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The vector field of normal modes in Pekeris waveguide is studied，and the expression of horizontal and vertical complex acoustic intensity are proposed，especially their characteristics are analyzed in this paper. For the single order mode，the form in the horizontal direction is traveling wave，the corresponding complex acoustic intensity is active，and the vertical form is standing wave，the corresponding complex acoustic intensity is reactive. But for the multi-order mode，either the horizontal or vertical complex acoustic intensity has both active and reactive component because of the interference between the normal modes. Only the active component can transport energy，but the reactive component also carries important information of sound field. The numerical analysis shows that，for the very low frequency sound field of point source，the sign of the active component of horizontal complex acoustic intensity and the reactive component of vertical complex acoustic intensity changed regularly with the change of the depth of acoustic source，so the target depths can be classified by comparing with a critical depth when the receiving transducer is placed in appropriate depths，especially the active component of horizontal complex acoustic intensity，can be used to tell whether the sound source is near the surface or underwater. Further more，the phase angle of complex acoustic intensity is discussed and its distribution regularities are analyzed.

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Pattern time delay shift coding scheme uses the time delay shift values of the Pattern to code the information whose anti-ISI (intersymbol interference) ability is related to the kind and duration of the Pattern. Spread spectrum communications is competent for long-range underwater acoustic communications for its spread spectrum gain, but the low communications rate seriously restricts its application. The proposed new scheme is adapted to the underwater acoustic environment, which integrates the spread spectrum with Pattern time delay shift coding and works at M-ary mode spread spectrum and several communications channels. The scheme could achieve good performance of the spread spectrum and also improve the data transfer rate. The results of lake trial show that the M-ary chaotic spread spectrum Pattern time delay shift coding scheme is robust and feasible.