Physical Parameters Of Hydrated Sediments Estimated From Marine Seismic Reflection Data

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Journal Article: Physical Parameters Of Hydrated Sediments Estimated From Marine Seismic Reflection Data

Gas-hydrates in marine sediments can be identified on multi-channel seismic data by an anomalous bottom simulating reflector (BSR), often associated with the base. of hydrate stability field. Physical parameters like porosity, density, thermal conductivity, temperature, geothermal gradient, hydrate saturation, electrical resistivity and heat flow provide useful inputs to understand several issues related to hydrates exploration. To determine these parameters by employing relevant techniques at depth below the sea water is not only difficult but also expensive. Here, we present case studies to derive these parameters with the help of BSR, identified on seismic sections from two completely different tectonic/geological areas of (i) the Makran (Arabian Sea) and (ii) the Cascadia (Pacific Ocean) margins. From the available velocity-depth model in the Cascadia margin, we determine the background velocity-depth function (without hydrates and freegas), which is used to estimate the variation of density, porosity and hydrate saturation with depth successively. The average bulk density, seafloor porosity and maximum hydrate saturation in the Cascadia margin are calculated as 1.70 g/cc, 63.8% and 22% respectively. From the porosity-depth function and the gradient of hydrate saturation, we determine the variation of thermal conductivity and resistivity with depth. The average resistivity, thermal conductivity and heat flow in the Cascadia margin are determined as 1.13 Omega-m, 1.02 W/m/K and 62.85 mW/m(2) respectively. Since the velocity model in the Makran accretionary prism across the BSR shows wide variation, we change the approach of deriving the above physical parameters. First, we determine the seafloor density from the seismic velocity and hence the seafloor porosity. Then the porosity-depth function is determined using Athy's law and the compaction factor available for the sediment. Porosity is then converted into density using an empirical relation between porosity and density. The remaining procedure is the same as that used for the Cascadia margin. Seafloor porosity, average density, resistivity, thermal conductivity, heat flow and maximum hydrate saturation are calculated as 54%, 1.98 g/cc, 1.96 Omega-m, 1.268 W/m/K, 43.55 mW/M-2 and 13% respectively, for the Makran region. The estimated physical parameters in both the margins match well with the available results. We also estimate errors in physical parameters, assuming +/- 5% error in the available velocity model.

R. Ghosh, M. Ojha, K. Sain and N. K. Thakur

Published Journal 
Current Science, 2006

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R. Ghosh,M. Ojha,K. Sain,N. K. Thakur. 2006. Physical Parameters Of Hydrated Sediments Estimated From Marine Seismic Reflection Data. Current Science. (!) .