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- Volume 35, Issue 9, 1987
Geophysical Prospecting - Volume 35, Issue 9, 1987
Volume 35, Issue 9, 1987
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PROCESSING SHEAR‐WAVE VSP DATA*
Authors M. KNECHT and H.A.K. EDELMANNABSTRACTShear‐wave arrivals in vertical seismic profiling recordings made with Vibroseis can be more accurately determined by wavelet processing and spectrum whitening. This method aids discrimination of shear‐wave arrivals against P‐wave disturbances. The data, thus improved, can be used to determine the main axis of shear‐wave polarization for each sounding depth in order to maximize shear‐wave amplitude by properly adding the two horizontal component signals vectorially.
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THE INTERPRETATION OF TRAVELTIME FIELDS IN REFRACTION SEISMOLOGY*
By E. BRÜCKLABSTRACTWe consider multiply covered traveltimes of first or later arrivals which are gathered along a refraction seismic profile. The two‐dimensional distribution of these traveltimes above a coordinate frame generated by the shotpoint axis and the geophone axis or by the common midpoint axis and the offset axis is named a traveltime field.
The application of the principle of reciprocity to the traveltime field implies that for each traveltime value with a negative offset there is a corresponding equal value with positive offset. In appendix A procedures are demonstrated which minimize the observational errors of traveltimes inherent in particular traveltime branches or complete common shotpoint sections.
The application of the principle of parallelism to an area of the traveltime field associated with a particular refractor can be formulated as a partial differential equation corresponding to the type of the vibrating string. The solution of this equation signifies that the two‐dimensional distribution of these traveltimes may be generated by the sum of two one‐dimensional functions which depend on the shotpoint coordinate and the geophone coordinate. Physically, these two functions may be interpreted as the mean traveltime branches of the reverse and the normal shot. In appendix B procedures are described which compute these two functions from real traveltime observations by a least‐squares fit.
The application of these regressed traveltime field data to known time‐to‐depth conversion methods is straightforward and more accurate and flexible than the use of individual traveltime branches. The wavefront method, the plus‐minus method, the generalized reciprocal method and a ray tracing method are considered in detail. A field example demonstrates the adjustment of regressed traveltime fields to observed traveltime data. A time‐to‐depth conversion is also demonstrated applying a ray tracing method.
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WEIGHTED STACKING FOR ROCK PROPERTY ESTIMATION AND DETECTION OF GAS*
Authors G.C. SMITH and P.M. GIDLOWABSTRACTAmplitude versus offset concepts can be used to generate weighted stacking schemes (here called geo‐stack) which can be used in an otherwise standard seismic data processing sequence to display information about rock properties.
The Zoeppritz equations can be simplified and several different approximations appear in the literature. They describe the variation of P‐wave reflection coefficients with the angle of incidence of a P‐wave as a function of the P‐wave velocities, the S‐wave velocities and the densities above and below an interface.
Using a smooth, representative interval velocity model (from boreholes or velocity analyses) and assuming no dip, the angle of incidence can be found as a function of time and offset by iterative ray tracing. In particular, the angle of incidence can be computed for each sample in a normal moveout corrected CMP gather. The approximated Zoeppritz equation can then be fitted to the amplitudes of all the traces at each time sample of the gather, and certain rock properties can be estimated. The estimation of the rock properties is achieved by the application of time‐ and offset‐variant weights to the data samples before stacking. The properties which can be displayed by geo‐stack are: P‐wave reflectivity (or true zero‐offset reflectivity), S‐wave reflectivity, and the reflectivity of P‐wave velocity divided by S‐wave velocity (or ‘pseudo‐Poisson's ratio reflectivity’). If assumptions are made about the relation between P‐wave velocity and S‐wave velocity for water‐bearing clastic silicate rocks, then it is possible to create a display which highlights the presence of gas.
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ENGINEERING GEOPHYSICAL STUDIES OF THE LOVIISA NUCLEAR POWER PLANT SITE, FINLAND*
More LessABSTRACTField studies have been performed in the power plant area of Loviisa in southern Finland to evaluate the suitability of the local bedrock for disposal of low‐level and intermediate‐level reactor wastes. The aim of the borehole geophysical studies was to evaluate the geometry and properties of fracture zones in relatively homogeneous granite.
