Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001geoji.145..647g&link_type=abstract
Geophysical Journal International, Volume 145, Issue 3, pp. 647-660.
Astronomy and Astrophysics
Astronomy
17
Continental Margins, Gas Hydrate, Heat Flow, Thermal Conductivity
Scientific paper
A prominent feature across some continental margins is a bottom-simulating reflector (BSR). This seismic reflection generally coincides with the depth predicted for the base of the gas hydrate stability field. Because the occurrence of gas hydrates is controlled by temperature and pressure conditions, it has been suggested that BSRs mark an isotherm and they have therefore been used to estimate the heat flow through continental margins; crucial parameters are the temperature at BSR depth and at the seafloor and the thermal conductivity structure between the BSR and the seabed. However, very often the required parameters are not available and therefore they have been derived from models for gas hydrate stability and empirical relationships to obtain thermal conductivities from seismic velocities. Here, we use downhole temperature, thermal conductivity, porosity and logging data from 10 Ocean Drilling Program (ODP) sites drilled into and through the gas hydrate field to investigate the quality of estimates. Our analyses and application of constraints to the Makran margin off Pakistan indicate the following. (i) The temperature at BSR depth could be approximated by a seawater-methane system, although capillary forces, chemical impurities or non-equilibrium conditions can lower (or increase) the temperature. If calibration by heat probe measurements is possible, errors of geothermal gradients are less than 10 per cent, otherwise uncertainties of 20 per cent (or even higher) may arise. In addition, seasonal variations of bottom water temperature have to be considered, because they may affect thermal gradients by up to ~10 per cent. (ii) The impact of typical quantities of low-thermal-conductivity gas hydrate on the bulk thermal conductivity is insignificant. (iii) The thermal conductivity profile between the BSR and the seabed can generally be approximated by a mean value. Thus, (iv) seabed measurements should be used instead of empirical relationships, which may produce errors of 5-30 per cent. Consequently, in addition to high-quality seismic data, a prerequisite should be a large data set of thermal conductivities and oceanographic data. Heat probe measurements are recommended to constrain geothermal gradients. In this case the uncertainty of heat flow is 5-10 per cent of the estimated heat flow. If these data are not available errors/uncertainties can reach 50-60 per cent of the calculated value.
Grevemeyer Ingo
Villinger Heinrich
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