Shusaku Goto

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

[1]xa0Characteristics 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.

Extremely high heat flow anomaly in the middle part of the Nankai Trough

Abstract Detailed heat flow measurements were carried out in the central part of the Nankai Trough off eastern Shikoku, where the fossil spreading center of the Shikoku Basin is subducting beneath the southwest Japan arc. The quality of temperature gradient values was examined for all the existing and new heat flow data and the average of new thermal conductivity data was used at all the stations. Reliable heat flow on the floor of the trough was found to be very high and rather uniform, with an average of about 200xa0±xa020 mW/m 2 . Taking account of the effect of recent rapid sedimentation on the surface heat flow, the heat flow from the deeper part is estimated to be even higher by 20–30%, almost twice as high as the value expected from the age of the subducting Shikoku Basin, about 15 m.y. Such a large heat flow anomaly cannot be attributed solely to advective heat transfer by pore fluid flows associated with subduction and accretion of the sediments brought into the trough, suggesting that the underlying Shikoku Basin lithosphere is anomalously warm in the study area. A possible cause of the anomaly is reheating of the lithosphere by post-spreading thermal activities along the fossil spreading center, which produced the Kinan Seamount Chain and continued until about 7 Ma. However, such reheating even at 6 Ma cannot produce a heat flow high enough to explain the observed values. A steep landward decrease in the heat flow is observed at the toe of the accretionary prism. The scale of this decrease indicates that its origin is not deep seated, and thus a part of the anomalous heat flow on the trough floor may result from some processes in relatively shallow layers. The thermal structure of the Shikoku Basin is a critical boundary condition for thermal models of the Nankai subduction zone and its seismogenic zone for large thrust earthquakes. More heat flow measurements need to be made around the study area in order to clarify the extent and origin of the high heat flow anomaly.

Heat flow distribution and thermal structure of the Nankai subduction zone off the Kii Peninsula

Detailed heat flow surveys were carried out in the central part of the Nankai Trough southeast of the Kii Peninsula (off Kumano) for investigation of the thermal structure of the subducting plate interface. At stations in the Kumano Trough (forearc basin) and its vicinity, long-term monitoring of temperature profiles in surface sediments was conducted because bottom water temperature variations (BTV) significantly disturb subbottom sediment temperatures. Heat flow values were successfully determined at seven stations by removing the influence of BTV from temperature records for 300 to 400 days. The surface heat flow data were combined with estimates from depths of methane hydrate bottom simulating reflectors to construct a heat flow profile across the subduction zone. Heat flow decreases from 90–110 mW/m2 on the floor of the Nankai Trough to 50–60 mW/m2 at around 30 km from the deformation front, while it is rather uniform, 40–60 mW/m2, in the Kumano Trough. The values measured on the Nankai Trough floor are concordant with the value estimated from the age of the subducting Philippine Sea plate, about 20 m.y., taking into account the effect of sedimentation. The obtained heat flow profile was used to constrain thermal models of the subduction zone. The subsurface thermal structure was calculated using a two-dimensional, steady state model, in which the frictional heating along the plate interface and the radioactive heat production are treated as unknown parameters. Comparison of the calculated surface heat flow in the Kumano Trough with the observed data indicates that the effective coefficient of friction is small, about 0.1 or less, and thus the shear stress on the plate interface is very low in this subduction zone.

Relations between the thermal properties and porosity of sediments in the eastern flank of the Juan de Fuca Ridge

The empirical relations of the thermal properties (thermal conductivity, heat capacity, specific heat, and thermal diffusivity) to the porosity and mineral composition of clay and sandy sediments recovered in the eastern flank of the Juan de Fuca Ridge are examined using the observed thermal properties, index properties, and mineral composition of the sediments. Observed thermal conductivity-porosity relations are explained using the geometric mean model. The observed relations of heat capacity and specific heat, respectively, to porosity are given by the arithmetic mean formula. A new model for the sediment thermal diffusivity-porosity relation is proposed based on models of thermal conductivity and heat capacity. This model, expressed by the geometric mean model with a correction function for the porosity and heat capacities of grain sediment and pore-filling fluid, explains the observed thermal diffusivity-porosity relations. These thermal property models are applicable to thermal properties of other sediment lithology types and are useful as standard models for estimating the thermal properties of marine sediment.

