The promise of elastic anisotropy

PROPERTIES:DYNAMICS An experimental system has been employed to introduce sets of fractures and microcracks oriented in a plane perpendicuiar to the minimum principal stress in cubic sandstone specimens. The results suggest that for sandstones with sets of oriented fractures and cracks, there is a relation between the velocity of shear waves propagating in a direction perpendicular to the plane of the cracks and the permeability, as the cracks are progressively closed by the application of stress. The relation appears to be rock type specific. The experimental system provides the opportunity to study the influence of changes in the intermediate principal stress on the mechanical (static, dynamic and failure properties) and transport (permeability and electrical resistivity) behaviour of porous sedimentary rocks. (from Authors)

958146 Dolomite reservoirs: porosity evolution and reservoir characteristics S. Qing Sun, American Association of Petroleum Geologists Bulletin, 79(2), 1995, pp 186-204. Systematic analyses of the published record of dolomite reservoirs worldwide reveal that the majority of hydrocarbonproducing dolomite reservoirs occurs in: 1) peritidal-domihated carbonate, 2) subtidal carbonate associated with evaporitic tidal fiat]lagoon, 3) subtidal carbonate associated with basinal evaporite, and 4) nonevaporitic carbonate sequence associated with topographic high]unconformity, platform-margin buildup or fault/fracture. Reservoir characteristics vary greatly from one dolomite type to another. This paper discusses the origin of dolomite porosity and demonstrates the porosity evolution and reservoir characteristics of different dolomite types. (from Author)

958147 Modemng of the compaction behaviour of fractured chalk M. Gutierrez, L. Tunbridge, H. Hansteen, A. Makurat, N. Barton & G. H. Landa, Publikasjon - Norges Geotekniske Institutt, 192, 1994, 8 pp; reprinted from: Prec. ISRM symposium EUROCK'94. Numerical modelling of the behaviour of the fractured chalk in the Ekofisk reservoir was performed using the distinct element code UDEC-llB. The non-linear fracture behaviour was modelled by the Barton-Bandis model, while the pore collapse of the rock matrix was modelled by NGI's cap plasticity formulation. The analyses were performed to investigate the effects of fractures, both natural and compaction induced on the compaction behaviour of the Ekofisk reservoir, and to determine the shear/normal displacement and permeability coupling of the fractures during pressure depletion and water injection. The results of the modelling could be used as a basis for explaining the observed behaviour of the Ekofisk field, provide possible guidelines to future actions in the development of the Ekofisk field, and as input to large-seale models of the Ekofisk field. (Authors)

958148 Intercorrelation of capillary pressure derived parameters for sandstones of the Tortel Formation, Hungary A. M. A. El Sayed, Geophysical Transactions - Eotvos Lorand Geophysical Institute of Hungary, 39(1), 1994, pp 7787. Porosity, permeability and capillary pressure data of 50 sandstone core samples obtained from the Tortel Formation have been used to evaluate reservoir quality. Throe types of reservoir rocks and capillary curves are outlined, and various correlation charts constructed to delineate porosity, permeability, pore throat size, recovery efficiency, height above free water level and capillary pressure at different water saturation values of the reservoir rock. The capillary pressure techniques utilized are typically favoured for geological and engineering

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applications to develop sandstone pay zones of the Tortel Formation. (Author)

Compression, swelling and consolidation 958149 Investigation of sediment compaction in the Pannonian Basin using 3D gravity modelling G. Papp & J. Kalmar, Physics of the Earth & Planetary Interiors, 88(2), 1995, pp 89-100. A 1D model was applied to the observed gravity field over a selected area of the Pannonian basin, and an extensive analysis of the sediment compaction was carried out based on a high-resolution digital depth model (DDM) and volume model of the sediments covering the Pannonian basin with an average thickness of 2 km. The obtained density contrast function resulted in apparent density values for the sediments higher than given by the reference model used in 3D gravity field model computations. This systematic deviation may be a consequence of the crustal thinning which is supposed to be a consequence of basin evolution. A crust-mantle model of the investigated area extending below the Moho discontinuity was used to support this assumption. (from Authors)

958150 Numerical modeling of freezing and swelling processes in the 'foundation - bed' system A. B. Fadeev, I. I. Sakharov & P. I. gepina, Soil Mechanics & Foundation Engineering, 31(5), 1995, pp 161-165; translated from: Osnovaniya, Fandamenty i Mekhanika Grunter, 5, 1994, pp 6-9. A method of modelling the process of soil swelling as the bed freezes through is investigated. An algorithm for FEM calculation and basic characteristic features of the developed software are presented. The potential of the software is illustrated in examples of the solution of a number of problems. (Journal summary)

958151 General solution of the spatial problem of the theory of interrelated seepage consolidation V. B. Shvets & V. G. Shapoval, Soil Mechanics & Foundation Engineering, 31(5), 1995, pp 177-180; translated from: Osnovaniya, Fundamenty i Mekhanika Grunter, 5, 1994, pp 19-21. An exact solution is obtained for the three-dimensional problem of the theory of seepage consolidation with axial symmetry, which is distinguished from familar approximate solutions by consideration of Poisson's ratio in computational equations. This makes it possible to predict more accurately the development of strains and stresses in saturated beds over time. (Journal summary)

Dynamic properties 958152 The promise of elastic anisotropy P. Armstrong & 10 others, Oilfield Review, 6(4), 1994, pp 3647. Elastic anisotropy has blossomed from a bothersome anomaly into a promising measure of reservoir properties. This phenomenon, in which waves travel with different velocities in different directions, indicates the presence of alignments such as cracks of fractures, tectonic stresses, depositional layers or ordered anisotropic grains. Formerly of concern only to theorists, elastic anisotropy can now be

