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- Volume 43, Issue 6, 1995
Geophysical Prospecting - Volume 43, Issue 6, 1995
Volume 43, Issue 6, 1995
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P‐ and S‐wave anisotropy of a synthetic sandstone with controlled crack geometryl1
Authors J.S. Rathore, E. Fjaer, R.M. Holt and L. RenlieAbstractA method is reported for the production of synthetic porous sandstones containing cracks of known dimensions and geometry with respect to the matrix. A synthetic sandstone was manufactured from Sand cemented with an epoxy glue. The cracks of known geometry were introduced into the material in the manufacturing stage, by emplacing thin metallic discs in the Sand‐epoxy matrix. These discs were chemically leached out of the consolidated porous sandstone. Acoustic anisotropy. and shear‐wave splitting were observed in the synthetic sandstones. For the dry sample the observed angular dependence of the P‐ and S‐ wave velocities (at 100 kHz) compares well, qualitatively, with the theoretical models of Hudson and of Thomsen. Quantitatively, however, the experimental data fits Hudson's model better. For the case of a saturated sample the experimental results are in excellent agreement with Thomsen's model. Hudson's model, on the other hand, predicts a different angular dependence for P‐waves. This demonstrates that the concept of fluid transfer between cracks and the ambient porosity can be a significant process. The results reported here are from the first successful experiment in which the theoretical models were tested on a porous material containing a known crack geometry.
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Phase‐shift migration with a variable‐length spatial transform–an algorithm for moderately varying lateral velocities1
More LessAbstractA migration algorithm appropriate for moderately varying lateral velocity changes is developed as an extension of phase‐shift migration by using a variable‐length spatial transform. This process significantly reduces the number of lateral wave‐numbers necessary to downward continue the data, and it replaces the spatial FFT with a simple recursion relationship. For a given frequency and position x, ten lateral wavenumbers are typically sufficient, and the migration algorithm produces accurate images when the velocity structure V (X, z) changes over a few depth intervals of thickness Δz, with lateral velocity gradients up to 1.4 to 1.0.
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On the application of geophysics in the exploration for copper and chrome ores in Albania1
Authors Alfred Frasheri, Ligor Lubonja and Perparim AlikajAbstractSome generalized results of geophysical exploration for copper sulphide and chromite ores in Albania are presented. The most important geophysical methods used are electrical prospecting, gravity, magnetics and electromagnetics. Physical properties of the ores, genesis and geological problems to be solved have determined the proper choice of any of these methods in the complex exploration.
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Monitoring soil contaminations using a contactless conductivity probe1
Authors Ming Huang, Ce Liu, Liang C. Shen and David ShattuckAbstractIn order to investigate the possibility of using low‐frequency electromagnetic waves to detect and monitor oil contamination of soils, a series of laboratory measurements were performed. A new measurement system to monitor the resistivities of soil and sand samples while samples are being contaminated by diesel oil is presented. The frequency used in measurements is 100 kHz. Since the measurement system is composed of coil‐type transmitters and receivers, there is no need for electrodes to be in contact with samples. The contamination process is simulated using diesel oil dripping on top of soil and sand samples. The conductivity distributions in samples along the sample length are recorded as a function of time. Water‐wet sand and soil samples were measured during diesel oil contamination. The measured data show that the conductivities of soil and sand samples change during the contamination process. The change in resistivity for measured samples before and after diesel oil contamination is in the range of 20% to 50%, giving a reflection coefficient change in the low‐frequency limit of 4.7% to 7%. This amount of change in the reflection coefficient makes it very challenging to detect and monitor oil contamination based on EM reflection from the contaminants. The results suggest that EM methods based on propagation and induction, such as tomography and borehole induction, could be used for this purpose.
