Calculation of Echosignal Temporal Structure From Objects of Complex

Form in an Irregular Waveguide

A0159 Tran Huy Dat Department of Acoustic, Institute of Hydromechanics, National Academy of Science of Ukraine

A0160 Oboznenko Igor Leonidovich Department of Acoustic, National Technical University of Ukrine

The analytical review of Complex Shaped Objects (CSO) reflection and

scattering models that can be used for (CSO)classification research is

provided. Several different approaches and methods for these tasks

were discussed. Multiflash CSO physical and mathematical models based

on Huygens Principle have been developed and explained for both static

and dynamic cases, including calculation of hydrodynamic phenomena

generated when the CSO is moving. The term LRE was introduced. A

distinction between LRE scattering and flash (optical-type) reflection

was clarified. The CSO physical and mathematical model considers

coherent, incoherent, and partially-coherent summation of separate LRE

echosignals. The calculations of temporal structure of CSO are

conducted. Temporal structure of some real CSOs are showed and

analyzed in the next condition in Atlantic Pacific: the radiator at

ranges of 36 and 200 km has been submerged to 10 meters and the

radiator beam pattern has been preset for an angular range of either

(-15° to +15°)or (-11° to 0°);all other angles are assumed to have

zero radiation levels; angular increments for the sound source have

been taken as 0.1°;other variants used different radiator beam

patterns.The bottom was treated as absolutely rigid. The nearest

receiver is at a distance of r=36km and the farthest receiver at

36.096 km (the second variant used ranges of r=200.014km and

r=200.55km. The distance between neighboring receivers is 1 m. The

figure shows that the echosignal length in the case of the homogeneous

waveguide and radiator beam pattern is 10 to 15 times greater than in

the case of the irregular waveguide. Along with range increase the

appearance of new rays was observed that caused the

appearance/disappearance of echosignals from some reflecting elements.

Also, calculations were carried out that simulated a moving object.

They showed that for assumed conditions the underwater object is

passing through the medium with various numbers of rays with different

grazing angles at the reception point as well as with different beam

ray tube sizes that result in a changing echosignal length due to

distance variation. By the figure may be concluded that, besides the

increase in echosignal length, the echosignal structure is also

changed due to the motion of an object within an irregular waveguide

Thus, at a distance of 200 km the echosignal duration may change by a

factor of approximately 127 when the object is moving toward or away

from the radiator. This change m signal duration amounts to a factor

of approximately 1.5 for smaller ranges (36 km). (Please note that

for an object moving at a depth of 40 m or more, there was no

attention given to echosignal appearance due to propeller rotation

—echosignal from propeller rotation plane, echosignal from CSO's

wake — that can be observed when the object is located at these

depths.) Similar situations will occur when the object moves away from

the radiation/reception point. In this case the echosignal will

involve the CSO stern part flash elements for upper and, partially,

for lower rays, deck house and deck house rudders. This will result

in some enhancement of homogeneous and irregular waveguide's

echosignals correspondence due to longer spatial base (deck house-

stern) in comparison to forward part dimensions (sonar-deck house).

However, in the case of the assumed sound velocity profile and

distances, the mentioned phenomenon was not observed because of an

absence of positive-angled ray tubes that have a width equal to stem-

deck house base. It should also be mentioned that several rays

entering the object zone are subjected to reflections from the sea

surface (several to tens of times). In the case of field experiments

in rough sea water, this may cause severe fluctuations of particular

elementary echosignals and, therefore, the echosignal active length

may become substantially shortened. However, it will still exceed the

echosignal length within the homogeneous waveguide with its tens of

milliseconds echosignal length. It also noted that spatial sound

attenuation has been neglected for the appearance of CSO echo signal

arrival structure.