Development of an Underwater Acousitic Profiler Equipped with Digital Recorder for Survey of Brackish Water Lakes

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In brackish lakes it is well known that sea water is overlain by fresh water. Their mutual boundary, which is recognized as the boundary of the haloline, sometimes appears on echo-sounding records. These phenomena are caused by the rapid change of acoustic impedance at the boundary of saline and fresh water, and has been termed the deep scattering layr (D.S.L.). The project to analyze this boundary using more sensitive echo-sounding equipment has been performed. This component was made by modification of a 200 kHz precision echo-sounder (Model 101, Senbon Denki Co., Ltd.). The receiving circuit was modified to use straight amplifiers to maintain the linearity and accuracy of signals. In addition, the recieving circuit was equipped with signal output for digital recording. The recorder consists of a 12 bit higt-speed A/D converter at 1MHz sampling and an EXABYTE tape drive using 8 mm tape (max.2.2GByte). Clear reflector in Lake Nakaumi could be observed in water about 6 m in depth with additinal refletors, correspoudig to changes in water quality, particularly changes in salinity. The newly developed underwater acoustic profiler equipped with a digital recorder can contribute much in understanding many phenomena of brackish lakes.