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Conventional and modern seismic investigations for rock quality determination at a dam site—a case history1
Authors J. Louis, T. Papadopoulos, G. Drakatos and P. PantzartzisAbstractRefraction seismics and modern tomographic surveys were employed at the Platanovrissi dam site on the Nestos river in northern Greece in an effort to investigate the dynamic elastic properties and rockmass condition of gneiss‐schist outcropping in the area under investigation. Six seismic refraction traverses were run on the ground surface, inside the exploratory tunnels and between tunnels and the ground surface. The results provided information about the low‐velocity surface layer parameters and the stress relief zones on the walls of all the tunnels in which seismic tomography tests were performed. Ultrasonic tests, conducted both in situ and in the laboratory, enabled the determination of the ratio Vp/Vs and the Poisson's ratio calculation. From the tomography data analysis and the geological interpretation of the results, three main velocity zones, ranging between 3500 and 4000, 3000 and 3500 and less than 3000m/s, were detected in each tunnel. High‐velocity zones were found to be compatible with the hard rockmass of gneiss‐schist material, while moderate velocities are closely related to jointed and fractured zones. Finally, low‐velocity zones were well correlated with weathered gneiss‐schist material.
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Use of traveltime skips in refraction analysis to delineate velocity inversion1
Authors H.C. Tewari, M.M. Dixit and P.R.K. MurtyAbstractFirst arrival refraction data does not normally provide any indication of the velocity inversion problem. However, under certain favourable circumstances, when the low‐velocity layer (LVL) is considerably thicker than the overlying higher‐velocity layer (HVL), the velocity inversion can be seen in the form of a traveltime skip. Model Studies show that in such cases the length of the HVL traveltime branch can be used to determine the thickness of the HVL and the magnitude of the traveltime skip in order to determine the thickness of the LVL. This is also applicable in the case of field data.
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Elastic anisotropy due to aligned cracks in porous rock1
By Leon ThomsenAbstractAll theoretical expressions which relate the characteristics of saturated aligned cracks to the associated elastic anisotropy are restricted in some important way, for example to the case of stiff pore fluids, or of the absence of equant porosity, or of a moderately high frequency band. Because of these restrictions, previous theory is not suitable for application to the upper crust, where the pore fluid is brine (Kf≅ K820), the equant porosity is often substantial (φp > 0.1), and the frequency band is sonic to seismic. This work removes these particular restrictions, recognizing in the process an important mechanism of dispersion. A notable feature of these more general expressions is their insensitivity, at low frequency, to the aspect ratio of the cracks; only the crack density is critical. An important conclusion of this more general model is that many insights previously achieved, concerning the shear‐wave splitting due to vertical aligned saturated cracks, are sustained. However, conclusions on crack orientation or crack aspect ratio, which were derived from P‐wave data or from shear‐wave‘critical angles’, may need to be reconsidered. Further, the non‐linear coupling between pores and cracks, due to pressure equalization effects, means that the (linear) Schoenberg‐Muir calculus may not be applied to such systems. The theory receives strong support from recent data by Rathore et al. on artificial samples with controlled crack geometry.
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Reflection points and surface points1
More LessAbstractA simple expression ties the midpoint of a surface spread to reflection points on a dipping plane. If we use two coordinate systems, an unprimed one with a z‐axis perpendicular to the surface and a primed one with a z‐axis perpendicular to the reflector, we have
where θ is the dip angle, φ is the profile angle, X is the source‐to‐receiver separation, and D is the depth of the reflector. The reflection point is (x, yp, D) and the surface midpoint is (xc, yc, 0).Using the expression, I show that if complete azimuthal coverage is required at a CMP position, the reflection points lie on an ellipse. Similarly, a fixed reflection point generates a circle of surface midpoints. A circle of CMP positions for fixed θ and φ becomes an ellipse of reflection points and a circle of reflection points becomes an ellipse of midpoints. A user can easily find the shape and location of the reflection area generated by a surface aperture.
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Finite‐difference elastic wave propagation in 2D heterogeneous transversely isotropic media1
More LessAbstractThe velocity‐stress formulation for propagation of elastic seismic waves through 2D heterogeneous transversely isotropic media of arbitrary orientation is presented. The equations are recast into a finite‐difference scheme and solved numerically using fourth‐order spatial operators and a second‐order temporal operator on a staggered grid. Absorbing, free‐surface and symmetry boundary conditions have been implemented. Test cases compare well with other published solutions. Synthetic seismograms are calculated over two idealized models: (i) vertical fractures in granite with a dolerite sill reflector and (ii) a dipping anisotropic shale. Comparisons with the isotropic counterparts show significant differences which may have to be accounted for in seismic processing in the future.
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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)