Three-dimensional tomographic inversion of combined reflection and refraction seismic traveltime data

Abstract

S U M M A R YA tomographic inversion method is presented for the determination of 3-D velocity and interface structure from a wide range of body-wave seismic traveltime data types. It is applicable to refraction, wide-angle reflection, normal-incidence and multichannel seismic data, and is best suited to a combination of these that provides good independent constraints on seismic velocities and interface depths. The inversion process seeks a layer-interface minimum-structure model that is able to explain the given data satisfactorily by inverting to minimize data misfit and model roughness norms simultaneously. This regularized inversion, and the use of smooth functions to describe velocities and depths, allows the highly non-linear tomographic problem to be approximated as a series of linear steps. The inversion process begins by optimizing the fit to the data of a highly-smoothed initial model. In each subsequent step, structure is allowed to develop in the model with successively greater detail evolving until a satisfactory fit to the data is obtained. Parameter uncertainties for the final model are then estimated using an a posteriori covariance matrix analysis. Smooth layer-interface models are parametrized using regular grids of velocity and depth nodes from which spline-interpolated interface surfaces and velocity fields are defined. Forward modelling is achieved using ray perturbation theory and a two-point ray tracing method that is optimized for a large number of closely-spaced shot or receiver points. The method may be used to generate 1-and 2-D models (from, for example vertical seismic profile data or 2-D surveys) in which the 3-D geometry of a survey is correctly accounted for. The ability of the method to resolve typical target structures is tested in a synthetic salt dome inversion. From a set of noisy traveltime data, the model converges quickly to a well-resolved final model from different starting models. The application of this method to real data is demonstrated with a combined 3-D inversion of refraction and reflection data which provide P-wave velocity constraints on the methane hydrate stability zone in the Cascadia Margin offshore Vancouver Island.

Gas hydrate concentration estimates from chlorinity, electrical resistivity and seismic velocity

“…Studies include 2D and 3D single channel and multichannel conventional and high-resolution seismic studies (Yuan et al, 1996Fink and Spence, 1999;Riedel et al, 2002;Chapman et al, 2002;Gettrust et al, 1999), vertical seismic profiling (MacKay et al, 1994), Ocean Bottom Seismometer (OBS) surveys (Spence et al, 1995;Hobro 1999), swath-bathymetry mapping, high-resolution subbottom profiling, heat-flow studies (Ganguly et al, 2000), piston coring with geochemical and geophysical sediment analyses (Novosel, 2002;Solem et al, 2002), ODP scientific drilling Leg 146 , seafloor electrical sounding (Edward, 1997;Yuan and Edwards, 2001), and ocean bottom video surveying with the remotely operated vehicle ROPOS .…”

2-D tomography of volcanic CO<sub>2</sub> from scanning hard-target differential absorption lidar: the case of Solfatara, Campi Flegrei (Italy)

“…All five scans were performed one-sided, i.e., from a single half space, as often the case in geophysical tomography problems (e.g., Hobro et al, 2003). As evinced above, this lack of angle diversity appears to have contributed to inconsistency of the inverse problem, leading to unrealistic models, which aggravated with increasing model grid number (non-uniqueness).…”

Study on the limitations of travel-time inversion applied to sub-basalt imaging

“…This causes that most of energy is reflected and/or travels along this layer. Furthermore, the heterogeneous structure of the basalt layers scatter 100 the higher seismic frequencies of the source signal (Pujol and Smithson, 1991;Hobbs, 2002). The lack of penetration and the multiple scattering within the basalt layer obscures the potential seismic events generated bellow the basalt.…”

“…The tomography is implemented as a joint interface and velocity inversion using the biconjugated gradient method. The TTT algorithm presented in Trinks (2003) and Trinks et al (2005) is based on the method by McCaughey and Singh (1997); Hobro et al (2003) with the advantage that allows for irregular parametrized-velocity grids using Delaunay triangulation. The velocities are defined at triangle vertices, then a linear interpolation is used to calculate the slowness squared within the triangle.…”