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- Volume 36, Issue 7, 1988
Geophysical Prospecting - Volume 36, Issue 7, 1988
Volume 36, Issue 7, 1988
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COMMON REFLECTION POINT DATA‐STACKING TECHNIQUE FOR CONVERTED WAVES1
Authors G. TESSMER and A. BEHLEABSTRACTFor converted waves stacking requires a true common reflection point gather which, in this case, is also a common conversion point (CCP) gather. We consider converted waves of the PS‐ and SP‐type in a stack of horizontal layers.
The coordinates of the conversion points for waves of PS‐ or SP‐type, respectively, in a single homogeneous layer are calculated as a function of the offset, the reflector depth and the velocity ratio vp/vs. Knowledge of the conversion points enables us to gather the seismic traces in a common conversion point (CCP) record. Numerical tests show that the CCP coordinates in a multilayered medium can be approximated by the equations given for a single layer. In practical applications, an a priori estimate of vp/vs is required to obtain the CCP for a given reflector depth.
A series expansion for the traveltime of converted waves as a function of the offset is presented. Numerical examples have been calculated for several truncations. For small offsets, a hyperbolic approximation can be used. For this, the rms velocity of converted waves is defined. A Dix‐type formula, relating the product of the interval velocities of compressional and shear waves to the rms velocity of the converted waves, is presented.
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PHYSICAL PROPERTIES OF SYNTHETIC SANDSTONE ROCKS1
More LessABSTRACTA novel process is used to coat glass spheres with a polymerizing thermo‐setting resin, only microns thick. Synthetic rocks of known grain size distribution and pore space characteristics are then made by heating the resin‐coated glass spheres under compression in a special mould. The dynamic Young's moduli of these rocks are found to be affected by the percentage of resin content (cement) and the synthetic diagenesis rather than the grain size and permeability.
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ULTRASONIC SPECTROSCOPY IN ROCKS: AN EXPERIMENTAL STUDY OF HIGHLY POROUS SYNTHETIC SANDSTONES1
More LessABSTRACTBroad‐band ultrasonic impulses and the attenuation spectral ratio technique have been used to investigate the characteristics of synthetic rocks for different saturants, by obtaining the attenuation coefficients in the frequency range of 0.1‐1.0 MHz.
The general trend of the data indicates that the experimental attenuation coefficient increases with an increase in the grain/pore size. Results show that large grains/pores can increase the attenuation coefficient in all mechanisms, and water‐saturated synthetic sandstone rocks result in a higher attenuation coefficient than similar oil‐saturated rocks.
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SEISMIC MODELLING: HIGH‐FREQUENCY RANGE1
More LessABSTRACTAn increase in the grain/pore size can cause the received echo signal to be distorted due to the preferential attenuation of the high‐frequency components of the transmitted acoustic signal. Therefore, a rock acts as a low‐pass filter and the property of the filter has a linear relation to the grain/pore size. The results have shown that P‐ and S‐wave attenuations can be used to characterize selected internal characteristics of a rock. For example, an increase in the differential stress can cause an increase in the attenuation, due to the sensitivity of the S‐wave to microcracks.
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OPTIMUM EXPRESSION FOR COMPUTATION OF THE GRAVITY FIELD OF A HOMOGENEOUS POLYHEDRAL BODY1
By V. POHÁNKAABSTRACTThe formula for the computation of the gravity field of a homogeneous polyhedral body is derived and transformed into the form which is most suitable when considering speed and simplicity of numerical calculation.
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STATISTICAL TECHNIQUES APPLIED TO BOREHOLE GEOPHYSICAL DATA IN GOLD EXPLORATION1
Authors T. I. URBANCIC and R. C. BAILEYABSTRACTGold, as a trace element, is not directly detectable by geophysical borehole logging techniques. Geophysical logging methods have therefore been used to infer the presence of structural features and alteration processes associated with gold. Since these features and processes differ with the style of gold mineralization, a unique relation between geophysical anomalies and gold cannot be established. However, in a particular environment, such an association can be established. We examined geophysical log data (self potential, induced polarization, resistivity, gamma, temperature, and temperature gradient) and drill core from the Barber‐Larder property in NE Ontario. The geophysically detectable alteration processes associated with gold were identified as sericitization and pyritization, by means of factor analysis. A linear discriminant function was constructed which allowed the zones of economic mineralization (nominally 0.05 oz/ton or 1.5 ppm) to be identified with a 75% success rate based on geophysical log data alone. This rate was achieved without sophisticated non‐linear data transformations.
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THE TIME‐DOMAIN ELECTROMAGNETIC RESPONSE OF POLARIZABLE BODIES: AN APPROXIMATE CONVOLUTION ALGORITHM1
Authors R. S. SMITH, P. W. WALKER, B. D. POLZER and G. F. WESTABSTRACTIt is now believed that the negative transients observed in coincident‐loop transient electromagnetic (TEM) measurements are caused by polarizable bodies (bodies whose conductivity increases as a function of frequency). Ordinarily the TEM response of polarizable bodies is obtained by calculating the frequency‐domain response at many frequencies and transforming it to the time domain via Fourier, Laplace or Hankel transforms. This is normally a computationally laborious task. However, for some simple non‐polarizable bodies the time‐domain response is analytical and can be computed easily. When these simple bodies are weakly polarizable an approximate response can be obtained by convolving the easily‐calculated, non‐polarizable response with the impulse response of the polarization. The approximate response is found to be very similar to the exact response for the polarizabilities normally seen in geological materials.
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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)