Migration methods for imaging different-order multiples

Multiples contain valuable information about the subsurface, and if properly migrated can provide a wider illumination of the subsurface compared to imaging with VSP primary reflections. In this paper we review three different methods for migrating multiples. The first method is model-based, and it is more sensitive to velocity errors than primary migration; the second method uses a semi-natural Green's function for migrating multiples, where part of the traveltimes are computed from the velocity model, and part of the traveltimes (i.e., natural traveltimes) are picked from the data to construct the imaging condition for multiples; the third method uses cross-correlation of traces. The last two methods are preferred in the sense that they are significantly less sensitive to velocity errors and statics because they use “natural data” to construct part of the migration imaging conditions. Compared with the interferometric (i.e., crosscorrelation) imaging method the semi-natural Green's function method is more computationally efficient and is sometimes less prone to migration artifacts. Numerical tests with 2-D and 3-D VSP data show that a wider subsurface coverage, higher-fold and more balanced illumination of the subsurface can be achieved with multiple migration compared with migration of primary reflections only. However, there can be strong interference from multiples with different orders or primaries when multiples of high order are migrated. One possible solution is to filter primaries and different orders of multiples before migration, and another possible solution is least squares migration of all events. A limitation of multiple migration is encountered for subsalt imaging. Here, the multiples must pass through the salt body more than twice, which amplifies the distortion of the image.     

Static elasticity of cordierite II: effect of molecular CO2 channel constituents on the compressibility

Two natural CO₂-rich cordierite samples (1.00 wt% CO₂, 0.38 wt% H₂O, and 1.65 wt% CO₂, 0.15 wt% H₂O, respectively) were investigated by means of Raman spectroscopy and single-crystal X-ray diffraction at ambient and high pressures. The effect of heavy-ion irradiation (Au 2.2 GeV, fluence of 1 × 10¹² ions cm^?2) on the crystal structure was investigated to characterize the structural alterations complementary to results reported on hydrous cordierite. The linear CO₂ molecules sustained irradiation-induced breakdown with small CO₂-to-CO conversion rates in contrast to the distinct loss of channel H₂O. The maximum CO₂ depletion rate corresponds to ~12 ± 5 % (i.e. ~0.87 and ~1.49 wt% CO₂ according to the two samples, respectively). The elastic properties of CO₂-rich cordierite reveal stiffening due to the CO₂ molecules (non-irradiated: isothermal bulk modulus K ₀ = 120.3 ± 3.7 GPa, irradiated: K ₀ = 109.7 ± 3.7 GPa), but show the equivalent effect of hydrous cordierite to get softer when irradiated. The degree of anisotropy of axial compressibilities and the anomalous elastic softening at increasing pressure agrees with those reported for hydrous cordierite. Nevertheless, the experimental high-pressure measurements using ethanol–methanol reveal a small hysteresis between compression and decompression, together with the noticeable effect of pressure-induced over-hydration at pressures between 4 and 5 GPa.