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.