Processing of seismic data in the presence of anisotropy
GEOPHYSICS, 1991
Processing techniques for extracting the polarization directions of the fast, and slower split sh... more Processing techniques for extracting the polarization directions of the fast, and slower split shear waves, together with their traveltime splitting from vector wavefield data, form the basis of prospective procedures for imaging details of the internal structure of subsurface rocks. A recently developed technique involving the independent rotation of the source polarization direction, and geophone axes, provides an effective processing tool for extracting the effects of anisotropy from shear‐wave data. It is theoretically capable of handling nonorthogonal polarization directions of split shear waves. It is further developed, and its performance examined using vertical seismic profiling (VSP) shear‐wave datasets for different anisotropic earth models, and computed using full‐wave modeling. The procedure works well for shear waves propagating through an earth model with a uniform crack strike with depth, but produces inaccurate estimates of polarization and time‐delay when the crack ...
Legacy streamer data and newer 3D ocean-bottom-cable data are cross-matched and analysed for time... more Legacy streamer data and newer 3D ocean-bottom-cable data are cross-matched and analysed for time-lapse analysis of geomechanical changes due to production in the Valhall Field. The issues relating to time-lapse analysis using two such distinctly different data sets are addressed to provide an optimal cross-matching workflow that includes 3D warping. Additionally an assessment of the differences between the imaging using single-azimuth streamer and multi-azimuth ocean-bottom-cable data is provided. The 3D warping utilized in the cross-matching procedure is sensitive to acquisition and processing differences but is also found to provide valuable insight into the geometrical changes that occur in the subsurface due to production. As such, this work also provides a demonstration of the use of high-resolution 3D interpreted warping to resolve the 3D heterogeneity of the compaction and subsidence. This is an important tool for Valhall, and possibly other fields, where compaction and subsidence (and monitoring thereof) are key factors in the reservoir management since the predominant observed production-induced changes are compaction of the soft, high-porosity chalk reservoir, due to pore-pressure reduction, and the resultant overburden subsidence. Such reservoir compaction could have significant implications for production by changing permeabilities and production rates. Furthermore the subsidence effects could impact upon subsea installations and well-bore stability. Geomechanical studies that have previously been used to model such subsidence and compaction are only constrained by observed surface displacements and measured reservoir pressure changes, with the geological overburden being largely neglected. The approaches suggested herein provide the potential for monitoring and assessment in three dimensions, including the probable heterogeneity and shearing, that is needed for full understanding of reservoir compaction and the resultant effects on the overburden to, for example, mitigate well-bore failures.
Processing of a nine‐component near‐offset VSP for seismic anisotropy
GEOPHYSICS, 1997
A convolutional sequence of matrix operators is offered as a convenient deterministic scheme for ... more A convolutional sequence of matrix operators is offered as a convenient deterministic scheme for processing a multicomponent vertical seismic profile (VSP). This sequence is applied to a nine‐component near‐offset VSP recorded at the Conoco borehole test facility, Kay County, Oklahoma. These data are corrected for tool spin and near‐surface anisotropy together with source coupling or imbalance. After wave‐field separation using a standard f‐k filter, each source and receiver pair for the upgoing waves is adjusted to a common reference depth using a matrix operator based on the downgoing wave‐field. The up‐ and downgoing waves are then processed for anisotropy by a similarity transformation, to separate the [Formula: see text] and [Formula: see text] waves, from which the anisotropic properties are estimated. These estimates reveal a strong (apparent) vertical birefringence in the near‐surface, but weak or moderate values for the majority of the subsurface. The target zone consists o...
Group velocities for first and second higher mode Rayleigh waves, in the frequency range 0.8-4.8 ... more Group velocities for first and second higher mode Rayleigh waves, in the frequency range 0.8-4.8 f k , generated from a local earthquake of magnitude 3.7 ML in western Scotland, are measured at stations along the 1974 LISPB line. These provide detailed information about the crustal structure west of the line. The data divide the region into seven apparently homogeneous provinces. Averaged higher mode velocity dispersion curves for each province are analysed simultaneously using a linearized inversion technique, yielding regionalized shear velocity profiles down to a depth of 17 km into the upper crust. Shear wave velocity is between 3.0 and 3.4 km s-' in the upper 3, km, with a slow increase to around 3.8 km s-'. P-wave models computed using these results agree with profiles from the LISPB and LUST refraction experiments.
