MAGNETOMETRY

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MAGNETOMETRY

The ISMAR group used a Marine Magnetics SeaSpy magnetometer. The towfish was kept 120 m off the stern, on the port side.The data was collected by the Marine Magnetics SeaLink software on the multibeam and transit lines, The software integrated positional data by the NMEA GGA and VTG strings delivered by the ship's DGPS receiver. Figure 11 shows the acquired lines. Intersecting lines will be used to check and minimize trends, day/night variations, spikes and errors wherever possible.

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**Figure 11:** Pattern of acquired magnetic lines.
$\begin{figure}\centerline{\epsfig{bbllx=20,bblly=10,bburx=550,bbury=610, file=IMG/MRS06_NAV_MAG.ps,width=12.5cm} } \end{figure}$

The following data processing steps were applied:

extraction of the navigation and total field ASCII data from the SeaLink data files
application of the IGRF 2010 (IAGA) to every point, to obtain anomalies
polar transport (GPS antenna offset, angle (hdg-180)) to get the towfish position and conversion of geographical coordinates to UTM-33 WGS84 coordinates
production of time and wiggle profiles and grids in geographical/metric coordinates for QC and analysis

An example of the data can be viewed in Figures 13 and 12

The INVG team towed two Geometrics Mod-G811 magnetometers in a gradiometer configuration 150+150 m off the stern on the starboard side. The system console received the positional and time data from the ship's DGPS via NMEA GGA sentences. The data were QC checked and processed by the OASIS GEOSOFT software.

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**Figure 12:** Line 01 IGRF2005 data.
$\begin{figure}\centerline{\epsfig{bbllx=20,bblly=40,bburx=550,bbury=610, file=IMG/MRS06_MAG_WIGGLE_11.ps, width=12.5cm}} \end{figure}$

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**Figure 13:** Line 01 IGRF2005 data.
$\begin{figure}\centerline{\epsfig{file=IMG/MRS06_MAG_TIME_11.eps,width=15cm}} \end{figure}$

Next: GRAVIMETRY Up: MATERIALS AND METHODS Previous: ON BOARD WATER ANALYSIS Contents

2006-11-01