Nonlinear refraction and reflection travel time tomography

Abstract

Abstract. We develop a rapid nonlinear travel time tomography method that simultaneously inverts refraction and reflection travel times on a regular velocity grid. For travel time and ray path calculations, we apply a wave front method employing graph theory. The first-arrival refraction travel times are calculated on the basis of cell velocities, and the later refraction and reflection travel times are computed using both cell velocities and given interfaces. We solve a regularized nonlinear inverse problem. A Laplacian operator is applied to regularize the model parameters (cell slownesses and reflector geometry) so that the inverse problem is valid for a continuum. The travel times are also regularized such that we invert travel time curves rather than travel time points. A conjugate gradient method is applied to minimize the nonlinear objective function. After obtaining a solution, we perform nonlinear Monte Carlo inversions for uncertainty analysis and compute the posterior model covariance. In numerical experiments, we demonstrate that combining the first arrival refraction travel times with later reflection travel times can better reconstruct the velocity field as well as the reflector geometry. This combination is particularly important for modeling crustal structures where large velocity variations occur in the upper crust. We apply this approach to model the crustal structure of the California Borderland using ocean bottom seismometer and land data collected during the Los Angeles Region Seismic Experiment along two marine survey lines. Details of our image include a high-velocity zone under the Catalina Ridge, but a smooth gradient zone between Catalina Ridge and San Clemente Ridge. The Moho depth is about 22 km with lateral variations.

Structural variations of arc crusts and rifted margins in the southern Izu‐Ogasawara arc–back arc system

“…To obtain a velocity structure with high resolution, we employed a tomographic inversion technique using first arrivals [ Zhang et al , 1998] and a diffraction stack migration approach using traced reflections [ Fujie et al , 2006]. Forward modeling using a layered model [ Zelt and Smith , 1992; Zelt and Ellis , 1988] was then used to fit the traveltimes and amplitudes of the reflections.…”

Crustal structure of the ocean-island arc transition at the mid Izu-Ogasawara (Bonin) arc margin

“…For the first step of the seismic modeling, the thickness of the top-most sediments is constrained by multi-channel seismic data (Park et al, 2002). Tomographic inversion of GeoCT II (Zhang et al, 1998) was then applied to the first arrival data. Although the crustal model of the Shikoku Basin could be estimated by the tomographic inversion, it was difficult to determine the deeper part of the island-arc crust only by the inversion owing to the sparse ray density.…”

Identification of cross-valley faults in the Maynardville Limestone, Oak Ridge Reservation, Tennessee, using seismic refraction tomography

“…A unique aspect of GeoCT-I is that the inversion algorithm seeks to minimize the difference between data and model Environ Earth Sci (2010) 60:1245-12561249 values for both the average slowness (travel time divided by raypath length) and apparent slowness (slope of the travel time curve), instead of just minimizing the misfit between modeled and observed travel times. GeoCT-I also utilizes Tikhonov regularization to minimize model ''roughness'' (Zhang and Toksöz 1998).…”