Abstract:
Seafloor compliance (SFC) is a non-intrusive geophysical method which is sensitive to the shear modulus of the sediments below the seafloor. A seafloor compliance analysis requires the computation of the frequency dependent transfer function between the vertical stress, produced at the seafloor by the ultra low frequency passive source-infragravity waves, and the resulting displacement, related to velocity through the frequency. The filtering nature of seawater to the gravity waves determines the magnitude and wavelength of the pressure signal at the seafloor; only waves with wavelengths greater or equal to the water depth are capable of palpitating the ocean bottom. The displacement of the oceanfloor is dependent on the elastic structure of the sediments and the compliance function is tuned to different depths, i.e., a change in the elastic parameters at a given depth is sensed by the compliance function at a particular frequency. In a gas hydrate system, the magnitude of the stiffness is a measure of the quantity of gas hydrates present. Gas hydrates contain immense stores of greenhouse gases making them relevant to climate change science, and represent an important potential alternative source of energy. Bullseye vent is an example of a gas hydrate system located in an area that has been intensively studied over the past 20 years and results suggest that this system is evolving over time. A partnership with NEPTUNE Canada (NC) has allowed us to investigate this possible evolution. We have obtained 222 days of simultaneously logged time-series of pressure and velocity data measured by the Scripps Institution of Oceanography differential pressure gauge (DPG), and the Gueralp CMG-1T broadband seismometer, respectively. These instruments are part of the NC's seismic network and are deployed near Bullseye vent, at SITE 889, in Cascadia margin. The data set spans the time period of 1st October, 2010 to 15th May, 2011. We performed a Fourier domain analysis on the data set, specifically focused on the gravity-waves band, 0.01Hz-0.03Hz, where they can deform the seafloor at water depths of 1256 metres. Within this frequency band, the well bounded transfer function shows a linear decrease of -2.52% over the 228 days period. 2011. This decrease in the transfer function represents a decrease in the compliance that is consistent with an increase in the hydrate concentration in the sediment at Bullseye. Using Lee and Waite 2008 formulation for modeling hydrate filled sediments, we then investigated the relationship between the observed decrease in the transfer function, and the percentage increase in the gas hydrate concentration of the sediments. To do this, we determined an elastic model of best fit to the experimental transfer function of day one of the data series. We then varied the best fit model's elastic parameters of the gas hydrate layers until we obtained a 2.52% decrease in the transfer function. From there, we computed the percentage change in gas hydrate concentrations that can cause the observed change in the elastic parameters. The result was that a 2.52% decrease in the transfer function reflects a 9% to 42% change in the hydrate concentration of the best fit hydrate model.