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1.1 Background Underwater acoustic instruments has been an indispensable tool to study the ocean. Echo sounder is one of the acoustic instrument used to remotely classify distributions of biological organisms such as fish and plankton (MacLennan and Simmonds, 1992; Furusawa, 2000). Knowledge of species, location, and behavior observed by the sounders are important for fisheries, fisheries management, and ecological studies. Echo sounders have also been used to characterize the sea bottom type such as rocks, sand, and mud (Stanton, 1994). The characterization of the sea bottom type are useful in applications such as fish habitat study, fishing port construction, geological studies, marine exploration, and mining. With the end of World War II, studies on underwater acoustics for sea bottom interactions began extensively. Urick (1954) defined the measure of backscatter strength as the ratio of scattered energy to incident energy per unit area, per unit solid angle and this quantity was expressed in decibels. Urick used the frequencies of 10 to 60 kHz to measure the backscattering from the harbor bottom. Mackenzie (1961) published the results for lower frequency scattering in deep water and introduced empirical Lambertian backscatter coefficient which was to be cited often in subsequent studies. McKinney and Anderson (1964) investigated potential scattering mechanism based on the interface relief and sediment particle. One of the methods to measure the bottom backscattering strength is to use the echo integration method (Aoyama et al, 1999). This method is used as the main tool for quantifying the abundance of marine fish or planktonic organisms. The echo integration output can be converted to biomass. For the general situation of arbitrary density, as with a school of fish, individual target echoes may not be resolvable. In this case, the echo voltage, after detection and application of suitable range compensation, is squared and summed over a defined interval. The resulting quantity is proportional to the volume or area backscattering coefficient. Division of this by the characteristic backscattering cross section for the target fish yields the numerical density. The echo integration technique has been originally used for fisheries surveys and at the present study we adopted it to measure backscattering strength (SS) of the sea bottom. Acoustic scattering by the sea bottom has been studied in order to either predict the performance of the echosounder system (Aoyama, et al. 1999) or to quantitatively map the

Underwater Acoustic Detection and Signal Processing Near the Seabed