A source of significant transverse arrivals from an isotropicanisotropic interface, e.g. the Moho

Guest, W. S.; Thomson, C. J.

doi:10.1111/j.1365-246x.1992.tb00579.x

Cited by 9 publications

(6 citation statements)

References 8 publications

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“…Finally, (7) shows that an 'anomalous' mode conversion from P-to-SH results from both shear-wave anisotropy, through the term proportional to dh, and from the term proportional to dy outside the dy. Guest and Thomson (1992) also discuss this effect. For small anisotropy and small aspect ratio, the second term in (7) is dominated by the first term.…”

Section: Modelling Resultsmentioning

confidence: 87%

P‐to‐S conversion for a thin anisotropic zone produced by vertical fracturing

Kelly

Baltensperger²,

McMechan

1997

Geophysical Prospecting

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Point‐source synthetic seismic responses for a thin, fractured bed are generated, interpreted and processed. The synthesis is carried out for a compressional source and multicomponent surface receivers. The anisotropy considered has hexagonal symmetry, with a horizontal symmetry axis, and represents oil‐ and gas‐filled, aligned vertical fractures for a broad range of fracture densities and aspect ratios. P‐to‐S reflected conversions recorded on the horizontal geophones show both kinematic and dynamic anomalies that increase with increasing fracture density and are only weakly dependent on aspect ratio. In contrast, the vertical component P‐wave reflections provide a much poorer diagnostic of fracturing. Analytic expressions for the eigenvalues and eigenvectors of a vertically fractured system are presented, that have the same simplicity as those for transverse isotropy. New linearized expressions for mode‐converted amplitudes are developed for small angles of incidence and are used to interpret the synthetic response.

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Section: Modelling Resultsmentioning

confidence: 87%

P‐to‐S conversion for a thin anisotropic zone produced by vertical fracturing

Kelly

Baltensperger²,

McMechan

1997

Geophysical Prospecting

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“…The source of large‐amplitude SmS arrivals has been explained by using different synthetic models such as a simple crustal structure [ Mori and Helmberger , 1996], a dipping Moho [ Kulhánek , 1990; Catchings and Kohler , 1996], or deep convex structure or deep lens [ Gao et al , 1996], and an isotropic crust/anisotropic upper mantle boundary [ Guest and Thomson , 1992]. From the proposed synthetic models given above, a simple crustal structure model lacking a Conrad discontinuity is not appropriate for the Sea of Marmara.…”

Section: Resultsmentioning

confidence: 99%

Enhanced ground motions due to large‐amplitude critical Moho reflections (SmS) in the Sea of Marmara, Turkey

Boztepe-Güney

2002

Geophysical Research Letters

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Aftershocks of the 17 August 1999, Mw = 7.4, Kocaeli earthquake were used to investigate the large‐amplitude SmS arrivals in the Sea of Marmara. We produced the event record sections from the transverse components of S and SmS arrivals at a distance of 10 to 200 km for the data recorded towards the east and west in the Sea of Marmara. For the data recorded towards the west, we observed that the amplitude of SmS phases enhances the ground motion by a factor of 2 to 3. The dipping Moho and/or anisotropy in the upper mantle or deep convex structure or deep lens are possible sources for the large‐amplitude SmS phases. Since the study area has a potential of creating damaging earthquakes in the future, the effect of large‐amplitude SmS phases on the ground motion should be considered in the studies of the seismic hazard of the region.

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“…It relies upon a known local shear-wave conversion at the top of the reservoir formation, which gives rise to a measurable transverse component associated with the reservoir anisotropy. A similar phenomenon has been studied for the case of a reflection at an isotropic-anisotropic interface by Guest and Thomson (1992), and subsequently applied to upper mantle anisotropy.…”

Section: At Sea With Seismic Anisotropymentioning

confidence: 90%

Estimation of reservoir fracturing from marine VSP using local shear‐wave conversion

MacBeth

Boyd²,

Rizer

et al. 1998

Geophysical Prospecting

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A marine VSP is designed to estimate the orientation and density of fracturing within a gas‐producing dolomite layer in the southern North Sea. The overburden anisotropy is firstly estimated by analysing shear waves converted at or just below the sea‐bed, from airgun sources at four fixed offset azimuths. Full‐wave modelling helps confirm that the background has no more than 3% vertical birefringence, originating from TIH anisotropy with a symmetry axis orientated perpendicular to the maximum horizontal compressive stress of NW–SE. This finding concurs with current hypotheses regarding the background rock matrix in the upper crust. More detailed anisotropy estimates reveal two thin zones with possible polarization reversals and a stronger anisotropy. The seismic anisotropy of the dolomite is then determined from the behaviour of locally converted shear waves, providing a direct link with the physical properties of its fractures. It is possible to utilize this phenomenon due to the large seismic velocity contrast between the dolomite and the surrounding evaporites. Two walkaway VSPs at different azimuths, recorded on three‐component receivers placed inside the target zone, provide the appropriate acquisition design to monitor this behaviour. Anisotropy in the dolomite generates a transverse component energy which scales in proportion to the degree of anisotropy. The relative amplitudes, for this component, between the different walkaway azimuths relate principally to the orientation of the anisotropy. Full‐wave modelling confirms that a 50% vertical birefringence from TIH anisotropy with a similar orientation to the overburden is required to simulate the field observations. This amount of anisotropy is not entirely unexpected for a fine‐grained brittle dolomite with a potentially high fracture intensity, particularly if the fractures contain fluid which renders them compliant to the shear‐wave motion.

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A source of significant transverse arrivals from an isotropicanisotropic interface, e.g. the Moho

Cited by 9 publications

References 8 publications

P‐to‐S conversion for a thin anisotropic zone produced by vertical fracturing

P‐to‐S conversion for a thin anisotropic zone produced by vertical fracturing

Enhanced ground motions due to large‐amplitude critical Moho reflections (SmS) in the Sea of Marmara, Turkey

Estimation of reservoir fracturing from marine VSP using local shear‐wave conversion

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