SIDESCAN SONAR IMAGES OF MUD VOLCANOS FROM THE MEDITERRANEAN RIDGE - POSSIBLE CAUSES OF VARIATIONS IN BACKSCATTER INTENSITY

A survey of a large held of mud diapirs on the Mediterranean Ridge sou th of Crete was carried out using both OKEAN (9.5 kHz) and MAK-1M (30 kHz) sidescan sonar systems during Leg-2 of the Third Training Through Research cruise of R/V Gelendzhik in June and July, 1993. High backsc atter intensities were recorded by both systems to different degrees o ver the mud diapirs and their immediate surroundings. Causes of variat ions in the backscatter intensity are considered to be the spatial dis tribution of mud breccia associated with the mud domes (both the buria l depth of the mud breccia beneath hemipelagic drape and its areal ext ent) and its physical properties, along with contributions from seaflo or roughness and the topography of the mud domes. Significant sources of volume scattering of the sidescan sonar sound signal are considered to be mud breccia inhomogeneities represented by millimetric/centimet ric clasts of various lithologies and free gas bubbles distributed thr ough the mud matrix. A good general relationship is established betwee n these sources and the observed backscatter variations through analys is of ground truth data from core samples, with 5 kHz deep-tow subbott om profiler records and seafloor video observations as well as the sid escan sonar records; but a fully quantitative assessment of this relat ionship would require acoustic modelling and additional ground truth d ata.