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 usin g three geothermal probes at a biological community area in western Sa gami Bay, Japan. The subbottom temperatures are affected by bottom wat er temperature variations (BTV), most of which are negligible deeper t han 50 cm below the sea floor. The steady-state geothermal gradient an d subbottom depth of each sensor were determined from the average temp eratures. 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 measurem ent of subbottom temperature can be a useful tool for determination of thermal gradient and diffusivity where BTV are significant. The avera ge thermal diffusivity is 2.3 x 10(-7) m(2)/s, which is consistent wit h typical values for marine sediments. The heat flow values are higher than 1 W/m(2) within 100 m of the community, and increase towards the community from 1 W/m(2) to 3.5 W/m(2). A transient temperature anomal y, starting on January 5th, 1992, appeared only at the lowermost senso r of the probe located 3 m to the east of the colony rim, with its amp litude up to 0.7 degrees C. This anomaly may be attributable to a very localized heat source at a depth of 35 cm, possibly a warm fluid inje ction within a very thin linear fracture. A complicated flow regime ma y have occurred around the probe.