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10 st / September 2014 www.sea-technology.com ian islands of Kauai and Oahu. The data-sampling rate of the vehicles was one sample per minute, with one-sample- per-second data available on select days. In addition, the vehicle measurements compared geomagnetic data from the Honolulu magnetic observatory (HON), located on Ewa Beach, Hawaii, and operated by the U.S. Geological Survey, with marine magnetic surveys from the U.S. National Geo- physical Data Center's (NGDC) GEODAS archives. The Wave Glider is the frst autonomous marine vehicle (AMV) that harnesses kinetic energy from wave action to produce forward propulsion in the ocean in an environmen- tally friendly manner. A hybrid sea-surface and underwater D irectional drilling uses measurement while drilling (MWD) to infer the inclination and azimuth of the drill bit. The accuracy of the mea- sured azimuth critically depends on the local di- rection and strength of the geomagnetic feld. For instance, lateral uncertainties in the bottomhole location can typically be reduced by 50 percent if the local geomagnetic feld was gauged accu- rately. However, to achieve this, both the map- ping of spatial crustal anomalies and the continu- ous monitoring of space weather disturbance feld variations during drilling operations are required. Traditionally, these tasks are carried out by aircraft, ships and onshore geomagnetic observatories. Well placement by MWD uses the direction of Earth's gravity and magnetic feld as a natural ref- erence frame. This requires accurate knowledge of the direction and strength of the magnetic feld in the wellbore. The natural magnetic feld can be divided into three contributions. The Main Field. This feld is generated by the geodynamo in the Earth's core. For practical pur- poses, the main feld is defned as the internal feld of spherical harmonic degree 1 to 15, ex- cluding time-varying felds with periods shorter than about two years. The Crustal Field. Crustal felds are caused by magnetic minerals in the crust. In practice, the crustal feld is defned as the static internal feld of spherical harmonic degree 16 and higher. The Magnetic Disturbance Fields. These felds are caused by electric currents in near-Earth space and corresponding "mirror-currents" induced in the Earth and oceans. Collecting Data With an Autonomous Marine Vehicle A study was completed by Schlumberger (Houston, Texas) in which the primary data were collected by two Liquid Ro- botics Oil and Gas (Houston, Texas) Wave Gliders equipped with towed marine magnetometers. The Wave Glider is a registered trademark of Liquid Robotics, Inc. (Sunnyvale, California). The gliders assessed an area around the Hawai- Marine Magnetic Surveying And Disturbance Field Monitoring Using Autonomous Marine Vehicles to Study Disturbance Fields By Benny Poedjono • Sudhir Pai • Stefan Maus The Wave Glider equipped with a tow magnetometer.