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Geometry of seismic line 10 in relation to lithology and several other seismic lines from the 2008 Geoscience BC Nechako Basin vibroseis survey; map area as indicated in Figure 1. The map is projected to UTM coordinates (NAD 27). The surface of the central portion of line 10 is dominated by the Ootsa Lake group, including rhyolite and other volcanic rocks erupted in the Eocene. Both flanks are overprinted by the Chilcotin basalt (Massey et al., 2005), characterized by variable observed seismic velocities most likely due to variable vesicularity and brecciation. To the north, the Hazelton Group volcanic rocks of the Stikine Terrane appear to plunge beneath line 10 toward the south. The 2D geometry for the model profile used in waveform inversion is shown by the red line. The receiver array for this study spans the model line; the part covered by the active source array is shown in blue.
Source publication
In order to improve the tractability of waveform tomography when applied
to field data acquired along a crooked-line, we implement 2.5-D forward
modeling and inversion. Waveform tomography combines conventional
velocity-model building (i.e. tomography) with full-waveform inversion
to reconstruct an image of subsurface acoustic velocity. For reasons...
Contexts in source publication
Context 1
... et al. (2003) identified the south-eastern portion of the basin as having the highest prospectivity. Figure 2 shows the study area in detail, highlighting the geometry of the seismic acquisition line and the surface geological units. Figure 3 presents a simplified depositional/emplacement history for the study area. ...
Context 2
... do not outcrop in the region of our study. The Hazelton Group, part of the Stikine Terrane, out- crops near the seismic line as shown in Figure 2 ( Massey et al., 2005). The near-surface is dominated by Eocene volcanic rocks of the Ootsa Lake and Endako groups, and by the younger Neogene Chilcotin basalt. ...
Context 3
... recent work by Calvert et al. (2011) indicates that brecciation may cause the Chilcotin basalt to possess lower P-wave velocities throughout much of the region. Quaternary deposits of differing types and vary- ing thicknesses overlie the older rocks (Figures 2 and 3). Recent work by others has increasingly found that the Nechako Basin is more appropriately thought of as a collection of related subbasins (J. ...
Context 4
... corresponding residuals represent the traveltime error resulting from the 2D geometry approximation (under the infinite-frequency approximation and in the velocity model of Figure 5a). The magnitude of these errors corresponds closely with the locations along the acquisition line in which the offset error is high (from the 2D projection; compare with the line geometry visible in Figure 2). We used the traveltime re- siduals presented in Figure 4b to static correct the data waveforms before carrying out full-waveform inversion. ...
Context 5
... high-amplitude model perturbations in the western part of the line can most likely be traced to problems in the initial data fit from the traveltime tomography model, which in turn are due to unre- solved geometry errors. The geometry correction incompletely ac- counts for approximation errors in the full-waveform inversion stage, which are significant toward the western end of the line (see geometry in Figure 2). The static correction improves the full- waveform inversion fidelity in the very near-surface (less than about 1 km depth) across most of the seismic line, but substantial model artifacts remain from the seismic traces with the largest crossline source-receiver offsets. ...
Context 6
... region of highest confidence in our work is the eastern portion of the line, due to the relatively low line-curvature. Figure 2 presents Figure 7. (a) The velocity model from Figure 5b is shown annotated with features of interest (described in main text). Color legend is identified by the right-angle arrow. ...
Context 7
... sharply defined high-velocity feature at E could be a volcanic plug. Another subbasin structure is seen farther west at F. However, the confidence of the full-waveform inversion is lower in this region, due to the poorer performance of the 2D geometry approximation (Figure 2). An 8-10 km wide synformal structure at G was initially interpreted to be an artifact of the ray tracing used in the initial model but subsequent comparisons with other studies have modified this view (see below). ...
Context 8
... a full- graben structure is interpreted between 16 and 25 km (H), and a half-graben east of 3 km (J; distances relative to Figure 7). They find evidence for post-Eocene extension in the Chilcotin basalt near the surface in the western portion of line 10 (Figure 2), leading to folding and brecciation. Additionally, recent reinterpretation of older seismic results ( Hayward and Calvert, 2011) indicates strike- slip faulting perpendicular to the seismic line in question. ...
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