Colin Zelt's abstracts

ABSTRACTS

Zelt, C. A. and R. M. Ellis, Practical and efficient ray tracing in two-dimensional media for rapid traveltime and amplitude forward modelling, Canadian Journal of Exploration Geophysics, 24, 16-31, 1988.

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An algorithm for tracing rays and calculating amplitudes in two-dimensional media based on a new velocity-model parameterization has been developed. The simple, layered, large-block parameterization in which velocity is an analytic function of position allows for computationally efficient ray tracing. The user's ability to specify simple kinematically similar ray families permits practical and rapid forward modelling of refraction data. In addition, the routine allows for S- wave propagation including converted phases, multiple and surface reflections, approximate attenuation, head waves, and a reverse ray-direction amplitude calculation important for the interpretation of common-receiver profiles. The source may emit both P- and S type rays and the ratio of P-wave to S-wave amplitude at the source may be specified. Amplitude calculations are based on zero- and first-order asymptotic ray theory. The velocity model is parameterized in terms of a sequence of quasi-horizontal layers, each layer separated by a boundary consisting of straight-line segments. Layer thicknesses may be reduced to zero to model pinch-outs or isolated bodies. Each layer is broken up laterally into a series of large trapezoidal blocks with vertical left and right sides and upper and lower boundaries of arbitrary dip. The velocity structure within each trapezoid is defined by a single upper and lower velocity such that the velocity varies linearly from the upper to lower boundary along a vertical path. The major attributes of the routine are illustrated with four examples: a comparison of efficiency and accuracy with the Spence et al. algorithm, a practical application to the interpretation of observed crustal refraction profiles from the Peace River Arch region, a complex subduction zone model to illustrate the degree of lateral inhomogeneity possible with the routine's model parameterization, and a demonstration of how the routine may be used to study the effects of near-surface velocity anomalies on CMP data.

Zelt, C. A. and R. M. Ellis, Seismic structure of the crust and upper mantle in the Peace River Arch region, Canada, Journal of Geophysical Research, 94, 5729-5744, 1989.

Four intersecting ~300-km-long reversed refraction lines within northwestern Alberta and northeastern British Columbia have been interpreted for crustal and upper mantle seismic velocity structure in the Peace River Arch (PRA) region of the Western Canada Sedimentary Basin. The data have been modeled using a two-dimensional ray trace forward modeling approach based on asymptotic ray theory to match travel times and amplitudes of first and coherent later arrivals. In addition, an inversion of first arrival travel times along a fan shot profile has been performed to constrain crustal thickness northwest of the arch in a region not sampled by the reversed profiles. The major features of the structural models are (1) weak to moderate lateral variations in crustal structure with no evidence of significant layering or thick low-velocity zones within the crust, (2) an average sub-basement RMS crustal velocity of 6.6 km/s, average upper mantle velocity of 8.25 km/s and average crustal thickness of 40 km, (3) regional variations in structure which appear related to the dominant N-S trending cratonic structure, including crustal thickness, upper crustal and upper mantle velocities and PmP character, and (4) subtle variations in structure that may be associated with the E-W trending Devonian axis of the PRA, including a shallowing of high lower crustal velocities, thickening of the crust, and an anisotropic PmP character beneath the arch and a weak trend in the dip of intracrustal reflectors away from the center of the PRA region. Evidence from the refraction and other geophysical data for the presence of a local crustal expression of the PRA is weak, but suggests a thermal as opposed to flexural origin for its anomalous vertical movements.

Zelt, C. A. and R. M. Ellis, Comparison of near-coincident crustal refraction and extended Vibroseis reflection data: Peace River region, Canada, Geophysical Research Letters, 16, 843-846, 1989.

