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

摘要

The discovery of natural gas offshore Cabo Delgado Province, Mozambique, East Africa has garneredrnmuch attention. The block’s operators are looking to transport the natural gas ashore via pipeline to whatrnis expected to be among the world’s largest LNG parks. The presence of sub-marine canyons includingrna carbonate escarpment make the geologic conditions complex.rnTo assess the feasibility of contouring or excavating the edge of the undersea cliffs, a seismic refractionrnsurvey was required to determine the P wave velocity structure of the escarpment. These measurementsrnare critical for determining equipment and methods for excavating and dredging work. Nearly all dredgingrnsurveys and operations are carried out in water depths of less than 50 meters. As a result, refractionrnequipment and professional expertise is limited to shallow water surveys.rnIn order to obtain velocity information at greater depths, new technology and survey techniques wererndeveloped.rnSurface-towed seismic arrays were not an option due to offset requirements necessary to achieverncritical refraction in deeper water. Furthermore, a surface-towed refraction array would be unlikely tornresolve thin sediment layers overlying the escarpment’s limestone. The logical option was to design arnrefraction spread capable of being towed near the seabed and able to resolve layering at the meter scale.rnThe result was the development and implementation of a deep-tow sparker source and hybrid hydrophonernstreamer and bottom tow cable array.rnThe key aspect of the equipment’s design was a decision to depart from pneumatic sound sources.rnAirguns were deemed unacceptable for a number of reasons:rn1. Airgun performance concerns at u0002100m depths.rn2. Concerns regarding the impact of the hydrostatic pressure on source characteristics and what affectrnthe bubble pulse would have in the data.rn3. Concerns regarding air-hose lengths needed to deployed to u0002100m depths.rn4. A larger vessel would be required since a shot interval of 3m would tax air production.rn5. Our commitment to minimize impacts on the environment and wildlife.rnA specially designed sparker as the source was deployed from a small vessel of opportunity. Thisrneliminated 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.rnThis introduces substantial noise into the data and can only be done safely on smooth, sandy/muddyrnsubstrates. The escarpment’s surface was highly irregular with boulders and ledges. Thus, a cable designrnwas employed where the active hydrophone was suspended just above the seafloor. This reduced noisernassociated with cable drag.rnIn 2015, the newly developed kit successfully acquired refraction data across the escarpment withrnexcellent environmental and operational performance. The tomography-software processed data producedrnuseful models of the escarpment’s P wave velocity structure.