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

Abstract

Seismic inversion is applied to generate physical property models (P-wave velocity and numerical attenuation) for four profiles in the Nechako-Chilcotin plateau region of south-central British Columbia, Canada. A newly developed method that combines three-dimensional (3-D) travel-time inversion and 2.5-dimensional (2.5-D) viscoacoustic full-waveform inversion was applied to generate the geophysical models from vibroseis data acquired along the preexisting crooked roads. These models are useful for the characterization of rock types in terms of their positions and thicknesses, which may be used in conjunction with geological ground truth to infer the extent of lithostratigraphic units in the subsurface. The velocity structures also may be used for future reprocessing of the seismic reflection data to derive improved images based on the better near-surface velocity models. The subsurface geology of the Nechako-Chilcotin plateau region is complex, resulting from multiple stages of tectonic compression and extension, contemporaneous with the deposition of sediments and volcanic material. Several basin structures are identified from the joint interpretation of the waveform tomography velocity models and post-stack time migration images. The combination of these results enables the extrapolation and characterization of geological structures to ϳ3 km depth, particularly within the Cenozoic volcanic units that dominate near-surface stratigraphy. Based on the seismic profiles, a fence-diagram geological interpretation that extends to ϳ3 km depth illustrates the complex structure of the Jurassic to Neogene stratigraphic sequence.Résumé : Des modèles de propriétés physiques (vitesse des ondes P et atténuation numérique) sont générés par inversion sismique le long de quatre profiles dans la région du plateau de Nechako-Chilcotin au sud-est de la Colombie Britannique. Une méthode récemment développée combine l'inversion tridimensionnelle (3-D) des temps de trajet et une inversion 2.5-dimensionnelle (2.5-D) des formes d'ondes viscoacoustiques. Cette méthode est utilisée pour générer les modèles géophysiques à partir de données sismiques acquises à partir d'une source vibroseis le long de routes curvilignes. De tels modèles sont utiles afin de caractériser la position et l'épaisseur des successions rocheuses; ainsi, combinés à des données géologiques de surfaces, ces modèles permettent de déduire l'étendue des unités lithostratigraphiques en sous-surface. Les structures imagées par les modèles de vitesse peuvent aussi être utilisées pour le retraitement de données de sismique réflexion: elles permettent d'obtenir des profils améliorés fondés sur des meilleurs modèles de vitesse en proche surface. La géologie du plateau de Nechako-Chilcotin est complexe, et résulte d'évènements tectoniques successifs de compression et extension durant le dépôt de matériel sédimen-taire et volcanique. Plusieurs bassins sont identifiés à partir de l'interprétation conjointe de modèles tomographiques et d'images de migration en temps. La combinaiso...

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