*Geological Survey of Japan, **Faculty of Science and Engineering Shimane University, ***Senbon Denki Co. Ltd.,****Clovertech Inc., *****Kawasaki Geological Engineering Co. Ltd., ******Y.O. Systems inc.
ABSTRACT
We have developed a long-term observation system for the study ofhalocline behaviour in brackish lakes and estuaries. The system consists of an acoustic observation system and a lake bottom thermometrv system using an optical fiber distributed sensor. The acoustic observation system measures acoustic reflection caused by the rapid change ofacoustic impedance at the boundary of the halocline in water. The lake bottom thermometry system detects movement of the lower part of the lake water by means of temperature change.
Basic specifications of the system are as follows:
(1)Acoustic observation system for the study of halocline behavior
Transducer: Ultrasonic frequency 200kHz. Beam angle approx. 6 degrees
Transmitter and receiver: Transmission pulse width 15 -60sms. Sensitivity adjustment 0 to 30dB
Systemcontroller and data logger: A/D converter l2bit. Sampling rate 1ms(max),CPU i80486DX-2 66MHz,RAM 16Mbytes, MO disk drive 230Mbytes
(2)Lake bottom thermometry s stem using an optical fiber distributed temperature sensor.
Temperature range: -200〜+500℃(depend on fiber type)
Number of channel: 4(loop type)
Long-term observations using the system were successfully carried out in Lake Nakaunu. We obtained records of the halocline movement from acoustic profiles. temperature distributions in the lake bottom waters from the thermometry system.
