This is the final paper in a series on the 3D multicomponent seismic experiment in Oman. In this experiment a 3D data set was acquired using three‐component geophones and with three source orientations. The data set will subsequently be referred to as the Natih 9C3D data set. We present, for the first time, evidence demonstrating that shear waves are sensitive to fluid type in fractured media. Two observations are examined from the Natih 9C3D data where regions of gas are characterized by slow shear‐wave velocities. One is that the shear‐wave splitting map of the Natih reservoir exhibits much larger splitting values over the gas cap on the reservoir. This increase in splitting results from a decrease in the slow shear‐wave velocity which senses both the fractures and the fracture‐filling fluid. Using a new effective‐medium model, it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect. Secondly, an anomaly was encountered on the shear‐wave data directly above the reservoir. The thick Fiqa shale overburden exhibits a low shear‐wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content. No such effects are evident in the conventional P‐wave data. This feature is interpreted as a gas chimney above the reservoir, a conclusion supported by both effective‐medium modelling and the geology. With this new effective‐medium model, we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves (S2), polarized perpendicular to the fracture orientation, whilst leaving the vertical compressional‐wave velocity largely unaffected. This conclusion has direct implications for seismic methods in exploration, appraisal and development of fractured reservoirs and suggests that here we should be utilizing S‐wave data, as well as the conventional P‐wave data, as a direct hydrocarbon indicator.

S U M M A R Y Lighthill and others have expressed the ray-theory limit of Green's function for a point source in a homogeneous anisotropic medium in terms of the slowness-surface Gaussian curvature. Using this form we are able to match with ray theory for inhomogeneous media so that the final solution does not depend on arbitrarily chosen 'ray coordinates' or 'ray parameters' (e.g. take-off angles at the source). The reciprocity property is clearly displayed by this 'ray-coordinate-free' solution. The matching can be performed straightforwardly using global Cartesian coordinates.However, the 'ray-centred' coordinate system (not to be confused with 'ray coordinates') is useful in analysing diffraction problems because it involves 2 X 2 matrices not 3 x 3 matrices. We explore ray-centred coordinates in anisotropic media and show how the usual six characteristic equations for three dimensions can be reduced to four, which in turn can be derived from a new Hamiltonian. The corresponding form of the ray-theory Green's function is obtained. This form is applied in a companion paper.

An S-wavefront from an isotropic region is expected to separate into two fronts when it passes into a gradually more anisotropic region. Standard ray expansions may be used to continue the waves in the anisotropic region when these two S-wavefronts have separated sufficiently. However, just inside the anisotropic region the two S-waves interfere with an effect that is stronger than the usual o-' corrections of the ray method. A waveform distortion can occur and this should be considered when modelling S-waves in, e.g., subduction zones with regions of isotropy grading into regions of anisotropy.The interference is studied here by local analysis of an integral equation obtained by the Green's function method. It is found that if the elasticity and its first two derivatives are continuous at the isotropy/anisotropy border, then zeroth-order ray theory may still be used to continue the incident wave into the anisotropic region. The incident displacement is simply resolved into two definite directions at the point where the anisotropy begins. These two directions are the limits of the unique eigenvectors on the anisotropic rays as the point of isotropy (onset of splitting) is approached. If the nth derivative of the elasticity is discontinuous at the isotropy/anisotropy border, then the scattering integral which describes the interference makes a correction to ray theory which is O ( W -"~+ ' ) in magnitude. Hence, the interference effect is stronger when the emergence of anisotropy is more gradual.Although the corrections are given by simple expressions, it is not reasonable to specify numerical velocity models up to such high-order derivatives. For a smooth interpolation scheme, such as cubic splines, it is more practical to monitor the splitting rays obtained by ray tracing and to use the best-fitting 'equivalent' high-order discontinuity. This will lead t o an estimate of the importance of the correction terms. An example is given for a subduction zone model involving olivine alignment in the mantle-wedge above the slab.

A three‐component refraction data set recorded over the East Greenland Shelf contained two anomalous shear wave arrivals. The direct‐S phase emerged at the receiver as an apparent SH wave even though the airgun source was purely compressional. We have concluded that this arrival is the result of anisotropy in the upper crust. By employing a number of fairly simple techniques we were able to highlight a second, delayed quasi‐shear arrival and interpret shear wave splitting. Modeling of the polarization, amplitude, and delay time of direct‐S suggested that the anisotropy is due to aligned subvertical fractures perpendicular to the local seafloor spreading direction. The second shear arrival of interest is a P to S conversion from the Moho (PmS). The relative strength of this arrival suggests that the Moho is a sharp transition in this region. PmS also contained a significant transverse component but attempts to identify shear wave splitting were inconclusive. However, modeling revealed that the transverse PmS displacement may be due to anisotropy in the upper mantle (alignment of olivine crystals with the spreading direction), which causes the converted S reflection to have a significant SH (transverse) component. An understanding of, and ability to model, anisotropic reflection and transmission effects are essential to these interpretations. Therefore we also outline an efficient method to obtain all of the reflection and transmission coefficients at boundaries between anisotropic media. This has been incorporated into a new ray tracing package, A‐TRAK, based on asymptotic ray theory, capable of modeling three‐dimensional inhomogeneous media separated by curved interfaces.

S U M M A R Y When a P-wave is incident on an isotropic-anisotropic boundary, the reflected S conversion will generally contain some transverse (SH) component. Numerical results show that the magnitude of this SH component is strongly related to the transmitted qP-wave and the form of the P-wave anisotropy (degree and orientation) in the lower medium, rather than the jump in shear wave velocities over the interface. Varying Poisson's ratio in the incident medium changes the amplitude of the reflected SV component, as one would expect, but has minimal effect on the reflected SH signal. Therefore, it is possible to obtain reflected S-waves that are almost purely transverse, even though the source is compressional and the medium of propagation is isotropic. This study of these indirect effects of anisotropy was prompted by anomalous transverse signals in a refraction data set, which is included for comparison.