Tomographic inversion of Rg wave group velocities for regional near‐surface velocity structure

Abstract

The tomegraphic inversion of group velocities of Rg waves is a viable technique for mapping lateral variations in the lithology of sedimentary basins. In order to determine a velocity structure, we first use the moving-window maximum entropy spectral analysis to measure the average Rg wave group travel times. Second, the group velocities within blocks (subregions)covering the area sampled by the Rg wave propagation paths are determined by tomegraphic inversion. Third, the vertical shear wave velocity profile within each block is determined from the inversion of group velocities for that block. Finally, a structural cross section is obtained by connecting blocks located along a profile. Rg waves recorded from mining explosions in Alabama were used in a test of the technique by modeling the Paleozoic sedimentary rocks in the Black Warrior Basin of northern Alabama. The high resolution of moving-window maximum entropy spectral analysis allows estimation of the Rg wave group velocities with a precision of better than 3 %. The usable frequency range of 0.5 to 1.5 Hz allows determination of structures to depths of about 2.4 km with layers of 0.2 km thickness. Blocks of widths of 20 km or less could be resolved with appropriate data. Shear wave velocities in the Black Warrior Basin range from 1.6 to 4.0 km/s. The layering in the velocity structure corresponds to major geologic units. Within units, an increase in velocity to the northeast corresponds to observed facies changes. INTRODUCTION The Rg waves observed on records of near-surface earthquakes and blasts are predominantly fundamental-mode Rayleigh waves. As such, their group velocities are controlled mainly by variations in the velocity structure of the upper few kilometers of the crust along their propagation path. Rg waves on short-period seismograms are most prominent at epicentral distances of 25 to 200 km along propagation paths characterized by smooth terrain and by low-velocity surface sediments or weathered rock. In this paper, we show that a detailed analysis of Rg wave dispersion can be a useful tool for the study of shallow structures. In particular, the range of distances of propagation and depth of penetration make Rg waves appropriate for the study of facies changes in sedimen• basins. Rg waves with periods of 0.5 to 2.0 s have been used to demonstrate regional variations of Rg wave dispersion in southern New England [Kajka and Dollin, 1985] and southeastern Maine [Kajka and Reiter, 1987]. Similarly, Woods et al. [1989] observed differences in the Rg wave dispersion properties of the Ozark Uplift and the Illinois Basin. Differences in dispersion were attributed to variations, from 0.3 to 1.0 kin, in the thickness of a single surface layer.These interpretations of Rg wave dispersion were based on clusters of propagation paths, each cluster consisting of paths principally in one structural region. In these studies, a tomegraphic inversion technique was not applied. In contrast. tomegraphic inversion techniques have been used extensively to study the la...

Shallow Seismic Velocity Structure of the Betic Cordillera (Southern Spain) from Modelling of Rayleigh Wave Dispersion

“…After isolating the fundamental mode Rayleigh wave, we used multiple filtering (Dziewonski et al, 1969Dziewonski and Hales, 1972;Herrmann, 1973) to obtain group velocity dispersion. In fact, our working scheme combines the beneficial aspects of several filtering techniques and is similar to the procedures followed by other authors (Al-Khatib and Mitchell, 1991;Kocaoglu and Long, 1993;Malagnini et al, 1995;Chourak et al, 2001Chourak et al, , 2003 based on the application of frequency-variable filters to surface-wave amplitude analysis . Figure 3 illustrates the effect of digital filtering to reduce noise and other undesirable effects; the filtered seismogram, the amplitude spectra before and after filtering, and the initial and final group velocity dispersions are shown.…”

Seismic Surface-Wave Tomography of Waste Sites - Final Report

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Shallow-depth shear wave velocity structure of the Southern Korean Peninsula obtained from two crustal-scale refraction profiles

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