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.
Ambient noise in the frequency range 1–20 Hz was measured for 1-day periods at each of four widely separated sites in the North Atlantic Ocean with freely drifting, surface-suspended hydrophones. The acoustic data were recorded aboard an attending research vessel following transmission over a radio link. Narrow-band spectral analysis was performed at 1-h intervals to form time series of noise spectral level for each site. Statistical analyses of the power-spectral time series included the determination of cumulative probabilities, standard deviation, skew, kurtosis, decorrelation times, and the wind-speed dependence of mean spectral level. Comparisons with data from bottom-mounted hydrophones and studies of inter- and intra-array coherence indicate that the data were, with few exceptions, uncontaminated by self-noise down to 1 Hz. A significant dependence of noise spectrum level upon local wind speed was observed in the 1.5- to 3-Hz frequency band at each site. At 4 Hz, the dependence upon wind speed was much weaker, suggesting that the noise sources at this frequency are more distant. Estimates of noise directionality and tests for normality were also made on the data between 1 and 5 Hz.
For the past 13 years, Defence Research Establishment Atlantic has been actively engaged in measuring properties of the low-frequency ambient noise field in the North Atlantic Ocean and Canadian coastal waters. Concurrently, there has been a strong emphasis on developing surface-suspended arrays that can reliably and inexpensively extend such measurements down to the infrasonic range. Special attention has been paid to hydrophone design and shrouding, and to minimizing the effects of sea-surface motion and current shear. The success of this program has led to the construction of small superdirective research arrays, and more recently, quiet large-aperture vertical and horizontal line arrays. Near-surface noise directionality measurements made in midocean with these arrays complement reported results from bottom-moored research arrays. The measurements of vertical directionality, in particular, show distinct trends, with arriving low-frequency noise becoming more concentrated towards the horizontal in the northern ocean, and more diffuse with decreasing frequency. Azimuthal anisotropies are less pronounced, with some bias towards busy shipping lanes and fishing grounds.
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