New Marine Seismic Refraction Technology and Survey Techniques: From Concept to Completion Offshore Mozambique, East Africa, 2015

Abstract

Abstract The discovery of natural gas offshore Cabo Delgado Province, Mozambique, East Africa has garnered much attention. The block's operators are looking to transport the natural gas ashore via pipeline to what is expected to be among the world's largest LNG parks. The presence of sub-marine canyons including a carbonate escarpment make the geologic conditions complex. To assess the feasibility of contouring or excavating the edge of the undersea cliffs, a seismic refraction survey was required to determine the P wave velocity structure of the escarpment. These measurements are critical for determining equipment and methods for excavating and dredging work. Nearly all dredging surveys and operations are carried out in water depths of less than 50 meters. As a result, refraction equipment and professional expertise is limited to shallow water surveys. In order to obtain velocity information at greater depths, new technology and survey techniques were developed. Surface-towed seismic arrays were not an option due to offset requirements necessary to achieve critical refraction in deeper water. Furthermore, a surface-towed refraction array would be unlikely to resolve thin sediment layers overlying the escarpment's limestone. The logical option was to design a refraction spread capable of being towed near the seabed and able to resolve layering at the meter scale. The result was the development and implementation of a deep-tow sparker source and hybrid hydrophone streamer and bottom tow cable array. The key aspect of the equipment's design was a decision to depart from pneumatic sound sources. Airguns were deemed unacceptable for a number of reasons: –Airgun performance concerns at >100m depths.–Concerns regarding the impact of the hydrostatic pressure on source characteristics and what affect the bubble pulse would have in the data.–Concerns regarding air-hose lengths needed to deployed to >100m depths.–A larger vessel would be required since a shot interval of 3m would tax air production.–Our commitment to minimize impacts on the environment and wildlife. A specially designed sparker as the source was deployed from a small vessel of opportunity. This eliminated the need for compressors, air-hoses, etc. A new streamer design and towing was also developed. During refraction surveying hydrophone streamers are frequently towed very near the seafloor. This introduces substantial noise into the data and can only be done safely on smooth, sandy/muddy substrates. The escarpment's surface was highly irregular with boulders and ledges. Thus, a cable design was employed where the active hydrophone was suspended just above the seafloor. This reduced noise associated with cable drag. In 2015, the newly developed kit successfully acquired refraction data across the escarpment with excellent environmental and operational performance. The tomography-software processed data produced useful models of the escarpment's P wave velocity structure.