Minerals Report - International Seabed Authority

4.3 Modern instruments of choice
The most popular current method for locating new sites of
hydrothermal activity is to use an integrated geophysical and geochemical
approach. First, one requires a pre-collected multibeam swath bathymetric
map of the given work area, to navigate by. Once that is obtained, an optimal
approach is to deploy a deep-tow sidescan sonar instrument (this author has
experience with both the WHOI DSL 120 system, USA and the SOC TOBI
30kHz system, UK) equipped with in situ real-time continuous optical
backscatter sensors. Towed at 100-300m above the seafloor, these instruments
are, thus, poised at exactly the right height off-bottom to intercept particleladen
“black-smoker” type hydrothermal plumes that may exist and,
whenever such signals are intercepted, can also provide a detailed geological
sidescan sonar image of the immediately underlying seafloor which hosts the
high-temperature hydrothermal field. Our experience has shown that,
allowing for a double-pass along any section of ridge crest, a one-month unit
of ship time devoted to these operations should be able to provide 100%
survey coverage of ca. 200km of ridge-crest with a return of ca. 6 new
potential sites of high-temperature hydrothermal activity (German et al., 1996,
1998, 2000).
This first survey of a ridge area will not take you immediately to an
active area of venting, however. More likely is that it will narrow down an
area of search from approximately 200km of previously unexplored ridgecrest
to an area 2-5km across within which the source field lies. To more
precisely locate the active site of venting requires detailed systematic lowering
of a conventional CTD-system along a grid-like pattern (e.g. Klinkhammer et
al., 1986) or, better, a tow-yo survey using (e.g.) the purpose-built BRIDGET
instrument, developed at Southampton Oceanography Centre, that can both
map out and sample a hydrothermal plume in 3-dimensions (Rudnicki et al.,
1995). Following identification of the Rainbow hydrothermal plume from
TOBI in 1996 (Fig.10) (7) a further 7 days of survey with BRIDGET in 1997
were sufficient to both i) map out the far-field dispersion of this plume over
20-30km downstream and ii) identify the source of venting for this
pronounced hydrothermal field to within ±100m on the seabed (Figure 11)
(21).
INTERNATIONAL SEABED AUTHORITY 391