19.7 The sonar imageInterpreting the bottom using sonarIt is important to understand how to correctly interpretthe bottom structure represented on-screen.The bottom usually produces a strong echo.The following images show how different bottomconditions are represented on-screen:D6855-3123Item Description1 A hard bottom (sand) produces a thin line.2 Asoftbottom(mudorseaweedcover)producesa wide line.3 A rocky or uneven bottom or a wreck producesan irregular image with peaks and troughs.The dark layers indicate a good echo; the lighterareas indicate weaker echoes. This could mean thatthe upper layer is soft and therefore allowing soundwaves to pass to the more solid layer below.It is also possible that the sound waves are makingtwo complete trips – hitting the bottom, bouncing offthe vessel, then reflecting off the bottom again. Thiscan happen if the water is shallow or the bottom ishard.Factors influencing the sonar displayThe quality and accuracy of the display can beinfluenced by a number of factors including vesselspeed, depth, object size, background noise andtransducer frequency.Vessel speedThe shape of the target changes along with yourspeed. Slower speeds return flatter, more horizontalmarks. Higher speeds cause the target to thickenand arch slightly, until at fast speeds the markresembles a double vertical line.Target depthThe closer the target to the surface, the larger themark on screen.Water depthAs water depth increases signal strength decreases,resulting in a lighter onscreen image of the bottom.Size of the targetThe larger the target, the larger the return on thefishfinder display. The size of a fish target is alsodependent upon the size of the fish’s swim bladderrather than its overall size. The swim bladder variesin size between different breeds of fish.Clutter / Background noiseThe fishfinder picture may be impaired by echoesreceived from floating or submerged debris, airbubbles or even the vessel's movement. This isknown as ‘Noise’ or ‘Clutter’ and is controlled by theSensitivity Settings. The system can automaticallycontrol some settings according to depth and waterconditions. You can also adjust the settings manuallyif required.Transducer frequencyThe same target will appear differently when usingdifferent transducer frequencies. The lower thefrequency the broader the mark.Recovering lost bottomIf the seabed floor (bottom) is lost then follow thesteps below to recover the bottom depth.From the fishfinder application:1. Ensure your vessel is in clear undisturbed water.2. If range is set to Manual, adjust the range to theknown, charted depth of your location. or3. If range is set to Auto then switch the rangeto manual and adjust the range to the known,charted depth of your location.4. Once bottom has been regained you can switchrange mode back to Auto.262 gS Series
