Thursday, July 9, 2015

CHIRP from a bottom mapping perspective

Maybe you've been hearing about this term in sonar circles called "Chirp" and noticing that most consumer sonar units now come with Chirp capability. Indeed, Chirp is a game changer for more precise definition of acoustic targets suspended from bottom (e.g., fish) and the technology is helping more anglers find fish in a wide range of aquatic environments (Figure 1).  But what does Chirp mean for mapping the bottom of waterbodies?  Does it provide any advantages or disadvantages over traditional 200 kHz frequency broadband sonar that is the foundation of Insight Genesis and BioBase
EcoSound mapping services?  Here we take a brief look at Chirp, explain what it is, and present some findings from preliminary tests in a couple of different lake environments.
Figure 1. Sample sonar screen grab from a Lowrance HDS Gen3 running high Chirp frequencies over numerous fish targets.

Compressed High Intensity Radar Pulse
Acoustic target separation is often tied to the pulse length or duration.  Longer pulses give better depth penetration, but less target separation.  For suspended targets, Chirp represents the best of both worlds by sweeping many short pulses of a range of frequencies within a relative long pulse burst.  Three Chirp ranges (Low 40-60 kHz; Medium 85-145 kHz; and High 130-210 kHz) typically accommodate most situations for locating fish targets throughout a range of depths.

Chirp Performance For Tracking Bottom in a Range of Conditions
It has been well established in recreational marine circles that Chirp is a benefit to mapping suspended targets, however differences in how bottom is declared with Chirp vs the traditional single 200 khz frequency (for which BioBase EcoSound and Insight Genesis was developed) is a question for which we recently sought an answer.

Tracking Bottom in Dense Submersed Aquatic Vegetation
Tests were conducted in Gray's Bay of Lake Minnetonka in August 2014.  Repeated transects were run over a dense bed of submersed vegetation with a Lowrance Elite 7 Chirp, and HDS Gen2 with a Chirp compatible SonarHub running both traditional 200 kHz and High Chirp frequencies.  Below is a plot showing the EcoSound depths from both the 200 kHz and High Chirp Frequencies (Figure 2).
Depth was significantly greater and was closer to truth based on sonar log verification with the traditional 200 kHz frequency than High Chirp.  However, the actual difference in depth declarations between the frequencies was only 6 inches on average.  Plant canopies apparently more quickly extinguish the Chirp signal than the 200 kHz.  Based on these findings and other tests, the 200 kHz frequency remains the recommended frequency for mapping aquatic plant dominated bottoms.
Figure 2.  Average depth declaration by BioBase over repeated transects in dense submersed vegetation (avg plant height of 3 ft) running both Chirp and 200 kHz frequencies.

Bottom Declarations Over Soft Bottoms
Frequently, we get questions regarding whether EcoSound can determine the depth of sediment.  The answer is yes, but only if the "true" bottom is known (e.g., "as built" bathymetry in a human-engineered system) or a desired bottom can be modeled (e.g. how deep does it need to be for waterway management objectives).  If a true bottom can be described, then Lowrance and EcoSound should tell you the precise depth where the water meets sediment.  Simple subtraction of map layers in ArcGIS with Spatial or 3D Analyst (Raster Calculator) will produce detailed sediment maps like produced by the Central Arizona Project.  A question that piqued our interests however, was the difference between 200 kHz and Chirp bottom declarations in relatively soft bottoms.  Because the Chirp signal appears to attenuate more quickly than 200 kHz, could Chirp be a solution for more precisely mapping the depth of the sediment water interface in softer, plant free bottoms?

The St. Johns River Water Management District at Lake Apopka FL helped address this question by sharing Chirp and 200 kHz frequency sonar logs over bare soft bottoms on Lakes Apopka and Griffen in Florida, USA.  Analysis of approximately 100 data points across both frequencies found almost no differences overall and deviated only one to three inches from true bottom determined from the sonar log (Figure 3).


Although the vast majority of user sonar logs have bottoms that are clearly defined visually and acoustically, there are some bottoms that are so soft and unconsolidated that they are source of philosophical discussions of where the "bottom" actually is.  Further research and testing is needed to determine whether Chirp can be a unique solution for better depth tracking in these and other special use cases where bottoms are highly flocculent.  The more rapid attenuation of the Chirp signal than the 200 kHz frequency in plant bottoms (which may present a similar acoustic signature to a soft bottom) suggests it could be a solution in a narrow range of use cases where the 200 kHz channel penetrates too far into the bottom.

Figure 3. Bottom depth declarations (solid line) with Chirp (Top) and 200 kHz (Bottom) over a soft bottom on Lake Apopka, Florida USA.  Although in this case, Chirp produced a slightly shallower depth (8.0 ft) than 200 kHz (8.7 ft), overall we found very small differences between frequencies in depths over moderately soft, plant-free bottoms.


Conclusion: Use 200 kHz for Mapping
EcoSound and Insight Genesis mapping algorithms were developed and optimized for the 200 kHz frequency.  Over plant-free bottoms, both 200 kHz and High Chirp should perform similarly and precisely map the sediment-open water interface.  However, in bottoms dominated by submerged vegetation, the 200 kHz frequency will produce the best depth declarations and clearest delineation of the vegetation-sediment interface.

2 comments:

  1. Hi,

    The above blog was very informative, thank you.
    I guess the tests were done using a standard HST-WSBL transducer?
    Do you intend to do similar tests with an Airmar TM150 transducer?
    With the new HDS Gen3 (CHIRP) units, these Middle Chirp TM150 transducers are rapidly becoming popular.

    It would be interesting to test this transducer against the HST-WSBL from a mapping perspective.
    When using a TM150 what would be best? Custom frequencies 95 kHz or 145 kHz or Middle Chirp?

    Kind regards,
    Danny Geysen
    Lowrance ProStaff Belgium/ The Netherlands.

    ReplyDelete
  2. Thanks for your question and glad you found the blog informative. Indeed, we ran the tests with the standard HST-WSBL transducer as that is our greatest install base. We did not run any tests with the TM150.

    The best way for you to get at your answer, would to probably do your own tests. But based on my understanding of the physics, I don’t expect much difference across frequencies or transducers in relatively clean semi-hard to hard bottoms. However, we have not found any better performing/bottom tracking frequency than 200 khz in vegetation.

    Outside of generating depth maps, I’m not sure Mid or Low Chirp will produce hardness or vegetation, and if so, we’ve never tested with these frequencies so we really can’t stand by the outputs. I know vegetation or hardness outputs are not created with non 200 khz single frequencies.

    Choosing the best frequency to run will depend on your situation. If you’re just out to map, run 200. If you are mapping and fishing in non veg bottoms where you don’t care about hardness maps, run the optimal chirp or single-frequency. If you are mapping and fishing in vegetation, run 200.

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