HIGH-RESOLUTION SEISMIC ANALYSIS OF SHALLOW GAS FEATURES: RÍA DE AROUSA (NW SPAIN)

The Ría de Arousa is located on the Atlantic margin of Galicia (NW Spain). This ría has an area of 230 km2 , with a main NE-SW longitudinal axis of 28 km, and a very variable width (from 4 to 15 km). The land geology is composed of Palaezoic metamorphic and granite rocks, cut by NE-SW, NW-SE and N-S trending faults. The water depth within the ría ranges from 75 m, at its entrance, to 8 m, at the most internal part.

A total of 312.1 km of seismic lines were acquired and interpreted in order to detect shallow gas signatures, both accumulations and escapes. The survey was performed as a rectangular grid with lines spaced 66 to 550 metres apart. The data were acquired using Uniboom and 3.5 kHz sub-bottom profiles. An echosounder and a side-scan sonar were also employed. Navigation and position fixing were performed using DGPS.

Gas accumulations and seeps in the sedimentary infill of the Ría de Arousa are clearly detectable from high-resolution seismic records, as well as in the side-scan sonar and echo sounder records. Two main types of gas accumulation have been distinguished: 1) acoustic blanking s.s (lateral extend ranges from 400 m up to 3856 m) and 2) acoustic curtains (convexe top and 100-300 m. of lateral extent).

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