Heat flow measurement in shallow seas through long-term temperature monitoring Hamamoto Hideki (Earthquake Research Institute,Univ. of Tokyo) Yamano Makoto (Earthquake Research Institute,Univ. of Tokyo) Goto Shusaku (Aso volcanological laboratory, Kyoto Univ.)

Research purpose To obtain heat flow data in shallow sea areas. Remove influence of bottom water temperature variation using results of long-term temperature monitoring

Conventional measurement method in deep-sea areas k ： Thermal conductivity ： Temperature gradient Q ： Terrestrial heat flow

κ ： Thermal diffusivity Amplitude decay Phase shift Thermal diffusion equation Propagation of bottom water temperature variation T （ 0,t ） T(z,t)

Pop-up heat flow instrument er recorder

pic tur e

Measurement station A D B C Kumano area Eurasian plate Philippine Sea plate Pacific plate

Sub-bottom temperature data CH1 CH2 CH3 CH4 Station B (water depth: 2055m)

Heat transfer between CH1 and CH2 CH1 CH2 CH ３ CH ４

Correction for the effects of bottom water temperature variation

Temperature gradient = 58 mK/m Heat flow c.a 60 mW/m 2 Thermal conductivity 1 W/m/K

A D B C Kumano area Eurasian plate Philippine Sea plate Pacific plate Measurement station

A (water depth 1040m) Time(days) After correction Raw data Temperature （℃）

Time(days) Temperature （℃） C(water depth 2008m) After correction Raw data

Time(days) Temperature （℃） D(water depth 2070m) After correction Raw data

Heat flow cross section Conventional prove Estimated from BSR

Monitoring of sea-bottom water temperature 1. Estimation of appropriate monitoring period and probe length 2. Determination of heat flow in combination with ordinary probe measurements Purposes

Measurement stations in Kumano Sediment temperature Bottom water temperature (in progress) Sediment temperature (in progress)

Bottom-water temperature records

Temperature record for two years

Spectrum analysis Period (days) Amplitude

Summary 1.Long-term temperature data over 220 days were obtained with pop-up heat flow instruments. 2. Heat flow values can be obtained by removing the influence of bottom water temperature variation. Long-term temperature monitoring may be a useful method for heat flow determination in shallow sea areas.

Summary 3. Dominant periods of bottom-water temperature variations are 150 to 200 days. Heat flow can be determined well from 250 to 300 days temperature records.

BSR (Bottom Simulating Reflector) BSR

Station A Temperature ( ℃ )

Station A (water depth1040m) Spectrum Amplitude

Thermal diffusivity CH1 CH2 CH3 CH4

Thermal conductivity (Hyndman’s equation )

Thermal conductivity of sediment sample The sample was obtained near the station B (about 15miles away).

Disturbance from sea-bottom water temperature variation Depth(m ) 0.01K Temperature

Example of temperature profiles Deep sea Shallow sea

Temperature Time Sea-bottom Sub-bottom Example of thermal diffusion Amplitude decays Phase delays

水温変動が海底下の温度分布に与える影響を計算 複数の地点で長期間の海底水温データが得られ た 海底水温が温度勾配に与える影 響 通常の方法による測定が可能かどうかを検 討

1.5m 2.5m D 地点 ( 水深 1040m) 海底水温が温度勾配に与える影響 G=50mK/m κ ＝ 2.4×10 -7 m 2 /s 2.5m 4.5m

B 地点 ( 水深 2026m) 海底水温が温度勾配に与える影響 1.5m 2.5m 4.5m G=50mK/m κ ＝ 2.4×10 -7 m 2 /s

* D 地点（水深 1040m ）では，温度勾配は通常の方法で は測 定できない * A ～ C 地点（水深 1230 ～ 2026m ）でも, プローブが４～ ５ m の 深さまで貫入しなければ温度勾配を求めるこ とが困難 長期観測が必要 海底下の温度データから水温変動の影響を取り 除いて熱流量を求める ここまでの結果 （プローブを 4 ～ 5 ｍ貫入させることは難しい場合が 多い）

CH2 CH3 CH4 CH5 CH6 CH7 CH Thermal diffusivity （ ×10 -7 ）（ m 2 / ｓ） CH1 と各センサー間の熱拡散率

CH2 CH3 CH4 CH5 CH6 CH7 CH thermal diffusivity （ ×10 -7 m 2 /s ） 隣り合った各センサー間の熱拡散 率

A 地点における計算値と実測値 計算値と実測値が一致しない

各センサー間の熱伝導率 (Hyndman の経験式 ) k: 熱伝導率 κ ：熱拡散率

C 地点の海底下の温度データ

日変化 年変化 日変化と年変化の温度プロファイ

Long-term temperature monitoring Long-term temperature monitoring system has been deployed with submersibles. ＊ Few chances for deployment and recovery. ＊ Submersibles can handle short probes only. (max. 1m) We have developed Pop-up temperature monitoring system which can be deployed from surface vessels. Demerits