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For the first pass in a shipwreck search, side-scan sonar is usually the better choice. It produces a high-contrast acoustic image in which a wreck’s hard surfaces, outline and seabed shadow can stand out clearly. Multibeam sonar is the better measurement system: it returns depth soundings across a swath, identifies the wreck’s least depth and builds a georeferenced three-dimensional seabed model. Professional surveys normally use side scan to find and recognize a contact, then multibeam to measure and map it.
What is the difference between side-scan and multibeam sonar?
Both systems send sound through the water and interpret returning echoes, but they answer different survey questions.
- Side-scan sonar asks, “What is lying on or above the seabed?” A towfish or hull-mounted side-scan unit sends fan-shaped pulses to both sides of the track. Echo strength is rendered as a two-dimensional image. Hard, protruding objects produce bright returns, while the area hidden behind them produces an acoustic shadow.
- Multibeam sonar asks, “How deep is the seabed at every measured point?” A hull-mounted transducer array sends many narrow beams in a swath beneath the vessel. Sound-travel time is converted into depth soundings, producing bathymetry and often an acoustic-backscatter layer.
That distinction explains why neither system is a universal replacement for the other. A side-scan image can make a wreck recognizable without reliably telling you its exact depth. A multibeam surface can measure a wreck and the surrounding terrain precisely, but its output is generally less like a photographic wreck picture than a high-frequency side-scan image.
NOAA notes that “most side scan systems cannot provide any depth information.” NOAA’s operational guidance is correspondingly direct: “If surveyors find a wreck or obstruction using side scan sonar, they will determine the least depth with multibeam sonar.”
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Which sonar is best for finding a shipwreck?
Use side scan first when the main job is reconnaissance: search a large area, detect small or visually distinctive objects, and decide whether an acoustic contact resembles a wreck. Use multibeam first when the main job is safe navigation, precise clearance, charting or a three-dimensional terrain model. Use both when you need discovery, identification and defensible measurements; that is the normal professional workflow.
| Survey need | Better first choice | Reason |
|---|---|---|
| Find unknown wrecks across a search area | Side-scan sonar | Efficient target detection and strong image-and-shadow signatures. |
| Recognize hull shape, debris field or orientation | Side-scan sonar | Acoustic texture and shadows show relief and hard surfaces clearly. |
| Determine least depth for navigation | Multibeam sonar | Produces depth soundings tied to a measured seabed surface. |
| Build a 3-D model of the wreck and seabed | Multibeam sonar | Creates georeferenced bathymetry over the vessel’s swath. |
| Confirm identity, ferrous content or archaeological condition | Combined survey | Use sonar for mapping, then magnetometer, ROV or diver observations for ground truth. |
How side-scan sonar reveals a wreck
Image formation
A side-scan towfish transmits a fan-shaped pulse to port and starboard. The receiver records the strength and timing of returning echoes and lays those measurements out as a strip image along the vessel’s track. Consecutive strips are assembled into a side-scan mosaic.
The image is not a photograph. Brightness represents returned acoustic energy, not visible color. Steel, exposed rock and other hard surfaces can create strong returns. A raised bow, mast, engine block or container can block sound from reaching the seabed behind it, leaving a dark shadow. The combination of a bright target and a proportionate shadow is often more informative than brightness alone.
Why shadows matter
Shadow length provides a visual clue to an object’s height above the seabed. A long, clean shadow can indicate substantial relief; a low, diffuse return may be debris, a natural outcrop or a partially buried object. Shadow geometry also helps an interpreter estimate orientation and distinguish a compact wreck from a scattered debris field.
Towfish altitude and detail
A towed fish can be flown close to the bottom, which improves detail on a deep target compared with a sensor fixed at the surface. The trade-off is positioning: the fish trails behind the vessel, and its exact position depends on cable length, layback, vessel motion, current and depth. A hull-mounted unit has a simpler position reference, but its distance from a deep seabed can reduce the detail available from a given frequency.
Frequency choices
Frequency determines a practical balance between coverage and detail. NOAA Ocean Exploration gives these examples:
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| Side-scan frequency | Typical trade-off | Useful situation |
|---|---|---|
| 50–100 kHz | Broad coverage with lower image resolution | Initial reconnaissance over a large search area. |
| 500 kHz–1 MHz | Narrower coverage with much greater detail | Inspection of a suspected contact, small features or a debris field. |
These are frequency examples, not universal performance guarantees. Actual range, resolution and image quality depend on the instrument, altitude, seabed, water conditions, vessel speed and processing settings. A practical survey often begins with a lower-frequency, wider-range pass and then repeats promising lines at higher frequency and closer range.
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Swath bathymetry
A multibeam echosounder (MBES) uses a hull-mounted transducer array to send many beams in a fan beneath the vessel. Each beam produces a depth sounding. As the vessel moves, overlapping swaths form a continuous bathymetric surface that can show the seabed rising over a wreck, dropping into scour or changing around a debris field.
NOAA describes typical multibeam coverage as approximately two to four times the water depth. The exact usable width changes with water depth, beam geometry, seabed slope, motion, acoustic conditions and the overlap required between survey lines. Wider swaths in deeper water can increase area covered per line, but they do not automatically deliver the same detail as a close, high-frequency side-scan pass.
Least depth and clearance
For navigation, the critical number is often the wreck’s least depth: the shallowest point from the chart datum or other stated vertical reference to the obstruction. Multibeam supplies the soundings needed to locate that point and model the surrounding seabed. A side-scan contact can show that an object exists and may indicate its shape, but it normally cannot establish the clearance required for a charting or navigation decision.
Backscatter and three-dimensional context
Many multibeam systems also provide acoustic backscatter. Backscatter can help separate harder and softer seabed materials, while the bathymetric grid shows height, depth and slope. Together they give a georeferenced surface that can be measured, compared between surveys and used to plan an ROV or diver inspection. The output is a terrain model rather than a literal picture of the wreck’s surface.
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| Comparison point | Side-scan sonar | Multibeam sonar |
|---|---|---|
| Primary measurement | Echo strength arranged as a two-dimensional image. | Sound-travel time converted into depth soundings. |
| Typical installation | Towed fish with transmission cable and topside processing unit, or a hull-mounted unit. | Hull-mounted transducer array with navigation and motion-reference equipment. |
| Best visual clue | Bright return plus acoustic shadow, texture and outline. | Elevation, depth change, relief and a measurable seabed surface. |
| Depth capability | Most systems cannot provide depth information. | Designed to measure depth and calculate least depth when correctly calibrated and referenced. |
| Detail strategy | Higher frequencies such as 500 kHz–1 MHz give narrower, more detailed coverage; 50–100 kHz gives broader, lower-resolution coverage. | Detail depends on beam configuration, frequency, range, line spacing, vessel motion and processing; the image is generally less photo-like than high-frequency side scan. |
| Coverage | Two-sided strip whose useful range is selected to balance search width and target detail. | Continuous swath beneath the vessel; typical coverage is about two to four times water depth, according to NOAA. |
| Positioning | A hull-mounted unit is directly referenced to the vessel; a towed fish requires layback and underwater-position estimation. | Directly integrated with vessel navigation and motion data, allowing georeferenced depth surfaces. |
| Best role in wreck work | Detection, recognition and target prioritization. | Least-depth determination, dimensions, seabed modeling and chart-quality measurement. |
| Typical follow-up | Multibeam for depth; magnetometer, ROV or diver work for identity and condition. | Side scan or optical inspection when the bathymetric shape alone cannot establish identity or archaeological detail. |
A professional shipwreck-search workflow
1. Define the search question
Separate “Where is the target?” from “How deep is it?” and “What is it?” A search for a reported loss over a broad area needs efficient detection. A harbor-clearance project needs defensible least depth. An archaeological project may need image detail, accurate coordinates, a measured site plan and evidence of preservation state.
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2. Plan lines, range and altitude
Lay out parallel search lines that provide the desired side-scan overlap over the expected target area. Select a range that balances swath width with the size of the smallest feature you need to detect. Set towfish altitude to keep the target in a useful part of the sonar geometry; flying too high can reduce shadow clarity, while flying too low can narrow coverage and increase bottom-contact risk.
3. Run the side-scan reconnaissance
Record vessel navigation, towfish position estimates, altitude, heading, speed and environmental conditions with the sonar data. Review the imagery during acquisition for gaps, excessive noise, weak shadows or contacts near the edge of the range. Mark candidate targets with coordinates and an image interpretation, not just a brightness value.
