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  • 1
    Electronic Resource
    Electronic Resource
    VSP traveltime inversion for anisotropy in a buried layer (2003)
    Oxford, UK : Blackwell Science Ltd
    Geophysical prospecting 51 (2003), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: We present a method for calculating the anisotropy parameter of a buried layer by inverting the total traveltimes of direct arrivals travelling from a surface source to a well-bore receiver in a vertical seismic profiling (VSP) geometry. The method assumes two-dimensional media. The medium above the layer of interest (and separated from it by a horizontal interface) can exhibit both anisotropy and inhomogeneity. Both the depth of the interface as well as the velocity field of the overburden are assumed to be known. We assume the layer of interest to be homogeneous and elliptically anisotropic, with the anisotropy described by a single parameter χ. We solve the function describing the traveltime between source and receiver explicitly for χ. The solution is expressed in terms of known quantities, such as the source and receiver locations, and in terms of quantities expressed as functions of the single argument xr, which is the horizontal coordinate of the refraction point on the interface. In view of Fermat's principle, the measured traveltime T possesses a stationary value or, considering direct arrivals, a minimum value, 〈inlineGraphic alt="inline image" href="urn:x-wiley:00168025:GPR0361:GPR_0361_m101" location="equation/GPR_0361_m101.gif"/〉. This gives rise to a key result -- the condition that the actual anisotropy parameter 〈inlineGraphic alt="inline image" href="urn:x-wiley:00168025:GPR0361:GPR_0361_m102" location="equation/GPR_0361_m102.gif"/〉. Owing to the explicit expression 〈inlineGraphic alt="inline image" href="urn:x-wiley:00168025:GPR0361:GPR_0361_m103" location="equation/GPR_0361_m103.gif"/〉, this result allows a direct calculation of 〈inlineGraphic alt="inline image" href="urn:x-wiley:00168025:GPR0361:GPR_0361_m104" location="equation/GPR_0361_m104.gif"/〉 in the layer of interest. We perform an error analysis and show this inverse method to be stable. In particular, for horizontally layered media, a traveltime error of one millisecond results in a typical error of about 20% in the anisotropy parameter. This is almost one order of magnitude less than the error inherent in the slowness method, which uses a similar set of experimental data. We conclude by detailing possible extensions to non-elliptical anisotropy and a non-planar interface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2478.2003.00361.x
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  • 2
    Electronic Resource
    Electronic Resource
    Scaled physical modelling of anisotropic wave propagation: multioffset profiles over an orthorhombic medium (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 107 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In this paper we report further results of scaled physical modelling experiments in the laboratory in which ultrasonic elastic waves are propagated through an anisotropic medium of orthorhombic symmetry. Whereas our earlier experiments consisted for the most part in sending and receiving on opposite faces of a small cube of phenolic laminate, these new results are from multioffset profiles run parallel and at 45d̀ to principal directions on a larger slab of phenolic.The variation of NMO velocity with offset (or angle of incidence) has been determined for compressional and transverse shear waves along profiles in the two principal directions on the 3-face (parallel to laminations) of the slab. These observed group velocities differ from the exact theoretical values by a maximum of about 1 per cent or less and also compare favourably with the theoretical velocities calculated from Thomsen's first-order equations, with maximum differences of about 2 per cent. Differences between the observed and theoretical velocities are attributed to some combination of finite transducer size (geometrical or effective path length effects, array attenuation effects, or interference with the otherwise free surface), sample inhomogeneity and/or anelasticity, and experimental error.The transmission shot gathers acquired for propagation in symmetry planes, and for source-receiver pairs with the same polarization, are similar in form to records acquired over a transversely isotropic medium. The effect of the shear-wave window and the variation of the hyperbolic NMO parameter with offset are clearly seen. Transmission records were also acquired in off-symmetry planes, namely along profiles at 45d̀ to principal directions. On these records, which include all nine possible pairs of source-receiver polarizations, we see clear shear-wave splitting at and near zero offset and more complicated wave effects with increasing offset, such as one or another wave phase dying out. This could be due to cusping of wave surfaces or rapid changes of amplitude and/or polarization with ray direction, possibly as consequences of nearby shear-wave singularities.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb01428.x
