[M-IS26] Gas hydrates in environmental-resource sciences

[MIS26-P10] Estimates of heat flow by means of long-term temperature monitoring in shallow-gas hydrate area off Sakata

*Shusaku Goto¹ (1.National Institute of Advanced Industrial Science and Technology)

Keywords:Heat flow, Long-term temperature monitoring, Bottom-water temperature fluctuation, Shallow-gas hydrate, Marine area off Sakata

Terrestrial heat flow, simply called heat flow, is the amount of thermal energy that passes through a unit area over a unit time, provides important constraints to infer subsurface geothermal regime. Heat flow is calculated as the product of the geothermal gradient and thermal conductivity of the earth material. Geothermal gradient in marine areas is generally determined from sediment temperatures measured by penetrating a geothermal probe (typically 3–5 m in length) with several sensors into sediment. This measuring technique is available in marine areas of stable bottom-water temperature (BWT) conditions, such as in deep-sea areas. In marine areas with BWT fluctuation with large amplitude (e.g., shallow-sea areas), sediment temperature near the seafloor temporally changes due to the BWT fluctuation, preventing an accurate measurement of geothermal gradient. Two measuring techniques are applied to obtain reliable heat flow data in such marine area: First technique is that sediment temperatures are measured at depths where the effect of BWT fluctuation has sufficiently attenuated using a long geothermal probe. The other is that bottom-water and sediment temperatures are simultaneously monitored over a long period of time and the effects of BWT fluctuations are removed from the sediment temperatures. Geothermal gradient is determined from the corrected sediment temperatures.
A small knoll off Sakata, called Sakata Knoll, is one of shallow gas hydrate areas in the Sea of Japan. Long-term bottom-water and sediment temperature monitoring conducted on the summit area of the knoll for a year revealed BWT fluctuation with large amplitude that perturbed sediment temperatures near the seafloor significantly (Goto et al., 2016).
In order to obtain reliable heat flow data and to infer the geothermal regime in Sakata Knoll, BWT and sediment temperature loggers were deployed at nine stations at and around the knoll in November 2020. These loggers were recovered in June 2021, and excellent long-term bottom-water and sediment temperature data were obtained. Using the obtained temperature data, values of heat flow were successfully estimated at these stations. In the presentation, characteristics of bottom-water and sediment temperature fluctuations are presented and distribution of heat flow in Sakata Knoll area are discussed.
This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).

Reference
Goto, S., Numanami, H., Satoh, M., Matsubayashi, O., Ishida, N., Matsumoto, R. (2016) Temperature measurements for near-surface sediment at cold seep site in Mogami Trough, Shallow Methane Hydrate Forum Abstract, Global Hall, Meiji University Surugadai Campus, Tokyo.