The microstructure of sediment-hosted hydrates: evidence from effective medium modelling of laboratory and borehole seismic data

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Description/Abstract

Much of our knowledge of hydrate distribution in the subsurface comes from
interpretations of remote seismic measurements. A key step in such interpretations is
an effective medium theory that relates the seismic properties of a given sediment to
its hydrate content. A variety of such theories have been developed; these theories
generally give similar results if the same assumptions are made about the extent to
which hydrate contributes to the load-bearing sediment frame. We have further
developed and modified one such theory, the self-consistent
approximation/differential effective medium approach, to incorporate additional
empirical parameters describing the extent to which both the sediment matrix material
(clay or quartz) and the hydrate are load-bearing. We find that a single choice of
these parameters allows us to match well both P and S wave velocity measurements
from both laboratory and in situ datasets, and that the inferred proportion of hydrate
that is load-bearing varies approximately linearly with hydrate saturation. This
proportion appears to decrease with increasing hydrate saturation for gas-rich
laboratory environments, but increase with hydrate saturation when hydrate is formed
from solution and for an in situ example.