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Fig. 4. AUV's sonar beam layout and data gathering process

Fuzzy logic methods were applied for identification of the pipeline (Pavin, 2006). The methods use the known pipeline parameters (diameter of its cross section and approximately known direction of its laying-out). The probability of pipeline correct identification depends on echo sounder and navigation data accuracy. These data compose AUV internal 3D terrain model. The main objective of identification system is an estimation of the pipeline presence probability in this or other place and determination of mutual position (offset) of AUV and the pipeline (fig. 4).

Offered method uses following information for the determination of pipeline exact position:

1. known seabed shape Ms,t (it is created from each echo-sounder s at each moment t when data updating occurred) during time interval dt;

2. echo-sounder system accuracy (beam directional diagram angle 8ES and distance accuracy AES);

4. known radius of the pipeline RPL;

5. 9pl, ^pl - supposed direction and pipeline inclination;

### 6. the pipeline is a cylindrical object.

For identification of pipeline location it is enough to identify its top part ("ridge"). In this case the axis of the pipeline settles down below "ridge". The most probable location of pipeline axis is where maximal pipeline existence probabilistic assessment P and coordinates of axes are concentrated along some line. Thus, for determination of the pipeline location it is enough to present the received coordinates as points on the plane xOz (fig. 5) then it is possible to use fuzzy logic methods of lengthy objects determination based on point's position on the plane.

Fig. 5. Map points with pipeline projections (left image) and fuzzy logic values (right image)

The belonging probability of some point to the pipeline surface is appreciated according to following fuzzy rules:

1. The probability of the point belonging to the pipeline surface is higher, than more the neighbor points are "on a surface of the pipeline": If L = "ON the pipe" then P := P + Ptrue;

2. If some point belongs to the pipeline surface then the neighbor points also lay on its surface, or are outside its borders. Otherwise the existence probability of the pipeline in the given point is reduced: If L = "IN the pipe" then P := P - Pfalse.

Values Ptrue and Pfaise take from modeling experiments, but supposed that Ptrue < Pfaise.. Fuzzy values "IN the pipe" and "ON the pipe" depend on echo-sounders accuracy, navigation accuracy and pipeline radius (fig. 5).