Heat flow measurement in shallow seas through long‐term temperature monitoring

[1]xa0It is difficult to obtain reliable heat flow values in shallow sea areas where the bottom water temperature is not stable. We attempted to measure heat flow in shallow seas by monitoring temperature profiles in sediments, using pop-up type instruments with 2 m long temperature probes. Long-term temperature records for over 200 days were obtained at four stations off southwest Japan. Analysis of the data showed the bottom water temperature variations (BTV) were propagated through sediments by thermal diffusion only. We could then remove the influence of BTV from the temperature records and determine the heat flow, though the effect of longer period components in BTV has possibly remained. The obtained heat flow values are consistent with those estimated from depths of gas hydrate BSRs in the vicinities of the stations. These results indicate that long-term temperature monitoring is a useful method for heat flow determination in shallow seas.

Inversion of needle‐probe data for sediment thermal properties of the eastern flank of the Juan de Fuca Ridge

[1]xa0An estimation method is introduced for measuring thermal conductivity, thermal diffusivity, and heat capacity of sediment from temperatures using a needle probe. This method is based on the continuous cylindrical heat-source model. Thermal conductivity is estimated from needle-probe temperature data using asymptotic approximation of the model. Heat capacity is then inverted from early time temperature data using a complete form of the model. Finally, thermal diffusivity is calculated from the estimated thermal conductivity and heat capacity. Numerical experiments with synthetic temperature data indicate that the estimation error of the sediments heat capacity is ∼12%. The error of thermal diffusivity is thought to be greater than that of heat capacity. We apply the method to needle-probe temperature data obtained during Integrated Ocean Drilling Program Expedition 301 carried out on the eastern flank of the Juan de Fuca Ridge and estimate the sediments thermal properties. The results indicate that thermal properties of sediments are strongly dependent on lithology and porosity. Examination results of the relations of heat capacity and thermal diffusivity to thermal conductivity show that these relations depend on the amount of quartz. The empirical formulas for these relations are provided, and they differ from those which are commonly used in thermal conductivity reduction schemes.

Reconstruction of the thermal environment evolution in urban areas from underground temperature distribution

It is possible to estimate the ground surface temperature (GST) history of the past several hundred years from temperature profiles measured in boreholes because the temporal variation in GST propagates into the subsurface by thermal diffusion. This geothermal method of reconstructing GST histories can be applied to studies of thermal environment evolution in urban areas, including the development of heat islands. Temperatures in boreholes were logged at 102 sites in Bangkok, Jakarta, Taipei, Seoul and their surrounding areas in 2004 to 2007. The effects of recent surface warming can be recognized in the shapes of most of the obtained temperature profiles. The preliminary results of reconstruction of GST histories through inversion analysis show that GST increased significantly in the last century. Existing temperature profile data for the areas in and around Tokyo and Osaka can also be used to reconstruct GST histories. Because most of these cities are located on alluvial plains in relatively humid areas, it is necessary to use a model with groundwater flow and a layered subsurface structure for reconstruction analysis. Long-term records of subsurface temperatures at multiple depths may demonstrate how the GST variation propagates downward through formations. Time series data provide information on the mechanism of heat transfer (conduction or advection) and the thermal diffusivity. Long-term temperature monitoring has been carried out in a borehole located on the coast of Lake Biwa, Japan. Temperatures at 30 and 40 m below the ground surface were measured for 4 years and 2 years, respectively, with a resolution of 1 mK. The obtained records indicate steady increases at both depths with different rates, which is probably the result of some recent thermal event(s) near the surface. Borehole temperatures have also been monitored at selected sites in Bangkok, Jakarta, and Taiwan.

