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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 122 (1995), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A model for a fractured rock is a transversely isotropic solid containing randomly spaced plane-parallel fractures with random compliance. A fracture constitutive relation is the linear slip model, in which the jump in motion across the fracture is proportional to the stress. I study the propagation at normal and oblique angles of incidence of a shear wave with polarization parallel to the fractures, obtaining the frequency-dependent excess fracture-normal slowness and attenuation. I calculate these quantities using some new methods that are not restricted to weak anisotropy or to low frequency or to any particular fracture constitutive relation. These are Monte Carlo (MC), averaged-multiple imbedding (AMI), path sum theory (PST) and a primary approximation (PA). Finite-frequency results from these dynamic theories are compared with results from quasi-static theory for a variety of rocks, all of which have the same quasi-static anisotropy of 9.5 per cent. I find that (1) quasi-static theory works well at finite frequencies for high-Q fractures if the fractures are stiff and closely spaced, but not if they are compliant and widely spaced; (2) the primary approximation works well if the interfracture distance distribution is exponential but not if it is narrow (periodic fractures) or broad (clumped fractures); (3) reducing the Q of the interfracture rock slightly increases the apparent anisotropy; (4) reducing the Q of the fracture filling material greatly reduces the apparent anisotropy; and (5) increasing the variance of either the fracture spacing distribution or the fracture compliance distribution increases the attenuation. A consequence of (4) is that estimates of fracturing derived from SH seismic data will be erroneously low if the fractures are low-Q. Elementary arguments suggest that overpressuring or lubrication increases the shear Q of natural fractures, thus making them more detectable with SH.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 118 (1994), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Synthetic seismograms are generated by an exact method that automatically separates stratigraphic effects from intrinsic effects. Then three different methods are used to derive rather simple expressions for the effective sound speed of a binary sediment for vertical propagation at wavelengths greater than the average layer thickness. A single-scatter theory (SST) is based on the notion that transmitted energy is reflected once less than absorbed energy. A path sum theory (PST) is used to explicitly sum the contribution of rays corresponding to multiple reflections. An averaged-multiple imbedding theory (AMI) is developed that explicitly averages multiple reflections over the distribution of layer thicknesses. Although each of these theories has a different ansatz they all agree remarkably well with numerical simulation and with each other. Unlike some earlier theories, they do not require that the impedance contrast of the layers decrease as the number of layers increases. They take account of the effect of intrinsic attenuation on multiples as well as the primary wave. They use the layer-thickness probability distributions directly rather than the autocorrelation of the reflectivity or the impedance variation. SST is shown to agree with earlier static theories of effective sound-speed in the limit as m ω→ 0. PST gives a stratigraphic (scattering) Q that vanishes as ω→ 0. Both PST and AM1 permit scattering losses due to spatial irregularities of the sand-shale interfaces, and they are more accurate than SST if reflection coefficients are large.
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 105 (1991), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We introduce two methods for inferring the direction of vertical fractures from marine seismic data. First we derive the necessary reflection and transmission coefficients for an interface between a liquid and an azimuthally anisotropic solid. Next we show that multicomponent ocean bottom seismometer data from surface airgun sources along two perpendicular shot lines can be rotated into the principal directions of azimuthal anisotropy to determine the orientation of vertical fractures. Finally we show that P-wave amplitude versus offset (AVO) depends on the orientation of the shot line with respect to the vertical fractures. Thus P-wave AVO can also be used to determine fracture orientation.
