1. A CLOSER LOOK AT BACKSCATTER UNCERTAINTIES
Lloyd C. Huff PhD
Member of THSOA Hydrographer Hall of Fame
LCHUFFCONSULTANCY, LLC

2. For the last 25 years, I have pushed to use echoes from multibeam sonars to characterize physical properties of the seabed.
My efforts have been frustrating because:
There have been difficulties in getting straight answers from sonar manufacturers,
There have been difficulties in getting consistent results using the available software for backscatter processing, and
The related literature has tended to be more about Bottom Segmentation, than Bottom Characterization.

3. Presented with scientific, or engineering measurements, one should always ask about their accuracy and precision.
When multibeam measurements of range, angle and backscatter, are used to “infer” information about the sea bed, one should address “error bars” for the primary measurements and for any derived parameters.
In September 1995, Rob Hare published a paper in the International Hydrographic Review addressing error budgets for multibeam measurements of Depth and Position. However, for many years the question of error bars for multibeam backscatter has largely been neglected.
In 2012, at an Oceanology International event, I opined that it was time for backscatter from multibeam sonar to be recognized for what it is:
ACOUSTIC REMOTE SENSING
And, therefore reports of multibeam backscatter should be assigned meaningful error bars.

4. Laudable efforts are finally afoot to address the total error budget for multibeam measurements of acoustic backscatter. More importantly, those efforts are beginning to produce results. An example being the 2018 paper by Mashoor Malik, et al in Marine Geophysical Research, titled: A framework to quantify uncertainties of seafloor backscatter from swath mapping echosounders .
I whole heartedly support the efforts, to address the total error budget for multibeam measurements of acoustic backscatter, but I also want to raise the bar.
In addition to needing an estimate of the total uncertainty of backscatter, we need to estimate the uncertainty as a function of off-nadir angle.

5. Error Bars
Confidence Bands
Uncertainties
Variance
One Sigma, Two Sigma
Each of the above representations has one, or more, problems when talking about backscatter.
I prefer to use the spread between the 10th and 90th percentile in the histogram of observed backscatter, expressed in decibels. That avoids getting hung up on whether the measurements exhibit characteristics of a Gauss, Rayleigh, or Rice distribution.
The spread of a histogram is easy to determine; just line up the measurements from the “smallest to the tallest”.

6. The approach works fine when discussing a group of existing samples. But what about anticipating the result before any measurements are taken?
You know there are models for anticipating the angular response of backscatter, given the acoustic frequency and the bottom material.

7. Given the previous predicted pattern of off-nadir backscatter response, I anticipate that the dB spread of backscatter measurements from the same bottom will present a similar, but “inverted” pattern.
I freely admit that this off-nadir pattern of backscatter uncertainties may be viewed as Lloyd’s Wild Ass Guess, but I am putting it out there to hopefully “kick-start the discussion”.

8. If you are intrigued about why I suggest the dB Spread ramps up at large off-nadir angles, or by any other aspect of this lightening talk, please visit my poster and we can have a discussion.

9. A CLOSER LOOK AT BACKSCATTER UNCERTAINTIES
Lloyd C. Huff PhD
Member of THSOA Hydrographer Hall of Fame
LCHUFFCONSULTANCY, LLC