Quantitative characterisation of geological processes using sidescan sonar imagery

In sidescan sonar imagery, textural analysis enables an objective approach, supplementing the interpreter with reliable quantitative results. Two textural indices are necessary to distinguish geological units: entropy, which measures the roughness of local acoustic textures, and local homogeneity which measures the medium-scale degree of organisation of the local texture. Extensively ground-truthed with submersible observations, this textural analysis technique has been applied to a wide range of resolutions and frequencies. Our presentation shows the application of textural analysis to contrasted portions of mid-ocean ridges: the Main Endeavour hydrothermal field (Endeavour Segment, Juan de Fuca Ridge); and regional-scale surface processes imaged by TOBI (Towed Ocean Bottom Instrument) along the Mid-Atlantic Ridge between 24/spl deg/N-30/spl deg/N and 36/spl deg/N-38/spl deg/N. Well-documented test areas provide good calibration which we can use for extrapolation to new areas of seafloor and increasingly complex combinations of geological processes. In some instances, textural analysis enables the detection of structures overlaid by sediments or with subtle acoustic differences. On a local scale, it is possible to appraise the differences between similar-looking types of extrusive volcanics, or document the weathering and aging of hydrothermal sites. Use of this technique at larger scales leads to quantitative regional assessments of the relations between volcanics, tectonics and hydrothermalism in mid-ocean ridge environments.