📸 Day 80: Side Scan Sonar Mosaicking and Target Identification
🖼️ DAY 80: SIDE SCAN SONAR MOSAICKING & TARGET IDENTIFICATION
⏱️ Estimated Reading Time: 15 Minutes | 🎓 Level: Professional Hydrographer / Sonar Analyst
From Raw Waterfall to Georeferenced Mosaic – Identifying Wrecks, Pipelines, and UXO
Instructor: Engr. Rokib Hossain | River Warrior Academy
📖 Table of Contents (Serialised)
- Why Mosaicking Transforms Raw Data into Intelligence
- Mosaicking Workflow: Slant Range, Gain, Georeferencing
- Target Identification: Wrecks, Pipelines, UXO, Geology
- Interactive Mosaic & Target Simulator
- Software Tools (SonarWiz, Hypack, Qimera, Open Source)
- Advanced Techniques: TVG, AGC, Speckle Reduction
- Case Study: Bay of Bengal UXO & Wreck Detection
- Mosaicking & Target ID Checklist
- Resources & References
- Frequently Asked Questions (with internal links)
- Action Items & Next Steps
Why Georeferencing is Critical: A side scan mosaic is only as accurate as the navigation and layback data feeding it. Even a 2‑degree heading error can shift features by several metres at 100 m range. Always cross‑check mosaic targets with MBES or magnetometer data. In the Bay of Bengal project, we discovered a 15‑m wreck that was completely invisible in individual lines but stood out clearly in the mosaic. This is the power of mosaicking—it reveals patterns that single lines hide.
1. Why Mosaicking Transforms Raw Data into Intelligence
A single side scan line is a narrow strip. A mosaic stitches multiple lines into a continuous, georeferenced image of the seabed – revealing features that would be invisible in individual tracks. Mosaicking is essential for:
- ✅ Pipeline and cable route inspection.
- ✅ Wreck search and identification.
- ✅ UXO (unexploded ordnance) clearance.
- ✅ Seabed habitat mapping.
- ✅ Pre‑dredge obstacle detection.
🧠 Golden Rule: A mosaic is only as good as the georeferencing. Incorrect layback or heading can shift features by metres – always validate with a known target.
🌊 River Warrior Pro-Tip: Bay of Bengal UXO Mosaic
A 15 km² mosaic revealed a cluster of five UXO candidates that were invisible in individual lines. The bomb disposal team cleared the area before dredging – potential disaster averted.
2. Mosaicking Workflow: Slant Range, Gain, Georeferencing
1️⃣ Import raw .xtf or .s7k files.
2️⃣ Apply slant range correction (convert slant to ground range).
3️⃣ Adjust gain / TVG for consistent brightness.
4️⃣ Georeference each line using layback, heading, and USBL (if towed).
5️⃣ Merge overlapping lines using feathering (blending).
6️⃣ Export final mosaic as GeoTIFF or KMZ.
💡 In SonarWiz, use the “Mosaic Wizard” – it automates most steps.
3. Target Identification: Wrecks, Pipelines, UXO, Geology
| Target type | Acoustic signature | Confirmation |
|---|---|---|
| Wreck (steel) | Bright return (metal) with long acoustic shadow. | MBES bathymetry, ROV video. |
| Pipeline (steel) | Linear bright line, periodic welds as bright spots. | Magnetometer, visual by ROV. |
| UXO (bomb/shell) | Small, strong dipole with sharp shadow. | Magnetometer, diver. |
| Rock outcrop | Irregular, rough texture, no clear shadow (if low relief). | SBP, grab sample. |
| Cable (fibre optic) | Faint, continuous thin line (non‑metallic). | ROV visual. |
🔍 Shadows are key: length of shadow = object height × (range / (towfish height)). You can estimate size.
📷 Target Size & Shadow Simulator
Estimate object height from shadow length (side scan):
Shadow length (m):
Towfish height above seabed (m):
Horizontal range to object (m):
Estimated object height: 2.13 m
Formula: height = towfish height × (shadow length) / (range – shadow length) (simplified).
