NW Rota 1 - Hydrothermal Plume Imaging
USNS 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

Hydrothermal Plume Imaging with Water Column Data.

survey lines over nw rota
As 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 Washington 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 indicate a 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 currents.

pie diagrams
80 ping pie diagrams corresponding to
3D imagery below.
rota JD 204 first pass
EM710 3D WC Imagery - S to N pass
0450 GMT J204 July 23 2009
(low threshold to identify the 3 streamers)
NW Rota 1
EM710 WC Imagery - NE to SW pass
1350 GMT J202 July 21st 2009
(stronger threshold to pick the mooring out from the DSL)
The surrounding figures illustrate the preliminary highlights of the vent imaging.

Coordinates of the base of the plume:
144.775463° N 14.600829° E ( 14° 36.0497' N -- 144° 46.5278' E)

It appears that there is a submerged mooring located due east of the summit. It consists of an upper unit (flotation?) suspended ~110m off the seabed and a mid-mooring object (instrument package?) that sits about 60m off the seabed. The dual, vertically aligned targets were found on all 5 passes over the summit in the same location (2 on JD202 and 3 on JD204), suggesting a permanent rather than an ephemeral object.

Coordinates of the apparent mooring:
144.776893° N 14.600853° E (14° 36.0512' N -- 144° 46.6136' E)
Detailed Imagery of Peak including locations of vent and mooring
peak map
plot showing the location of the vent and the mooring
peak profile
topographic profile along
section indicated in map to left.
(corrected metres)

JD202 NE to SW pass
JD202 NW to SE pass
Mooring - JD202 NE to SW pass
** WC imagery is sonar relative
(excluding draft and refracted ray path, using nominal 1500m/s).

Mooring - JD202 NW to SE pass

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. The EM710 is forced to use an FM pulse and thus, for duty-cycle reasons, can 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 rate).

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.

EM710 detail over NW Rota 1 peak
(single FM swath)
EM122 detail over NW Rota 1 peak
(dual CW swath)

While the EM710 is working adequately at these depths, at greater depths, it 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 intermediate system.

created on board - July 2009,  stripped down August - John E. Hughes Clarke