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- Volume 1, Issue 2, 1983
First Break - Volume 1, Issue 2, 1983
Volume 1, Issue 2, 1983
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The Nature of Seismic Reflections from Coal Seams
Authors V.J. Hughes and B.L.N. KennettThe Coal Measures exhibit a strongly cyclic sequence of bedding, with thin coal seams interleaved with thicker layers of country rock, generally shales or sandstones. The coal itself is an unusual rock type as it has both a low seismic velocity and a low density in comparison with the surrounding rocks. These seismic properties generate a high impedance contrast across any coal-country rock interface and so the reflection coefficient at normal incidence lies between 0.3 and 0.5 with typical values around 0.35. For such a cyclic system with rapidly alternating impedance contrasts, O'Doherty and Anstey (1971) have shown that as the observation point is taken deeper into the layering, the composite transmission effect of the 'peg-leg' multiples in the thin beds can quite quickly become larger than the directly transmitted arrival. Such a mechanism allows a large part of the incident energy to be transmitted through the cyclic stack but with slight delay due to the multiple paths in the thin coal seams. The apparent attenuation of seismic waves in such cyclic structures is frequency dependent (Spencer, Edwards and Sonnad 1977, Schoenberger and Levin 1974,1978). The attenuation will reach a maximum at a frequency which is inversely proportional to the thickness of the layers. Of recent years, there has been considerable interest in the use of seismic methods for coal exploration particularly in the development of mine sites. As a result a number of studies have been made of the seismic response of the coal measures. Dresen and Ullrich (1976) have made scaled down simulations of the coal measures for model seismic experiments. Van Riel (1965) and Rüter and Schepers (1978) have constructed synthetic seismograms for normal incidence plane waves for impedance models of coal sequences. These calculations were performed in the time domain and so the thinnest layers which could be resolved were between 0.75 and 1.00 m. Even for a small number of coal seams, interference effects give very complex reflection seismograms. The restrietion of the minimum thickness of coal seams can be removed by working in the frequency domain. This is the approach adopted by Fertig and MüIler (1978, 1979), who have calculated seismograms at large offsets from a point source with particular interest in conversions from P and S waves. This work allowed the construction of the full response of the Coal Measures including geometrical spreading but the algorithm used by Fertig and Müller did not allow them to analyse the way in which internal multiples contributed to the theoretical seismograms. In this paper, we also make use of theoretical seismograms for offset receivers calculated in the frequency domain, but are able to use the reflection matrix methods discussed by Kennett (1979) to isolate the contribution of the multiples to the final seismograms. The major effect arises from multiples within the coal seams and then the actual disposition of the seams gives rise to complex interference phenomena.
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Seismic Near-Vertical Reflection Studies of the Earth's Crust in the Federal Republic of Germany
Authors R. Meissner and E. LeuschenThe first observations of near-vertical reflections from horizons within the crystalline basement were made by Junger (1951), Dohr (1957) and Dix (1965). Dohr's observations, obtained by standard techniques of seismic reflection methods for prospecting, were sporadic at first and gave a rather diffuse picture of the reflectivity of the continental Earth's crust in Germany. In the 1960s the sporadic measurements developed into a more general application of long-running magnetic tapes, e.g. recording (analogue) up to 12 secs two-way travel time during regular surveys of prospecting crews. In spite of many pessimistic predictions regarding the possibility of receiving any signals at all from the 'homogeneous' basement, the results showed certain clusters of true reflections at various depths of the Earth's crust (Dohr and Fuchs 1967). Especially strong arrivals were obtained at various sites from the Mohorovićić boundary (= Moho) at 10 secs two-way travel time, equivalent to a depth of about 30 km. From the rather short reflecting segments of the first surveys and from the increase of reflectivity towards the base of the crust just on top of the Moho, the concept of reflecting 'lamellas' as the transition from crust to mantle was born (Meissner 1966).
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Starjet - A New Implosive Source
By H. Le BoulchA seismic recording is always affected by the spatial and temporal specifications of the source. In order to obtain the highest possible resolution at all depths, Starjet was designed to create a sharp impulse, short and clean, which remains identical in all directions and can be measured with great accuracy. Starjet is based upon the implosion of several different sized steam bubbles through a system of compact subarrays set perpendicular to the profile. The recording of signatures produced by each subarray can be made at each shot.
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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)