Seismic measurements in Suva Harbour (Fiji)

Seismic reflection measurements in Suva Harbour have shown the existence of a mud layer between 50 and 140 m thick covering the bottom of the harbour entrance and the inner harbour. Average compressional wave velocities in the mud increase with increasing thickness of the mud layer, and vary from 0 · 54 km/sec to 1 · 10 km/sec, indicating a steep velocity gradient in the mud. The low mud velocities can be explained by the presence of free carbon dioxide resulting from decaying organic matter; the existence of CO 2 in the mud has been proved by chromatographic analysis. Taking into account a decrease of porosity and gas Concentration with increasing thickness of the mud layer (and water depth) using data from Lake Mead, the observed average velocities can be explained by a CO 2 fraction which reaches a maximum value of about 0·2% at a hydrostatic pressure ot 3·8 atm and which decreases by about two orders of magnitude at 80 m below the top of the mud layer. The uppermost layer of the mud contains no gas.