The Pennsylvania State University, Applied Research Laboratory, State College,

Data acquired by an autonomous underwater vehicle (AUV) towing a source (1600– 3500 Hz) and a horizontal array of hydrophones have been analysed to image discrete, isolated or even a small cluster of scatterers within the sediment, to determine shallow migration paths of hydrocarbons in a mud volcano system of the Malta Plateau. An algorithm based on a semblance function was applied to the acoustic data to highlight scatterers rather than interface reflections. The resulting scatterer map, obtained along the AUV track, generated a pseudo-three-dimensional coverage of the study area, with a horizontal and vertical resolution of roughly 3–5 m and 5–10 m, respectively. This map was combined with high-resolution bathymetric and backscattering seafloor maps obtained from previous explorations. This integrated dataset provides new evidence for the role of fault zones as a preferential path for gas/fluid migration and reveals the intermittent activity of seeping gas. The data show, in particular, that gas bubble slugs, i.e. discontinuous gas columns, rise through Plio-Quaternary sediments along a complex system of conduits terminating at the surface into quiescent mud volcanoes. The gas flux is facilitated by the regional stress field that results in dilatant conditions on the mapped fault zones.

Submarine mud volcanoes occur in many parts of the world’s oceans and form an aperture for gas and fluidized mud emission from within the earth’s crust. Their characteristics are of considerable interest to the geology, geophysics, geochemistry, and underwater acoustics communities. For the latter, mud volcanoes are of interest in part because they pose a potential source of clutter for active sonar. Close-range single-interaction scattering measurements from a mud volcano in the Straits of Sicily show scattering 10–15 dB above the background. Three hypotheses were examined concerning the scattering mechanism: 1 gas entrained in sediment at/near mud volcano, 2 gas bubbles and/or particulates emitted in the water column, 3 the carbonate bio-construction covering the mud volcano edifice. The experimental evidence, including visual, acoustic, and nonacoustic sensors, rules out the second hypothesis at least during the observation time and suggests that, for this particular mud volcano the dominant mechanism is associated with carbonate chimneys on the mud volcano. In terms of scattering levels, target strengths of 4–14 dB were observed from 800 to 3600 Hz for a monostatic geometry with grazing angles of 3–5°. Similar target strengths were measured for vertically bistatic paths with incident and scattered grazing angles of 3–5° and 33–50°,