A Study On The Characteristics Of Scattering From Target Nearby Sea Bottom Below The Critical Angle

To meet the demand of large-scale and high-efficiency military requirements for the detection of mines in shallow water,traditional mine detective technology is faced with a great challenge.In accordance with this request,detective methods should head for increasing the cognitive rate of mines near sea bottom with low-frequency and small grazing angle.As for the design of overall model-based sonar system,the marine environment should also be taken into consideration.So the sonar system will be more tolerant for the model mismatch,especially targets in the vicinity of sea bottom.Therefore,the study on the characteristics of scattering from a target nearby interface below the critical angle is of very important significance.In this paper,forward scattering field spatial distribution of both the submerged and buried targets in different sand bottom with small grazing angle is measured in laboratory experiments.The results of the experiment show that in the reflecting wave area,the scattering signal of shallow buried target is seriously influenced by the reflecting signal of interface.Pseudo-color images of received signals processed by the matched filter are used to describe the spatial distribution of scattering filed. According to the experiment results we find that besides the scattering beam in the reflection direction,there are also some other beams in other directions.In the buried mine detection experiment,the targets under different interfaces are detected by monostatic hydrophone with different grazing angles.With the grazing angles in the range of 90-50 degree the target under the flat interface can be detected.The target scattering signal is most obvious when the grazing angle is 70 degree and when the grazing angle decreases to 40 degree,the signal can not be distinguished.With 40 degree grazing angle incidence the target under the sinusoid interface can be detected. At 30 degree and 70 degree grazing angles the target under the random fluctuation interface can be detected. OASES model based on perturbation theory,and SCATT model based on virtual sound source method are applied to calculate sound transmission and scattering fields of targets nearby interface,by the numerical results the phenomenon in the experiment are explained.In addition to the phenomenon of the experiment,we also find that under the interface under the fluctuation seabed there is a critical depth. Above the critical depth,the attenuation sound filed plays the leading role.Below the critical depth,there is mainly sound scattering field.The sound intensity of scattering fields nearby interface below the bottom is decided by RMS value of interface roughness.And the sound intensity attenuation versus the increase of depth had some relation with correlation length of roughness.In the scattering field the intensity of backward scattering is the weakest.In scattering field of target in submerged and semi-buried conditions,the angle between main forward scattering beam and sand bed is in accordance with the grazing angle.For buried target with subcritical angle incidence,the angle between main forward scattering beam and sand bed is larger than the grazing angle,and these two meet the snell law.The scattering beams of buried target almost focus in the area above targets between the main forward scattering beam and backward scattering beam.Multiple scatterings between the target and the interface can increase the scattering beams in the scattering fields. Adding layers in the sea bed,the intensity of multiple scatterings can be increased. When the source beam incidences on the head of horizontal buried oval target,the backward scattering field is very weak.To sum up,detecting the target nearby interface with small grazing angle,it is important to choose a bistatic way,especially for buried targets.The receiving hydrophones should be placed on a mobile platform to receive scattering signals in different positions.