A 10-km-long uncorrelated Vibroseis data set was processed to test the feasibility of using industry reflection data for deep crustal imaging in the Peace River region and elsewhere in the Western Canada Sedimentary Basin. These results were compared with a crustal refraction interpretation of a line ~10 km to the north. The Vibroseis data were extended from a conventional 3 s to 14 s two-way travel time using the self-truncating-sweep method. The reflection section contains a prominent eastward-dipping event at ~9.4 s (29 km depth) which corresponds to the zero-offset two-way travel time of an intracrustal boundary in the refraction model. This may represent the Riel discontinuity imaged at similar depths in southern Alberta. A series of near-vertical incidence reflections over 1-1.5 s terminating at 13 s corresponds well with the refraction Moho at 12.8 s (41 km depth). This suggests that locally, the crust-mantle boundary is a complex, possibly layered transition zone of 3 5 km thickness.

Zelt, C. A. and R.M. Ellis, Crust and upper mantle Q from seismic refraction data: Peace River region, Canadian Journal of Earth Sciences, 27, 1040-1047, 1990.

Crustal refraction data from the Peace River region of Alberta, Canada have been analysed using the spectral ratio method to obtain Q. A total of 1205 first and later arrivals corresponding to turning and reflected P-waves within the crust and upper mantle were studied. Source spectra were estimated from near-offset traces assuming typical sedimentary Q values. The large scatter of measured spectral ratios restricted the resolution to a three-layer model of the crust and upper mantle with Q constant in each layer. This model was obtained using a linear inverse method since the measured spectral ratios and known travel times in each layer are linearly related through the attenuation (Q-1) in each layer. A weighted L1 norm was minimized using linear programming, the weights being a measure of the certainty of each spectral ratio. The inversion was performed using the 25% most certain spectral ratios, regardless of magnitude or sign. Model bounds which account for the scattered data were estimated. The results suggest that Q is between 200-500 in the upper crust and Q is greater than 600 in the lower crust and upper mantle. This model is generally consistent with nearby crustal Q studies.

Zelt, C. A. and R. B. Smith, Seismic traveltime inversion for 2-D crustal velocity structure, Geophysical Journal International, 108, 16-34, 1992.

A method of seismic traveltime inversion for simultaneous determination of 2-D velocity and interface structure is presented that is applicable to any type of body-wave seismic data. The advantage of inversion, as opposed to trial-and-error forward modeling, is that it provides estimates of model parameter resolution, uncertainty and non-uniqueness, and an assurance that the data have been fit according to a specified norm. In addition, the time required to interpret data is significantly reduced. The inversion scheme is iterative and is based on a model parameterization and a method of ray tracing suited to the forward step of an inverse approach. The number and position of velocity and boundary nodes can be adapted to the shot-receiver geometry and subsurface ray coverage, and to the complexity of the near-surface. The model parameterization also allows ancillary amplitude information to be used to constrain model features not adequately resolved by the traveltime data alone. The method of ray tracing uses an efficient numerical solution of the ray tracing equations, an automatic determination of take-off angles, and a simulation of smooth layer boundaries that yields more stable inversion results. The partial derivatives of traveltime with respect to velocity and the depth of boundary nodes are calculated analytically during ray tracing and a damped least-squares technique is used to determine the updated parameter values, both velocities and boundary depths simultaneously. The stopping criteria and optimum number of velocity and boundary nodes are based on the trade-off between RMS traveltime residual and parameter resolution, as well as the ability to trace rays to all observations. Methods for estimating spatial resolution and absolute parameter uncertainty are presented. An example using synthetic data demonstrates the algorithm's accuracy, rapid convergence and sensitivity to realistic noise levels. An inversion of refraction and wide-angle reflection traveltimes from the 1986 IRIS-PASSCAL Nevada, U.S.A. (Basin and Range province) seismic experiment illustrates the methodology and practical considerations necessary for handling real data. A comparison of our final 2-D velocity model with results from studies using other 1-D and 2-D forward and inverse methods serves as a check on the validity of the inversion scheme and provides estimates of parameter uncertainties that account for the bias introduced by the modeling approach and the interpreter.

