Improvements in the Observation of Shear Waves*

Dohr, G.; Janle, H.

doi:10.1111/j.1365-2478.1980.tb01221.x

Cited by 17 publications

(7 citation statements)

References 3 publications

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“…The range of incidence angles over which the shear-wave particle motion is linear becomes severely limited for high values of a at the free surface, as in the case of plane-wave motion. This is important because high values of a i n sediments, or near the surface of the crust in general, is the rule rather than the exception (AssumpGfio 1978;Dohr & Janle 1980). We alter the effective curvature of the incident wavefront on the free surface of the isotropic half-space model 1A by varying the dominant source frequency, while keeping the spatial dimensions fixed, and examine the effect on the particle motions over a wide range of angles of incidence in Fig.…”

Section: Dependence On Poisson's Ratio and Source Frequencymentioning

confidence: 99%

Shear-wave polarizations on a curved wavefront at an isotropic free surface

Booth

Crampin

1985

Geophysical Journal International

292

200

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We present polarization diagrams of the particle motions at the free surface of an isotropic half-space generated by incident shear waves from a local buried point source. The reflectivity technique is used to calculate synthetic seismograms from which the particle motions are plotted. The particle motions are examined over a range of epicentral distances in a uniform isotropic half-space for different source frequencies and polarization angles, and for different Poisson's ratios. The particle motions due to a curved wavefront possess different characteristics from those generated by plane wavefronts at corresponding incidence angles. A curved wavefront generates a local SP-phase: a P-headwave which propagates along the free surface, and arrives shortly before the direct S-wave. These two arrivals give rise to cruciform particle motions in the sagittal and horizontal planes, which could be misinterpreted as anisotropy-induced shear-wave splitting. An examination of the particle motion in the transverse plane, mutually orthogonal to the sagittal and horizontal planes, can be used to discriminate between isotropic and anisotropic interpretations. The amplitude of the SP-phase is enhanced when it propagates in a low-velocity surface layer overlying the source layer, and may then become the dominant phase on radial-component seismograms. The presence of even a single surface layer may introduce considerable complexity into the seismogram, and we examine the effects of layer thickness, velocity contrast, and source depth on the corresponding polarization diagrams. Reliable information on the source and propagation path characteristics of shear waves from a buried local point source can only be obtained from free-surface records if they are recorded within a very limited epicentral distance range.

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Section: Dependence On Poisson's Ratio and Source Frequencymentioning

confidence: 99%

Shear-wave polarizations on a curved wavefront at an isotropic free surface

Booth

Crampin

1985

Geophysical Journal International

292

200

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“…For some years we tried to use conventional shots in a deep borehole which produced mainly compressional waves (P-waves). We used the reflected S-waves from the deep layered structure as a secondary source of S-waves (Dohr and Janle 1980). The energy of these waves depends strongly on the angle of incidence at the deep interface.…”

mentioning

confidence: 99%

Shear Waves by an Explosive Point‐source: The Earth Surface as a Generator of Converted P‐s Waves*

Fertig¹

1984

Geophysical Prospecting

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FERTIG, J. 1984, Shear Waves by an Explosive Point-Source: The Earth Surface as a Generator of Converted P-S Waves, Geophysical Prospecting 32, 1-17.The most common source of seismic energy is an explosion at some depth in a borehole.The radiated waves are reflected not only at the subsurface layers but also at the free surface.The earth's surface acts as a generator of both P-and S-waves.If the source depth is much less than the dominant wavelength the reflected waves resemble closely the waves generated by a single force. Theoretical seismograms were computed with different methods to look for the relevance of the surface-reflected waves. The numerical experiments show reflected shear waves even for small shotpoint-receiver distances. Due to their polarization these waves can be detected most easily on in-line horizontal geophones. The existence of these waves was examined during a conventional survey in Northern Germany. Conventional data analysis shows a large variability in the up/u, ratio. The method used here produced a shear-wave section with a rather good signal-to-noise ratio down to 4 s S-wave reflection time., I i I N T R O D U C T I O NThe general formulation for the displacements in an elastic medium leads to very complicated formulae. The general solution of these equations is unknown. Only simplifications of the problem or approximations of the medium lead to equations which we can solve analytically, or at least approximately. We have the most complete knowledge for waves propagating in a homogeneous, infinite, isotropic, and ideally elastic medium. In a medium with so many contraints we have a wavepropagation of two independent kinds of waves. The most important one is the P-wave which is faster than the S-wave. Both propagation velocities depend on the elastic parameters of the medium. These velocities describe the kinematical aspects of wave-propagations. The causes of these waves can be described by single forces.

