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The 2000 hydroacoustic survey of Lake Matano, Sulawesi is used to illustrate the mechanism of formation of an artificial, but acoustically-observed density layer in the midst of the water column. Such a spurious signal resembling the true acoustic target can be formed under certain combinations of the lake morphology and the echosounder settings by way of recording the residual echo from the previous ping during the subsequent ping-recording interval. The. generic physical mechanism of the phenomenon is described and the necessary conditions for its formation stated. The echosounder settings sufficient to prevent the interference during the field surveys are suggested, and their application illustrated with an example. The phenomenon described is explained by the physics of sound propagation and hence, the algorithms here developed apply to other water bodies and hydroacoustic equipment.