Dimitri M. Donskoy
Davidson Lab., Stevens Inst. of Technol., 711 Hudson St., Hoboken, NJ 07030
Alexander M. Sutin
Andrew I. Potapov
Inst. of Appl. Phys., Nizhnii Novgorod, Russia
A parametric echo-sounder is used for sea sub-bottom profiling and target (cables, mines, etc.) finding. It generates a modulated high-frequency signal (pump signal). Due to the nonlinearity of water, part of the pump energy is converted into a low-frequency wave named the secondary signal. The high-frequency pump signal reflects mostly from a bottom surface and has high signal-to-noise ratio. The low-frequency secondary signal penetrates deeper under the bottom and reflects from sub-bottom layers and buried objects. However, the level of these reflected signals is low, often below noise level. In order to enhance the signal-to-noise ratio for these reflected secondary signals, the coherent average technique can be employed with the use of the reflected pump (high-frequency) pulses as reference signals. The feasibility of this approach is based on the following considerations. (1) The pump and the secondary signals are always correlated. (2) Average from one pulse to another does not depend on deviation in distance between the transducer and the bottom (due to the pitching, drifting, etc.) because the reference signal (reflected from the bottom pump signal) always follows this deviation. (3) The front of the pump signal is much sharper as compared with the front of the secondary signal. Therefore coherent averaging of the secondary signals can be done precisely. (4) For the ``frozen'' sub-bottom structure time delays between the first reflected signal and the all following reflected signals are fixed; therefore the coherent accumulation will take place for all of these signals. The proposed method was proved during feasibility field test performed at Barents Sea.