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- Volume 1, Issue 3, 1983
First Break - Volume 1, Issue 3, 1983
Volume 1, Issue 3, 1983
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Velocity Filtering of Seismic Reflection Data
Authors P.A.F. Christie, V.J. Hughes and B.L.N. KennettThe term 'velocity filtering' may be applied to any process which seeks to separate coherent energy incident upon a seismic array by using the apparent velocity of propagation across the array as a discriminant. The pie-slice process was carried out using a limited multitrace operator in the time-distance (t-x) domain. In order to economise on the convolutions involved, a typical operator consisted of 21 time-points in length, covered 12 traces and resulted in a rejection level about 20 dB down with a less than ideal response function. While the pie-slice or fan filter may be suitable for stacked profiles, we wish to propose the velocity filtering of unstacked data in the frequency-wavenumber (w-k) domain as a more suitable alternative. Providing certain precautions are taken in handling the dataset, several advantages of this procedure are obtained: -the number of output traces is equal to the number of input traces, -the ideal filter response is more closely achieved, -no filter coefficients need be computed beforehand, -implementation of the filter is by straightforward multiplication, -interactive examination of the data in the (w-k) domain permits greater flexibility in the design of the filter response, -since the eye follows group velocity in the (t-x) domain, the presentation in (w-k) space allows a better estimate to be made of phase veloeities and the degree of aliasing, -other wavefield operations may be performed on the data while in (w-k) space. Velocity filtering in the (w-k) domain is particularly important for the high-resolution, single geophone data recorded in the United Kingdom by the National Coal Board and examples of such data are presented in the paper. One of the objects of successful filter design is to achieve maximum separation between those signals which are to be retained and those which are to be suppressed. This may be realised if we can find a suitable representation or domain in which the data can be manipulated. The mute and low pass filter are simple examples of one-dimensional filters using time of arrival and frequency content respectively as discriminants. The two-dimensional velocity filter, as its name suggests, uses the apparent (horizontal) velocity of coherent energy across a seismic aperture to discriminate between wanted and unwanted signals. As such, the concept is not new and indeed the pie-slice or fan filter has been in use for many years now. However, until recently, the implementation of the velocity filter has been performed in the time-distance or (t-x) domain. The advent of truly fast, fast Fourier transtorms has enabled the advantages of working in the more natural (w- k) domain to be realised. The purposes of this paper are to discuss the implementation of the (w-k) algorithm and to point out its advantages with specific regard to the type of high-resolution data recorded by the National Coal Board in the United Kingdom.
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Seismic Impedance
By K. HelbigGeophysical exploration methods depend on differences in physical properties: a gravity anomaly indicates that density is distributed anomalously, a magnetic anomaly points to anomalous magnetisation. These properties are known as the diagnostic properties of the corresponding methods. The most important method of exploration geophysics is reflection seismics, and the corresponding diagnostic property is the seismic impedance: a seismic wave is reflected at the interface between two strata only if they differ in their seismic impedance. Nothing else will do: they can differ in colour, age, density, velocity, porosity, fluid content -all to no avail. If they do not also differ in seismic impedance, the seismic waves will take no notice of the interface. Seismic impedance is thus the most important property rocks possess-at least if we take the (admittedly somewhat special) standpoint of the reflection seismologist. What then hides behind this scientifically sounding name?
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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)