Thermal response of sediment with vertical fluid flow to periodic temperature variation at the surface

Characteristics of thermal responses of sediment with vertical fluid movement to periodic temperature variation at the surface were examined using a one‐dimensional analytical solution. The amplitude of the thermal response decays exponentially, and the phase is delayed linearly with increasing depth, but they depend on the direction and velocity of vertical fluid flow, thermal diffusivity of fluid‐saturated sediment, and period of surface temperature variation. To examine general characteristics of the thermal response, we defined two nondimensional parameters related to thermal diffusivity of fluid‐saturated sediment, vertical fluid flow velocity, period of the surface temperature variation, and specific penetration depth at which the amplitude of the thermal response decays to e −1 of that at the surface. Analysis using these nondimensional parameters shows that there are three heat transport regimes for downward flow: (1) heat transport strongly governed by advection, (2) heat transport strongly governed by conduction, and (3) transition between these regimes. For upward flow, there are also three heat transport regimes: (1) balance of heat transports by advection and conduction, (2) heat transport strongly governed by conduction, and (3) transition between these regimes. The analytical solution is used to estimate the downward fluid velocity and thermal diffusivity of sediment from temperatures measured by long‐term temperature monitoring at a site of seafloor hydrothermal circulation.