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

Abstract

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.

Correlation Migration Using Gaussian Beams of Scattered Teleseismic Body Waves

“…Vertical and radial component ray/Born synthetics have been generated and are shown in Figure 3, where the scattering is incorporated by using point scatterers within a background two-layer model similar to that used by Shragge et al (2001) for their test migrations. More complete finite-difference simulations have been performed by Nowack et al (2004), and similar inversion results were found for the Gaussian beam imaging. The source is a plane P-wave incident from beneath the structure at a 20Њ angle from the vertical with a Gaussian pulse width of 1 sec.…”

Application of Gaussian-Beam Migration to Multiscale Imaging of the Lithosphere beneath the Hi-CLIMB Array in Tibet

“…We have chosen the GB approach to calculate the wave fields (Nowack et al, 2006(Nowack et al, , 2007; Appendix A) for the migration and imaging of teleseismic P-S conversions, and this choice distinguishes our method from alternatives such as those of Bostock and Rondenay (1999) and Bostock et al (2001), whose algorithms are based on geometric rays. A key advantage of the GB approach is its ability to handle triplicate (caustic) arrivals in the wave field without any special treatment.…”

“…This approach assumes one-dimensional (1D) structures for all calculations to construct two-dimensional (2D) images and then typically applies some arbitrary scheme to smooth the resulting product by averaging values from neighboring cells (bin averaging). In contrast to recent developments in imaging techniques (Bostock et al, 2001;Nowack et al, 2006Nowack et al, , 2007, the CPP approach utilizes only a fraction of the information in the scattered wave field. The difference in methodology distinguishes the current study from other recent work (Hetényi et al, 2007).…”

“…This formulation is similar to that of Hill (2001) originally designed for prestack migration of seismic reflection data but is now extended to scattered SV waves associated with upgoing, incident P waves from distant earthquakes. To help visualize how GB migration works, Figure A1b shows an example using synthetic seismograms calculated for a model of thickened crust, with a sharp overhang in the Moho between depths of 40 and 55 km (Nowack et al, 2007). The target is illuminated by a planar P wave whose initial angle of incidence is 15°counterclockwise from the vertical.…”

An Energy-Conserving Discontinuous Multiscale Finite Element Method for the Wave Equation in Heterogeneous Media

“…Other solutions emphasize the analysis of reflected waves, especially from artificial sources of seismic waves, and such measurements provide some of the most important information for applications such as hydrocarbon exploration [Beylkin (1985); Bleistein et al (2001); de Hoop and Bleistein (1997); Lambaré (2003)]. Recent research has also been extending the application of such methods to apply them to seismic waves generated by natural sources to image the structure of earth's crust and mantle [Bostock and Rondenay (1999); Nowack et al (2007); Wittlinger et al (2004)]. …”