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PROPERTIES:DYNAMICS

measured in ultrasonic, sonic, crosswell and borehole seismic experiments. This article examines how elastic anisotropy is being used to enhance reservoir description and impact reservoir development decisions. (Authors)

958153 Plane waves in a semi-infinite fluid saturated porous medium R. De Boer & Zhanfang Liu, Transport in Porous Media, 16(2), 1994, pp 147-173. The field equations governing the propagation of waves in an incompressible liquid-saturated porous medium are investigated and a general solution is presented. It has been revealed that coupled longitudinal and transverse waves propagate in the porous medium. The propagation of transverse waves in the fluid phase is completely due to the interaction between the solid and fluid phases. The dispersion relationship and attenuation features are discussed. The reflection of the plane harmonic waves at the plane, traction-free boundary, which shows the influence of the dissipation on the velocity, and the attenuation coefficients of the reflected waves is studied. (from Authors)

958154 Boundary element method with exterior collocation in twodimensional elastodynamic problems T. M. Fu, J. G. Xiong & Y. K. Cheung, Earthquake Engineering & Structural Dynamics, 24(1), 1995, pp 99-108. The boundary clement method with exterior collocation was applied to two-dimensional clastodynamic problems. The stability of the numerical solution was discussed and the collocation rule of source points outside the region studied was investigated in the frequency domain by means of the computed error in the boundary displacement and the condition number of the coefficient matrix for two typical wave propagation problems. The achieved results are helpful to the practical application of this method to earthquake ground motion analysis. (Authors)

958155 Three-dimensional response of a layered cylindrical valley embedded in a layered half-space J. E. Luco & F. C. P. De Barros, Earthquake Engineering & Structural Dynamics, 24(1), 1995, pp 109-125. An indirect boundary integral method to obtain the threedimensional response of an infinitely long, layered, viscoelastic valley of arbitrary cross-section embedded in a layered viscoelastic half-space is presented. Additional comparisons with previous three-dimensioual results obtained by a hybrid finite element-boundary integral method for cylindrical valleys subjected to obliquely incident waves show large differences. However, the results obtained here for an infinitely long valley appear to be in some agreement with earlier results for an elongated prelate semi-ellipsoidal valley and with results obtained by a discrete wavenumber boundary element approach. An extensive bibliography on the dynamic response of valleys is also presented. (from Authors)

958156 Basin effects analysis from a dense strong motion observation network Kuo-Liang Wen, Han-Yih Peng & Liang-Fang Liu, Earthquake Engineering & Structural Dynamics, 24(8), 1995, pp 1069-1083. Forty-three free-field accelerometers have been installed in the Taipei Basin. Three events with magnitudes of 5.7, 5.4 and 6.2 that triggered more than ten stations are used here to study the characteristics of the basin effects on ground motions both in the time and frequency domains. From this it

is clear that vibrations of different frequencies have different amplifications in the Taipei Basin. In fact, the frequency responses show the variation can even be up to about 7 times. The horizontal peak ground acceleration and spectral ratio contours in the low-frequency band are closely correlated with the geological structure in the Taipei Basin. The analysis of the response spectra also shows this correlation phenomenon and the relation between the damage from an earthquake and the basin effects. (from Authors)

958157 Pseudo relative velocity spectra of earthquake ground motion at high frequencies M. D. Trifunae, Earthquake Engineering & Structural Dynamics, 24(8), 1995, pp 1113-1130. Presents a method for extrapolation of pseudo-relative velocity spectral amplitudes of strong earthquake shaking to short periods (0.01 < T < 0.04 see). The extrapolated spectra can be used as a physical basis for defining design spectral amplitudes in this higher-frequency range. The analysis implies that for typical strong motion accelerations, particularly on sedimentary sites in California, the peak ground accelerations are projected to be unaffected by frequencies higher than those recorded. Consequently, in California, the high-frequency pseudo-acceleration spectra can be approximated from the recorded absolute peak accelerations, (Author)

958158 Vertical increase in seismic velocity with depth in shallow crystalline bedrock O. Okko, Journal of Applied Geophysics, 32(4), 1994, pp 335345. There arc numerous examples of the increase in seismic velocity with the depth of crystalline bedrock due to the lithological confining stress that compresses the open fractures in the shallow bedrock. The resistivity logs obtained in 500-1000 m deep boreholes correspond well with the open fractures observed along the core samples down to a depth of 150-200 m. In the corresponding sonic logs, however, the seismic velocity is constant below the casing at depths greater than 40 m. The reflection seismic soundings suggest an increase in the velocity at the very surface of the crystalline bedrock. Reflections from shallow depths of less than 100 m show higher velocities than the direct refracted P-wave at the outcropped surface. In the full waveform sonic log there is an increase in the P-wave velocity within the uppermost 15-20 m. Very slow tube waves are also typical for this depth interval. (Author)

958159 Determination of shallow refractor properties by 3D-CMP refraction seismic techniques T. Ruhl, First Break, 13(2), 1995, pp 69-77. The determination of shallow refractor properties is a very important task in processing of land seismic reflection data, because inhomogeneities in the near-surface low-velocity layers can severely distort the seismic image of the subsurface. Static corrections aim at compensating for these anomalies in order to obtain a correct structural image of deeper reflectors. In engineering and environmental geophysics the target is the near-surface structure itself. There is a strong need for highresolution methods of structural imaging and determination of the velocity distribution. This paper presents a simple, robust and very promising method to evaluate first breaks from land seismic surveys, the so-called 'CMP refraction seismic technique'. The main idea of this technique is to carry out a common midpoint sorting of traveltimes of refracted arrivals. This allows a simple, straightforward and accurate determination of shallow refractor properties. Having corn-