Geology of the Scotian Margin - Crustal structure

In 2001, the research ship CCGS HUDSON was used to collect new data to investigate the crustal structure of the continental margin and deep sedimentary basin offshore Nova Scotia. Three lines of wide-angle seismic reflection / refraction data were collected by towing an array of pressurized airguns (total volume 104 L) over a series of sensitive receivers placed on the seafloor. Source energy from the airguns was reflected or refracted through the layers of rock beneath the seafloor and returned energy was recorded digitally on ocean bottom seismographs (OBSs) spaced 10 to 40 km apart.

Lines 1 and 2 span the eastern and central parts of the margin, respectively, and are coincident with deep multichannel seismic profiles acquired in the late 1980s. Line 3 crosses the southwestern end of the margin, where a nearby multichannel line reveals seaward-dipping reflectors (SDRs), believed to be of volcanic origin, that are coincident with a prominent magnetic anomaly, the East Coast Magnetic Anomaly (ECMA).

Preliminary interpretation is done using forward ray tracing (Zelt and Smith 1992, Geophys. J. Int. 108, pp. 16-34) to develop a model of velocity structure. Rays are traced from the OBS position to the surface (shot positions) and the travel times are compared with the recorded data. The velocity and depth of each layer can be determined by examining the arrival times and slope of the refracted rays that turn in the layer, and the rays that reflect from its upper surface. Close to the receiver the "first arrivals" are waves that travelled directly from the source to the receiver, or waves that refracted through the sedimentary layers or upper crust. Beyond a certain distance, however, rays that travelled deeper, into higher velocity material, are faster than shallower, slower rays and arrive earlier at the receiver.

This figure shows various ray paths through a velocity model (at bottom) and the corresponding arrivals on the data plot (top). Within 8 km of the receiver position, the earliest arrivals are the direct waves. Beyond this distance, most of the first arrivals are from waves refracted through the sedimentary and crustal layers.