MB survey

By default MB, MG and SB profiles were acquired simultaneously. When the sea-state was force 6 and above (Beaufort scale) the magnetometer was not deployed to avoid unsafe operations on the deck. In such conditions, not all SB profiles were recorded due to very poor signal to noise ratio. Survey speed during the MB/MG routes (towing the magnetometer) was 6 to 7 knots, this ensured sufficient spatial coverage and production. Prior to the start of the MB acquisition and each 24 hours, CTD probes were deployed. On Unicorn smt the CTD probe was lost and the remaining area was surveyed without water column data. At Funchal a smaller SVP probe (usable for water depths < 1000 m) was embarked and used for the last 4 measurements. During the 2nd leg of acquisition geophysical operations mostly consisted of the infilling of the preliminary grid obtained from Seine and Unicorn Seamounts during the 1st leg. Route lines, CTD casts location and the preliminary contour maps realtive to the three smts are illustrated in the following figures. The route lines spacing and their heading was chosen depending on the average depths, production/coverage strategy and wave direction. On Ampére smt. line spacing was ranging from 2000 to 400 m (over the summit). On Unicorn the lesser bathymetric range allowed an uniform 2000 m spacing while on Seine smt spacing varied from 2000 to 400 m. All this resulted in a sea-floor coverage varying from 50-75% (Ampére and Unicorn smts) to

100% (Seine smt). A preliminary processing of the raw data acquired by the RESON 8160 was carried out on-board with CARIS HYPS© software allowing to generate 50x50 m grid files. The quality of the data varied from very good to acceptable, excluding swaths acquired at great water depths and under rough sea conditions.

**Figure 9:** Ampere smt. Navigation lines (MB, CH and some MG). In background the regional bathymetry from Gebco Digital Atlas. Coordinates UTM zone 28.
$\begin{figure}\centerline{\epsfig{ file=IMG/ampere_nav.ps,width=12.5cm}} \end{figure}$

**Figure 10:** Ampere smt. preliminary bathymetry. Contour interval 100 m Coordinates UTM zone 28.
$\begin{figure}\centerline{\epsfig{ file=IMG/contour_ampere.eps,width=12.5cm}} \end{figure}$

**Figure 11:** Unicorn smt. Navigation lines (MB and CH). In background the regional bathymetry from Gebco Digital Atlas. Coordinates chilometric UTM zone 28.
$\begin{figure}\centerline{\epsfig{ file=IMG/unicorn_nav.ps,width=12.5cm}} \end{figure}$

**Figure 12:** Unicorn smt, preliminary bathymetry. Contour interval 100 m Coordinates chilometric UTM zone 28.
$\begin{figure}\centerline{\epsfig{ file=IMG/unicorn_bathy_pre.ps,width=12.5cm}} \end{figure}$

**Figure 13:** Seine smt navigation lines (MB, CH and MG). Blue lines are the infilling routes over the top of the smt. In background the regional bathymetry from Gebco Digital Atlas. Coordinates chilometric, UTM zone 28.
$\begin{figure}\centerline{\epsfig{ file=IMG/nav_seine.eps,width=12.5cm}} \end{figure}$

**Figure 14:** Seine smt, preliminary bathymetry. Contour interval 50 m. Coordinates chilometric, UTM zone 28.
$\begin{figure}\centerline{\epsfig{ file=IMG/MB_bathymetric_seine.eps,width=15cm}} \end{figure}$

**Figure 15:** Seine smt. Shaded relief map of the DTM at the end of the 1st leg. DTM cell size is 50x50m.
$\begin{figure}\centerline{\epsfig{ file=IMG/seine_shaded_preliminary.ps,width=15cm}} \end{figure}$

**Figure 16:** Seine smt. Shaded relief map of the DTM after infilling and aboard pre-processing. DTM cell size is 50x50m.
$\begin{figure}\centerline{\epsfig{ file=IMG/seine_shaded_final.ps,width=15cm}} \end{figure}$