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- Volume 4, Issue 7, 1986
First Break - Volume 4, Issue 7, 1986
Volume 4, Issue 7, 1986
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What is DMO?
More LessI would like to have entitled this article 'What is Dip Moveout?' but then I would have to admit that there is more than one answer. If we turn to the text book on Applied Geophysics by Telford et al. (1976), we find the original classic definition has to do with the effect of dip on the timing of events on shot records. But the dip moveout I will be describing here is completely different and so I will refer to it only by the acronym DMO. DMO is a migration process which transforms the prestack data set so that each common midpoint (CMP) gather of traces actually contains events from the same depth point, as defined by the normal incidence ray. That this is not the case in the absence of DMO is illustrated in Fig. 1, which shows how the reflector point is dispersed further up dip for increasing offset. For constant velocity, a necessary and linked property of DMO is that, after its application, events with every and any dip stack with the same moveout velocity. For a complex overburden such a transformation requires knowledge of the depth model so that refraction of the ray paths can be taken into account. But it turns out, somewhat surprisingly, that a first-order approximation of DMO is widely applicable, which is not only structurally independent but is also virtually velocity independent. That is, to a very good approximation, all the required information for DMO is contained within the time dips and shooting geometry. Some implementations of DMO incorporate the subsequent stack. As will become apparent this denies to the geophysicist important information regarding the offset dependenee of his data. Accordingly we will confine our attention to DMO processes which can be separated from normal moveout (NMO) and stack.
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Curved ray seismic tomography: application to the Grand Etang Dam (Reunion Island)
Authors J.F. Cottin, P. Deletie, H. Jacquet-Francillon, J. Lakshmanan, Y. Lemoine and M. SanchezCurved ray seismic tomography is used regularly by Compagnie de Prospection Géophysique Française (CPGF) for dam and nuclear site investigation. Measurements are made between tunnels, from hole to surface or, as here from hole to hole. The hydroelectric power development proposed for the Marsouins river (Réunion Island) involves a large capacity dam. Thc only suitable site is Grand Etang, a temporary lake situated in a deep valley, fillcd with heterogeneous materials, several hundred metres thick. During field investigations, a recent basaltic flow 5-3O m thick has been detected at a dcpth of 20-40 m, interbedded with silt and scoria. The following are successively examined in this article: -the data processing programme which takes into account curved ray paths and, after several iterations ends up with an optimum solution; - the hole-to-hole tornography surveys made between 13 drill holes on a 600 m profile; - the overall results, with thc limits and the discontinuities of the basaltic flow, showing that there is no link between the recent flow and older basalts of the valley borders. The hydroelectric development project on the Marsouins river, Reunion Island in the Indian Ocean is situated in a volcanic environment. It includes thc existing Takamaka dam on the river itself; it also involves a large capacity dam for flow regulation, for which the only suitable site is in an adjacent valley at Grand Etang (Fig. 1). This small temporary lake is situated in a deep valley, filled with several hundred metres of heterogeneous material including a recent basaltic flow. The Grand Etang water level varies notably according to rainfall; it does not have any free outlet and empties only by leakage through various pervious levels. Before undertaking the design of the dam and if necessary its impervious cut-offs, it seemed neccssary to secure better knowledge of lithology, structure and permeability of different horizons. In 1981-1982, preliminary surveys included surface resistivity soundings, seismic refraction and drilling. At the projected dam site, a recent basalt flow, 5 to 30 m thick and lying on silt, was discovered under 20 to 40 m of silt and scoriaceous overburden. Figure 2 shows a geological section along the valley. In order to complete information on this flow, a line of 13 drill holes, 40 to 60 m deep, were drilled in 1982-83 along the dam axis at an average spacing of 50 m. Gamma-ray logs were run in these holes. In order to determine the continuity of the flow between the holes and to appreciate the extent of weathering, a hole-to-hole seismic tomography survey was carried out. It should be noted that a previous surface seismic refraction survey had been unable to 'see' through the high velocity basalt flow.
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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)