Mud Volcanoes And Pockmarks: Seafloor Engineering Hazards Or Geological Curiosities?

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Abstract
Two potential geologic hazards, mud volcanoes and pockmarks, have been documented with increasing frequency since worldwide requirements for offshore hazard, environmental and engineering surveys have become more exacting. Terrestrial and marine mud volcanoes, as researched in the Caspian Sea area, form a drilling hazard and at the same time present one of the most reliable indicators of hydrocarbon producing structures. Mud volcanic eruptions of plastic clays, usually containing rock fragments, are driven by thermal waters and gas (mainly methane and nitrogen). Gas which does not escape to the surface frequently charges near-surface, permeable horizons to form high pressure gas pockets which can result in blow-outs during drilling unless operations are carefully controlled. Periodic reactivation of mud-volcanic activity also forms a potential hazard to oil field structures. Foundation stability may be threatened through liquefaction and gas escaping through gryphons poses a fire hazard. An engineering geophysical survey in the Northern Aegean Sea revealed that the seafloor offshore Thasos Island contains an abundance of pockmarks. These nearly circular depressions in the silty, alluvial seafloor sediment average 20 meters in diameter and attain depths up to three meters. Analysis of the data collected led to the conclusion that the pockmarks resulted from the escape of either artesian water or gas upwards through the thick alluvial sediment of the subbottom. The evidence suggests that artesian water is the most likely cause. From an engineering hazard viewpoint, the pockmark risk for this particular case was judged to be slight regardless of which of the two origins is chosen, provided that a relief mechanism for excess pore water or gas pressure is allowed for. INTRODUCTION: Occurances of mud volcanoes and pockmarks on the seafloor have been documented more and more frequently since the intensive use of side scan sonar began in the late 1960's. They have been described from both research and applied surveys. This paper will discuss the geological origin of these features and evaluate their significance as seafloor hazards based on two engineering surveys. In the case of mud volcanoes, their value as an exploration tool will be pointed out. As a basis for the discussion of mud volcanoes, the Caspian Sea occurances will be used; for pockmarks, an example from the northern Aegean Sea will be presented. MUD VOLCANOES: Mud volcanoes, in contrast to magmatic volcanoes, are of limited distribution throughout the world, with occurrences having been recorded in Rumania, Italy, Iran, Iraq, New Zealand, India, the Arakan Yoma Coast, the Malaysian Islands, Java, Borneo, the Gulf of Mexico, Trinidad, Venezuela, Colombia and the USSR. However, nowhere in the world are there as many or as well documented mud volcanic occurrences as in the Azerbaijan trend which continues into the Southern Caspian area (Figure 1). Mud volcanoes are plano-conical features exhibiting relief of up to 500 meters on land. Their base diameters range from less than one to over three kilometers. Marine. mud volcanoes in the Caspian Sea are similar to their terrestrial counterparts with the exception of height.

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