Converted‐wave azimuthal anisotropy in a carbonate foreland basin

Mattocks, B.; Li, Jianchao; Roche, Steven L.

doi:10.1190/1.2148304

Cited by 16 publications

(9 citation statements)

References 6 publications

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“…All of the methods used to analyse seismic anisotropy indicated a sediment column with properties consistent with a HTI symmetry. The orientations of S1 , the fast S ‐wave polarization, were determined by analysis of the variations in amplitude and traveltime ‘pull‐up’ of the radial component, as also described by Mattocks et al (2005) and Bale et al (2009), and the sense of rotation on particle motion plots for each OBS receiver. Transverse component records and particle motion plots suggested a consistent S1 orientation at each OBS that did not appear to change significantly within the depth range investigated.…”

Section: Resultsmentioning

confidence: 99%

Detection of seismic anisotropy using ocean bottom seismometers: a case study from the northern headwall of the Storegga Slide

Exley

Westbrook

Haacke

et al. 2010

Geophysical Journal International

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S U M M A R YAzimuthal seismic anisotropy has been identified from the analysis of S-waves generated by P to S mode conversion in the Pleistocene sediments that form the northern headwall of the Storegga Slide, which were investigated with a seismic experiment employing a seabed array of ocean-bottom seismometers and a grid of airgun shots. The principal technique used to detect the anisotropy was azimuthal stacking of the radial and transverse horizontal geophone components, after the application of moveout, to show the variations in amplitude, phase and cumulative traveltime effects of S-waves, ultimately providing information that identified the 'fast' and 'slow' S-wave polarization orientations. Particle-motion analysis was used to corroborate the results and provide further information on the magnitudes of cumulative S-wave splitting. A 2-D ray-traced inversion of the traveltimes of pre-critical P and PS arrivals provided a velocity model from which the variation with depth of V p , V s and anisotropy could be compared with lithological and stratigraphic data from a borehole at the centre of the OBS array. Increased anisotropic response was observed to be coincident with high velocity units, which have high mica but low water content and are interpreted to be of glacial origin.The analysis of azimuthal seismic anisotropy shows clear evidence for horizontal transverse isotropy or an orthorhombic symmetry. The distribution in orientations of the fast plane of symmetry is broadly bimodal (E-W and NE-SW) across the OBS array. The E-W group showed correlation with the headwalls of old, buried slides and other faults visible within coherency attributes calculated from an accompanying 3-D seismic data set and with the strike of some of the headwalls of slides shown in multibeam bathymetry. However, the pattern of headwall fractures shown in the bathymetry is complicated and reticulate, and the NE-SW orientation is also well represented. We infer that the cause of the anisotropy is the presence of vertical to sub-vertical, fluid-filled fractures and micro-cracks, partially held open by high pore-fluid pressure. The fracture orientations are controlled primarily by the present-day gravitationally induced down-slope stress, which is mediated by the heterogeneous nature of sub-surface, causing local changes in the orientation of the principal stresses at the margins of incipient or failed slides. The fractures, if connected, are likely to increase vertical permeability within the sediment column significantly, and influence the distribution of gas hydrate within the strata.

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Section: Resultsmentioning

confidence: 99%

Detection of seismic anisotropy using ocean bottom seismometers: a case study from the northern headwall of the Storegga Slide

Exley

Westbrook

Haacke

et al. 2010

Geophysical Journal International

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“…Since its amplitude fluctuates regularly with the orientation of the polarization of the S-type transducer, the arrival of the C 1 -wave was identified in the polarized experiment (Fig. 2) (Mattocks et al 2005;Bale et al 2009). However, our laboratory data failed to identify and isolate the C 2 -wave event due to the entanglement of the arrivals of C 2 -and S 1 -waves.…”

Section: Polarized Experimentsmentioning

confidence: 99%

“…On the other hand, it is widely recognized that a polarized shear wave splits into two shear waves in an anisotropic medium. 2) (Mattocks et al 2005;Bale et al 2009). The one that travels with a faster velocity tends to polarize parallel to the isotropic plane of a transverse isotropy Figure 5 The dotted curve and the dashed curve show the azimuthal variations of P-and C 1 -waves reflectivity strengths.…”

Section: Polarized Experimentsmentioning

confidence: 99%

“…1b). At three offset intervals, specifically 5 mm, 10 mm, and 15 mm, acoustic energy was initiated by a compressional P-type transducer, and the reflections were collected by a polarized S-type transducer (Mattocks et al 2005;Bale et al 2009). Commencing from the in-line polarization, reflections were acquired by rotating the active S-type transducer at an angular interval of 45°between each successive observation (Fig.…”

Section: Polarized Experimentsmentioning

confidence: 99%

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Azimuthal variation of converted‐wave amplitude in a reservoir with vertically aligned fractures − a physical model study