Of the single‐hole methods, the dipmeter method was used to determine the orientation of individual fractures. The sonic log was used to evaluate the openness (width or thickness) of fractures. Cross‐hole methods such as the seismic and the mise‐à‐la‐masse method were used to determine the geometry and continuity of fractured zones between boreholes.
It was, in general, impossible to evaluate the continuity of single fractures. However, fracture sets can be identified based on the dipmeter data. The continuity of fracture zones can be evaluated with the combined results of single‐ and cross‐hole methods.
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THREE‐DIMENSIONAL INTERPRETATION OF GRAVITY DATA FROM SEDIMENTARY BASINS USING AN EXPONENTIAL DENSITY‐DEPTH FUNCTION*
By H. GRANSERABSTRACTIn mapping the topography of the basement of deep sedimentary basins by gravity modelling, the accuracy can be improved by incorporating an exponential increase in density with depth. For calculating the gravity effect of a three‐dimensional (3D) structure with such an exponential density‐depth relation a frequency‐domain forward algorithm based on series expansion is presented, the numerical evaluation of which can be performed efficiently by fast Fourier transform. The algorithm can be applied in a recursive procedure to give the inverse solution in terms of basement relief.
The inversion procedure is satisfactorily tested on a 2D synthetic example and a 3D field example of gravity data from the western margin of the Pannonian Basin in eastern Austria, where up to 2.2 km of Tertiary sediments overlie an igneous or metamorphic basement. The results are confirmed by basement intersections in several wells.
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FORWARD MODELLING FOR FREQUENCY DOMAIN MARINE ELECTROMAGNETIC SYSTEMS*
By M. GOLDMANABSTRACTAn algorithm for the accurate evaluation of rapidly oscillating integrals is described. The method is based on deformation of the integration path into the complex plane of the integration variable. Numerical integration is then carried out along appropriate cuts where the oscillating factor is transformed to the decaying factor. Contrary to standard methods, the proposed technique permits accurate evaluation of numerically divergent integrals.
The algorithm is especially useful in forward modelling for high‐frequency electromagnetic methods and, in particular, for the new marine electromagnetic system based on measuring signals on the sea bottom at high induction numbers.
Results of calculations using both the proposed and standard methods are compared with available analytical solutions.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 71 (2022 - 2023)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 68 (2020)
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Volume 67 (2019)
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Volume 66 (2018)
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Volume 65 (2017)
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Volume 64 (2015 - 2016)
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Volume 63 (2015)
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Volume 62 (2014)
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Volume 61 (2013)
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Volume 60 (2012)
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Volume 59 (2011)
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Volume 58 (2010)
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Volume 57 (2009)
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Volume 56 (2008)
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Volume 55 (2007)
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Volume 54 (2006)
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Volume 53 (2005)
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Volume 52 (2004)
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Volume 51 (2003)
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Volume 50 (2002)
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Volume 49 (2001)
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Volume 48 (2000)
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Volume 47 (1999)
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Volume 46 (1998)
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Volume 45 (1997)
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Volume 44 (1996)
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Volume 43 (1995)
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Volume 42 (1994)
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Volume 41 (1993)
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Volume 40 (1992)
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Volume 39 (1991)
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Volume 38 (1990)
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Volume 37 (1989)
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Volume 36 (1988)
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Volume 35 (1987)
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Volume 34 (1986)
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Volume 33 (1985)
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Volume 32 (1984)
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Volume 31 (1983)
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Volume 30 (1982)
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Volume 29 (1981)
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Volume 28 (1980)
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Volume 27 (1979)
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Volume 26 (1978)
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Volume 25 (1977)
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Volume 24 (1976)
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Volume 23 (1975)
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Volume 22 (1974)
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Volume 21 (1973)
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Volume 20 (1972)
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Volume 19 (1971)
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Volume 18 (1970)
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Volume 17 (1969)
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Volume 16 (1968)
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Volume 15 (1967)
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Volume 14 (1966)
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Volume 13 (1965)
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Volume 12 (1964)
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Volume 11 (1963)
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Volume 10 (1962)
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Volume 9 (1961)
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Volume 8 (1960)
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Volume 7 (1959)
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Volume 6 (1958)
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Volume 5 (1957)
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Volume 4 (1956)
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Volume 3 (1955)
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Volume 2 (1954)
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Volume 1 (1953)