4. Software Tools (SonarWiz, Hypack, Qimera, Open Source)
| Software | Mosaicking features | Target ID tools | Link |
|---|---|---|---|
| SonarWiz (Chesapeake) | Full mosaic, TVG, blending, export | Target annotation, length/area measurement | SonarWiz |
| Hypack Side Scan | Real‑time mosaic, waterfall display | Target marking, report generator | hypack.com |
| QPS Qimera (SSS module) | Georeferencing, mosaic, export | Side scan viewer, basic measurement | qps.nl/qimera |
| Open source: MB‑System | Command‑line mosaic (mblevitus) | Basic, scriptable | MB‑System |
5. Advanced Techniques: TVG, AGC, Speckle Reduction
- Time‑Varied Gain (TVG): Compensates for signal spreading – essential for even brightness across swath.
- Automatic Gain Control (AGC): Local contrast enhancement – improves target visibility.
- Speckle reduction (median filter): Reduces granular noise while preserving edges.
- Feathering (weighted blending): Smooths boundaries between overlapping lines.
📌 For UXO detection, use AGC with a small window (5‑10 pings) to enhance small targets.
6. Case Study: Bay of Bengal UXO & Wreck Detection (2026)
Project: Pre‑dredge clearance for a new port, 5 km² area.
- Data: 450 kHz side scan, line spacing 40 m, 120 line km.
- Processing: Slant range correction, TVG, AGC, mosaic (SonarWiz).
- Targets identified:
- 3 shipwrecks (20‑40 m length) – confirmed by MBES and ROV.
- 5 UXO candidates (sharp dipoles) – removed by bomb squad.
- 1 abandoned pipeline – avoided during dredging.
- Outcome: No dredge equipment damage. The mosaic was delivered as GeoTIFF to the port authority.
📈 The cost of the survey ($45k) was recouped tenfold by avoiding a single pipeline strike.
7. Mosaicking & Target ID Checklist
- Raw side scan data quality checked (gain, range, no dropouts).
- Slant range correction applied (towfish depth profile).
- TVG/AGC adjusted for consistent brightness.
- Georeferencing: layback, heading, USBL validated.
- Mosaic generated with blending (feathering) to avoid lines.
- Targets digitised with measurements (length, width, shadow).
- Cross‑referenced with MBES and magnetometer data.
- Target list exported (coordinates, dimensions, confidence).
- Mosaic exported as GeoTIFF and KMZ.
- Report includes target map and representative images.
Click items to track progress (saved in browser).
8. Resources & References
| Resource | Description | Link |
|---|---|---|
| SonarWiz Training Videos | Mosaic creation, target annotation | SonarWiz Training |
| NOAA Side Scan Sonar Guide | Theory and interpretation | NOAA PDF |
| UKHO Side Scan Guidelines | Target classification standards | UKHO |
📌 Master Side Scan with These Hubs:
9. Frequently Asked Questions (with internal links)
How do I distinguish a wreck from a rock outcrop?
Wrecks produce a strong, often linear bright return with a sharp, long shadow. Rocks are irregular, with diffuse shadows. Use MBES to confirm shape. Day 33 advanced analysis covers feature discrimination.
What is the minimum target size detectable with 450 kHz side scan?
Approximately 0.1 m (cable) in good conditions, 0.5 m for reliable detection. Depends on range and sediment. Day 23 side scan integration discusses frequencies.
Why does my mosaic show banding (stripes)?
Incorrect TVG or gain mismatch between lines. Adjust TVG globally, or use histogram matching. Day 36 image processing covers gain adjustment.
Can I create a mosaic from hull‑mounted side scan?
Yes, same workflow, but towfish height is zero (mounted on hull). Slant range correction still needed. Day 23 compares towfish vs hull‑mounted.
How do I export a georeferenced mosaic for Google Earth?
Export as GeoTIFF, then convert to KMZ using QGIS or Global Mapper. SonarWiz can export KMZ directly. Day 76 final product explains formats.
10. Action Items & Next Steps
- 📌 Use the target size simulator: compute height for shadow length 6 m, towfish height 3 m, range 12 m.
- 📌 If you have side scan data, create a small mosaic and digitise at least three targets.
- 📌 Compare the mosaic with a magnetometer grid (if available) – do the targets coincide?
- 📌 Proceed to Day 81: Magnetometer Surveys – Ferrous Detection.
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