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Summary A new hovercraft-based technology for gravity and magnetic surveys in transition zones was developed in YMG GraviMag, the Research and Exploration Subdivision of the State Scientific Centre Federal Geological Enterprise Yuzhmorgeologia (YMG), Gelendzhik, Russia. The technology was tested at the Azov Sea transition zone in Southern Russia. Afterwards, the technology was used on two surveys in Northern Russia, in 2007 and 2008. The acquired gravity and magnetic data showed good quality and has sufficient resolution for detailed geological interpretation. Introduction Detailed gravity and magnetic surveys in transition zones with water depths starting from 0 m present a complicated and technologically challenging task and usually are very expensive. Conventional shipborne surveys are limited by water depths, as the vessels cannot approach too close to shore due to safety reasons, and the survey lines have to be stopped early to allow safe space for line change circulation. Surveying with land gravity meters installed on tripods has low production rate, and airborne gravity surveys cannot provide sufficient resolution. In 2006, YMG has been contracted to perform gravity and magnetic surveys in the transition zone of Pechora Sea in Northern Russia. More than 50% of the survey area was shallower than 1 m. Strong currents and tides caused sea level changes up to 1.5 m. The area could not be covered with conventional shipborne survey methods. In search for an effective solution, YMG-GraviMag considered using an air-cushion vehicle (ACV), or hovercraft, as a survey vessel. Technology test survey To evaluate the possibility of gravity and magnetic data acquisition from a hovercraft at shallow sea within tens of kilometers from the shore, a test survey has been conducted at Taganrog Bay of the Azov Sea in Southern Russia. The marine gravity meter system GRIN-2000 (Yuzhmorgeologia, Russia) had been installed onboard a hovercraft “Hivus-10” (Figure 1) built by Aerohod Ltd. (Nizhniy Novgorod, Russia). Magnetic data was acquired with a towed SeaSPY marine magnetometer (Marine Magnetics Inc., Canada). The selected hovercraft model “Hivus-10” has the overall length 7.55 m, width 3.30 m, height 2.55 m. Its cruising speed is 50 km/h (max 70 km/h) over the water and 70 km/h (max 90 km/h) over the snow. It is able to work with the wind up to 15 m/s and waves up to 1.2 m. The test survey operations were performed from a shore base. The hovercraft crew consisted of 4 men, including the survey geophysicist, navigator, mechanic and the vessel driver. The test demonstrated the possibility to acquire online gravity data with the wave height up to 0.3 m, with simultaneous acquisition of magnetic data with a towed magnetometer. The test also showed that in order to perform offshore surveys at a significant distance from a shore base or from a support vessel, the hovercraft must be equipped with radar, an auxiliary generator for survey equipment, additional fuel tanks and personal safety and survival equipment. The test also showed that a special platform was required for the gravity meter to damp the high frequency vibration. Such platform has been developed in YMG-GraviMag.