Salt water intrusion in the Mekong River, Vietnam, was studied using a towing type echo-sounder (Model SC-3) and water quality meter (TPM CLOROTEC) observation instruments during spring tide of the low flow season in May, 2005. Observations were taken at two stations located at points 26 and 8 km from the river mouth. Measurements were taken at hourly intervals for 24 hours during one week, using several instruments set on the river bottom. A joint towing observation system using echo-sounder and water quality meters has been proved its usefulness for analyzing the time-space behavior of salt intrusion in the Mekong River. Precise measurements of tide-related phenomena including water level, bottom currents and water quality (temperature, salinity, turbidity and chlorophyll-a) at two stations revealed detailed movements of the saline water-mass.

宍道地溝帯は宍道湖から鳥取市沖合までの全長160 km以上に及ぶ半地溝で, 日本海形成時のリフト帯縁辺に相当すると推定される.この中軸部にあたる美保湾の水深26 mの海底で, 更新世の安山岩体(全岩K-Ar年代 : 0.9±0.3 Ma;ジルコンのF・T年代 : 0.16±0.04 Ma)が発見された.岩体は直径が約500 mの円錐台で, 氷期の海退期に陸上で形成されたと考えられる.全岩組成はSiO2=60-61 wt%, Al2O3=22.8-18.6 wt%で, 著しくAlに富む特異な火山岩である.岩石には斑晶とその包有物や分解生成物として斜長石, Cr古銅輝石-紫蘇輝石, Cr透輝石-普通輝石, ピジョン輝石, 普通角閃石, アルカリ長石, 石英, チタン磁鉄鉱, チタン鉄鉱, ヘモイルメナイト, F金雲母, カミングトン閃石, スピネルなどの多種多様な鉱物が含まれる.宍道地溝帯中軸部の鮮新世以降の火山体は, 地溝帯に伴う深部断裂帯に沿って形成されたと推定される.

We have developed several new measurement systems for wide area and long-term measurement of the brackish water area (estuaries and coastal lagoons) using ultrasonic waves, optical fiber temperature sensor high-precision salinity and temperature sensors. They are (1) an underwater acoustic reflection profiling system for survey of the halocline, (2) an underwater acoustic reflection measurement system for long-term observation of halocline behavior, (3) a thermometry system using optical fiber distributed temperature sensor and (4) a CT multi-sensor temperature salinity measurement system. Long-term observations using these systems were successfully carried out in the estuaries and coastal lagoons. We are acquiring basic data for preservation and creation of the hydrosphere environment.