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  • 3
    Electronic Resource
    Electronic Resource
    Azimuthally varyingP-wave travel-time residuals in Fennoscandia and lateral inhomogeneity (1973)
    Springer
    Pure and applied geophysics 105 (1973), S. 741-758 
    Details
    ISSN: 1420-9136
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary To the residual data ofEnayatollah [11] are fitted sine curves with back-azimuth as the independent variable. The result is a clear regional trend towards apparently earlyP-wave arrivals from the north relative to those from the south at any one station for δ≥55o. Explanations are plausible, but not entirely satisfactory, on the basis of systematic epicentral mislocation, or of lateral inhomogeneity inP-velocity structure of the Fennoscandian crust and/or uppermost mantle. A zone of lateral inhomogeneity deeper in the mantle, for example a dipping 600-km discontinuity, consistent with the residual observations, is more likely than a shallower emplacement considering the physical implications of extending such a structure to underlie all or most of Fennoscandia. More fundamental inhomogeneous velocity structure throughout most or all of the mantle may have to be considered to enable epicentral determination and travel-time prediction within the accuracy that present instruments would theoretically permit.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00875824
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  • 4
    Electronic Resource
    Electronic Resource
    Slowness and azimuth at the Uppsala array (1973)
    Springer
    Pure and applied geophysics 105 (1973), S. 759-769 
    Details
    ISSN: 1420-9136
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary Relative arrival times at the Uppsala tripartite seismograph array station are used to determinedT/dΔ and ray azimuth for some 200 compressional phases. Corrections, although very small, are applied for station elevations, telephonic transmission delays and array geometry. The computeddT/dΔ and ray azimuths are further corrected to remove the effect of lateral heterogeneity immediately beneath the array station. Errors indT/dΔ and azimuth are considered from a partly theoretical, partly empirical approach. They amount to about 0.18 sec/deg indT/dΔ and 1.6° in azimuth. Standard deviations in epicentral locations are empirically determined to be about 2° due to slowness error and less than 1.5° due to azimutherror; or about 250 km overall. These figures compare favourably with other, more costly, arrays.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00875825
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  • 5
    Electronic Resource
    Electronic Resource
    P-wave velocity anomalies in the earth's mantle from the Uppsala array observations (1974)
    Springer
    Pure and applied geophysics 112 (1974), S. 597-617 
    Details
    ISSN: 1420-9136
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary Distribution of compressional-wave velocities in the mantle is determined fromdT/dΔ measurements using the Uppsala seismograph array station (UPSAS). Short-period vertical-component seismograms from 181 events in the epicentral distance range 16°–100° have been used. The velocity distribution shows anomalous variations at depths of 750, 1500, 1800, 2300 and 2550 km. Evidence of lateral heterogeneity beneath the northern part of the Asian continent, in the depth range 1700–2300 km, is discussed. Computed travel times, based on this velocity-depth relation, are tested by an examination of travel-time residuals from the Long Shot and Milrow explosions on Amchitka, Aleutian Islands.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00877296
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  • 6
    Electronic Resource
    Electronic Resource
    Lateral inhomogeneity in the crust and upper mantle fromP-wave amplitudes (1972)
    Springer
    Pure and applied geophysics 101 (1972), S. 102-154 
    Details
    ISSN: 1420-9136
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary Azimuths and angles of incidence have been calculated from short-periodP-wave amplitude components of 197 events recorded at Umeå, Sweden, near the center of Pleistocene glaciation in Fennoscandia. Anomalies in both sets of data indicate significant departure from a spherically symmetricP-velocity structure. A model incorporating five parameters-strike and dip of equivelocity planes, velocity at the Earth's surface, and at the top and bottom of the heterogeneous zone-is assumed and the parameters are optimized by a computer search. Results, both from the two independent sets of data-incidence-angles and azimuths-as well as from a combined setdirection consines-are mutually consistent and indicate rising equivelocity surfaces, or laterally increasingP velocity, to the ENE in the upper 100 or 200 km of the Earth in the neighbourhood of Umeå. An added refinement, consistent with the results, is an asthenosphere layer pinching out to the ENE, i.e. towards the point of present-day maximum uplift. A surfaceP-wave velocity of about 5.9 km/sec is indicated by the data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00876778
