MAPPING OF SHALLOW GAS FEATURES IN THE RÍA DE MUROS (NW SPAIN)

The Ría de Muros is located on the Atlantic margin of Galicia (NW Spain). This ría has an area of 125 km2 , with a main NE-SW longitudinal axis of 12.5 km, and a very variable width (from 1.3 to 7.9 km.). The land geology is composed of Palaeozoic metamorphic and granite rocks, cut by NE-SW, NW-SE and N-S trending faults, and its water depth ranges from 51 m, at the ría entrance, to 5.5 m, at the most internal part.

A total of 169 km of seismic lines were acquired and interpreted in order to detect shallow gas signatures. The survey was performed as a rectangular grid with lines spaced 1.6 to 2.2 metres apart. The data were acquired using Bubble Pulser, 3.5 kHz sub-bottom profiler, and a side-scan sonar. Navigation and position fixing were performed using DGPS.

Gas accumulations in the sedimentary infill of the ría are clearly detectable from high-resolution seismic records, as well as in the side-scan sonar records. Three main types of gas accumulations have been distinguished: 1) acoustic blanking s.s. (having kilometric extent), 2) acoustic curtains (convex top and hundred of meters of lateral extent), and 3) acoustic turbidity.

Mapping of these features reveals the presence of a large gas field where gas accumulates from the close seabed surface up to 17 ms. When gas accumulates near the seafloor it is very frequent the appearance of pockmark fields, that can be easily recognised in the Side-Scan Sonar records.

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