We present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore‐offshore large‐aperture seismic and gravity data. High velocities obtained within a highly reflective half‐graben that was imaged on coincident seismic reflection data demonstrate the dominantly mafic composition of the graben fill and constrain its total thickness to be at least 30km. Strong wide‐angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (55–60km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high‐velocity lower crust interpreted to result from syn‐rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half‐graben. A second region of thick lower crust is found approximately 100km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre‐rift Moho is probably within or above the high‐velocity lower crust, whereas the post‐rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.

We have developed a simple but practical methodology for updating subsalt velocities using wave-equation, migration-perturbation scans. For the sake of economy and scalability (with respect to full source-receiver migration) and accuracy (with respect to common-azimuth migration), we use shot-profile, wave-equation migration. As input for subsalt-velocity analysis, we provide waveequation migration scans with velocity scanning limited to the subsalt sediments. Throughout the migration-scan sections, we look for the best focusing or structural positioning of characteristic seismic events. The picking on the migration stacks selects the value of the best perturbation attribute (alpha-scaling factor) along with the corresponding position and local dip for the chosen seismic events. The associated, locally coherent events are then demigrated to the base of the salt horizon. Our key observation is that this process is theoretically equivalent to performing a datuming to a base of salt followed by subsalt migration of the redatumed data perturbed-velocity profiles. Thanks to this implicit redatuming of shot profiles, no ray tracing through the salt body is required. Thus, the events picked on the subsalt-velocity scans only need to be demigrated to the base of salt. For the event demigration we use 3D specularray tracing up to the base of the salt horizon within a predefined range of reflection angles. Event demigration produces model-independent data -time and time slope -that are then kinematically migrated using the current tomographic-inversion working model. To find a finalvelocity model that will flatten best the remigrated events on common image point (CIP) angle gathers, we use the same set of demigrated observation data as the input data set for several nonlinear iterations of 3D tomographic inversion.

The Grenville Front, which marks the orogenic boundary between the Archean Superior Structural Province and the much younger Grenville Province to the southeast, is one of the major tectonic features of the Canadian Shield. Within Canada, it is approximately 1900 km in length extending from the north shore of Lake Huron across Ontario and Quebec to Labrador. In 1986, a major coincident onship near‐vertical reflection and onshore wide‐angle reflection/refraction experiment (GLIMPCE–Great Lakes International Multidisciplinary Program on Crustal Evolution) was conducted along a series of lines across the Great lakes. One of the lines, line J, ran across Georgian Bay and Lake Huron for a distance of 350 km and crossed the Grenville Front Tectonic Zone (GFTZ). The seismic signals from the air gun array source were well recorded by the onshore stations up to distances of 250 km with a seismic trace spacing of 50–62.5 m. The GFTZ had a profound effect on the nature of the reflector patterns observed on the onshore seismic sections. Data recorded by the stations on the east end of the line indicate that the crustal P phases are very complex and form a “shinglelike” pattern of reflected waves. Data recorded by stations at the center and at the western end of the line show that the Pg phases are normal and lack the shinglelike appearance. This character of arrivals was also observed on the corresponding S wave sections. A combined P and S wave forward modeling analysis shows that the GFTZ is composed of bands of reflectors dipping at angles of 20°–35° extending to the lower crust. These reflectors were also well imaged on the coincident near‐vertical reflection data. Reflectors under the Britt domain to the east of the GFTZ have a shallower dip than those along the zone. The structure of the crust under the Manitoulin terrane to the west of the GFTZ is laterally homogeneous with a major intracrustal reflector at a depth of 17–20 km below the surface. Poisson's ratio is slightly higher to the east of the Grenville Front compared to the region to the west. The travel times of the PmP signals indicate that the Moho may be deeper under the GFTZ than under the surrounding regions. Our results give added support to tectonic theories that the Grenville Front owes its origin to a continental collision process.

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A 3-D prestack Kirchhoff migration is implemented and successfully applied to a 3-D data set from the Ouachita frontal thrust zone in southeastern Oklahoma. The algorithm uses data decomposition and operates efficiently both in multiprocessor vector and parallel environments. Image time maps are precomputed once for each survey point, regardless of how many times it is a source or receiver, and stored in random access disk files. These files, and the data traces, are read asynchronously during migration. Image time computation is done by two‐point ray tracing to avoid interpolation between irregularly spaced rays or points on wavefronts. The same software can be used to depth migrate all input data geometries (stacked sections, common‐source or common‐receiver, or common‐offset or common‐midpoint gathers). The output images from the Ouachita field data show structural closure on folded thrust sheets of Wapanucka limestone, and repetition of lithology caused by lateral displacement on the thrusts.