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 111 (1992), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The computation of exact synthetic seismograms for azimuthally anisotropic (AA) models with a frequency-slowness integration method requires two horizontal slowness integrations. However, a single slowness integration that is exact for azimuthally isotropic media requires much less computation time, and has therefore been considered for the AA case as well. As a single slowness integration leads to traveltime and amplitude errors, it should be used for AA media only if these errors are negligible. In this paper we discuss the errors, and outline how they can be estimated. The main contribution to the single-integration traveltime error comes from incorrect group (ray) velocities in the single-integration case. The single-integration group velocities are always greater than or equal to the true group velocities, causing traveltimes to be too small. If there are cusps in the group-velocity surface, not only may the traveltimes be wrong, but there may also be arrivals missing from the seismograms. In layered AA media a minor contribution to the single-integration error arises from not allowing the ray to leave the sagittal plane. The resulting traveltime error is opposite in sign, but much smaller than the traveltime error caused by incorrect group velocities. Amplitudes may be incorrect even though traveltimes are accurate. However, this can only be the case for certain isolated sagittal planes, such as symmetry planes; in other sagittal planes amplitude errors and traveltime errors go together. To decide whether single-integration amplitudes will be accurate one should compute the sagittal velocity curves for single and double integration for a range of azimuths. If the velocity errors are insignificant, then the amplitudes will be accurate for all sagittal planes.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 120 (1995), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The width of a seismic pulse increases monotonically with distance and with Q-1. Estimates of Q from pulse width measurements are often not robust for oscillatory arrivals or for impulsive arrivals in the presence of noise. We present a method to estimate Q from two arrivals using measurements of any signal attribute, ɛ, that is sensitive to propagation loss. The propagation loss is defined as the change in ɛ divided by the difference in traveltime between the arrivals. The first data arrival is used as the reference wavelet. The Q-gram method is based on propagating the reference wavelet with a plane-wave Q-propagator for various values of Q-1. The Q-propagator includes a dispersion relation and the measured difference in traveltime between the data arrivals. The plot of synthetic propagation loss between the reference and propagated wavelets, versus Q-1, is called a Q-gram. The Q-gram, together with the measured propagation loss of the data, gives the Q of the data.The averaged instantaneous frequency f̄ and the averaged instantaneous pulse width f̄ make good signal attributes. Tests on synthetic seismograms show that the Q-gram method, using either f̄ or τ for ɛ, is applicable to both impulsive and oscillatory arrivals and is relatively robust with regard to noise, phase changes and signal clipping.We apply the Q-gram method to horizontal-component airgun ocean-bottom seismometer (OBS) data using the basement-converted shear-wave reflection, PS, as the first arrival and PSSS as the second arrival. We estimate Qβ, the effective sediment shear-wave Q, with an f̄-type Q-gram and a τ-type Q-gram for the PS and PSSS sediment shear-wave reflections. The data indicate that Qβ∼ 75 ± 15, in agreement with results from the application of the spectral-ratio method using windows that exclude interfering arrivals identified by means of the instantaneous frequency.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 104 (1991), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We consider a medium consisting of homogeneous layers separated by curved interfaces. In order to evaluate the response of a single generalized ray, the source and receiver wavefields are expanded in a series of plane waves. The coupling of these plane waves at each point of the surfaces of material discontinuity is determined by means of a Kirchhoff integral using generalized reflection and transmission coefficients. The resulting integral, called the multifold phase space path integral (PSPI) consists of a series of integrals over ray parameters and over interfaces touched by the generalized ray on its way from the source to the receiver. This approach is a generalization of the multifold configuration space path integral (CSPI) to which it reduces by successive application of the stationary phase point method over the ray-parameter integrals.The PSPI like the CSPI automatically includes diffractions from corners. In addition classical head waves are included, although for curved interfaces the head waves are only approximate. 2-D synthetic seismograms are converted to equivalent approximate point-source responses by assuming cylindrical symmetry about source and/or receiver. The waveforms and amplitude of PSPI synthetic seismograms compare well with those computed by generalized ray theory for a 1-D model, and with finite difference synthetics for a 2-D model.
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 97 (1989), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Type of Medium: Electronic Resource
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  • 8
    Publication Date: 2016-07-06
    Description: Predictive habitat suitability models are powerful tools for cost-effective, statistically robust assessment of the environmental drivers of species distributions. The aim of this study was to develop predictive habitat suitability models for two genera of scleractinian corals (LeptoserisandMontipora) found within the mesophotic zone across the main Hawaiian Islands. The mesophotic zone (30–180 m) is challenging to reach, and therefore historically understudied, because it falls between the maximum limit of SCUBA divers and the minimum typical working depth of submersible vehicles. Here, we implement a logistic regression with rare events corrections to account for the scarcity of presence observations within the dataset. These corrections reduced the coefficient error and improved overall prediction success (73.6% and 74.3%) for both original regression models. The final models included depth, rugosity, slope, mean current velocity, and wave height as the best environmental covariates for predicting the occurrence of the two genera in the mesophotic zone. Using an objectively selected theta (“presence”) threshold, the predicted presence probability values (average of 0.051 forLeptoserisand 0.040 forMontipora) were translated to spatially-explicit habitat suitability maps of the main Hawaiian Islands at 25 m grid cell resolution. Our maps are the first of their kind to use extant presence and absence data to examine the habitat preferences of these two dominant mesophotic coral genera across Hawai‘i.
    Electronic ISSN: 2167-8359
    Topics: Biology , Medicine
    Published by PeerJ
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  • 9
    Publication Date: 1975-04-10
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
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  • 10
    Publication Date: 2013-09-01
    Print ISSN: 0921-8181
    Electronic ISSN: 1872-6364
    Topics: Geosciences , Physics
    Published by Elsevier
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