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  • 7
    Electronic Resource
    Electronic Resource
    Slowness and azimuth at the Uppsala array (1973)
    Springer
    Pure and applied geophysics 109 (1973), S. 1623-1637 
    Details
    ISSN: 1420-9136
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary From the Uppsala tripartite array,dT/dΔ and azimuth are determined for 181P and 24PKP phases. Near-surface lateral inhomogeneity is found from azimuth deviations to be small, equivalent to 〈3° dip on the Moho. Slowness, corrected for this structure, is divided into two azimuthal groups: from an ‘Asian’ group are indicated higherP velocities in the lower mantle and a lowerP-velocity gradient in the uppermost mantle, probably a low-velocity layer; the remainingdT/dΔ, from N, W and S azimuths, indicate a more normal velocity profile. There is evidence for a narrow zone of rapid transition between these two structural groups along the Arctic-Soviet continental margin. The few core phases indicate aBolt, rather than aJeffreys, type of core model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00876089
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  • 8
    Electronic Resource
    Electronic Resource
    Strike and dip of crustal boundaries — A method and its application (1971)
    Springer
    Pure and applied geophysics 88 (1971), S. 60-74 
    Details
    ISSN: 1420-9136
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary If for a point on the earth's surface, the apparent (moveout) seismic velocities have been determined for a number of profiles in different directions, a possibility is offered to calculate true strike and dip of crustal boundaries. In the present paper the theory is developed, partly for a sloping Mohorovičić discontinuity only, partly for sloping Conrad and Mohorovičić discontinuities with parallel strikes. The theory for the sloping Mohorovičić discontinuity is applied to field measurements at a place about 30 km west of Uppsala performed in June 1969. With a crustalP-wave velocity of 6.3 km/sec, a good fit to the observations is obtained for aPn-velocity of 8.55 km/sec and a downdip of Moho of 7.2° in the direction S 3.8° E. These numerical values are not the most important output of this paper. On the other hand, the testing of the method appears far more important, and also that improvements can be suggested in the experimental procedure in future applications of this method.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00877893
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  • 9
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    Using virtual reality to estimate aesthetic values of coral reefs (2018)
    Royal Society
    Details
    Publication Date: 2018-04-19
    Description: Aesthetic value, or beauty, is important to the relationship between humans and natural environments and is, therefore, a fundamental socio-economic attribute of conservation alongside other ecosystem services. However, beauty is difficult to quantify and is not estimated well using traditional approaches to monitoring coral-reef aesthetics. To improve the estimation of ecosystem aesthetic values, we developed and implemented a novel framework used to quantify features of coral-reef aesthetics based on people's perceptions of beauty. Three observer groups with different experience to reef environments (Marine Scientist, Experienced Diver and Citizen) were virtually immersed in Australian's Great Barrier Reef (GBR) using 360° images. Perceptions of beauty and observations were used to assess the importance of eight potential attributes of reef-aesthetic value. Among these, heterogeneity, defined by structural complexity and colour diversity, was positively associated with coral-reef-aesthetic values. There were no group-level differences in the way the observer groups perceived reef aesthetics suggesting that past experiences with coral reefs do not necessarily influence the perception of beauty by the observer. The framework developed here provides a generic tool to help identify indicators of aesthetic value applicable to a wide variety of natural systems. The ability to estimate aesthetic values robustly adds an important dimension to the holistic conservation of the GBR, coral reefs worldwide and other natural ecosystems.
    Keywords: environmental science
    Electronic ISSN: 2054-5703
    Topics: Natural Sciences in General
    Published by Royal Society
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  • 10
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    Frequency-dependent velocity analysis and offset-dependent low-frequency amplitude anomalies from hydrocarbon-bearing reservoirs in the southern North Sea, Norwegian sector (2017)
    Society of Exploration Geophysicists
    Details
    Publication Date: 2017-10-06
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
    Published by Society of Exploration Geophysicists
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