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TOPIC: bottom density module questions

bottom density module questions 2 months 4 weeks ago #12566

  • ADM1953
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One of my primary species target are grouper in the NE Gulf of Mexico. Our area the gulf has a lot sand bottom with patches of flat limerock where the fish hangout. In an attempt help me identify the rock areas, I added the bottom hardness module to RM. I've been using it for about a week and am a little confused. I have using the Peak and E2 options to generate my track colors and get very different t result for the same track segments with the 2 choices. 3 pictures are attached. The firs i sth epaulet I am using for hardness and blue is the hardest bottom. Th next is a screen grab of the Peak choice. The track segment between the waypoints marked with an "X" show the greatest density and a screen grab of the downscan pane is shown. The hardness is subtle bu tone can seen greater density well below th enrich tree bottom echo over the 240 ft of interest.

The next picture shows the segment ID with E2 as the hardest bottom shown between the 2 pin designated waypoints. the areas just to left of the picture is the same areas as shown in the Peak figure. As I understand the options, peak is an average the harness features and E2 uses the strength of the second echo to determine harness. I am surprised that the 2 options provide such wildly different harness estimates over the same bottom. Does the vegetation the E2 image attenuate the signal? E2 shows the hardest bottom devoid of vegetation, but I know form diving that vegetation in our areas grown on rocks, not sand.

When we use sonar to find fish, we rarely seen fish arches as the grouper hide under rocks and in little crevices. The rule is fish the structure not fish arches. Any help understanding this issue edible appreciated.

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bottom density module questions 2 months 3 weeks ago #12567

  • Matt
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Peak SV is a measure of the highest intensity in the first echo return (an average of the highest values). It is meant as an approximate replacement for E2 in the case where users don't have a fully recorded second echo return, and as an approximation of the old DrDepth hardness value which it is generally quite close to. E2 should be preferred if you have recordings with sufficient range.
Any of the layer values can be badly affected if the depth is inaccurately recorded, so always worth opening the depth correction window to ensure that this is not the case - it is also possible that weeded areas could register as "harder" than unweeded because of this, if the depth is recorded as too shallow and weed/open water are included in the hardness calculations.
If you have it, it would be interesting to see the second echo sonar returns for your examples.
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