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- Volume 5, Issue 3, 1987
First Break - Volume 5, Issue 3, 1987
Volume 5, Issue 3, 1987
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The plus-minus method for rapid field processing by portable computer of seismic refraction data in multi-layer groundwater studies
More LessThe major advantage of Hagedoorn's 'plus-minus' method of interpreting seismic refraction sections is that it is simple and quick. It is most frequently used when one shallow interface has to be investigated. But in groundwater surveys more interfaces are often present and greater depths must be reached. In such cases a practical approach is to combine the plus-minus and intercept-time methods. Modem portable computers contribute to maintaining the attractiveness of the plusminus method, allowing data processing in the field to keep pace with data acquisition. A computer program is described that not only allows progress to be monitored rapidly in the field, but also creates graphs and tables suitable for direct reporting. The program also enables long plus-minus profiles to be continuously plotted. Field examples from North Yemen are presented to illustrate the application of the program.
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What can the polarisation of shear waves tell us?
More LessThe direction of polarisation of seismic shear waves and the time shift between shear-wave arrivals of different polarisation contain important clues to the internal structure of the medium through which the waves have passed. For instance, the direction of polarisation can be used to infer the average orientation of cracks (and thus the direction of tectonic stresses), while the time shift depends not only on the orientation of the ray path with respect to the plane of the cracks, but also on the product of crack density and path length through the cracked medium. Before the direction of polarisation and the time shift can be used for this purpose, one must ascertain that these parameters are not contaminated by spurious effects. Disturbances that must be corrected for occur at the free surface, but even in vertical seismic profiles (VSPs) disturbances might occur due to transmission through interfaces: unless the shear waves are polarised parallel or perpendicular to the 'plane of propagation' that contains the ray and the normal to the interface (or unless the angle of incidence is zero), the direction of polarisation changes at transmission due to the different transmission coefficients of the two component waves. For incidence at and beyond the critical angle, the transmission coefficient of the component parallel to the plane of propagation is complex, resulting in a time shift. The effects at the free surface and at individual interfaces are relatively small (though not negligible). However, passage through several interfaces can lead to the accumulation of time shifts. In the limit this accumulation results in two shear waves with different veloeities, i.e. to anisotropy.
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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)