音波探査による中海・宍道湖の塩分躍層の検討とその意義

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 halocline and thermocline, sometimes appears on echo-sounding records, These phenomena are caused by the rapid change of acoustic impedance at the boudary of saline and fresh water, and has been termed the deep scattering layer (D.S.L), although there has been little interest shown in it. A project to analyse this boundary using more sensitive echo-sounding equipment has been performed, with financial support from the Ministry of Education, Government of Japan in fiscal years 1992 and 1993. The acoustic profiling component was made by modifying a 200 kHz precision echo sounder (Model 101, Senbon Denki Co. Ltd,). The development of such an acoustic profiler with a digital recorder is also reported from a mecha-nical viewpoint by Nishimura et al. in the present volume. The surverys were done in November 1992. February 1993 and August 1993 along the same teansects from Daikon-shima ls. southwest to Yonago Bay in Lake Nakaumi, and in February, 1993 and October, 1993 in Lake Shinji. Clear reflectors could be observed in water about 6 m in depth with additional reflectors, corresponding to changes in waterquality, particularly changes in salinity. In lake Nakaumi, seasonal variations in the depth of reflectors were also recognized. In lake Shinji, clear refrectors could he also observed, corresponding to changes in salinity. The digital recorder was first used in Lake Nakaumi in August 1993, when detailed and clear profiles were recorded in color. This was also in lale Shinji in October, 1993. These records are collectively shown in the color figures. The degital and color records can provide us much more informations than the conventional analog records. Thus, newly developed underwater acoustic profilerequipped with a digital recorder can contribute much in understanding many phenomena of brackish lakes.
Key words : acoustic impedance, brackish water, echo-sounding survey, wedge-like saline layer