NW Rota 1 - Hydrothermal Plume Imaging
Pathfinder (TAGS 60) EM710/EM122 Assessment
July 17th to 25th (JD 198-206), 2009
|John E. Hughes Clarke
Ocean Mapping Group
Dept. Geodesy and Geomatics Engineering
University of New Brunswick, CANADA
|Mel Broadus and Rebecca Martinolich
Systems Engineering Division
U.S. Naval Oceanographic Office
Stennis Space Center, MS, USA
August 2009 -
Compacted extract of assessment report
with Water Column Data.
part of an ongoing investigation into the potential of the water
column imaging capability of the EM710 and EM122, a series of
reportedly active volcanic edifices were imaged in transit between the
Farallon de Medinilla and the NW Guam area.
Minor venting and curious mid water targets were encountered in the
vicinity of East Diamante. But the most impressive imagery was found
over the crest of the NW Rota 1 seamount. This area has long been the
focus of intense scientific study. Most recently the RV Thomas
undertook a detail ROV and Tow-Yo CTD survey of the crest in April 2009
(Bill Chadwick, Chief Scientist).
Reportedly there are periods of actively extruding lava, seismic
activity and near continuous hydrothermal venting. Certainly, the 5
passes obtained over the vent on two days (JD 202 and JD204) clearly
plume extending at least 500m above the peak of the seamount. The shape
of the plume varies from day to day and the upper part breaks up into
two or three streamers which are clearly advected by the prevailing
Comparing EM710 and
EM122 water column imaging in 500m of water.
550m depth is within the EM710 bottom detection range, but it is
close to the limit of sufficient S/N for water column imaging.
EM710 is forced to use an FM pulse and thus, for duty-cycle reasons,
only achieve single swath capability in these depths. Also, a maximum
swath angle of
only about +/- 50-55° is possible.
In contrast, at 550m depth the EM122 is not even beginning to be S/N
limited for bottom detection. It is still only using CW pulses and thus
still has enough duty-cycle to achieve dual swath. Also, unless
otherwise constrained, it will be perfectly capable of obtaining
+/-70° sectors (doing so though, drastically reduces the ping
The two animations below represent the same 40 transmit cycles of the
EM710 (left) and EM122 (right) over the crest of the seamount. As the
EM122 is in dual ping mode, there are 80 swaths achieved, whereas only
40 swaths are present for the EM710 in the same time. The first pings
of each cycle for each of the two sonars are synchronized to avoid
interference. The animation rate of the EM122
has been doubled so that the two time series are seen in phase.
As can be seen, the 122 is not seeing the noise floor (the main reason
that the volume scattering appear to be increasing with range for the
710). Also it is responding differently to the plume characteristics,
presumably reflecting the ~ 5 x longer wavelengths used.
over NW Rota 1 peak
(single FM swath)
over NW Rota 1 peak
(dual CW swath)
While the EM710 is working adequately at these depths, at greater
would not be usable for water column imaging purposes. The EM122,
however, would be capable of providing
this same capability (in angular not absolute resolution) down to
several thousand metres. The EM302 would, of course, represent an
created on board - July
stripped down August - John E.