抄録
On this paper, we solve the two problems on a multi-frequency echo sounder, which are frequency dependency of acoustic reflection loss Lp on marine sedimentary bottom such as soft mud and depth differences on echo-gram for each frequency, by using the critical multi-layered model.
In order to explain the first problem we set up some multi-layered models, a basic model is composed of sea water, the 1st sedimentary layer and the 2 nd layer, and an advansed model is very thin surface layer with low volume concentration on the 1 st layer. A correspondence between absolute value of complex reflectivity |R(f)| and calculated reflectivity converted from Lp in situ is examined. The calculated reflectivity is composed with echoes from each boundary and layer, so it shows that the main reflection point is changed to shallow boundary according to increase of frequency. For each model, frequency dependency of |R(f)| is in good response to converted values.
The second problem seems to be related to deepening of reflection point by sounding frequency, so we set up practical 4-layered model. The layer model is composed of a basic layer concluded 20% porous organic particles in mineral as the 1 st layer and very thin surface layer of thickness 1 centimeter covered on the 1 st layer. For this model there are some peaks on auto-correlation coefficients C(τ) obtained from |R(f)|, which expresses propagating time difference at each layer. The depth of each layer calculated from the peak agrees with it of echo sounding records in each frequency of 200, 100, 24 kilohertz because of deepening of main reflection point in inverse proportion to frequency. It can be also verified by layer's physical properties estimated from each peak of C(τ).
