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- Volume 5, Issue 10, 1987
First Break - Volume 5, Issue 10, 1987
Volume 5, Issue 10, 1987
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The relationship of pre-stack apparent velocity filtering to the symmetry of the CMP stack response
By G. HampsonWearing my processing geophysicist's hat, I, like others, often ask questions to which there appear to be no answers readily available. One such question that had re-occurred to me was 'what is the relationship between pre-stack apparent velocity filtering and the common midpoint (CMP) stack response?' The elements of any answer were roughly -potential reduction in spatial resolution; -reduction in coherent noise; -apparent velocity in common source or geophone space is twice that observed on the CMP stack. During the summer of 1983 Walt Lynn presented some material at Western Geophysical's London office (as part of a week long 'Wave Equation Update School' lecture course given by Jon F. C1aerbout) entitled 'Dip filtering for noise suppression'. Part of this graphical material demonstrated how apparent velocity filtering could be viewed in source-geophone space as removing ciearly definable parts of a pre-stack diffraction. Upon this basis the following text is an attempt to answer my question by doing three things: • Setting down the necessary equations. • Showing how certain symmetry conditions affect the CMP stack. • Computing synthetic examples. The text will consider a point scatterer in the subsurface, how it responds to the CMP method and what happens when pre-stack apparent velocity filtering is included. The most important conclusion will be that certain apparent velocity filter symmetry conditions are necessary if the CMP stack is to respond similarly to positive and negative dips. The analysis will commence by describing the prestack traveltime surface that is associated with the point scatterer, which is sometimes called Cheop's Pyramid. Equations for apparent velocity on the commonly used axes of Cheop's Pyramid will be derived which show how certain areas of the surface are rejected by apparent velocity discrimination. The idea of symmetry will be described and a set of rules will be presented in order to test whether the pre-stack apparent velocity filter will prejudice dip direction on the CMP stack. Some computed examples will be presented along with a brief description of the computational method used. Finally a potential problem with 'marine' three-dimensional (3D) surveys that can be caused by pre-stack apparent velocity filtering will be briefly discussed.
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Compact formats for seismic data storage
By S. KravisSeismic data are usually stored using a 32-bit floating-point format for each data sample after conversion of the field tapes to whatever internal format is used by the processing system. This procedure ensures that there is no possible loss of dynamic range as field data are always recorded with less than 32 significant bits, and also minimizes the amount of work that the central processing unit (CPU) has to do, as data may be transferred without conversion from the mass storage device. However, the increasing power of modern CPUs offers the possibility of using a compact format of less than 32 bits per sample in the mass storage medium, and using the CPU to expand the data to 32-bit floating-point format once it has been read in. This procedure reduces the input/output (I/O) load at the cost of increasing the CPU load, which is not unreasonable as CPU costs are at present decreasing much faster than I/O costs. Compact storage has a number of operational benefits: if more data can be fitted on to each tape, the number of tape changes per shift is reduced proportionately, and smaller tape libraries are required. Disc capacity is almost invariably a scarce resource, and any increase in seismic data storage capacity at zero capital cost will be met with broad smiles from the system manager. A further benefit accrues to processing centres which do not have operators on duty for 24 hours per day to mount and dismount tapes. The amount of work which can be done during the unattended period is often limited by the amount of data which can be stored on disc. Tape drives are often a bottleneck in system productivity and they are seldom used at their maximum speed. If processing can be done from disc, the tapes are used as spooling devices and can operate much closer to their maximum capacity. The actual increase in storage capacity will be smaller for tapes than for disc, as most internal tape formats are unblocked; they have an end-of-record gap consisting of half an inch of blank tape between each data trace. For a high density, 6250-bpi, 9-track tape, the data samples of a 1500-sample trace using 32 bits per sample will only occupy 1 inch of tape, so that each trace requires 1.5 inches of tape. Using a 16-bit format for the data samples would reduce the amount of tape needed to 1 inch, thus raising the number of traces fitting on to a reel by 50%. For disc, the storage capacity would be nearly doubled by using 16-bit data storage.
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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)