Chang

Tseng

2016

Geophysical Prospecting

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The existence of fractures not only provides space for oil and gas to reside in but also creates pathways for their migration. Accurate description of a fractured reservoir is thus an important subject of exploration for geophysicists and petroleum engineers. In reflection seismology, a reservoir of parallel vertical fractures is often considered a transversely isotropic medium with its symmetry axis horizontally oriented and its physical properties varying in azimuth on the horizontal symmetry‐axis plane. In the history of fractured reservoir exploration, azimuthal variation in the P‐wave amplitude, velocity, and fractional difference of the split S‐waves have been popular seismic attributes used to delineate characteristics and extract information from the reservoir. Instead of analysing the reflection signatures of P‐wave and S‐wave, the objective of this study is to demonstrate the azimuthal variation of the converted wave (C‐wave) amplitude in a fractured reservoir. To facilitate our objective, both common offset and end‐on shooting reflection experiments were conducted in different azimuths on the horizontal symmetry‐axis plane of a horizontal transverse isotropic model. In the acquired profile, reflections of P‐wave, PS1‐wave (C1‐), and a mixture of PS2‐ (C2‐) and S1‐waves were observed and identified. Thereafter, the laboratory observations were Hilbert transformed to compute the reflectivity strength of the relative events. Results show that the reflectivity strengths of both P‐ and C1‐waves are consistently weakened from the direction of the layering strike to the layering normal. However, the azimuthal variation of the C1‐wave amplitude is more significant than that of the P‐wave and can be considered another effective seismic attribute for orienting the fracture strike of a reservoir that consists of vertically aligned fractures.

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“…Li, 1998;Mattocks et al, 2005;Sil et al, 2009). The P-wave path could be expected to experience smoothly varying velocity with azimuth.…”

Section: Azimuthal Anisotropy and Shallow Ps Reflectionmentioning

confidence: 99%

Resolution enhancement in shallow 3D-PS seismic reflection

Strong¹

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This thesis aims to advance methodology for acquiring and processing 3D multicomponent seismic data for shallow (< 300 m) exploration targets. The primary focus is to improve seismic resolution, and hence geological interpretation, for coal-scale targets.The coal industry is a significant contributor to the energy security and economy of Australia and the world. Conventional P-wave seismic methods are widely used in this industry, providing economic, safety and environmental benefits. There is potential for expanding these benefits by including multicomponent procedures. In this thesis, the primary focus is on converted-wave (PS) reflection, which is a logical extension to the standard approach. This has theoretical potential to provide extra geological information. It has also been proposed that there may be resolution advantages for shallower targets such as coal.A valuable starting point for understanding resolution in shallow P and PS reflection is via visco-elastic finite-difference simulation. This provides a useful indication of the reflection response of different targets, and can include the influence of different processing flows. For typical coal-scale environments, modelling with reasonable anelasticity assumptions suggests that PS resolution is unlikely to be superior to P resolution, even with idealised acquisition and processing. In real-world situations, achieving good PS resolution may be even more challenging. There are a number of factors across the acquisition and processing flow which incrementally influence resolution.Survey design is intrinsically more complex for PS surveys than for P, primarily because of ray-path asymmetry. In addition, phase and amplitude effects require careful analysis, and finite-difference modelling provides a useful tool. Such modelling suggests that for shallow surveys in particular, it may be possible to incorporate longer relative offsets, compared to the petroleum scale. An examination of bin fold, and offset/azimuth distribution, suggests that the natural bin size of a PS survey is mostly dependent on the receiver spacing, and is generally larger than for P waves. This favours the use of higher receiver densities for multicomponent surveys. These observations can be more important at the shallow scale, again because of greater ray-path asymmetry and potentially higher V P /V S value.One of the most critical steps in the shallow PS processing sequence is correcting for the S-wave receiver statics. Three relatively conventional approaches have been evaluated. A surface-consistent inversion approach has been shown to fail for shallow targets, or in the presence of strong noise. For our 3D data set, PPS refraction analysis provided the preferred solution. In other cases where PPS refractions are poorly defined, our robust statistical approach may be useful for determining short-wavelength statics, although additional long-wavelength control would then be needed.ii In exploration seismology, surface waves are commonly considered to be noise. However, the d...

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Converted‐wave azimuthal anisotropy in a carbonate foreland basin

Cited by 16 publications

References 6 publications

Detection of seismic anisotropy using ocean bottom seismometers: a case study from the northern headwall of the Storegga Slide

Detection of seismic anisotropy using ocean bottom seismometers: a case study from the northern headwall of the Storegga Slide

Azimuthal variation of converted‐wave amplitude in a reservoir with vertically aligned fractures − a physical model study

Resolution enhancement in shallow 3D-PS seismic reflection

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