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- Volume 40, Issue 8, 1992
Geophysical Prospecting - Volume 40, Issue 8, 1992
Volume 40, Issue 8, 1992
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MULTIPARAMETER GEOPHYSICAL LOGGING AT THE YAVA LEAD DEPOSIT: A STATISTICAL APPROACH1
Authors ANNE CINQ‐MARS, C. J. MWENIFUMBO, P. G. KILLEEN and M. CHOUTEAUAbstractThe present study describes multiparameter geophysical logging carried out at the Yava sandstone lead deposit, Nova Scotia, Canada. Statistical analysis of the multiparameter data set shows that the spectral gamma‐gamma ratio log (SGG ratio) is the most useful technique for characterizing the disseminated sulphide mineralization.
Principal component analysis (PCA) indicates that the apparent chargeability (IP parameter) responds to the presence of clay minerals in the sandstone in addition to disseminated sulphides, so that the induced polarization method (IP) does not accurately delineate the disseminated galena content as was originally assumed in the preliminary log interpretation. PCA has also confirmed that the SGG ratio and density are related to lead content and that lithological variations can be delineated with natural radioactivity and resistivity.
The zinc content of the deposit was poorly characterized by geophysical logs. Sphalerite occurrences seem to be localized as narrow bands (< 1 cm) which were not geophysically detectable.
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OPTIMAL ESTIMATION OF CRACK‐STRIKE1
Authors COLIN MacBETH and GARETH S. YARDLEYAbstractThe accuracy of estimating crack‐strike from the algebraic equivalent of a popular technique, the dual source cumulative technique (DCT), for analysing shear‐wave splitting in seismic experiments is evaluated for earth models permeated by different alignments of micro‐cracks. A complementary analysis is performed using another analysis procedure, the dual‐independent source‐geophone technique (DIT), to investigate any benefits of the alternative formulation. The investigation considers synthetic vertical seismic profile (VSP) and reflection data for an earth model with two layers over a half‐space, and three different classes of crack‐strike variation with depth: uniform crack‐strike, an abrupt change of crack‐strike between the upper and lower layer, and a continuous increase over both layers. The synthetic data for zero‐offset and near‐offset VSPs and a reflection profile are computed using a full‐wave modelling package in which equivalent anisotropic media simulate distributions of aligned vertical, parallel, water‐filled microcracks. Estimates from the two techniques agree for the constant crack‐strike model, but differ for the VSP data with crack‐strike changes. The asymptotic behaviour of the two angular parameters θG and θS from DIT suggest that it may be used to determine crack‐strike under appropriate circumstances in these VSPs, when the time‐delay between the split shear‐waves for the layer of interest exceeds the peak period of the wavelet. In this limit, θG tends to follow the crack‐strike change with θS tending to a constant value, whereas DCT will give a misleading value between the upper and lower crack‐strike. Although the behaviour of DIT is not understood in all cases, θG and θS values from the VSP data always appear to diverge near the point where an abrupt crack‐strike change takes place. This could be used as a qualitative indicator for layer stripping. Both techniques agree for the reflection data as the recorded data matrix is necessarily symmetric, but still give misleading results for deeper layers in the presence of crack‐strike changes. This study suggests that more care should be taken when designing and analysing experimental configurations for detecting crack properties in reservoir rocks, to consider the response and resolution limits of the analysis techniques. A note of caution is offered to those who directly interpret polarization estimates as crack‐strike.
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REFLECTION AMPLITUDES AND MIGRATION AMPLITUDES (ZERO‐OFFSET SITUATION)1
Authors R. BORTFELD and M. KIEHNAbstractThe effect of wave‐equation migration on amplitudes is determined. This effect is derived for zero‐offset traces and for second‐order approximations of the traveltimes. Three steps are followed: firstly, the amplitudes of zero‐offset traces are established; secondly minus half the traveltimes are used as input for downward continuation in migration (forward in space and time); thirdly, the amplitudes of the migrated events are determined by downward continuation (at zero‐traveltimes).
Layered models (piles of homogeneous layers) with smooth interfaces are used. The determinants of the 2 × 2 matrices B0 obtained for these models are responsible for the main effect on migration. The migration result primarily depends on the overburden as the inverse of det (B0). Drastic effects can occur over small distances. For weakly reflecting media, it is confirmed that wave‐equation migration gives “correct” results (but the input data must be multiplied by V0T0), i.e. amplitudes proportional to the reflection coefficient. For any velocity changes, the inverse of det (B0) will, in general, give inaccurate migration amplitudes and inaccurate lithological interpretations. In a simple step, true amplitude migration, or exact migration, is derived from our results.
It is assumed that no focus phenomena are present. The effect of buried foci is discussed briefly.
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FIXED LOOP SOURCE EM MODELLING RESULTS USING 2D FINITE ELEMENTS1
By PIERRE VALLAAbstractAmong electromagnetic sounding techniques, the Mélos method possesses the specific feature of including an apparent resistivity computation. This acts as a normalizing scheme so that 2D modelling results can be obtained without accounting for a true 3D source. However, in order to get reliable numerical modelling results for a 2D magnetic dipole source, improved algorithms are required in order to apply the standard finite‐element technique: quadratic basis functions must be used in place of linear basis functions, and a more sophisticated method than conventional ones is necessary for properly solving the resulting system of linear equations.
Such modelling results have been used to study theoretical responses for the Mélos method in the search for conductive bodies in mineral exploration. Two sets of models are presented and discussed. They show that the typical Mélos response to a conductive target is a bipolar anomaly on the apparent resistivity pseudo‐section, with a conductive pole at low frequency which is centred above the target.
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INTEGRATED INTERPRETATION OF MARINE ENGINEERING GEOLOGICAL AND GEOPHYSICAL DATA ON THE PRINCIPLES OF EXPERT SYSTEM TECHNOLOGY1
Authors E. V. KOVALEVSKY and V. I. KHARCHENKOAbstractWe describe an approach to the construction of an engineering geological expert system for identification of sub‐bottom soils in accordance with some predefined nomenclature. The following principles of integrated interpretation of engineering geophysical and geotechnical data are presented: Firstly, the transformation of physical data (compressional‐ and shear‐wave velocities, compressional‐wave attenuation coefficients, electrical conductivity, etc.) for each of the medium points into subjective probabilities for the soil belonging to each type listed in the nomenclature, and secondly, the extrapolation of local geotechnical data (primarily drilling data) to the surrounding space by means of diffusion of the initial membership function distribution, resulting in the same set of probabilities for soil types at each point in the medium under consideration. Aggregation of the fuzzy information obtained, sufficient for reaching a conclusion for most points in the medium, is carried out by means of Bayesian summation. An example is given of integrated interpretation of real data obtained from four different sources (compressional‐ and shear‐wave velocity sections Vp(x, z) and Vs(x, z), and two boreholes) related to the same profile.
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COMMENT ON ‘THE RESOLUTION OF NARROW LOW‐VELOCITY ZONES WITH THE GENERALIZED RECIPROCAL METHOD’ BY DERECKE PALMER1
More LessAbstractThe following discussion concentrates on one of the synthetic examples used by Palmer (1991) to justify his method of seismic refraction interpretation (Fig. 2, p. 1035) and on his field data and interpretation of the first field example (a collapsed doline) which appeared on pp. 1050–1053.
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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)