Zelt, C. A., D. A. Forsyth, B. Milkereit, D. J. White, I. Asudeh and R. M. Easton, Seismic structure of the Central Metasedimentary Belt, southern Grenville Province, Canadian Journal of Earth Sciences, 31, 243-254, 1994.

Crust and upper mantle structure interpreted from wide-angle seismic data along a 260-km profile across the Central Metasedimentary Belt of the southern Grenville Province in Ontario and New York state shows (i) relatively high average crustal and uppermost mantle velocities of 6.8 and 8.3 km/s, (ii) east-dipping reflectors extending to 24 km depth in the Central Metasedimentary Belt, (iii) weak lateral velocity variations beneath 5 km, (iv) a mid-crustal boundary at 27 km depth, and (v) a depth to Moho of 43-46 km. The wide-angle model is generally consistent with the vertical-incidence reflectivity of an intersecting Lithoprobe reflection line. The mid-crustal boundary correlates with a crustal detachment zone in the Lithoprobe data and the depth extent of east-dipping wide-angle reflectors. Regional structure and aeromagnetic anomaly trends support the southwest continuity of Grenville terranes and their boundaries from the wide-angle profile to two reflection lines in Lake Ontario. A zone of wide-angle reflectors with an average apparent eastward dip of 13 degrees has a surface projection that correlates spatially with the boundary between the Elzevir and Frontenac terranes of the Central Metasedimentary Belt and resembles reflection images of a crustal-scale shear zone beneath Lake Ontario. A high-velocity upper-crustal anomaly beneath the Elzevir-Frontenac boundary zone is positioned in the hanging wall associated with the concentrated zone of wide-angle reflectors. The high-velocity anomaly is coincident with a gravity high and increased metamorphic grade suggesting northwest transport of mid-crustal rocks by thrust faulting consistent with the mapped geology. The seismic data suggest (i) a reflective, crustal-scale structure has accommodated northwest-directed tectonic transport within the Central Metasedimentary Belt, (ii) this structure continues southwest from the exposed Central Metasedimentary Belt to at least southern Lake Ontario, and (iii) crustal reflectivity and complexity within the eastern Central Metasedimentary Belt is similar to that observed at the Grenville Front and the western Central Metasedimentary Belt boundary.

Zelt, C. A. and D. A. Forsyth, Modeling wide-angle seismic data for crustal structure: southeastern Grenville province, Journal of Geophysical Research, 99, 11687-11704, 1994.

A modeling methodology for obtaining two-dimensional (2-D) crustal structure from wide-angle seismic data is applied to data from the southeastern Grenville Province. Pre-modeling steps include (1) assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal-to-noise ratio, (2) using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity, and (3) identifying data unsuitable for 2-D modeling. The goal of the travel time inversion-amplitude modeling approach is to obtain a minimum-structure and minimum-parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide-angle data. Each step of a layer-stripping procedure involves a series of inversions in which a one-dimensional or simple starting model is improved with additional velocity and/or interface nodes until a satisfactory trade-off between travel time fit, parameter resolution and complete ray coverage of all source-receiver pairs is achieved. Using zero vertical-velocity gradient layers and head waves during preliminary first-arrival inversion can (1) decrease the number of intermediate models, (2) allow greater lateral heterogeneity to be imaged, and (3) simplify incorporation of amplitude modeling constraints into the final model. Using amplitude-distance curves allows quantitative modeling of the relative amplitude and offset variations of phases. Discrepancies between observed and calculated reflection amplitudes are used to infer fundamental, non step-like velocity changes at layer boundaries. Later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that ``float" within the model without an associated velocity structure. These reflectors provide a spatial image like that obtained from vertical-incidence reflection data, as opposed to a velocity image. The model of Grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data. A crustal-scale zone of wide-angle reflectors with an average easterly apparent dip of 13 degrees defines a major Grenvillian terrane boundary.