ABSTRACT: Identification and characterization of seabed pockmarks in the St. Lawrence Estuary.

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Pockmarks are crator-like depressions that have been identified on various seabeds. Created by the seepage of fluids upward through the sediments, their formation is due to a variety of structural, stratigraphic or man-made mechanisms such as faulting, differences in grain size between strata, or pressure changes in surficial sediments. The pockmarks located in the St. Lawrence Estuary, similar to those located on seabeds around the world, vary in shape and size, ranging from 80 meters to 300 meters in diameter and up to 15 meters in depth. They are mainly located on the northwest channel shelf as isolated occurrences as well as on the channel floor as isolated, grouped, coalesced and linear arrangements. Pockmarks seem to have an irregular distribution, the majority being present in the deeper parts of the channel. The objectives of this study are to (1) present a new approach to semi-automatically extract pockmarks from multibeam data and (2) analyse the distribution and classify the morphology of pockmarks located in the St. Lawrence Estuary. The first step uses Geographic Information Systems (GIS) to extract the pockmarks from the bathymetric data. This is done by analysing several seabed characteristics, such as fractal dimensions, curvature and slope. The most informative characteristics in determining whether or not a feature can be classified as a pockmark were determined on the basis of a logistic regression. These characteristics were then input into a Maximum Likelihood Classifier to discriminate pockmarks from other natural seabed features (e.g. wave forms, outcrops) or dataset acquisition artefacts. The second phase uses the result of the Maximum Likelihood Classification to analyse the spatial distribution, morphology and levels of activity of pockmarks by using a combination of GIS functions, such as density and directional distribution functions, and zonal statistics of the pockmark characteristics, such as range of size and slope values. Hovland and Judd’s morphological classification scheme identifies several forms of pockmarks and was the basis for this classification.The extraction and classification methods have been tested on different areas of the St. Lawrence Estuary seabed to validate the approach. Sample areas have been selected to include pockmarks located at various depths, on different types of substrate, seabed morphology and at different scales. This method provides a way to semi-automatically identify and distinguish pockmarks from the surrounding seabed while extracting the statistics needed to adequately classify them

Bibliography of Canadian Geomorphology