Compressional and shear wave velocities (Vp and Vs) have been measured to 10 kbar in 17 granulite facies rocks and 15 eclogites. The former included quartzo-feldspathic, gabbroic, and garnet granulites as well as mangerites from the Adirondack region. The eclogites included the three types described by Coleman et al. from California, Norway, South Africa, and Tasmania. The mean ranges of the values of (∂Vp/∂P)T and (∂Vs/∂P)T in the 8- to 10-kbar range are slightly higher (0.015–0.022 and 0.008–0.012 km/s kbar) for eclogites than for granulites. Velocity-density systematics, based on the 10-kbar data, is evaluated in the light of Birch's law, D. L. Anderson's seismic equation of state, Wang's C-ρ relation, and the K-V relationship of O. L. Anderson and Nafe and D. L. Anderson and O. L. Anderson. On close analysis, there is a distinction in Wang's relations for ∼ 21 and ∼ 22. Most of the granulites and eclogites have values of ∼22; Birch's relationship for these and previously studied basalts ( ∼ 22) is Vp = −1.85 + 2.87ρ, where r2 = 96%. For Vs the best fit of the data for granulites and eclogites ( ∼ 22) is Vs = −0.33 + 1.40ρ, where r2 = 88%. The best-fit equation for Vs that includes the calcium oxide effect in all the granulites and eclogites is Vs = −0.63 + 0.21(21 − ) + 1.56ρ + 0.016 [CaO]. On the basis of the laboratory results, it is shown that elastic properties of garnet granulite with ∼ 22 are compatible with those of the 7.1- to 7.8-km/s crustal layer. Analysis of crust-mantle seismic data in various regions, and the present laboratory results, shows that the assumption of ∼ 22 for the lower crust as well as for the upper mantle fits better than ∼ 21. On the other hand, the ∼ 21 model fits better for some geophysically anomalous regions in the western United States.
The elastic-wave velocities in three spinel lherzolite xenoliths from the Massif Central rift zone (France) indicate that the high field seismic velocity (8.4 km/s) found parallel to the rift, at a depth of 40 km in the upper mantle beneath the Massif Central, can only be explained by a preferred orientation of the olivine a axis parallel to the rift. This is not predicted by two-dimensional models of mantle flow beneath a rift. Horizontal asthenospheric flow in lithospheric fractures associated with rifting would explain the olivine orientation and the high upper mantle velocity parallel to the rift axis.
Longitudinal and transverse wave velocities are reported for several directions at pressures to 10 kb in two samples of dunite from the Twin Sisters peaks, Washington. For both dunites, elastic-wave propagation is controlled to a large extent by olivine fabric. Dunite A, which has a strong concentration of olivine a crystallographic axes and girdles of b and c axes, is uniaxial in elastic properties. The longitudinal wave velocity at 10 kb for propagation parallel to the olivine a axes maximum is 8.76 km/sec. For propagation normal to the a axes maximum, longitudinal wave velocities are low (Vp=7.98 km/sec at 10 kb) and two transverse waves (Vs=4.41 and 4.69 km/sec at 10 kb) are clearly transmitted through the rock. Dunite B, with strong concentrations of all three olivine crystallographic axes, is similar in elastic properties to orthorhombic crystals with a high longitudinal wave velocity (9.15 km/sec at 10 kb) along the olivine a axes maximum and a low longitudinal wave velocity (7.83 km/sec at 10 kb) along the olivine b axes concentration. Elastic stiffnesses and compliances were computed from the velocities, and the physical properties of isotropic aggregates of the two dunites were calculated using the Voigt and Reuss averaging techniques. Primarily because of the presence of accessory minerals in the dunites, the Voigt and Reuss velocities are lower than values computed from olivine single-crystal data. The high pressure gradient (∂Vp/∂P=17.0 km sec−1 mb−1) observed for the longitudinal velocity of dunite B at 8 kb is interpreted as being due to the effect of grain boundary cracks.
Migration is seldom applied to shallow seismic reflection data because migration on microcomputer<br>is slow and final seismic sections are often only minimally improved. Black et.al. (1994) show that for<br>small values of velocity and traveltime, the horizontal and vertical displacements of a reflector point after<br>migration may not be large enough compared to the trace spacing and time sampling interval to make a<br>noticeable change on a migrated section. They do mention, however, that migration may be useful in<br>shallow seismic survey which requires high resolution.<br>Two cases showing significant improvement of a shallow seismic section after migration, due to<br>increased resolution or decreased noise, are presented here. The first case is a common-midpoint section<br>over buried sand channels. Before migration, the section showed two contiguous channels at a depth of<br>about 60 m in a sequence of horizontal beds . Improved lateral resolution after migration revealed a third<br>channel. This case illustrates the possiblevalue of migration even when the reflectors are horizontal. The<br>second case is a common-offset survey over a channel cut in Precambrian bedrock and overlain by glaciofluvial<br>sands and gravels. Migration, by collapsing the diffraction noise, resulted in a clearer picture of<br>the channel and revealed the presence of two terraces.
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