Estimation of thermal gradient and diffusivity by means of long- term measurements of subbottom temperatures at western Sagami Bay, Japan

Bottom water and subbottom temperatures were monitored for 1 year using three geothermal probes at a biological community area in western Sagami Bay, Japan. The subbottom temperatures are affected by bottom water temperature variations (BTV), most of which are negligible deeper than 50 cm below the sea floor. The steady-state geothermal gradient and subbottom depth of each sensor were determined from the average temperatures. The influence of BTV was been eliminated using the iterative, non-linear, least-squares method (Gauss-Newton method), so that the thermal diffusivities are determined with reasonable accuracy for each sensor of the probe. The result indicates that the long-term measurement of subbottom temperature can be a useful tool for determination of thermal gradient and diffusivity where BTV are significant. The average thermal diffusivity is 2.3 × 10−7 m2/s, which is consistent with typical values for marine sediments. The heat flow values are higher than 1 W/m2 within 100 m of the community, and increase towards the community from 1 W/m2 to 3.5 W/m2. A transient temperature anomaly, starting on January 5th, 1992, appeared only at the lowermost sensor of the probe located 3 m to the east of the colony rim, with its amplitude up to 0.7°C. This anomaly may be attributable to a very localized heat source at a depth of 35 cm, possibly a warm fluid injection within a very thin linear fracture. A complicated flow regime may have occurred around the probe.

Estimate of heat flux and its temporal variation at the TAG hydrothermal mound, Mid-Atlantic Ridge 26°N

[1]xa0From August 1994 to March 1995, three 50-m-high vertical thermistor arrays designated “Giant Kelps” (GKs) were deployed around the central black smoker complex (CBC) at the TAG hydrothermal mound, Mid-Atlantic Ridge (26°08′N, 44°49′W). These were designed to monitor the temporal variability of the vertical temperature distribution in the hydrothermal plume. One small high-temperature probe “Hobo” was also deployed in one of the black smoker vents of CBC. Over the observation period, two typical characteristics are recognized in plume temperatures measured with GKs: (1) the amplitudes of temperature anomalies decrease with increasing height above the top of CBC; (2) maximum temperature anomalies on the upper thermistors occurred periodically and nearly simultaneously across the array about every 6 hours. Conversely, maximum temperature anomalies on the lower thermistors occurred periodically every 12 hours, indicating that the location of the plume discharged from CBC was forcibly moved by the change in direction of tidally modulated current flow. The heat flux from CBC was estimated from temperatures measured by GKs based on a model of buoyant hydrothermal fluid rising in a stable, stratified density environment. The estimated heat flux from CBC gradually decreases from about 86 to 55 MW over the ∼7 months of measurement, with a mean rate of decrease of 0.17 MW d−1. Since the black smoker effluent temperature measured with Hobo was almost stable over the measurement period, a plausible cause of the decrease is a reduction in the volume of hydrothermal fluid provided to the CBC (in which case the estimated mean rate of decrease in volume flux of CBC is 8.9 m3 d−1). Estimated heat flux, temperature anomalies observed by Hobo, and diffuse flow and subbottom temperature anomalies recorded by other long-term monitoring instruments before, during, and after ODP Leg 158 indicate that the drilling probably affected the fluid flow pattern within the mound but had little effect on the total heat flux from CBC.

Distribution of radioactive heat production around an active fault and in accretionary prisms of southwest Japan

Abstract Contents of radioactive heat-producing elements (U, Th and K) were measured on core samples from a borehole drilled into the shear zone of the active Nojima fault, southwest Japan. The heat production rate is rather uniform in a granite layer below 560xa0m with an average of 1.6xa0μW/m 3 . In the lower part of the granite layer, which is close to the fault zone, the U/Th and U/K ratios seem to be lower than those in the upper part. A possible cause of this anomaly is redistribution of U by groundwater flows in highly fractured layers. Through monitoring of the temperature profile in the borehole, a local anomaly, which may be associated with groundwater flow along a fracture zone, was detected in the uppermost part of the hole. The conductive heat flow was found to increase with depth, which is possibly due to downward fluid flow in the surrounding formations. Measurements of heat production were also made on rock samples from Permian to Cretaceous accretionary prisms in southwest Japan. The heat production of sandstone and mudstone, major components of the accretionary prisms, is about 1.5xa0μW/m 3 on average, which is similar to the average value for granitic rocks in southwest Japan. These values are significantly higher than those reported in accreted terranes in the Cascadia subduction zone.