摘要:

AbstractA reflection response function for a 1D discretized earth model can be obtained using ray‐theory and Z‐transforms with the Goupillaud model. This is usually done by taking the source function as a plane wave impinging normally on the layered earth. Two important problems have been tackled with this basic idea. The first, extraction of the source wavelet, and the second, a description of the free‐surface related problems.In the Goupillaud model, the one‐way traveltime in each layer is taken to be the same time interval At, which is also the time unit for the Z‐transform. The two‐way traveltime in any layer is 2Δt, corresponding to a multiplication byZ2. The reflection impulse response therefore contains only even powers ofZ. The convolution of the reflection response with the wavelet yields a seismogram whoseZ‐transform contains both odd and even powers ofZ. However, even though the seismogram contains more coefficients than unknowns, the wavelet cannot be extracted, because the coefficients are not independent: later coefficients are functions of earlier ones, which does not make sense physically. To overcome this physical problem for the reflection seismogram, the two‐way traveltime through the layer should be Δt.It is then impossible to extract the wavelet, as there are fewer coefficients in the seismogram than unknowns.Szaraniec has proposed a modification to the Goupillaud model, known as the odd‐depth model, that includes the free surface and a top layer whose two‐way traveltime Δtis half the two‐way traveltime 2Δtof all the other layers. Using what Szaraniec calls the fundamental identity of the odd‐depth model, it is possible to extract the source wavelet from the seismogram. We show that this fundamental identity holds only if reflection coefficients of deeper interfaces are functions of the reflection coefficients of shallower interfaces; that is, for extremely improbable geologies.Neither of these approaches offers a solution to the deconvolution problem. It is better to obtain the source signature from measurements in the field. Only Szaraniec's model offers the possibility of tackling the problem of the free surface but because of an inherent flaw in the model, it f

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00862.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe use of relaxation mechanisms has recently made it possible to simulate viscoelastic (Q) effects accurately in time‐domain numerical computations of seismic responses. As a result, seismograms may now be synthesized for models with arbitrary spatial variations in compressional‐ and shear‐wave quality factors (Q9, andQs, as well as in density (ρ) and compressional‐ and shear‐wave velocities (Vp, andVs).Reflections produced byQcontrasts alone may have amplitudes as large as those produced by velocity contrasts.Qeffects, including their interaction withVp,Vsandp, contribute significantly to the seismic response of reservoirs. For band‐limited data at typical seismic frequencies, the effects ofQon reflectivity and attenuation are more visible than those on dispersion.Synthetic examples include practical applications to reservoir exploration, evaluation and monitoring.Qeffects are clearly visible in both surface and offset vertical seismic profile data. Thus, AVO analyses that neglectQmay produce erroneous

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00863.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThis study investigates the propagation of borehole Stoneley waves across permeable structures. By modelling the structure as a zone intersecting the borehole, a simple 1D theory is formulated to treat the interaction of the Stoneley wave with the structure. This is possible because the Stoneley wave is a guided wave, with no geometric spreading as it propagates along the borehole. The interaction occurs because the zone and the surrounding formation possess different Stoneley wavenumbers. Given appropriate representations of the wavenum‐ber, the theory can be applied to treat a variety of structures, including a fluid‐filled fracture. Of special interest are the cases of permeable porous zones and fracture zones. The results show that, while Stoneley wave reflections are generated, strong Stoneley wave attenuation is produced across a very permeable zone. This result is particularly important in explaining the observed strong Stoneley wave attenuation at major fractures where it has been difficult to explain the attenuation in terms of the single planar fracture theory. In addition, by using a simple and sufficiently accurate theory to model the effects of the permeable zone, a fast and efficient method is developed to characterize the fluid transport properties of a permeable fracture z

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00864.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractP‐wave velocity anisotropy determined from a cross‐hole survey at the Imperial College borehole test site compares favourably with that measured in the laboratory on core from the holes. The holes penetrate a layered sequence of sandstones, shales and carbonates of the Namurian Upper Limestone Group. The Laboratory measurements of the vertical and horizontal velocities of core samples indicate that the shales exhibit P‐wave anisotropies of over 20% but that the sandstones and limestones are only slightly anisotropic. These discrete measurements have been used in combination with wireline data to produce a log of P‐wave anisotropy. Including the anisotropic information vastly improves the match between observed and synthetic traveltimes from the cross‐hole data set. This implies that there is little frequency dependence of intrinsic P‐wave anisotropy.Inversion of the cross‐hole traveltimes highlights the need for good angular coverage in order to resolve the anisotropy parameters. The observed P‐wave anisotropy of the field data is due to the combined effect of sedimentary layering and the intrinsic anisotropy of the rocks. The intrinsic anisotropy is found to be the

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00865.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractOne simulation and two field examples from New Jersey illustrate resolution improvement in geoelectrical soundings applied to groundwater exploration. Layered‐earth parameter resolution is derived from data obtained with the commonly used methods of resistivity, induced polarization (IP) and transient electromagnetic (TEM) soundings. Resolution improvement is achieved by simultaneous inversion of two or more data sets and by constraining parameters of the inverse problem.A quantitative analysis showing the contribution of IP data to the resolution of geo‐electric sections is presented. Simultaneous inversion of simple IP data with conventional resistivity and resistivity‐TEM data sets resulted in improved parameter resolution. IP data improved resolution in three ways: (1) by decoupling correlated layered‐earth parameters, (2) by adding information to a geological interpretation about a second physical property, namely chargeability, and (3) by increasing the electrical information av

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00866.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractA depth migration method is presented that uses Radon‐transformed common‐source seismograms as input. It is shown that the Radon depth migration method can be extended to spatially varying velocity depth models by using asymptotic ray theory (ART) to construct wavefield continuation operators. These operators downward continue an incident receiver‐array plane wave and an assumed point‐source wavefield into the subsurface. The migration velocity model is constrained to have longer characteristic wavelengths than the dominant source wavelength such that the ART approximations for the continuation operators are valid.This method is used successfully to migrate two synthetic data examples:1a point diffractor, and2a dipping layer and syncline interface model.It is shown that the Radon migration method has a computational advantage over the standard Kirchhoff migration method in that fewer rays are computed in a main memory impleme

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00867.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY