Carl Spencer

Lithoprobe crustal reflection cross section of the southern Canadian Cordillera, 1, Foreland thrust and fold belt to Fraser River Fault

Seismic reflection data from the south central Canadian Cordillera covering the interval from the easternmost metamorphic core complexes near Arrow Lakes to the Fraser River fault system along the Fraser River reveal a highly reflective and complex crust. The base of the crustal reflectivity, interpreted as the reflection Moho, is clearly delineated by a continuous sharp boundary that is essentially planar and slopes uniformly over a distance of 250 km from about 12.0 s in the east to about 10.5 s in the west. This virtual lack of relief at the base of the crust contrasts sharply with surface structures that involve 25 km or more of structural relief. Some of these surface structures can be readily correlated to structures that are outlined by the reflection data and that can be followed into the middle and lower crust. Even though part of this area was subjected to large amounts of Eocene extension, the crust is not divisible into transparent upper and reflective lower layers as it is in parts of the U.S. Cordillera. Three structural culminations, the Monashee complex, the Vernon antiform, and the Central Nicola horst, are interpreted on the basis of the reflection configuration and the surface geological relationships to have formed initially during Jurassic to Eocene compression and then to have been modified and exposed during early and middle Eocene extension. An example of a compressional structure observed on the profiles is the Monashee decollement, which can be traced from the surface westward into the lower crust. Extension is manifested along a variety of normal faults, including the regionally extensive low angle Okanagan Valley-Eagle River fault system, moderately dipping faults such as the Columbia River and Slocan Lake faults, and high-angle faults such as the Quilchena Creek and Coldwater faults. Both Jurassic to Eocene compressional shear zones and early to middle Eocene extensional shear zones are listric into the lower crust or Moho under the Intermontane belt.

Crustal structure of the Midcontinent rift system: Results from GLIMPCE deep seismic reflection profiles

Interpretation of Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) seismic reflection profiles indicates that the Midcontinent (Keweenawan,1100 Ma) rift system of volcanic rocks and postvolcanic and interbedded sedimentary rocks extends to depths as great as 32 km (about 10.5-s reflection time) along profiles crossing western, central, and eastern Lake Superior and the northern end of Lake Michigan. This area may overlie the greatest thickness of intracratonic rift deposits on Earth. Times to Moho reflections vary along strike from 11.5 to 14 s (about 37-46 km depth) in the west, to 17 s (about 55 km) in the center, and 13 to 15 s (about 42-49 km) in the eastern end of Lake Superior. The prerift crust, however, was thinned 25-30 km beneath the central rift (compared with its flanks), providing evidence for crustal extension by factors of about 3-4. The Midcontinent rift system differs from Phanerozoic rifts in having total crustal thicknesses equal to or greater than the surrounding (presumably unextended) regions.

Lithoprobe crustal reflection structure of the Southern Canadian Cordillera 2: Coast mountains transect

The Lithoprobe seismic reflection transect across the southern Coast Mountains of the Canadian Cordillera images fundamental crustal structures presumably related to collision of the Intermontane and Insular composite terranes, and deep levels in the upper plate of the offshore Cascadia subduction belt. The eastern part of the Coast Mountains are characterized by east dipping upper crustal reflectors that project to exposed faults and east dipping lower crustal reflectors; they are truncated by subhorizontal to west dipping middle and upper crustal reflectors. These geometric relationships are interpreted to have formed during an early phase of primarily west directed contraction that created the east dipping structures of the upper and lower crust, and a later phase of east directed shortening caused by wedging of the Intermontane belt into the lower and middle crust of the tectonic stack. Subsequently, the Coast belt may have been displaced eastward on contractional faults that ascend from the lower crust beneath the Intermontane belt and surface in the Omineca and Foreland belts. Extensional faults bounding the east flank of the Coast Mountains and west flank of the central Nicola horst in the Intermontane belt flatten into the middle and lower crust of the intervening region and geometrically outline crustal boudinage. Within the western Coast Mountains, east dipping reflections spanning the middle crust to upper mantle are traced updip to Vancouver Island and the underlying Cascadia subduction zone. The C reflector on Vancouver Island is believed to separate Wrangellia from underlying accreted terranes and is correlated to the mainland where it forms the upper boundary of a reflective lower crustal wedge that flattens into the Moho. If the Moho is not a young feature, then some accreted material appears to have wedged into the continental framework above the crust-mantle boundary, possibly causing shortening in the overlying crust and creating midcrustal ramps observed on the reflection data. The structurally lower E reflections, interpreted as shear zones, originate at the subduction contact offshore and project landward into sub-Moho reflections within the upper plate on the Mainland. The region between the E reflector and the descending oceanic plate is interpreted to be subducted lower continental crust and mantle.

The resolving power of seismic amplitude data : An anisotropic inversion/migration approach

A description of the theory and numerical implementation of a 3-D linearized asymptotic anisotropic inversion method based on the generalized Radon transform is given. We discuss implementation aspects, including (1) the use of various coordinate systems, (2) regularization by both spectral and Bayesian statistical techniques, and (3) the effects of limited acquisition apertures on inversion. We give applications of the theory in which well-resolved parameter combinations are determined for particular experimental geometries and illustrate the interdependence of parameter and spatial resolutions. Procedures for evaluating uncertainties in the parameter estimates that result from the inversion are derived and demonstrated.

Quasi-Monte Carlo integration over for migration inversion

In this paper, we analyse the discretization of the generalized radon transform/amplitude versus scattering angles (GRT/AVA) migration - inversion formula by means of quasi-Monte Carlo methods. These methods are efficient, in the sense that they require sparsely sampled measurements only, and accurate, which we have shown by theory and examples. Another feature of Monte Carlo methods is their ability to suppress effectively coherent noise associated with undesired wave phenomena in the inversion procedure. As examples, we carried out the associated integrations over and , and consistently found that quasi-random sequences achieve a prescribed accuracy with significantly fewer nodes.

Pre- and poststack migration of GLIMPCE reflection data

Abstract GLIMPCE deep Seismic reflection profiles across the Midcontinent Rift System beneath Lake Superior reveal a central asymmetric rift with an enormous thickness of volcanic and sedimentary rocks. True amplitude cmp-processing, poststack and prestack migration and forward modelling are used to improve images of steeply dipping faults, unconformities and other discontinuities in the deep Seismic data. With prestack migration important steeply dipping structural features of the Lake Superior data set are revealed for the first time. Improved images of high-angle normal faults, later reactivated as reverse faults, provide key structural information for understanding the evolution of the rift system. Results illustrate that structural interpretations of complex deep reflection records, such as those recorded by GLIMPCE, should always be based on migrated data. Furthermore, depth-migrated sections provide useful starting models for forward velocity modelling of Seismic data.

Linearity, resolution, and covariance in GRT inversions for anisotropic elastic moduli

This paper is concerned with the linearized inversion of elastic wave data using the Generalized Radon Transformation to give anisotropic medium parameters. Assumptions of linearity are examined by comparing linearized reflection coefficients calculated using the Born approximation with full plane-wave reflection coefficients. In typical sand/shale models we have found that the linearity assumption is valid only to approximately 60 degrees from the normal. Linear dependencies between the scattering patterns produced by individual moduli result in an ill-posed inverse problem. Utilizing P-wave data, we find that for the Transversely Isotropic case similarities in the C55 and C13 scattering directivity mean that they cannot be distinguished. C11 is best observed at wide-angle and hence estimates made using limited aperture data are subject to large error. Quasi-Monte Carlo techniques are adapted to carry out the 4D inversion integral.

A new method for the migration velocity analysis of noisy seismic data

Abstract We present a method for the determination of optimum migration velocities that is especially suited to datasets, such as deep crustal seismic data, in which both incoherent and coherent noise are present. Slowness weights are evaluated for each point in a time section in order to separate coherent and incoherent noise from signal, which can be identified by its characteristic slowness. These weights are calculated from slant stacks in a single pass through the data. Thereafter the weights may be summed along suites of diffraction curves corresponding to different velocity models to produce coherency estimates for each subsurface point as a function of velocity. After contouring these functions optimum migration velocities may be picked. Synthetic examples are presented illustrating the improved efficiency and velocity sensitivity of this method over previous ones, and showing the applicability of the technique to pre-stack, post-stack, and converted phase migration. A real example taken from a marine survey is used to demonstrate the utility of applying the method to deep crustal data.

A Resolution analysis for anisotropic inversion / migration

We consider spatial and parameter resolution aspects of G(eneralised) R(adon) T(ransform) / A(mplitude) Versus scattering) A(ngles) inversion in anisotropic media. Particular attention is paid to the proper treatment of amplitudes within the distorted Born approximation, viz., by taking into account the radiation patterns of the contrast sources corresponding to the medium perturbations giving rise to the images. By treating the amplitudes correctly, the spatial resolution can be improved even if one leaves the parameter interpretation aside. On the other hand, the parameter resolution, through a singular value analysis, provides a hierarchy of parameter combinations with a decreasing number of symmetries.

Combining Perturbation And Bending Approaches to Seismic Ray Tracing

Perturbation theory is used to combine so called “bending” and “perturbation” approaches to ray tracing. A sin le set of perturbation equations, encompassing both approaches, is developed using fractional arc length along a ray as a convenient parameterization. An examination of the properties of solutions to the equations leads to a fast, stable scheme for ray bending. The method is illustrated with examples showing ray tracing in 2D pseudo-random media. It is found that in a typical problem up to 100 rays per second can be traced using a high performance UNIX workstation.