Waveform tomography in 2.5D: Parameterization for crooked‐line acquisition geometry

Abstract

[1] A method for 2.5D viscoacoustic waveform tomography that can be applied to generate 2D models of velocity and attenuation from inversion of refraction waveforms on land seismic reflection data acquired along crooked roads is developed. It is particularly useful for typical crustal reflection surveys. First-arrival travel time tomography is applied using a 3D method, but with constraints on the intermediate 3D velocity model; the result is the starting model for the next step. A 2.5D frequency-domain full-waveform inversion stage parameterizes 3D geometry in the seismic source and receiver arrays, with the assumption that the velocity and attenuation models are homogeneous in the out-of-plane direction. This approach results in superior results compared to a strictly 2D approach when the acquisition line is crooked, with a moderate increase in computational cost. A case study using data acquired in the Nechako Basin in south-central British Columbia, Canada, exemplifies and validates the procedure. The velocity model derived from 2.5D waveform tomography is compared with that from a previous study in which 2D waveform tomography was applied to the same data set and with results from 3D travel time tomography. The resolution and accuracy of the velocity model from 2.5D waveform tomography are demonstrated to be greater than those from travel time or 2D waveform tomography. A model of viscoacoustic attenuation, which was not possible in the 2D case, is also generated. These models are interpreted jointly to highlight features of geological interest, such as a sedimentary basin, basement rocks, and faults, from surface to about 3 km depth.

New geophysical models for subsurface velocity structure in the Nechako–Chilcotin plateau from 2.5-D waveform tomography

“…Heuristic techniques may also be applied to accommodate amplitude variations between field seismic records and the synthetic records produced during FWI. One example of this is log-linear Amplitude Variation with Offset (AVO) correction, which was applied for the Line 10 dataset discussed in our case study; see Smithyman and Clowes (2013). These techniques may affect the recovered attenuation structures.…”

“…The 2.5-D FWI method characterizes each seismic profile with a cross-sectional (x, z) model and a cross-line direction (y). The cross-line direction is assumed to be the direction of geological strike, and should be chosen based on a priori knowledge of the large-scale geology of the site in question (where possible; see Smithyman and Clowes 2013). Figure 1b indicates the in-line and cross-line directions chosen for each seismic profile, which define the direction of assumed homogeneity.…”

“…FWI fits the phase of the data waveforms within a selected frequency band, which yields models of seismic velocity with improved resolution (Smithyman and Clowes 2013). Additionally, FWI is applied on the data waveforms, including amplitude information, which enables inversion for numerical attenuation (parameterized as the inverse of the seismic Q factor at a centre frequency of 10 Hz).…”

“…This method yields high-resolution images of in-situ P-wave velocity and numerical attenuation through nonlinear inversion of the seismic data waveforms. Synthetic waveforms are simulated using a viscoacoustic method in 2.5-D (Song and Williamson 1995;Pratt 1999;Smithyman and Clowes 2013), which enables direct comparison with the field data waveforms that propagate in three dimensions. The seismic inversion grid is optimally aligned to cross geological strike if a dominant strike direction is known a priori, which reduces significantly the approximation errors inherent in the 2.5-D method.…”

“…Full-waveform inversion (FWI) is applied in the frequency domain (using a modified version of FULLWV ;Pratt 1999;Smithyman and Clowes 2013) to update the P-wave velocity model from travel-time inversion. FWI fits the phase of the data waveforms within a selected frequency band, which yields models of seismic velocity with improved resolution (Smithyman and Clowes 2013).…”