SIDE SCAN SONAR MAPPING AND SHALLOW GAS OCCURENCES IN A RÍA ENVIRONMENT: RÍA DE PONTEVEDRA (NW SPAIN)

The Ría de Pontevedra is located on the Atlantic margin of Galicia (NW Spain). It belongs to the so called Rías Baixas and has an area of 64.5 km2. It shows a funnel-shaped in plan view having a main NE-SW longitudinal axis of 28 km and an open mouth with 12 km width. The bathymetry shows a typical valley geometry, with its deepest part located along the central axes, where depth decreases gradually from outer to inner parts (50 m at the entrance of the ría and 5 m in the most internal part). According to the geological map, the country rock in the ría consists mainly of post Hercynian orogenic plutonic and Paleozoic metamorphic rocks with a N-S and NNE-SSW structural trend.

Side Scan Sonar (Klein model 595 operating at 100 and 500 kHz) was used to perform a detailed map of the Ría de Pontevedra. High resolution single channel reflection profiles (Uniboom, EG&G Geopulse and ORE 3.5 kHz subbottom profiler), Atlas Deso 20 echosounder and Side Scan Sonar were employed to characterised the presence of shallow gas in the ría. Positioning was achieved by using a differential GPS.

High resolution seafloor mapping of the Ría de Pontevedra is presented. Four different types of gas accumulations in the ría have been identified: (1) acoustic blanking s.s., (2) acoustic curtains, (3) acoustic columns and (4) acoustic turbidity. Mapping this features reveals the presence of a gas field in the inner part of the ría, where gas accumulates from the closed seabed up to 10 ms. Evidences of gas escapes from the seafloor (acoustic plumes and pockmarks) are also recorded.

Contribution to: PGIDT00PXI30105PR, REN2000-1102 MAR and 437 IGCP Spanish Projects.