Application of image analysis for mapping of sedimentary facies in a shallow lagoon: Case study, south sound, Grand Cayman, British West Indies

Abstract

Facies are areas on the seafloor that are characterized by the same sediment and biota. They are key elements of any sedimentological assessment of shallow coastal lagoons. They are also natural barometers of change. Thus, changes in facies distribution over time may signal changes in the environment that are driven by natural or anthropogenic causes. Monitoring shallow coastal lagoons is a time consuming and expensive process. The accuracy of conventional facies maps, which are difficult to produce, is a function of the skill of the mapper and the time that is spent compiling the map. Image analysis of black and white or colour air photographs, however, offers an easier and more economical method of producing such maps. Image analysis techniques are generally applied to remote sensing space-borne data sets, and were used to produce facies maps of South Sound, a shallow water lagoon on Grand Cayman. Ground truthing showed that the implemented techniques are capable of producing thematic maps of facies changes with high accuracy. Analysis of air photographs taken in different years allows the assessment of changes in the total area covered by each facies as well as changes in their spatial characteristics (sea floor landscape changes). Image analysis of South Sound, for example, showed that the Thalassia (sea-grass) banks expanded in area by 17% between 1971 and 1992 whereas the area covered by the Sand Facies was substantially reduced. This information provides added insights into the dynamic sedimentological behaviour of the lagoon and opens new venues for further expanding the use of conventional aerial photography as a tool for regular mapping of shallow coastal lagoons.