Underwater explosions in the Kirkcaldy Bay region of the Firth of Forth, Scotland, recorded by th... more Underwater explosions in the Kirkcaldy Bay region of the Firth of Forth, Scotland, recorded by the British Geological Survey's LOWNET array (Crampin et al.) generate seismograms which are characterized by a long dispersed wavetrain. On the assumption that these waves are caused by the shape of the dispersion curves, phase and group velocities are analysed to determine the structural parameters of a model of the Kirkcaldy Bay region to which they are most sensitive. The shear-wave velocity and thickness of the sea-bottom sediments are found to be most influential in determining the shape of these dispersion curves. Simple synthetic seismograms constructed for a solid model of the Kirkcaldy Bay region (formed by excluding the water and sediment layers) confirm that the extended wavetrain cannot be caused by the source. The source effects increase the duration of the seismogram by only a few seconds, compared to over 30 s for the effects of dispersion, and are thus not significant. It is demonstrated that, for surface wave studies intended to probe the deeper solid basement using underwater explosions, care must be exercised in selecting a shot location free from significant sediment thickness because these sediments effectively screen structure to between 10 and 100 times their own depth. On the other hand the Kirkcaldy Bay surface wave seismograms also illustrate the detail with which sediment structure may be determined for acoustic and geological studies; for example a 40-m thick sea-bed sedimentary layer with shear-wave velocity of about 0.2 km s-l is resolved overlying a 1.4 km s-l solid layer of 260 m thickness.
Multicomponent seismic data collected using directional sources are degraded by the wave excitati... more Multicomponent seismic data collected using directional sources are degraded by the wave excitation process due t o inaccurate control of the ground motion, unequal activation strengths or ground couplings between differently oriented sources, and misalignment of the pad. These acquisition uncertainties are exacerbated by the complicated near-surface scattering present in most seismic areas. Neither group of effects should be neglected in multicomponent analyses that make use of relative wavefield attributes derived from compressional and shear waves. Thesc effects prevent analysis of the direct and reflected waves using procedures based on standard scalar techniques or a prima fucili interpretation of the vector wavefield properties, even for the seemingly straightforward case of a near-offset vertical seismic profile (VSP). Near-surface correction, using a simpl'e matrix operator designed from the shallowest recordings, alleviates many of these interpretational difficulties in near-offset VSP data. Results from application of this technique to direct waves from a nine-component VSP shot at the Conoco test-site facility, Oklahoma, are encouraging. T h e technique corrects for unexpected compressionalwave energy from shear-wave vibrators and collapses near-surface multiples, thus facilitating further processing for the upgoing wavefield. T h e method provides a simple and effective processing step for routine application to near-offset VSP analyses.
Rayleigh waves in the frequency range 0.7-2.5 Hz are recorded at stations on the Scottish LOWNET ... more Rayleigh waves in the frequency range 0.7-2.5 Hz are recorded at stations on the Scottish LOWNET array, from local underwater explosions in the Kirkcaldy Bay region of the Firth of Forth. The fundamental mode is identified by comparing theoretical and observed particle motion plots derived from seismograms at the Edinburgh station, this being used as a guide for selection at the other stations. The selected signals are then analysed to obtain estimates of the group velocity dispersion characteristics along isolated event-station paths, and this information is interpreted in two ways: (a) The group velocity curves are inverted to obtain shear velocity profiles of each path, these penetrating to depths of between 1.2 q-d ^k m into the crust. The models show an increase of shear velocity with depth. Shear velocity in the upper 400 m of the crust, which includes the surface weathering, lies between 1.4 and 2.1 km sC1. The regional variation of shear velocity correlates with the surface geology, which consists of Carboniferous sediments, Old Red Sandstone and Dalradian rocks. There are wide differences in the velocities of the deepest layer, ranging from 1.9 to 3.5 km s-l. (b) The variation of group slowness with the geological expression is utilized in a linear scheme to obtain pure-provincial group velocities for several geological provinces. The dispersion curves corresponding to the Carboniferous sediments and the Firth of Forth regions decrease smoothly from 1.6 km s-l at 0.7 Hz to 1.2 km sC1 at 1.8 Hz. The group velocities for the Old Red Sandstone formation are about 0.5 km s-l faster. These results also demonstrate that the water layer and seabed sediments do not have a significant effect on the surface wavt propagation.
High-frequency (_<5 Hz) coda waves for velocities of arrival less than 3 km/s, recorded on vertic... more High-frequency (_<5 Hz) coda waves for velocities of arrival less than 3 km/s, recorded on vertical component instruments and generated from a local earthquake in Scotland, are analyzed to ascertain their cause. The adaption of existing velocity modela and scattering from near-surface irregularities in Scotland such as mountains and lochs are considered as possible causes of the observed behavior. The former mechanism is not feasible, as it implies a significant alteration of the velocities in the upper 2 km crust, contradicting previous seismic surveys in the area. An analysis of the effects of scattering is performed using a formalism derived from the Born approximation. The scattered wave field is computed for interactions between first six Rayleigh and Love modes. The general character of the synthetic seismograms for these scattered waves agrees with the observations on a qualitative basis. The apparent absence of the fundamental mode energy from the records is also explained by the synthetic seismograms. The calculations imply that scatterers with a scale length of less than 300 m are applicable to these data from the northernmost stations but around 2 km for the more southem areas. It is thought that the scale length relates to the size of a region on the slopes of the mountains or lochs where there is a sharp gradient. This study emphasises the effectiveness of linear scattering theory in accounting, on a qualitative basis for many of the observed features of the apparently complex coda waves. these velocities. These observations were attributed to complexities arising in the upper 500 m of the crust. The aim of the present study is to afford an explanation as to the cause of this extended portion of the seismographs generated by KEQ and to elucidate the nature of the generating mechanism.
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