A New Avenue for the Detection and Quantification of Near Surface Fluid Flow

In recent years there has been a growing need to refine methods for quantifying the volume of gas released into the water column from marine sediments via seeps/vents. This demand is fueled by both a need to better understand the natural carbon cycle and a requirement to "measure, monitor and verify" offshore Carbon Capture and Storage (CCS) sites. Traditional methods of flux quantification are flawed, being energy intensive techniques that provide single snapshot measurements. As a result, emission estimates across larger areas are poorly constrained, often ranging across several orders of magnitude, are rarely cross validated and almost never account for temporal variation.

Recent studies have demonstrated how the acoustic signal of such seeps, recorded in the water column using hydrophones, can be used to passively measure the gas flux. Such passive acoustic flux inversion (PAFI) has already provided fresh insight into the variability of natural seeps, with changes being correlated to tides, surface seiches and even day/night temperature cycles. Our research is now attempting to take this passive acoustic inversion technique a step further.

We believe with modern technology it is possible to develop a method to quantify the volume of gas traveling through the subsurface (along pathways in unconsolidated sediment) based on passive seismic data. Such passive seismic flux inversion (PSFI) would grant scientists fresh insight into fluid flow in near surface.

This will not be an easy task and will require inputs from several disciplines, notably seismology, marine acoustics, and material science. So far work has focused on

Understanding the mechanics behind an individual bubble fractures propagation in sediment with observations being made using slow motion photography, CT imaging, radiography, and micro-seismometers.

Observing the seismic signature of controlled artificial seeps using geophones, hydrophones and fiber optic cables.

However, work needs to be done bridging the gap between these small- and large-scale observations. We hope to initiate a dialogue about how to advance this research and identify possible collaborations as well as other near-surface geophysical techniques that could be harnessed to study/quantify gas migration in the near surface.