Reversed refraction surveys were conducted along and across the Lomonosov Ridge as part of the 1979 Lomonosov Ridge Experiment. Interpretation of the strike profiles indicates a 5‐km‐thick upper crustal layer with a velocity of 4.7 km/s overlying a 15‐ to 20‐km‐thick layer of 6.6 km/s material. An upper mantle velocity of 8.3 km/s is indicated by a few pn arrivals. High‐amplitude reflection events recorded from this boundary can be successfully modeled by a transition zone of rapidly changing velocity over a depth interval of 5 km. Ray trace modeling of the dip profiles suggests a root structure extending to about 28‐km depth flanked by crust thinning to a depth near 13 km beneath the Makarov Basin and a more gradual thinning to near 16 km beneath the Fram Basin. The similarity between the crust of the Lomonosov Ridge and that beneath the Barents and Kara seas supports the suggestion that the ridge is a slice rifted from the Baltic Shelf.

Several countries are actively investigating the feasibility of using crystalline rock bodies for the ultimate disposal of radioactive nuclear waste. As part of the concept assessment phase in the Canadian nuclear fuel waste management program, a multidisciplinary research investigation is being conducted across the Lac du Bonnet batholith in southeastern Manitoba; no radioactive materials, other than sealed sources or tracers, are to be emplaced within this rock body. The results of a high‐resolution seismic reflection survey, together with information from a number of deep boreholes, demonstrates that major subhorizontal fractures occur at depths of up to 800 m within the batholith. An integrated interpretation of the seismic data with selected borehole logs shows that warm water is flowing up the major fractures from depth. A natural corollary of these results is that a three‐dimensional seismic reflection survey combined with an appropriate exploratory drilling program has the potential for delineating blocks of a rock body that may be relatively unfractured.

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A synthesis of refraction data recorded in 1972 and 1973 in the central Sverdrup Basin with other geophysical data shows major features which correlate well with the regional geological structure. The record sections from the Arctic Archipelago show little coherent secondary energy compared with those from other areas of Canada. Normalization of the sections to remove effects of varying shot size and instrument gain has revealed a significant loss of amplitude and coherence of the upper and mid-crustal phases of the seismic energy on traversing a major northeast-trending structure between Melville and Lougheed Islands. The upper mantle phase (Pn), however, is not abnormally attenuated in its travel beneath the area. The aeromagnetic data reveal a major series of dykes or minor graben, a likely cause of scattering and attenuation of the seismic energy travelling within the crust. These seismic effects and the focal depths of earthquakes suggest that lateral heterogeneities in the crust may extend to near-mantle depths in this area. The age dates available suggest fracture or dyke development progressed from south to north beginning in the Early Cretaceous. The correlation of the recorded seismicity with these structures provides one of the better examples of an active, intraplate tectonic feature.East of King Christian Island (KCI) the refraction results concur with gravity and regional geology in suggesting a major change in crustal and upper mantle structure. Models derived using ray theory indicate a crust which thins from near 40 km beneath the eastern Sabine Peninsula to 32 km west of KCI. East of KCI the Moho may lie at 40 km beneath a complex crustal structure. The average crustal compressional wave velocity is between 5.9 and 6.4 km s−1 and the mean upper mantle velocity is 8.2 km s−1. The present study does not support the existence of a distinct mid-crustal layer with a velocity of about 7.3 km s−1.

The results of three seismological investigations of the crust in the seismically active La Malbaie Region are reported. First arrivals from a reversed reflection-refraction profile within the Charlevoix structure indicate a uniform P, velocity of 6.08 f 0.04 kmls. Secondary arrivals forming discontinuous, sometimes arcuate segments on the record sections are interpreted as subcritical reflections from structural contortions related to impact of the Charlevoix meteorite. A coherent event at 14s may signal reflection from the M-discontinuity near 45 km depth.Analysis of travel times from calibration shots recorded by a network of stations spanning the St. Lawrence River in terms of time-distance profiles, travel time residuals, and numerical models provides excellent support for a model of the Precambrian-Paleozoic contact striking along the north shore and dipping to the southeast about 20" beneath the wedge of 5.5 kmls sediments.Unreversed profiles obtained by recording timed Thetford Mines blasts across a similar network suggests a comparable deep structure beneath the north and south shores, with average crustal velocitiesof 6.8and 6.7 kmls, respectively. A minor mid-crustaldiscontinuity is suggested, and Moho depth is estimated at 42-43 km.The 6.2 kmls upper crustal velocity found for the crater and immediately surrounding area is anomalously low compared to the 6.4-6.5 kmls reported for the eastern shield region. Structural lineations gleaned from satellite imagery suggest an elongated area of impact-related disruption much larger than previously recognized. This expanded area of crustal weakening encompasses and may partially explain the zone of continuing microseismic activity, as well as the low upper crustal velocity of the La Malbaie Region.On rapporte les resultgts de trois etudes sismologiques de la croDte dans la region sismiquement active de La Malbaie. Les premieres arrivkes d'un profil renverse reflexion-refraction a l'interieur de la structure de Charlevoix indiquent une vitesse P,uniforme de 6.08 +_ 0.04 kmls. On interprete les anivees secondaires formant des segments discontinus, quelquefois arques sur des portions de l'enregistrement, comme des reflexions subcritiques des contorsions structurales reliees a l'impact meteoritique de Charlevoix. Un episode coherent de 14 s peut signaler une reflexion de la discontinuite-M a environ 45 km de profondeur. L'analyse de la duree des trajets a partir d'explosions de calibration enregistres par un reseau de stations s'etendant de part et d'autre du fleuve Saint-Laurent en termes des profils temps-distance, des valeurs residuelles de la durte des trajets, et de modeles numeriques fournit un excellent appui pour un modele de contact Precambrien-Paleozo'ique dont la direction est parallele a la rive nord avec un pendage au sud-est a environ 20" sous un coin de sediments dont la vitesse est de 5.5 kmls.Les profils non renverses obtenus par enregistrement des explosions de dates connues a Thetford Mines a travers un reseau semblable suggerent une structure p...

Crustal-scale seismic refraction data obtained in the Beaufort Sea during 1976 reveal a 4–5 km thick sedimentary layer overlying an oceanic crust that thickens rapidly as it approaches the continental terrace of Alaska. A synthetic seismogram analysis of multiple reflected, water-wave events indicates that the upper sedimentary layer has a compressional velocity of 1.8 km/s and a shear velocity of about 0.2 km/s. An oceanic layer 2A of 4.3 km/s, a layer 3A of 6.6 km/s, and a layer 3B of 7.6 km/s overlying an anisotropic (3%) upper mantle with a median velocity of 8.3 km/s are interpreted. The direction of maximum upper-mantle velocity appears to be approximately north–south in the area surveyed, suggesting that a rotation of the Northwind Ridge–Chukchi Plateau away from the Barrow–Martin Point sector of Alaska may have occurred.

The seismic structure of the British Columbia continental margin has been investigated using four reversed refraction profiles. The profile across strike extended 350 km from the volcanic arc on the continent to the deep ocean of the Juan de Fuca Plate; the three profiles along strike were located on Vancouver Island, on the continental shelf, and in the deep ocean on the Juan de Fuca Plate. Interpretation of the profile along Vancouver Island yields a well constrained model for the upper crust with velocity increasing from ~5.3 km/s at the surface to ~6.4 km/s at 2 km depth to ~6.75 km/s at 15.5 km depth where the velocity increases sharply to ~7 km/s. The velocity structure of the deep crust and the crustal thickness are poorly constrained. Four possible velocity functions, based on ambiguous first arrivals and (or) secondary phases interpreted as Moho reflections, are presented. The preferred one includes a deep crustal low velocity zone with a crustal thickness of 37 km; models with a constant 7.1 km/s deep crust require thicknesses of 52 km. Preliminary results from the profile across strike show the dip of the basement towards the continent steepens from approximately 1.4° immediately west of the continental rise to ~4° beneath the rise. Sediment velocities increase as the sedimentary layer thickens towards the shelf. The Moho, with velocity near 8 km/s, appears to dip at similar angles in this region; the dip is ~6° from the edge of the shelf to the central portion of Vancouver Island; here there is an abrupt thickening of the continental crust by about 10 km with a flat-lying Moho to the east. This suggests a contact between subducting oceanic Moho and continental Moho. A small positive velocity gradient is required in the mantle.Two short reflection lines, one using explosives and the other a large air gun fired in an inlet, were recorded on a land-based multichannel reflection system. These were run to test the feasibility of obtaining coherent reflections to upper mantle depths in this complex geological environment, and of acquiring deep reflection data in coastal areas with an air-gun source. The preliminary explosion section showed reflections near 4.4 and 6.8 s. The depths of these reflections correspond closely to the 15.5 km crustal refractor and the top of the subducting oceanic lithosphere, respectively. Dip on the deeper reflector is close to that estimated from the refraction profile. Without stacking or velocity filtering, the air-gun recordings on a line adjacent to the explosion profile show arrivals of energy at similar times.

Three seismic surveying techniques have been employed in a study of the Superior–Churchill boundary zone in southwestern Manitoba and southeastern Saskatchewan. Two reversed refraction – wide angle reflection profiles, one north–south within the Superior tectonic province and one east–west traversing part of the Superior tectonic province, the boundary zone, and part of the Churchill tectonic province, were used to obtain information on the gross velocity structure of the crust over a large region. Preliminary results from these surveys suggest that the crust beneath the north–south profile is typical of previously published crustal models of the western Superior Province, while the crust beneath the east–west profile is similar to that reported for the Churchill Province in eastern Alberta and western Saskatchewan. Generally, the upper and middle crustal sections in the two tectonic provinces are quite similar, while the lower crust in the Churchill Province has a distinct ~7 km/s layer that is not observed in this part of the Superior Province. In addition, there is a marked thickening of the crust within the boundary zone from ~41 km in the Superior Province to ~46 km in the Churchill Province.A 72 km length of fourfold common reflection point coverage was collected in order to determine the fine structure of the crust over a relatively small region. Reliable stacking velocities that may be used for future processing of the common reflection point data were obtained from an expanding spread reflection survey. Various data processing techniques, including common reflection point stacking, linear and nonlinear velocity filtering, and velocity spectral analysis, have been successful in enhancing reflections from the middle and lower parts of the crust. From the preliminary results of the two reflection surveys, it may be concluded that those parts of the crust which are shown as relatively simple layers in the refraction derived models, may be quite complex when viewed on a smaller scale.