Monday, August 18, 2014

Humminbird 698SI survey

Steve and I mounted the Humminbird on a jetski and used it last Friday to check out an eelgrass meadow in Clark's Cove, New Bedford.  It worked well.

Here's the jetski with the Humminbird.  The transducer is on a trolling motor mount so we could easily move it up and down to try and avoid noise.  We could also pop it out of the water for high speed travel.  The set up was fine, though noise is an issue.  With the transducer about 2-3 feet down in the water, the signals were acceptable.  We collected with a 120 foot range and used an enhancement setting of 9.

The processing workflow was straightforward.  I first checked the lines in HumViewer, a free viewer that plays back the sonar data.  This helped me identify which files I wanted to fully process.  Since we were just doing a test run, we didn't keep careful track of when we pressed record!  It also turned out to be very useful because the viewer shows the sounder data as well, and our target habitat, eelgrass, is clearly visible in the sounder data.  I discovered that when we were directly over the eelgrass, it was actually difficult to see in the sidescan image, but it was very easy to see in the sounder data.  The edge of the eelgrass meadow was clear.  The images below are the unprocessed data.  Depth and range are in meters.

The next step was to slant range and beam angle correct the data.  These processing steps remove the water column and make corrections so the sonar image is georeferenced and gain corrected across the range.  I used SonarTRX and it was easy, quick, and resulted in images we could interpret.  I've used Caris HIPS/SIPS in the past, and the batch processing capabilities will probably send us back to Caris, especially for larger surveys.  But the simple and intuitive workflow and easy export to Google Earth (and ArcGIS) made me an instant fan of SonarTRX.

You can see that in some lines it was pretty hard to see the vegetation nearer the sonar -- the best images are at about 1/2 range, or 60 feet.  Even in the 2nd image it might be interpreted as having two beds.  However, as I mentioned earlier, the sounder sonar (downward looking dual beam of 200/83 kHz) clearly showed that there was vegetation under the jetski.

I think the next step is to tweak some of the image settings to try and draw out the vegetation signal more clearly in the sidescan images.