ALBERT

Beschreibung: We present high-resolution resistivity imaging of gas hydrate pipe-like structures, as derived from marine controlled-source electromagnetic (CSEM) inversions that combine towed and ocean-bottom electric field receiver data, acquired from the Nyegga region, offshore Norway. Two-dimensional CSEM inversions applied to the towed receiver data detected four new prominent vertical resistive features that are likely gas hydrate structures, located in proximity to a major gas hydrate pipe-like structure, known as the CNE03 pockmark. The resistivity model resulting from the CSEM data inversion resolved the CNE03 hydrate structure in high resolution, as inferred by comparison to seismically constrained inversions. Our results indicate that shallow gas hydrate vertical features can be delineated effectively by inverting both ocean-bottom and towed receiver CSEM data simultaneously. The approach applied here can be utilised to map and monitor seafloor mineralisation, freshwater reservoirs, CO2 sequestration sites and near-surface geothermal systems.

Beschreibung: 3D reflection seismic data were acquired using the P-Cable system of the National Oceanographic Centre, Southampton, UK during cruise 178 Leg 2 onboard RRS Charles Darwin between the 5th and 8th of April 2006. The responsible PI's was C. Berndt, Southampton Oceanography Centre, Southampton, UK. The aim of this cruise was to map submarine landslides on the eastern slopes of the Eivissa Channel, western Mediterranean Sea located between the islands of Ibiza-Formentera and the Spanish mainland. Berndt et al. (2012) used the acquired data to study repeated slope failure linked to fluid migration, while Lafuerza et al. (2012) studied geotechnical aspects of slope stability using this as additional data. Acquisition parameters: The source during seismic acquisition consisted of four 40 in3 Bolt 600B air guns spaced 0.75 m apart and tower at a depth of 1.5 m about 20 m behind the stern of the vessel (Berndt et al., 2012). The air guns are fitted with wave shape kits that emit approximately 10 in3 of air prior to the main volume to reduce the bubble pulse. The air pressure is 2000 psi, and the gun controller triggers the guns to figure every 7 seconds. The data were collected with 11 single-channel analogue streamers that were towed 10 m apart. The seismic cube in the Eivissa Channel covers an area of ca. 14 km2 (ca. 6.4 EW x 2.2 NS km) located at 306091.83 4280497.41; 305951.42 4278353.92; 312321.94 4277936.57 in UTM zone 31N. 3D seismic processing: Data were frequency filtered from 45 to 220 Hz and binned at 10 m bin interval before a Stolt time migration with a migration velocity of 1500 ms-1 was carried out. The resolution of the data is approximately 5-6 m vertically and for the 10 m inline and crossline spacing the horizontal resolution is 10-15 m (Berndt et al., 2012). Seismic data acquisition was performed between 10:05 PM on the 5th of April until 08:30 PM on the 7th of April 2006 (CD178 cruise report). The seismic cube is located at water depths of 550 to 825 m from east to west. Raw data is available here:doi:10.1594/PANGAEA.943523.