Mapping marine benthic biotopes using acoustic ground discrimination systems

Abstract

This paper describes the use and appraises the performance of acoustic ground discrimination systems (AGDS) for marine benthic biotope mapping for those who are more familiar with satellite or airborne remote sensing. The operation of AGDS is described and the nature of the data produced compared with more conventional remotely sensed data. Analysis of the AGDS data involves the interpolation of track point data to create areal coverages that can then be interpreted using classification and image processing techniques. AGDS data have been shown to be variable both within and between surveys and systems which affects the performance of interpretation. Additionally, the marine environment imposes limitations on the way that ground sampling can be undertaken and the spatial extent of information that can be obtained. In summary, the main issues that contribute to variability and uncertainty of biotope maps based on AGDS-based methodology are: (1) errors in position fixing of both AGDS and the ground samples; (2) inconsistent performance of the AGDS system; (3) single frequency of AGDS and its effect on discrimination; (4) tracking intensity of AGDS survey; (5) heterogeneity of the seabed at the scale of resolution of the system; (6) lack of vista for designating training sites and the reliance on buffers around ground samples; (7) unsuitable biotope categories. Variability due to AGDS hardware is often beyond the control of surveyors, but quality control procedures can at least alert operators to poor quality data. Performance of AGDS surveys also depends on intensity of both tracking and ground sampling in relation to heterogeneity of the sea floor. This relationship can be investigated through exploratory spatial data analysis, particularly the variogram. The performance of biotope maps can be measured using error and similarity matrices. The accuracy of the biotope maps resulting from AGDS surveys is modest when compared with the performance of satellite or airborne remote sensing and the similarity between maps derived from different datasets can be low. It is misleading to base expectations of the performance of AGDS on satellite or airborne remote sensing systems. AGDS are low resolution systems with limited powers for discrimination between different ground types. However, they afford the opportunity to map benthic biotopes in environments where electromagnetic spectra-based systems cannot operate. Thus, AGDS is a useful tool for benthic ecologists to study the spatial distribution of biotopes on the sea floor although the limitations of AGDS for mapping may preclude their use for precise and accurate survey required for some monitoring applications.