Pseudo-3D seismic imaging of intertidal and sub-tidal zones at Waterside Beach, Bay of Fundy

The feasibility of a novel dual profiler, pseudo-three-dimensional technique for seismic profiling in shallow-marine environments has been investigated by application of sub-bottom imaging to a transgressive shoreline in the Bay of Fundy at Waterside, New Brunswick. The survey site is characterized by a 10 m tidal range, short-period swell and water depths of 2 to 30 m, overlying highly variable sea-bottom sediments ranging from gravel bars to sandy muds. Two IKB Seistec(TM) sub-bottom profilers were towed side by side, 12 m apart, fired alternately, and positioned by dual frequency global positioning systems capable of giving centimetre accuracy in three dimensions. The intention was to efficiently acquire sufficient data density and positioning precision to allow heave and tidal effects to be removed and geological cross-line dips to be resolved, thereby producing high resolution subsurface maps without the cost of acquiring a full three dimensional dataset.;The seismic data revealed a bedrock unit as well as four seismically and sedimentologically distinct, post-glacial deposits with a vertical resolution on the order of 0.20 m and depths of penetration of up to 12 m sub-bottom. These units include mixed sand and gravel beach deposits, salt-marsh, laminated offshore sandy mud sediments, and relatively steeply dipping reflectors, which likely represent glacio-marine sediments. Additionally, gravel bars that rise over 6 m above the surrounding beach were imaged as components of the four main deposits. A pseudo three dimensional bathymetric map allowed detailed study of beach surface morphology and imaged sand bars as well as subtle tidal creek deltas. The toe of the beach and a sub-tidally exposed zone of glacio-marine sediments were inferred from the seismic data and identified on the bathymetric map.;All the objectives, with the exception of heave compensation by global positioning system heights and the production of a three dimensional seismic data cube, were met. The dual profiler pseudo three dimensional approach still holds great potential as a strategy for acquisition of seismic data in shallow-water environments without the expense of acquiring a full three dimensional seismic dataset.;Tidal variations were effectively measured by filtering out short-period heave effects in the post-processed kinematic global positioning system heights obtained for one of the profiler as it was towed through the water. The precision of the tidal corrections determined in this way to bring all seismic lines to a common datum is revealed by the near perfect line levelling that was achieved between all intersecting port and starboard seismic lines. This allowed the production of high resolution pseudo three dimensional maps of the beach surface and subsurface layers. Post-processed kinematic global positioning system heights sampled at a rate of 2 Hz also proved adequately precise to track vertical heave effects related to waves/swells on the sea surface. However, heave removal by conventional water-bottom smoothing proved more robust for most of the data, as global positioning system update rate and reliability issues were encountered.