Repeat, high-resolution, multibeam, bathymetric surveys of the Monterey Canyon floor were conducted with MBARI’s mapping AUVs (autonomous underwater vehicle). Each AUV carried a Reson 7125 multibeam echosounder (vertical precision of 0.15 meter and horizontal resolution of 1.0 meter). An inertial navigation system combined with a Doppler velocity logger allowed the AUV to fly pre-programmed grids at three knots, while maintaining an altitude of 50 meters above the seafloor, to obtain a nominal line spacing of 130 meters.

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The axial channel and canyon flanks between 200 and 540 meters water depths were surveyed six times during the CCE. Fourteen out of fifteen turbidity currents captured by the array of CCE instruments passed through this region of the canyon (time of recorded turbidity events). These events caused moorings and instruments to move down canyon at least 30 times. Difference grids obtained by subtracting one survey from the next, show the canyon experienced erosion and deposition of up to +/- 3 meters within a continuous, clearly defined ~200 meter wide swath running along the canyon axis. Sediment gravity flows caused massive remolding of thick sections of the canyon floor in <540 m water depth as a consequence of displacement or fluidization of entire slabs of the seabed during these events. The floor and lower flanks of the canyon from 1350 to 1880 m water depths were also mapped with AUVs six times during the CCE (datasets on IEDA). Between these repeated surveys, three sediment gravity flow events extended beyond 1850 meters water depth. Unlike in the upper canyon where events caused wholesale reorganization of geomorphological features, changes to the lower canyon morphology involved a more moderate re-sculpting of the features. Navigation issues The AUV has no access to GPS fixes when it is below the sea surface so it depends on a Doppler velocity logger and an inertial navigation system to dead reckon. The AUV position can drift during its descent and while mapping. In principle, by repeating surveys we can detect change by differencing grids. However, the precision of the dead-reckoning navigation is inadequate given the resolution of the grids. In order to make accurate difference grids, successive surveys must be spatially co-registered using features that are identical, and unmoving.  A significant effort was made to co-register surveys together at each site using MBnavadjust, part of MBSystem. The surveys in the CCE Shallow site are all co-registered together, but some of the surveys in the CCE Deep site couldn’t be co-registered without creating navigation artifacts. However, pairs of surveys that straddled the two largest turbidity events were co-registered. Grid pairs at the CCE Deep site include, start (Topo1m_CCEDeep_20151105m1_UTM.grd) and end (Topo1m_CCEDeep_20170323m1_UTM.grd) of experiment, before (Topo1m_CCEDeep_20151202m1_UTM.grd) and after January 15, 2016 (Topo1m_CCEDeep_20160322m2_UTM.grd) and before (Topo1m_CCEDeep_20160418m4_UTM.grd) and after September 1, 2016 (Topo1m_CCEDeep_20161206m2_UTM.grd). Data are projected in UTM zone 10N.

Surveys: