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Method Selection: Optical, Sonar or Combined
A compact decision aid for planners, operators and expert witnesses: which method fits which inspection task? This page complements the method comparison in ROV vs. Diver and the technical foundations in Sonar Foundations with a task-based matrix.
Three Selection Modes at a Glance
In practice, method selection reduces to three configurations, each with distinct strengths.
Optical (HD video + stills)
Standard method of ROV inspection. Sufficient with clear visibility, defined asset geometry, and visual damage documentation.
Sonar (Side-Scan / Multibeam / Profile)
Acoustic capture. Required in turbid water, large-area search, or geometric surveying — see Sonar & Scanning.
Combined (Optical + Sonar)
Both methods in parallel. Recommended with variable visibility, safety-critical structures, or when both geometry and damage imagery are required.
Method Selection by Task
This matrix shows the typical recommendation per inspection task. It does not replace a briefing — it structures method selection as a starting point.
| Inspection Task | Optical | Sonar | Combined |
|---|---|---|---|
| Quay-wall visual inspection, clear water | recommended | optional | optional |
| Quay wall in turbid harbour water | limited | possible | recommended |
| Bridge-pier structural inspection | recommended | complementary | for scour surveying |
| Scour formation / bathymetry at pier base | insufficient | recommended | for image + geometry |
| Industrial basin, clear water | recommended | optional | optional |
| Cooling / storage basin with turbidity | limited | possible | recommended |
| Large-area search task (Search & Recovery) | unsuitable at scale | recommended | sonar leads, optical verifies |
| Habitat mapping / sediment classification | limited | recommended | for verification |
| Underwater survey of large surfaces | expensive | recommended | for detail verification |
| Evidence preservation after incident | recommended | complementary | with structural damage |
| Monitoring follow-up (time series) | identical to baseline | identical to baseline | identical to baseline |
When Optical?
An optical ROV inspection is the right choice when:
- visibility in the work area is typically above 1–2 m
- the task is primarily visual damage documentation — cracks, growth, corrosion, joint condition
- asset geometry is known and does not need to be re-surveyed
- HD video, position-referenced stills and a findings report cover the required deliverable
- the report recipient expects an image-based format (engineering office, asset owner, expert witness)
Typical examples: quay-wall inspection in clear water, bridge-pier visual inspection, lock-wall documentation.
When Sonar?
Sonar becomes the primary method when:
- visibility is so limited that optical methods cannot deliver usable images
- a surface or geometric capture is required — bathymetry, sediment map, point cloud
- a search task over a larger area can be resolved faster with acoustic coverage
- scour at pier base, sediment accumulation, or build-up must be surveyed
- results are to be ingested into a GIS, CAD, or point-cloud system
Method-level foundations — multibeam, side-scan and profile sonar — are documented in Sonar Foundations.
When Combined?
The combination of optical and sonar is the right choice when:
- visibility in the work area is variable (e.g. tide- or current-dependent)
- both image and geometry are required as deliverables
- the task is safety- or expert-witness-relevant and redundant data paths are desired
- a sonar hit (e.g. search task) must be visually verified afterwards
- a monitoring baseline must guarantee both visual and geometric comparability for future cycles
The combined method underlies most monitoring and recurring inspections and demanding insurance and expert-witness missions.
What Drives Method Selection
Visibility in the work area
Experience values for the water body, season, suspended sediment. When uncertain, the combined method is recommended.
Asset type and geometry
Linear structures (quay walls, piers) are well suited to optical work. Surfaces, basins and sediment require sonar.
Reporting and review recipient
Authority and expert-witness contexts often expect both imagery and geometric evidence. Insurers tend to focus on imagery.
Deliverable requirement
Findings report with image references → optical. GIS/CAD data or point cloud → sonar or combined.
Repeatability / time series
The method of the baseline determines the method of follow-ups — comparability requires identical methodology.
Permit and safety context
In protected areas, sensitive installations, or with live operations, method selection may be constrained by permits.
Frequently Asked Questions on Method Selection
When is a purely optical ROV inspection enough?
With sufficient visibility (typically > 1–2 m) and a clear task such as structural inspection, damage documentation, or visual condition assessment. HD video and targeted stills are sufficient for an audit-grade record in these cases.
When is sonar mandatory?
In strongly turbid water, with high suspended sediment, large-area search tasks, or geometric surveys of large surfaces. Sonar delivers where optical methods reach their limit.
When are optical and sonar combined?
When both geometric capture and visual damage documentation are required, or when visibility conditions are variable. The combination provides redundant, complementary data sets.
Who decides the method?
Method selection is agreed during the briefing between client and ScanSustain — based on objective, asset type, expected visibility, required deliverables and reporting format. The ScanSustain process describes this briefing step in detail.
Related Pages
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