摘要:

ABSTRACTA model of parallel slip interfaces simulates the behaviour of a fracture system composed of large, closely spaced, aligned joints. The model admits any fracture system anisotropy: triclinic (the most general), monoclinic, orthorhombic or transversely isotropic, and this is specified by the form of the 3 × 3 fracture system compliance matrix. The fracture system may be embedded in an anisotropic elastic background with no restrictions on the type of anisotropy. To compute the long wavelength equivalent moduli of the fractured medium requires at most the inversion of two 3 × 3 matrices. When the fractures are assumed on average to have rotational symmetry (transversely isotropic fracture system behaviour) and the background is assumed isotropic, the resulting equivalent medium is transversely isotropic and the effect of the additional compliance of the fracture system may be specified by two parameters (in addition to the two isotropic parameters of the isotropic background). Dilute systems of flat aligned microcracks in an isotropic background yield an equivalent medium of the same form as that of the isotropic medium with large joints, i.e. there are two additional parameters due to the presence of the microcracks which play roles in the stress‐strain relations of the equivalent medium identical to those played by the parameters due to the presence of large joints. Thus, knowledge of the total of four parameters describing the anisotropy of such a fractured medium tells nothing of the size or concentration of the aligned fractures but does contain information as to the overall excess compliance due to the fracture system and its orientation. As the aligned microcracks, which were assumed to be ellipsoidal, with very small aspect ratio are allowed to become non‐fiat, i.e. have a growing aspect ratio, the moduli of the equivalent medium begin to diverge from the standard form of the moduli for flat cracks. The divergence is faster for higher crack densities but only becomes significant for microcracks of aspect ratios approachin

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02181.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThree‐component recordings permit the construction of particle trajectories. These three‐dimensional pictures of particle motion show successive predominant directions of polarization and allow wave modes with distinct polarization directions to be recognized. A polarization selection, called ‘spatial directional filtering’, can be accomplished by several methods; four techniques are described. The application of these polarization filters to a noise shot, offset VSPs and CDP stack are also presented. This type of filtering is shown to cancel waves with undue polarization and to enhance the signal‐to‐

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02182.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTMedia containing aligned cracks show anisotropy with respect to elastic wave propagation. There are several models describing the wave propagation in cracked media, most of them only valid for cracks with small aspect ratios. One of these models (Crampin's model) is compared with a model valid for all aspect ratios (Nishizawa's model). The elastic constants and the group velocities are compared for both dry and liquid‐filled inclusions with aspect ratios ranging from 0.0001 (flat cracks) up to 1 (spheres). The difference between both models is small for small aspect ratios but becomes larger for increasing aspect ratios. At a crack density of 0.05 both models give‐within an error of 5%–the same results for aspect ratios up to 0.3. Therefore Crampin's model can be applied to a large range of cracked media even if the aspect ratio of the inclusions is not small. The variation of the anisotropy as a function of the aspect ratio can be studied using Thomsen's dimensionless parameters δ, E and y. They show how inclusions with large aspect ratios result in elliptical anis

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02183.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTMigration of zero‐offset data from a point diffractor gives an elliptic or hyperbolic ‘smile’ when the velocity is too high or too low, respectively (over‐ and undermigration).The phase‐shift within the smile is positive (approximately +π/4) when the velocity is too high, and negative (approximately –π/4) when the velocity is too low. The phase‐shift is calculated from the stationary phase approximation of the Kir

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02184.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe theory by which the Surface Integral Equation method may be applied to the solution of electromagnetic transmission boundary value problems is presented. For a 3D target of arbitrary electrical property contrast with its host medium excited by an arbitrary time‐harmonic source, two integral equations are derived which need to be simultaneously solved for tangential electric and magnetic source density on the target's surface. If the target is 2D, though still excited by an arbitrary source (the 2½ D case), the problem is best solved in the transform domain for a number of different wavenumbers in the target's strike direction. Then a set of four simultaneous scalar integral equations needs to be solved for the components of the surface source density transforms in the target's strike direction and in the direction of the tangent vector to the target's cross‐sectional contour.Examples are presented in which the 2½D problem is solved numerically using the method of moments with piecewise linear basis functions. Although the results generally compare well with analytical solutions, or solutions obtained numerically by other means, errors appear in the calculation of the real response of these targets to excitation by a magnetic dipole source at low frequencies. This is attributed to ill‐conditioning of the system resulting from a non‐unique solution at zero

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02185.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY