A Sonar Phenomena: Crosstalk
Crosstalk occurs when objects send back bright echoes that are picked up not only in the main lobes of the beams aimed at the object, but also in the side lobes of beams not aimed at the object. The display will show an arc of “crosstalk spots” about the object. The shape is an arc because the returns are picked up at the same time in the adjacent beams and thus the mapping will show the same range for these returns. If the bright return comes from only one (or a few) adjacent beams, the arc will not be solid but contain up to 11 spots depending on the intensity of the primary return, as shown in figures 1 and 2. If the bright return extends through at least 8 beams (HF) or 4 beams (LF), then the crosstalk will show up as a continuous arc as seen in figure 3. Crosstalk can be reduced by changing the aspect angle of the offending object so no surface sends back an extremely bright return. The image threshold may also be raised to eliminate the appearance of crosstalk.
 Figure 1: Eleven Bands of Crosstalk |
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 Figure 2: Metal Plate |
 Figure 3: Crosstalk Arc |