Thursday, January 7, 2016

Helpful Lowrance Hints: Depth Tracking

We promote BioBase as an automated "easy-button" solution for creating aquatic maps, but unfortunately, mobile acoustic data collection is not something you can push a button and forget about and expect perfect results.  Like using most other sophisticated instrumentation, users need to monitor that the instruments are performing as expected and sometimes make adjustments if they aren't.

As such, we often stress the importance of monitoring your sonar and GPS display while logging sonar making sure you get a clear view of bottom with a continuous digital depth reading (Figure 1).  If you can't see a digital depth reading on your screen, add it (here's how). If you're a first timer not familiar with sonar of "fish finders" you should spend some time familiarizing yourself visually interpreting a sonar display prior to mapping "for real."

Figure 1. Example of a clear 200 kHz ("Sonar;" left) and 455 kHz Downscan ("Structure;" right) signal. Depth (white line) should track pretty well through moderately dense plant canopies.  Smooth and accurate depth tracking through plant canopies occurs during times of the year when plants are elongating vertically.  As the growing season progresses, horizontal and "matting" growth proliferates and high levels of plant biomass can obscure bottom readings (see Fig. 2). 
Figure 2. Example of periodic losses of depth in the vegetation canopy.  Extreme changes and periodic losses of depth are automatically smoothed or removed from BioBase.  Users can verify and edit their output in the main trip viewer in BioBase EcoSound, loading the data and using Trip Replay.

Figure 3.  We’ve frequently get calls from users wondering why parts of mapped water bodies were missing from their BioBase map after upload.  Often in these circumstances, depth was lost as some point in the trip and the user did not notice and continued driving their path recording null data.  This stresses the critical need to monitor your sonar display and ensure continual depth tracking.
Mapping Extremely Shallow Water: Tricks for a Tricky Environment
Areas of waterbodies less than 2 ft (< 0.75 m), are difficult to characterize with acoustics.  Target separation between the bottom and the face of the transducer is extremely small and these environments are very acoustically noisy due to wind mixing.  Accordingly, by default BioBase requires a minimum depth of 2.4 ft (0.76 m) to create vegetation and bottom hardness outputs. Reading depth alone is not as difficult and you may find your Lowrance unit providing reliable depths as shallow as 1 ft (0.3 m).  But, as stated earlier, this is a tricky environment and the acoustic signal can easily be lost and difficult to recover, especially in Auto Range.

Alternative 1: Fix the Range (Shallow Water Only)
Fixing the Sonar Range to 5 ft in very shallow waters can be a tool to establish a known zone where the sonar should search for a bottom signal (Figure 3).  This may ensure a more consistent bottom lock.  But, Range should be reset to Auto if mapping in variable or deep waters to ensure the 3200 bytes of data are in the optimal range for bottom typing.  Never change range during logging or it could corrupt the .sl2/.sl3 file.

Alternative 2: Use a Kayak

Figure 4. Sit-on-top kayak mounts of HDS.  Kayak's are light and will not draw as much water.  You may be able to bring your transducer.
Alternative 3: Use a Jet-ski pulling a Catamaran
Figure 5. See how EnvirolinkSouth Ltd in New Zealand maps shallow waters with a Jet-ski pulling a Catamaran with a transducer and Point-1 GPS tethered by long cables to the Jet Ski with HDS Display.  Ensure cavitation behind the jets do not create noise under the transducer.

Alternative 4: Use a Robotic Boat (especially if you have to map rivers with Hippos)!

Figure 6. Autonomous Platypus Lutra boats equipped with Lowrance can help you stay alive if Hippos are in your way or if you want to map waters difficult to access with a boat. Ask a Platypus rep about Boat Rental/BioBase EcoSound Bundles.