Synthetic Seismograms and Wide-angle Seismic Attributes from the Gaussian Beam and Reflectivity Methods for Models with Interfaces and Velocity Gradients

— The effects of interfaces and velocity gradients on wide-angle seismic attributes are investigated using synthetic seismograms. The seismic attributes considered include envelope amplitude, pulse instantaneous frequency, and arrival time of selected phases. For models with interfaces and homogeneous layers, head waves can propagate which have lower amplitudes, as well as frequency content, compared to the direct arrivals. For media with interfaces and velocity gradients, higher amplitude diving waves and interference waves can also occur. The Gaussian beam and reflectivity methods are used to compute synthetic seismograms for simple models with interfaces and gradients. From the results of these methods, seismic attributes are obtained and compared. It was found that both methods were able to simulate wide-angle seismic attributes for the simple models considered. The advantage of using the Gaussian beam method for seismic modeling and inversion is that it is fast and also asymptotically valid for laterally varying media.     

Modeling of Wide-Angle Seismic Attributes Using the Gaussian Beam Method

In this study, observed seismic attributes from shot gather 11 of the SAREX experiment are used to derive a preliminary velocity and attenuation model for the northern end of the profile in southern Alberta. Shot gather 11 was selected because of its prominent Pn arrivals and good signal to noise ratio. The 2-D Gaussian beam method was used to perform the modeling of the seismic attributes including travel times, peak envelope amplitudes and pulse instantaneous frequencies for selected phases. The preliminary model was obtained from the seismic attributes from shot gather 11 starting from prior tomographic results. The amplitudes and instantaneous frequencies were used to constrain the velocity and attenuation structure, with the amplitudes being more sensitive to the velocity gradients and the instantaneous frequencies more sensitive to the attenuation structure. The resulting velocity model has a velocity discontinuity between the upper and lower crust, and lower velocity gradients in the upper and lower crust compared to earlier studies. The attenuation model has Q _p ^-1 values between 0.011 and 0.004 in the upper crust, 0.0019 in the lower crust and a laterally variable Q _p ^-1 in the upper mantle. The Q _p ^-1 values are similar to those found in Archean terranes from other studies. Although the results from a single gather are non-unique, the initial model derived here provides a self-consistent starting point for a more complete seismic attribute inversion for the velocity and attenuation structure.     

Imaging Offsets in the Moho: Synthetic Tests using Gaussian Beams with Teleseismic Waves

We carry out a sequence of numerical tests to understand conditions under which rapid changes in crustal thickness can be reliably imaged by teleseismic body waves. Using the finite-difference method over a 2-D grid, we compute synthetic seismograms resulting from a planar P-wavefield incident below the grid. We then image the Moho using a migration scheme based on the Gaussian beam representation of the wavefield. The use of Gaussian beams for the downward propagation of the wavefield is particularly advantageous in certain geologically critical cases such as overthrusting of continental lithosphere, resulting in the juxtaposition of high-velocity mantle material over crustal rocks. In contrast to ray-based methods, Gaussian beam migration requires no special treatment to handle such heterogeneities. Our results suggest that with adequate station spacing and signal-to-noise ratios, offsets of the Moho, on the order of 10 km in height, can be reliably imaged beneath thickened crust at depths of about 50 km. Furthermore, even sharp corners and edges are faithfully imaged when precise values of seismic wave speeds are available. Our tests also demonstrate that flexibility in choices of different types of seismic phases is important, because any single phase has trade-offs in issues such as spatial resolution, array aperture, and amplitude of signals.     

Geologische Forschungen in Ostafrika

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Modeling Seismic Attributes of Pn Waves using the Spectral-Element Method

To investigate the nature of Pn propagation, we have implemented the spectral-element method (SEM) for vertically and laterally varying media with and without attenuation. As a practical measure, essential features of the Pn waves are distilled into seismic attributes including arrival times, amplitudes and pulse frequencies. To validate the SEM simulations, we first compare the SEM results with reflectivity calculations of Braile and Smith (Geophys. J.R. Astr. Soc. 40, 145–176, 1975) and then to the asymptotic results of ?erveny and Ravindra (Theory of Seismic Headwaves, University of Toronto Press, pp. 235–250, 1971). Models with random, laterally varying Moho structures are then simulated, where the amplitude and pulse frequency characteristics are found to be stable to small Moho interface perturbations. SEM calculations for models with different upper-mantle velocity gradients are next performed where it is found that interference effects can strongly influence the Pn amplitudes and pulses frequencies. For larger-scale, laterally varying structures, SEM models similar to that found along the Hi-CLIMB array in Tibet are then performed. It is observed that large-scale structures, along with small-scale structures, upper-mantle velocity gradients and attenuation, can all significantly affect the Pn attributes. Ambiguities between upper-mantle velocity gradients and attenuation are also found when using Pn amplitudes and pulse frequency attributes. These ambiguities may be resolved, to some degree, by using the curvature of the travel times at longer regional distance, however, this would also be complicated by lateral variability.     

Slant-stack velocity analysis for one-dimensional upper mantle structure using short-period data from MAJO

In this paper, regionalP-wave upper mantle structure is investigated using slant-stack velocity analysis of short-period earthquake data recorded at station MAJO (Matsushiro, Japan). Shallow earthquakes from 1980–1986 within 35° of MAJO are used to construct a common receiver gather. Processing of the wavefield data includes focal depth and static time corrections, as well as deterministic deconvolution, in order to equalize pulse shapes and align wavelets on the first arrivals. The processed wavefield data are slant stacked and interatively downward continued to obtain a regional upper mantle velocity model. The model includes a low velocity zone between 107 and 220 km. Beneath the LVZ, the velocity increases smoothly down to the discontinuity at 401 km. In the transition zone, the velocity model again increases linearly, although there is some suggestion of further complexity in the downward continued wavefield data. At the base of the transition zone, a second velocity discontinuity occurs at 660 km, with a linear velocity gradient below. In addition to slant-stack analysis, travel times and synthetic seismograms are computed and compared with the processed and unprocessed wavefield data.