach heights of 180 m. Aramco has conducted various exploration activities in this area for several decades. Hoke (1967) describes some of the early refraction work. Robinson and Al-Husseini (1982) detail the field methodology for modern seismic reflection prospecting. Because of the regular nature of the desert floor, near-surface statics are minimal except in the presence of sand, where one-way static corrections can exceed 100 ms and where seismic continuity, which is usually excellent, decreases markedly. The problem faced by data processing has always been one of improving the stack response underneath sand dunes. It was originally thought that poor coupling and unresolved statics were the primary contributors to degradation. The treatment of statics has received considerable attention and it can now be shown that the loss of stack response is not due to incorrect statics. This article demonstrates that, contrary to previous thoughts, the level of seismic energy actually increases dramatically within sand dunes; but that this energy is trapped by the dune, severely reducing penetration into the subsurface underneath the sand. This observation leads to the design of a frequency-wavenumber (F-K) domain filter which provides a degree of signal enhancement to the field records.
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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)