Monochromatic infra‐sound waves recorded offshore Ecuador: possible evidence of methane release

Resumen

ABSTRACT Methane is known to be a greenhouse gas but it is difficult to estimate its significance. We have identified a new approach for monitoring fluxes of methane from large areas of sea floor. During spring 1998, we conducted an offshore passive seismic experiment using 13 three‐component digital Ocean Bottom Seismometers. During this 3‐week experiment, we recorded unusual sustained and intense monochromatic infrasound waves (6–7 Hz). The origin of the noise is tentatively interpreted as pressure‐waves resulting from oscillating clouds of bubbles. Such clouds must have a characteristic size of ∼ 10 m, at depths of 5–15 m, and must contain more than 10 10 bubbles of 1 mm radius to produce the observed frequency. The presence of a Bottom Simulating Reflector in the study area suggests that gas seepage may occur at the seafloor, near 500 m depth. Then, bubbles of gas, namely methane, could form in the water column. Although highly speculative, Ocean Bottom Seismometer records could potentially be useful for monitoring gas seeps and/or related unusual behaviour of the seabed.