1. Mosaic artifacts reduction in SIPS Backscatter
Eli Leblanc, Travis Hamilton and Michael Redmayne US Hydro 2019

2. Introduction
Increasing attention paid to backscatter as data collected for Hydrographic purposes is being reused
Presence of artifacts can drastically compromise mosaic usability
Improvements to address artifact issues were released in HIPS & SIPS 11.1
AVG adaptive normalization range
Beam pattern accounting for acquisition mode changes and sectors
Collect once use many times increasing focus on backscatter
Hydrographic surveys still collected with bathymetry as the primary
Post processing packages are instrumental in creating usable products from the backscatter (mosaics)
Artifacts hinder both visual interpretation and image processing algorithms
One way that some vendors approach the problem is to applying a lot of averaging to cover the issues.
With SIPS we are focused on properly correcting the underlying data.

3. Angular varying gain AVG
Normalization
range dB
Required to make usable geographical representation
Compensates for the angular dependency
Signal response is function of the incident angle
Estimate the dB vs incident angle curve over a stack of pings
Calculate a reference level using average over a range of angles, then normalize the curve
gravel
sand
The seafloor backscattering strength is intrinsically dependent on the angle of incidence, leaving a sediment specific across-track signature on the mosaic. The consequence of this dependence is that a georeferenced representation of BS displays a strong along‐track banding that hinders both visual interpretation and image processing algorithms. A compensation for angular dependence is therefore necessary to make a usable geographical representation of BS.
AVG steps for reference.
1. Select a subset of data to calculate the lookup table from;  
2. Bin all signal samples from that data subset into angular bins, typically 1°‐wide;
3. Compute the average signal level per angle bin;
4. Calculate a “reference level” as the average value over an angle interval (30deg to 60deg);
5. Create the lookup table as the difference of the average angular response and the BS level at the reference angle.
mud
Incident angle (deg)

4. Standard swath Narrow swath AVG Inadequate normalization
Normalization range
Normalization range
Inadequate normalization
Normalization
range
30°
60°
Normalization range
75%
99%
In SIPS the previous approach was to used a fixed deg range. This range was being used as an attempt to capture the outer 25% of data samples, as this was the range that presented an optimal normalization value.
When little or no data is found in the standard normalization range (for example, on narrow swath systems, or in deep water), the computed reference level is not reliable and the normalization can cause along-track banding artifacts.
To overcome this, the default normalization range is replaced by the angle equivalent to the outer 25% of data sample, ~17-37° in this case.

5. Adaptive normalization range
AVG
Adaptive normalization range
Fixed normalization
Adaptive normalization
Solution used to avoid artifact automatically
Take fixed values as start point, but can diverge if angle distribution is not standard
Fixed values other than can still be entered
In 11.1 we introduced an adaptive normalization range, by dynamically adjusting the normalization range to account for varying swath widths
As a general rule, when the AVG normalization is set to adaptive, the range follows the outer 25% of data samples
The adapative AVG is also useful beyond deep water. Steep slopes can cause there to be not enough data in the typical degree range, so the adaptive normalization will also handle this case.
Implementation in HIPS allows user to still use fixed values if they do not want to apply the adaptive normalization

6. Beam pattern correction
Compensates for the angular signature of the sonar that transmits and receives varying amount of energy across the swath
SIPS Backscatter approach
Based on the assumption that contributions from different bottom types at different incident angles will oscillate around a mean value for one angular sector
dB vs angle curve estimated over a large quantity and variability of data
A beam pattern correction is used to correct for angular variation in transmission power and receiver sensitivity of a particular system.
The approach taken for this correction with SIPS backscatter is designed to be automated, and avoid sensitivities due to not having a completely homogenous seabed to determine a bp from.
SIPS uses the assumption that contributions from different bottom types at different incident angles will oscillate around a mean value
So we estimate the bp by averaging over a large quantity and variability of data by allowing the entire survey to contribute to the beam pattern.

7. Acquisition modes Beam pattern
Pulse length and waveform changes the quantity and the pattern of energy transmitted across the swath
Along-track intensity shifts
Badly compensated across-track
However it is not always appropriate to apply a single beam pattern to the entire survey.
The angular signature of a single system will vary if the pulse length or waveform changes.
In this example, there is a dominant acquisition mode, used by the majority of the pings, and it will have enough weight to make the beam pattern similar to what it would be if the secondary mode was not used. That explains those areas of the mosaics look like they’re compensated properly, and others don’t.
The pulse length as an impact on both the energy that gets transmitted and the angular signature, which explains that we see some artifacts both along and across-track.

8. Acquisition modes Beam pattern Solution
Distinct beam patterns
Combined beam pattern
Beam pattern
Acquisition modes
Solution
Distinct beam pattern for each acquisition mode
With the new improvements we assign each sector of a swath to a beam pattern based on The combination of sector #, and pulse length / waveform for the whole ping.
We automatically determine the sector # and acquisition mode of each beam from the datagrams, then group all beams which are the same together, and use that group to create a beam pattern which is subsequently used to correct that group.

9. Conclusion
Presence of artifacts can drastically compromise mosaic usability
With HIPS & SIPS we are pushing to properly resolve common sources of artifacts introduces
AVG adaptive normalization range
Beam pattern accounting for acquisition mode changes and sectors
Stay tuned as we release a second round of improvements in 11.2

10. Stop by booth 17 to learn more!
Michael Redmayne