Paul E. Kepkay
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M. Sc. Thesis
Preliminary Investigation of Free Gas as the Control of a Sub-Bottom Acoustic Reflector in the Fine-Grained Sediments of Halifax Harbour and St. Margaret's Bay, Nova Scotia.
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A preliminary, acoustically-based model of free gas in the fine-grained sediments of Halifax Harbour and St. margaret's Bay has been constructed from laboratory and field measurements of compressive velocity.
Laboratory observations of bubble nucleation in a water-saturated mud show that compressional velocity reduction due to small interstitial bubbles (a maximum of 60 mm in diameter) is controlled by the approximation of a bubble-screen. Prior to this state, velocity reduction is not apparent due to first arrivals propagating through largely bubble-free areas, and contemporaneous with this state, velocity reduction is small. Both observations are not in agreement with the predictions of emulsion theory.
Velocity profiles of gassy muds in the field show that an acoustic reflector recorded in Halifax Harbour and St. Margaret's Bay is coincident with a relatively small velocity contrast between gassy muds overlying substantially less gassy muds. The existence of gas bodies below this interface is not excluded.
Models of reflection coefficients at vertical incidence show that a sub-bottom reflector up to five times the reflectivity of the sediment-water interface can be developed by density, rather than by large bulk modulus (and therefore velocity), contrast between gassy and less gassy muds.
The often impenetrable nature of the reflector is not apparent in simple reflectivity models, and may be due to dispersive wave propagation in gassy sediment, perpendicular to vertical incidence.