RIASSUNTO
Abstract
This paper describes a study that integrated geographic information system (GIS) habitat mapping, habitat sensitivity analysis, and direct observational data to develop an efficient means of locating shallow water seafloor features warranting environmental protection during seismic operations off northern Mozambique. Data collected during the study were then used to develop a mitigation plan to minimize potential impacts associated with proposed geophysical survey activities. This study combined proven survey techniques for rapid assessment and large-scale mapping of sensitive seafloor features.
Preliminary seafloor habitat categorization and mapping of the project area was conducted using high-resolution satellite imagery and a review of relevant published scientific literature. An operation-specific environmental sensitivity index (ESI) was applied to habitat types, and selection criteria were developed for habitats warranting environmental protection (sensitive habitats) from proposed geophysical survey activities. Sensitive habitats were protected from potential impacts from the seismic sound source by mitigation buffers. A subsequent field survey, consisting of a combined side-scan sonar survey of selected geophysical survey lines and rapid assessment groundtruthing of selected seafloor features, was conducted over a 21-day period, surveying 250 km of seafloor in water depths between 2 and 15 m. The side-scan sonar data were reviewed during collection to identify seafloor features that potentially met sensitive habitat criteria. The locations of seafloor features of interest were obtained for subsequent examination by diver scientists utilizing rapid assessment methods for selection criteria verification. Seventy-nine targets were selected for examination during the field survey. The results of the field survey were used during the design of the geophysical survey, with protective areas placed around sensitive habitats.
The study demonstrates an integration of survey techniques that collectively provide a time-efficient and cost-effective means to identify and map environmentally sensitive shallow-water seafloor features within a large and environmentally complex geographical area. This study additionally presents the application of an operational-specific ESI based on habitat characteristics which are sensitive to impact producing factors associated with offshore oil and gas exploration activities. This combined technical approach is a practical means to minimize environmental impact while meeting industry engineering and logistic constraints.
Introduction
A two-dimensional (2D) and three-dimensional (3D) geophysical survey program was proposed within shallow waters of northern Mozambique (Fig. 1). The project area comprises the northern section of the Quirimbas Archipelago, which stretches a distance of approximately 400 km from the Tanzanian border at the mouth of the Rovuma River southward to the town of Pemba. The continental shelf of the project area is very narrow and incised in some places with submarine canyons that run westwards from deepwater between a band of offshore islands, banks, and shoals towards the coast. Apart from these canyons, isobaths generally lie parallel to the mainland coast. Offshore of the fringing reefs, water depth increases sharply in many places; gradients of 1:1 (45º) may be found in water depths exceeding 200 m.