Seismic refraction surveys are used to define the large-scale geometry of Earth’s crust and the physical properties of its subsurface layers. Seismic reflection surveys reveal fine-scale geometry, and, with refraction data, provide a more complete representation of subsurface geology.
Surveying in marine waters, however, is a challenge. Until the mid-1970s, sea-surface acoustic sensors (sonobuoys) used in seismic refraction surveys offshore eastern Canada had serious limitations. Drifting sonobuoys created uncertainty in positions, instruments were lost, and shear waves, useful in distinguishing rock types, could not be recorded, as they cannot travel through fluid. The challenge was to develop sensors that could withstand great pressure when lowered to abyssal depths, record the required data, and then float to the surface for retrieval.
The solution was the ocean bottom seismometer (OBS) developed in the late 1970s by Geological Survey of Canada staff at the Bedford Institute of Oceanography. Recording seismic energy with an OBS eliminated wave noise. Coupling the OBS to the seabed improved signal-to-noise ratio. Compression waves and shear waves could be captured allowing discrimination between rock types. Further refinements included conversion to digital instruments in the 1990s.
Category: Equipment and Instrumentation