We investigate the sedimentary and volcanic structure of the Tuamotu Plateau with multichannel seismic, seismic refraction, and gravity data along a ship track crossing the plateau near 15øS. The volcanic basement of the central portion of the plateau is capped with a 1 to 2km-thick sediment layer composed of two compositional sequences. The uppermost sequence, with semblance-derived P wave velocities of 1.6-1.9 km/s and thicknesses of 0.2-0.9 km, is composed of pelagic sediments. The underlying sequence, with velocities 2.5-3.5 km/s and thicknesses of 0.5-1.5 km, is composed of limestone and volcaniclastic sediments. Sonobuoy refraction data show the upper 1 km of the volcanic basement to have velocities 4.5-5.5 km/s. The gravity data indicate that the platform is compensated by an elastic lithosphere with effective thickness 5+5 km and that the volcanic thickness is 9-10 km thicker than normal oceanic crust with a volume of 2.0-2.6x106 km 3. The inferred eruption rates of 0.1-0.13 km3/yr are comparable to those of the Hawaiian and Marquesas island chains but substantially less than those of many oceanic plateaus. Radiometric and paleontological ages for the plateau and geomagnetic dates of the surrounding seafloor indicate that the northwestern portion of the plateau formed -600 km off the axis of the paleo-Pacific-Farallon spreading center, on lithosphere of age ~ 10-20 Ma. Linear volcanic ridges and scarps bounding deep sediment-filled basins, however, are similar to features of oceanic plateaus which formed at or near accretionary plate boundaries. We suggest that these volcanic ridges and the gross plateau like morphology were formed by magma that was channelled along the lithospheric discontinuities left behind by a southward propagating rift segment of the nearby spreading center. We attribute the formation of the northwestern portion of the Tuamotu Plateau to the passage of two hotspots during times 50-30 Ma as they migrated beneath the Pacific plate but remained west of the Tuamotu propagator. IntroductionHotspots in the mantle, are thought to be the sources of many crustal anomalies in the world's ocean basins. Oceanic hotspot features can be separated into two classes: island chains and oceanic plateaus. The morphological characteristics that distinguish these two classes may reflect differences in the tectonic environments at which they formed and/or the mantle sources which produced these melt anomalies. Ocean island chains are composed of discrete volcanic edifices with geographic age distributions reflecting the motion of the lithosphere with respect to the hotspot reference frame [Duncan and McDougall, 1976; Clague and Jarrard, 1973; Morgan, 1972]. As demonstrated by isotopic and paleontological ages that Copyright 1995 by the American Geophysical Union. Paper number 95JB00071. 0148-0227/95/95JB-000710505.00 are less than magnetic ages of the underlying lithosphere [Jarrard and Clague, 1977; Henderson, 1985], most island chains were formed midplate, far from oceanic spreading centers. As a result...

Seismic data are routinely used to infer in situ properties of earth materials on many scales, ranging from global studies to investigations of surficial geological formations. While inversion and imaging algorithms utilizing these data have improved steadily, there are remaining challenges that make detailed measurements of the properties of some geologic materials very difficult. For example, the determination of the concentration and orientation of fracture systems is prohibitively expensive to simulate on the fine grid and, thus, some type of coarse-grid simulations are needed. In this paper, we describe a new multiscale finite element algorithm for simulating seismic wave propagation in heterogeneous media. This method solves the wave equation on a coarse grid using multiscale basis functions and a global coupling mechanism to relate information between fine and coarse grids. Using a mixed formulation of the wave equation and staggered discontinuous basis functions, the proposed multiscale methods have the following properties.• The total wave energy is conserved.• Mass matrix is diagonal on a coarse grid and explicit energy-preserving time discretization does not require solving a linear system at each time step.• Multiscale basis functions can accurately capture the subgrid variations of the solution and the time stepping is performed on a coarse grid.We discuss various subgrid capturing mechanisms and present some preliminary numerical results.

A model for attenuation of acoustic waves in suspensions is proposed which includes an energy loss due to viscous fluid flow around spherical particles. The expression for the complex wavenumber is developed by considering the partial pressures acting on the solid and fluid phases of the suspension. This is shown to be equivalent to the results of the Biot theory for porous media in the limiting case where the frame moduli vanish. Unlike earlier applications of the limiting case Biot theory, however, a value for the attenuation coefficient is developed from the Stokes flow drag force on a sphere instead of attempting to apply a permeability value to a suspension. If the fluid and solid particle velocities have harmonic time dependence with angular frequency w, the attenuation in this model is proportional to w 2 at low frequencies and approaches a constant value at high frequencies. The predicted attenuation is very sensitive to the radius and density of the spherical particles. Accurate modeling of observed phase velocities from suspensions of spherical polystyrene particles in water and oil and successful inversion for kaolinite properties using attenuation and velocity data from kaolinite suspensions at 100 kHz show that this viscous dissipation model is a good representation of the effects controlling the propagation of acoustic waves in these suspensions. Attenuation predictions are also compared to amplitude ratio data from an oil-polystyrene suspension. The viscous effects are shown to be significant for only a limited range of solid concentration and frequency by the reduced accuracy of the model for attenuation in a kaolinite suspension at 1 MHz.

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It is important to develop fast yet accurate numerical methods for seismic wave propagation to characterize complex geological structures and oil and gas reservoirs. However, the computational cost of conventional numerical modeling methods, such as finite-difference method and finite-element method, becomes prohibitively expensive when applied to very large models. We propose a Generalized Multiscale Finite-Element Method (GMsFEM) for elastic wave propagation in heterogeneous, anisotropic media, where we construct basis functions from multiple local problems for both the boundaries and interior of a coarse node support or coarse element. The application of multiscale basis functions can capture the fine scale medium property variations, and allows us to greatly reduce the degrees of freedom that are required to implement the modeling compared with conventional finite-element method for wave equation, while restricting the error to low values. We formulate the continuous Galerkin and discontinuous Galerkin formulation of the multiscale method, both of which have pros and cons. Applications of the multiscale method to three heterogeneous models show that our multiscale method can effectively model the elastic wave propagation in anisotropic media with a significant reduction in the degrees of freedom in the modeling system.