Comparison of airborne LiDAR and shipboard acoustic data in complex shallow water environments: Filling in the white ribbon zone

Shallow coastal areas of oceans, seas and lakes pose significant remote sensing and navigational challenges due to the transient nature of bathymetric features that characterize these areas (e.g., shoals and shoaling). Seabed data/ maps often represent these areas as white ribbon zones, where sparse data coverage presents a particular challenge to defining and enforcing sustainable use of marine resources. Given the lengthy perimeter of coastlines, there is a need for cost-efficient methods for producing high resolution maps of shallow areas. The research described here compares airborne light detection and ranging (LiDAR) with shipboard acoustic surveys around the Kvarken Archipelago of the northern Baltic Sea. The Kvarken Archipelago includes a unique, brackish shallow water marine environment with a number of moraines. The landscape of the area is characterised by fields of ridged, washboard-like De Geer moraines. The moraines combined with an ongoing glacio-isostatic uplift typical of the area, lead into succession of the elevated structures from the seafloor to land, where present seabed structures will rise on land in future. The ongoing modification of the environment by both climatic and geologic forces contributes to complex geomorphic and ecological feedback systems operating on a range of spatial and temporal scales. Airborne LiDAR can provide high resolution remote sensing data on shallow coastal areas. The technology is especially effective in areas with very shoal waters, like those found along the Finnish coast. LiDAR enables continuous detection of topographic (elevation) and bathymetric (depth) data and can thus generate continuous maps of transition zones between continental and marine areas. Environmental conditions such as water clarity and ice cover can affect LiDAR depth detection capabilities: these conditions require complementary accuracy assessment strategies. As LiDAR cannot determine detailed information on seafloor surface substrate, and anything on subsurface substrate type, geological conditions are determined through the application of acoustic-seismic. methods, sediment sampling and underwater video. Data from LiDAR surveys provide a detailed topographic and bathymetric map of the study area up to water depths of 14 m. The seabed bathymetry model shows a series of elevated, narrow, elongate features that appear throughout the study area. These parallel ridges represent the uppermost geomorphic features of the seafloor. Interpretation of acoustic bathymetric surveys indicates that these ridges are De Geer moraines. The general orientation of these moraines differs from that associated with older features of the northern Kvarken Archipelago. Eroded remnants of older De Geer moraines are overprinted at a high angle by the younger landforms. De Geer moraines in the study area thus represent two phases of deglaciation and finally the eastward ice-sheet/ice edge retreat. The combined approach of using LiDAR and acoustic bathymetric data provides new insights into past glacial dynamics of the Kvarken Archipelago and the Gulf of Bothnia. (C) 2017 Published by Elsevier B.V.