2:15 PM - 2:30 PM
[SCG47-03] Long-term monitoring of sediment temperature profile in an area with fluid discharge in Lake Biwa
★Invited Papers
Keywords:heat flow, Lake Biwa, fluid discharge, heat transport, long-term temperature monitoring
Fluid discharge including emission of gas bubbles has been observed around the deepest part of Lake Biwa through visual surveys with AUV and ROV and acoustic imaging with echo sounders (Kumagai et al., 2021). These observations indicate the existence of upward flow of pore water and gas through bottom sediments, which should transport heat advectively and affect the surface heat flow distribution. We conducted closely-spaced measurements of temperature profiles in surface sediment with a 3-m long probe in the vicinity of a site where prominent venting activity has been reported, around 35o20'N, 135o06'E. We found that temperature gradient, essentially proportional to heat flow, is extremely high at some sites, indicating the existence of upward fluid flow through sediment. The high heat flow sites are concentrated in an area with a diameter of about 200 m. It suggests that significant fluid venting activity is confined in a very small area.
In Lake Biwa, temporal variation of bottom water temperature (BWT) is very large, exceeding 1 K even in the deepest part of the lake, where the high heat flow area is located. The influence of BWT variation propagates downward through sediments and the propagation process depends on the thermal property of sediment and the Darcy velocity of pore fluid. Analysis of a long-term record of temperature distribution in sediment provides information on vertical fluid flux (Goto et al.,2005). We attempted long-term monitoring of sediment temperature profile in the high heat flow area using a 2-m long probe along which six small temperature recorders are attached. The instrument was deployed on October 5, 2022 and recovered on March 25, 2023. The obtained temperature data show very high temperature gradient typical of the high heat flow area and downward propagation of BWT variation. We can estimate the effective thermal diffusivity of sediment from the temperature variations at the six depths assuming that vertical heat transport through sediment is purely conductive. The estimated thermal diffusivity is extraordinarily high for soft surface sediment, which may be attributed to downward flow of pore fluid. It indicates that downward flow through sediments coexists with upward flow in the high heat flow area in the vicinity of active fluid venting sites.
In Lake Biwa, temporal variation of bottom water temperature (BWT) is very large, exceeding 1 K even in the deepest part of the lake, where the high heat flow area is located. The influence of BWT variation propagates downward through sediments and the propagation process depends on the thermal property of sediment and the Darcy velocity of pore fluid. Analysis of a long-term record of temperature distribution in sediment provides information on vertical fluid flux (Goto et al.,2005). We attempted long-term monitoring of sediment temperature profile in the high heat flow area using a 2-m long probe along which six small temperature recorders are attached. The instrument was deployed on October 5, 2022 and recovered on March 25, 2023. The obtained temperature data show very high temperature gradient typical of the high heat flow area and downward propagation of BWT variation. We can estimate the effective thermal diffusivity of sediment from the temperature variations at the six depths assuming that vertical heat transport through sediment is purely conductive. The estimated thermal diffusivity is extraordinarily high for soft surface sediment, which may be attributed to downward flow of pore fluid. It indicates that downward flow through sediments coexists with upward flow in the high heat flow area in the vicinity of active fluid venting sites.