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“…In practice, several configurations of sources and receivers have been used depending on the purpose of the survey. These include (1) three-component acquisition (vertical source recorded by vertical, Hl, and H2 receivers, or zz, zx, and zy components) in VSPs for correlating seismic horizons (Hardage, 1991); (2) vertical and H2 (or SHwave) surveying (zz and yy components) (Conoco Group Shoot, Ensley, 1984;Winterstein and Hanten, 1985;Tatham and McCormack, 1991); and (3) vertical and Hl (or SV-wave) surveying (zz and zx components) for mapping geologic structure and lithology (Dohr, 1985).…”

Section: Four-component Geometrymentioning

confidence: 99%

14. Case Studies of Multicomponent Seismic Data for Fracture Characterization: Austin Chalk Examples

Li¹,

Mueller²

1997

Carbonate Seismology

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Shear wave studies of multicomponent seismic data were done along the Austin Chalk trend in Texas. Six surface seismic lines of four-component shear wave data from Pearsall and Giddings fields and three zero-offset vertical seismic profiles (VSPs) from three sites with different production rates were studied to demonstrate the applications of shear wave splitting for fracture reservoir delineation. The three seismic lines (l-3) from Pearsall field formed a classic experiment for studying shear wave splitting. They have different (line) azimuth with respect to the regional fracture strike (parallel, perpendicular, and at -4O", respectively), are in areas with different hydrocarbon production, and have different split shear wave behavior. The anisotropy along line 1 is small, which correlates with the absence of nearby commercial production. There is an increasing trend in anisotropy from line 2 to 3, which correlates with line 2 being close to and line 3 being within the producing Austin Chalk acreage. The trend of anisotropy variation along line 3 also correlates with the distribution of producing oil wells along that line. In particular, production from the three horizontal wells (Wl-W3) drilled nearby correlates with the variation in shear wave polarization, time delay, and amplitude. Line 4 from Giddings field had a horizontal well drilled on it, and mud logs were obtained for identifying fracture zones. The fracture swarms identified by the mud logs are coincident with the dim spots identified from the section of the slow split shear wave. Lines 5 and 6, also from Giddings field, demonstrate classic S, dim spot and S, versus S2 mistie behavior, respectively.The three VSPs, from three wells with different production rates, show different shear wave responses. VSP 1, from a nonproductive well, shows minimal shear wave time delay and no amplitude anomalies. VSP 2, from a water-producing well, shows some amount of splitting but no anomalies in shear wave attributes. VSP 3, from an oil-producing well, shows clear shear wave splitting and anomalies in shear wave amplitudes. Amplitude corrections are necessary for preserving and recovering shear wave anisotropy information associated with the target, and the linear transform technique (LTT) simplifies the processing sequence for examining anisotropy in shear wave reflection data. Stacked polarization logs can be used for identifying local variations in polarization (often associated with local variations in crack geometry) and for better imaging of subsurface structure, as in line 3.

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Improvements in the Observation of Shear Waves*

Cited by 17 publications

References 3 publications

Shear-wave polarizations on a curved wavefront at an isotropic free surface

Shear-wave polarizations on a curved wavefront at an isotropic free surface

Shear Waves by an Explosive Point‐source: The Earth Surface as a Generator of Converted P‐s Waves*

14. Case Studies of Multicomponent Seismic Data for Fracture Characterization: Austin Chalk Examples

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