Joint seismic/electrical effective medium modelling of hydrate-bearing marine sediments and an application to the Vancouver Island margin

Abstract

Remote determination of the hydrate content of marine sediments remains a challenging problem.
In the absence of boreholes, the most commonly used approach involves the measurement of Pwave
velocities from seismic experiments. A range of seismic effective medium methods has
been developed to interpret these velocities in terms of hydrate content, but uncertainties about
the pore-scale distribution of hydrate can lead to large uncertainties in this interpretation. Where
borehole geophysical measurements are available, electrical resistivity is widely used as a proxy
for hydrate content, and the measurement of resistivity using controlled source electromagnetic
methods shows considerable promise. However, resistivity is commonly related to hydrate
content using Archie’s law, an empirical relationship with no physical basis that has been shown
to fail for hydrate-bearing sediments. We have developed an electrical effective medium method
appropriate to hydrate-bearing sediments based on the application of a geometric correction to the
Hashin-Shrikman conductive bound, and tested this method by making resistivity measurements
on artificial sediments of known porosity. We have adapted our method to deal with anisotropic
grains such as clay particles, and combined it with a well-established seismic effective medium
method to develop a strategy for estimating the hydrate content of marine sediments based on a
combination of seismic and electrical methods. We have applied our approach to borehole
geophysical data from Integrated Ocean Drilling Program Expedition 311 on the Vancouver
Island margin. Hydrate saturations were determined from resistivity logs by adjusting the
geometric factor in areas of the log where hydrate was not present. This value was then used over
the entire resistivity log. Hydrate saturations determined using this method match well those
determined from direct measurements of the methane content of pressurized cores.

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ORCID for T.A. Minshull: orcid.org/0000-0002-8202-1379

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