AUV- and ROV-based Quantitative Mapping at Axial Seamount
MBARI began using a Dorado class Autonomous Underwater Vehicle (AUV) for 1-m lateral resolution mapping on Axial Seamount in 2006. The Mapping AUV is equipped with a 200 kHz 1°x1° multibeam, 110 kHz chirp sidescan, and a 1-6 kHz or 2-16 kHz chirp subbottom profiler. The multibeam data are tide-corrected using BPR records, and the navigation is adjusted so that topographic features in overlapping or crossing swathes match. The achieved vertical precision is 10 cm.
Mapping AUV surveys conducted from 2006 through 2009 completely mapped the summit caldera floor and parts of the upper rift and caldera rim. These 1-m resolution maps enabled detailed ROV sampling of lavas and mapping on a flow-by-flow basis that resulted in three papers on the geology of the summit (Clague et al, 2013), morphology and emplacement of the 1998 lava flows (Chadwick et al, 2013), and the petrology and geochemical evolution of the seamount (Dreyer et al, 2013). A key component of the geology and petrology papers---ages of many flows based on radiocarbon dating of foraminifera recovered from short cores on top of the flows—provided constraints on the number of flows during specific time periods to calculate eruption frequencies, and a stratigraphic framework for the observed changes in lava compositions.
The summit lava flows emplaced by the 2011 eruption were almost entirely located within the 2006-2009 MBARI Mapping AUV coverage. Repeated surveys of this area during 2011 allowed us to map the extent and thickness of new lava flows through differencing of the pre- and post-eruption surveys. The repeat survey data processing requires vertical adjustments to fully coregister the unchanged regions surrounding the new flows. The new flows were reliably detected wherever the thickness exceeds 20 cm. These results were mostly published in Caress et al (2012), but subsequent coverage of portions of the flow that were not captured in 2011 were added during surveys in 2014. The 2011 and 2014 surveys also served to re-establish a 1-m resolution baseline in advance of the next eruption at Axial Seamount.
In a different application of repeat mapping, the navigation of successive surveys are coregistered horizontally without vertical correction, allowing relative vertical changes to be only so relative vertical deformation can be obtained by equating depths at regions most distal from the summit caldera. A difference map extracted from surveys done in August 2011 and August 2014 show a maximum relative uplift (inflation) of the caldera floor of 1.8 m during the three-year period. We have now also conducted a similar repeat survey of on the Alarcon Rise axis at the mouth of the Gulf of California, but data processing is still underway. The two surveys, run in April 2012 and again in March 2015, should be able to determine if the elevated section of the ridge segment has inflated at less or more than 7 cm/yr.
The Mapping AUV CTD sensor has proven to be useful for identifying previously unknown hydrothermal vent sites. Potential temperature and salinity anomalies observed during 50-m altitude surveys have indicated that chimney structures resolved in the 1-m resolution bathymetry are actively venting at sites on the Alarcon Rise and in the Pescadero Basin, both in the southern Gulf of California. The combined observations made these sites a priority for ROV dives that subsequently explored and sampled the new vent fields.
MBARI is developing a capability for low altitude surveys combining topography with photographic imagery. During our 2014 Mapping AUV surveys we used ROV Ventana at ASHES vent field to survey an 80-m by 80-m area with multibeam sonar and stereo cameras from 2 m altitude, resulting in coregistered 5-mm resolution photomosaic and 5-cm resolution bathymetry. Test surveys in Monterey Bay have also used a LIDAR to achieve 1-cm lateral resolution topography, with repeated surveys of a chemosynthetic clam community resolving individual animals, changes between surveys, and the trails left on the seafloor as these animals move (Caress et al, 2014). Similar repeated surveys at Axial hydrothermal vent fields could resolve changes to the vents and the communities that live around them. MBARI intends to ultimately field a low altitude survey AUV, but in the near future our low altitude surveys will continue to be ROV based.
Caress, D.W., D.A. Clague, J.B. Paduan, J.F. Martin, B.M. Dreyer, W.W. Chadwick, A. Denny, D.S. Kelley (2012), “Repeat bathymetric surveys at 1-metre resolution of lava flows erupted at Axial Seamount in April 2011 ”, Nature Geoscience, 5, pp. 483-488, http://dx.doi.org/10.1038/NGEO1496.
Caress, D. W., B. Hobson, H. J. Thomas, R. Henthorn, E. J. Martin, L. Bird, M. Risi, G. Troni, C. K. Paull, S. Rock, J. A. Padial, M. M. Hammond (2014), “Repeated 1-cm Resolution Topographic and 2.5-mm Resolution Photomosiac Surveys of Benthic Communities and Fine Scale Bedforms in Monterey Canyon”, Abstract OS31A-0979 presented at 2014 Fall Meeting, AGU, San Francisco CA.
Clague, D. A., B. M. Dreyer, J. B. Paduan, J. F. Martin, W. W. Chadwick, D. W. Caress, R. A. Portner, T. P. Guilderson, M. L. McGann, H. Thomas, D. A. Butterfield, R. W. Embley (2013), “Geologic history of the summit of Axial Seamount, Juan de Fuca Ridge”, Geochemistry, Geophysics, Geosystems, http://dx.doi.org/10.1002/ggge.20240.
Chadwick, W. W., D. A. Clague, R. W. Embley, M. R. Perfit, D. A. Butterfield, D. W. Caress, J. B. Paduan, J. F. Martin, P. Sasnett, S. G. Merle, A. M. Bobbitt (2013), “The 1998 eruption of Axial Seamount: New insights on submarine lava flow emplacement from high-resolution mapping”, Geochemistry, Geophysics, Geosystems, http://dx.doi.org/10.1002/ggge.20202.
Dreyer, B. M., D. A. Clague, and J. B. Gill (2013), Petrological variability of recent magmatism at the Axial Seamount Summit, Juan de Fuca Ridge, Geochem. Geophys. Geosyst., 14, 4306–4333, doi:10.1002/ggge.20239.
Helo, C., D. A. Clague, D. B. Dingwell, J. Stix (2013), High and highly variable cooling rates during pyroclastic eruptions on Axial Seamount, Juan de Fuca Ridge, J.Volcanology and Geothermal Res., 253, 54-64, doi: 10.1016/j.jvolgeores.2012.12.004.
Portner, R. A., D. A. Clague, C. Helo, B. M. Dreyer, J. B. Paduan (2015), Constrasting styles of deep-marine pyroclastic eruptions revealed from Axial Seamount push core records, Earth Planetary Science Letters, in press, doi: 10.1016/j.epsl.2015.03.043.