Seismic reflection methods are being developed at the University of Manitoba to aid in determining fine crustal structure in the Precambrian of Manitoba and northwestern Ontario. Present‐day environmental concern as well as mineshaft conditions necessitate the detonation of several smaller charges repeated, say, I times and followed by ‘vertical’ stacking. To obtain the familiar √I improvement in signal‐to‐noise (S:N) amplitude ratio applying the straight‐sum (SS) method, one assumes, among other things, that both S:N ratio and signal variance are the same on all traces. Dropping these assumptions, as we must for our data, it becomes necessary to apply weighting coefficients to optimize the S:N ratio of the stacked trace. We still assume the signal shapes to be the same for repeated shots, so for the jth trace on the record of the ith shot we model the time series as: tij=ai (sj+nij); where ai is a scaling factor. The proper weights wi are then shown to be proportional to σsi/σ2ni where σ2 is variance, or to γi/ai where γi is S:N power ratio. Applying the weighted‐stack (WS) method gives S:N amplitude ratios which are, on average, 55% of the optimal ratios expected from WS theory compared with only 24% for the SS method. The 45% shortfall in WS performance is ascribed mainly to trace‐alignment (or time‐delay) errors. Varying noise levels on individual traces, slight dissimilarity of signal shape, and correlated noise may also contribute to a lesser extent (in decreasing order of significance). This WS method appears to strike a good practical balance between S:N improvement and processing efficiency.

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.

An expanding spread seismic reflection survey has been conducted across the Snake Bay–Kakagi Lake greenstone belt in northwestern Ontario. Receiver and shot arrays with multiple shots per location helped to maintain a high signal to noise ratio in most of the data. Distances between the shots and receivers ranged from 1.04–8.48 km and the total charge per shot location varied from 26–86 kg. After computer processing the data, numerous coherent reflections were observed from near vertical and near horizontal discontinuities.Prominent early reflections were used to map a granite–greenstone contact to the south of the profile and a section of the Long Bay fault zone to the northeast of the profile. A noticeable absence of reflections from the Aulneau granite batholith–greenstone contact suggests that this contact dips westwards, towards the centre of the batholith.From the later reflections a model of the deep crust beneath the Snake Bay–Kakagi Lake greenstone belt was derived. This model, which represents a lateral extension of the Aulneau crustal model, consists of a three-layered crust. The top crustal layer is 19 km thick with Pg and Sg velocities of 6.2 and 3.5 km/s respectively, the middle layer is 3 km thick, and the lower layer extends to the Mohorovicic discontinuity at 38 km depth.

A sub-critical seismic reflection survey has been conducted across the Aulneau granite batholith in northwest Ontario. Shot to receiver distances ranged from 2.23 to 71.9 km and the total amount of explosives used for each shot-receiver configuration ranged from 54 to 327 kg. Multiple shots with vertical stacking, frequency filtering and time varying velocity filtering were used to increase the signal to noise ratio for each trace. Reflections from shallow vertical faults and deep horizontal crustal discontinuities were observed on the processed records. From these data a model of the crustal structure in the region of the Aulneau batholith has been derived. The model has a number of near vertical faults and an essentially three layered crust in which the upper layer is 16 km thick with P-wave and S-wave velocities of 6.1 and 3.6 kmls, respectively, in the top part of the layer. The average P-wave velocity throughout the layer is 6.2 kmls. This layer probably represents the relatively homogeneous part of the batholith. The middle layer is 5 km thick beneath the shot point and thins to the southeast. It has a P-wave velocity of 6.9 kmls. The lower layer extends to the Mohorovicic discontinuity at 38 km depth and has an interval velocity of 7.2 kmls. The proposed crustal model is generally supported by other geophysical data collected in northwest Ontario.On a procede 2 un leve de sismique-reflexion sous le niveau critique a travers le batholite granitique d3Aulneau dans le nord-ouest de I'Ontario. On afait varier les distances entre les points d'impact et les recepteurs de 2.23 a 71.9 km et la quantite totale d'explosifs utilisee pour chaque configuration source-recepteur allait de 54 a 327 kg. On autilise des sources multiples configuration verticale, un filtrage de frequence et un filtrage des vitesses en variant le temps pour augmenter le rapport signallbruit de chaque trace. On a observe sur les donnees traitees des reflexions de failles verticales peu profondes et des discontinuites horizontales dans la croiite en profondeur. A partir de ces donnees, on a Clabore un modele pour la structure de la croDte dans la region du batholite d'Aulneau. Le modele possede un certain nombre de failles presque verticales et une croDte composee de trois couches dans laquelle la couche superieure a une epaisseur de 16 km avec des vitesses d'ondes P de 6.1 kmls et des vitesses d'ondes S de 3.6 kmls au somrnet de la couche. La vitesse moyenne des ondes P dans cette couche est de 6.2 kmls. La couche superieure represente probablement la portion relativement homogene du batholite. La couche mediane a une epaisseur de 5 km au point d'impact et s'amincit vers le sud-est. La vitesse des ondes P y est de 6.9 kmls. La couche inferieure vajusqu'a la discontinuite de MohoroviCic a une profondeur de 38 km et a une vitesse de 7.2 kmls. Ce modele de croiite est compatible avec d'autres donnees geophysiques recueillies dans le nord-ouest de I'Ontario. Can. J. Earth Sci., 15,301-315 (1978) [Traduit par le journal]

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