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- Volume 5, Issue 9, 1987
First Break - Volume 5, Issue 9, 1987
Volume 5, Issue 9, 1987
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Simultaneous reflection - refraction acquisition
By R. GarottaThe deeper geological targets of exploration projects are not always detected by seismic reflection. This may be due to weak or progressive impedance contrast or multiple reflections from the overburden. In some cases, seismic refraction can provide deeper information and complement reflection results. When a vibrating source is used, refraction data can be obtained together with conventional reflection data by using simultaneous vibrating techniques. Similar techniques were described by Garotta (1983) for simultaneous recording of several reflection lines, which is especially attractive for 3D acquisition, and by Garotta (1985) for simultaneous recording of conventional and shallow reflection data. Simultaneous recording of several offset VSPs was also described by Naville (1984). In simultaneous reflection and refraction profiling, refraction data acquisition must not in any respect adversely affect the quality of the reflection data, and the production of the conventional reflection crew must not be reduced by more than a few percent. The reflection recording spread can be used for refraction as: 1. 8 to 10 Hz geophones have low enough responses to record refracted energy; 2. refraction events from deeper strata usually have wavelengths of several hundred metres, and thus are not affected by the most comonly used geophane patterns, of some tens of metres; 3. modern reflection spreads are most of ten 4-6 km long, which is a reasonabIe length for a refraction spread; 4. although the usual reflection trace spacing is shorter than for refraction recording, this is not really a drawback.
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The use of the Sheargun® and the Sismopressiomètre in seismic studies
Authors Y. Bertrand, P. Bozetto, J. Lakshmanan and M. SanchezThere is a growing need for earthquake engineering site investigation, particularly for foundation studies of nuclear power plants. Consequently, geophysicists have developed equipment to generate and record shear waves in the ground. It has become clear during recent years that the shear wave velocity and quality factor (Q) are important parameters for the engineering classification of a lithological formation. The main parameter used in earthquake engineering is the shear modulus G related to shear wave velocity Vs by G = pV2s where p is the bulk density. Comparison of various sources, such as sparkers, explosives and purely mechanical devices, has shown us that the latter are the most efficient in creating strong shear waves at reasonable distances. Compagnie de Prospection Géophysique Française (CPGF) has built, jointly with Techniques Louis Ménard, a sonde clamped pneumatically to the walls of a borehole, the Sheargun; it is equipped with a hammer which strikes on an anvil to produce vertically polarised shear waves. The striking system is pneumatically operated from the surface. The strike instant is read with a precision of around 10 microseconds on a signal produced by a ceramic sensor and sent uphole after conversion into logical data within the sonde itself. The receiving device, lowered in a nearby borehole is a tridirectional geophone, also pneumatically clamped to the borehole wall, the Sismopressiomètre. The range of energy produced by the strike varies from 50 to 400 J, according to the diameter of the Sheargun used. The energy is then sufficient to allow a reading accuracy of arrival times (for distances from 10 to 30 m) compatible with the sampling rate of the fastest seismic recorders.
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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)