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

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.

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 0 30 60 90 Incident angle (deg)

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 30-60 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.

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 30-60 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 30-60 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

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.

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.

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.

Conclusion Presence of artifacts can drastically compromise mosaic usability With HIPS & SIPS we are pushing to properly resolve common sources of artifacts. 11.1 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

Stop by booth 17 to learn more! Michael Redmayne Michael.redmayne@teledyne.com