4. Re-survey promising contacts
Run additional lines at a suitable heading and, where useful, at higher frequency or shorter range. A second look from another direction can show whether an apparent wreck shape persists or is an artifact of grazing angle, seabed texture or processing. This stage narrows the list before expensive high-precision mapping.
5. Map the target with multibeam
Use overlapping MBES lines to cover the wreck and a surrounding area of seabed. Process sound-speed, motion, navigation and vertical-reference information so the resulting surface is georeferenced. Extract dimensions, orientation, relief, scour and the shallowest measured point. The result is the basis for a clearance or site map, subject to the survey’s uncertainty and quality controls.
6. Add ground truth when identity or condition matters
Use a magnetometer when the question includes ferrous content or a buried steel target. Use an ROV or diver video when visual confirmation, artifact distribution, biological growth or archaeological condition matters. These observations answer questions sonar alone cannot settle, such as whether a low-relief contact is a wreck, natural rock, modern debris or a dispersed site.
When side scan should come first
- Large or uncertain search area: Side scan lets a survey team screen broad corridors and prioritize contacts before collecting dense bathymetry.
- Small or visually distinctive features: Masts, exposed frames, cargo, propellers and debris can produce recognizable returns and shadows even when a bathymetric bump is subtle.
- Deep targets: A towfish can be brought close to the bottom, improving image detail while the surface vessel remains above deep water.
- Archaeological reconnaissance: Image texture and shadow relationships help an interpreter distinguish a coherent hull from scattered material before selecting targets for detailed mapping.
Side scan is not a substitute for a depth survey. Treat its image as a detection and interpretation layer, then obtain multibeam or another appropriate depth measurement before making navigation or engineering decisions.
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When multibeam should come first
- Navigation safety: The priority is the shallowest obstruction and its clearance, not a visually persuasive image.
- Charting or engineering: A georeferenced surface is needed for dredging, route planning, cable work, mooring design or change detection.
- Known target with uncertain relief: If the wreck’s location is already established, MBES can efficiently quantify height, footprint, scour and surrounding slopes.
- Three-dimensional documentation: A bathymetric model supports measurements and repeat surveys even when the side-scan appearance changes with heading or processing.
MBES can identify a wreck in some cases, especially when its relief and dimensions are distinctive. Identification is less certain when the target is low, buried, heavily dispersed or similar in shape to natural seabed features. Side scan, magnetometer or optical inspection may still be needed.
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Deployment and data-quality pitfalls
Confusing brightness with identity
A bright side-scan return is not automatically a wreck. Rock, scrap, a pipeline or a hard seabed patch can produce strong echoes. Check the shadow, repeat the contact from another heading and compare the shape with the expected target description.
Assuming a side-scan image contains a reliable depth
Across-track distance in a side-scan image is not the same as a surveyed depth surface. Slant-range geometry, layback and seabed slope affect apparent position. Use MBES or another depth-capable survey method for least-depth decisions.
Flying the fish too high or too low
Excessive altitude can make a target small and shadows weak. Insufficient altitude can reduce coverage and increase the chance of striking the seabed. Plan altitude from the target’s expected height, the selected range and the site’s hazards, then monitor the towfish throughout the line.
Ignoring towfish layback
The vessel’s GPS position is not automatically the towfish’s position. Cable length, depth, current and turns can move the fish substantially behind the vessel. Record and model layback, and report the positioning uncertainty with the contact.
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Multibeam quality depends on accurate vessel position, heading, heave, pitch, roll, sound-speed information and vertical referencing. Poor calibration or inadequate overlap can create false ridges, gaps or depth bias. Quality-control the swaths before reporting a least depth.
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Choosing coverage over resolution without stating the trade-off
A broad, low-frequency side-scan pass is efficient for reconnaissance but may miss small details that a close, high-frequency pass would reveal. Conversely, a narrow, detailed survey takes more lines and time. State the target size, required resolution and search range when selecting settings.
How much seafloor can each system cover?
Side-scan coverage is set by the selected range on each side of the towfish or hull. Lower frequencies such as 50–100 kHz are used for broad, lower-resolution coverage; 500 kHz–1 MHz trades width for detail. Survey speed, line spacing and the need for overlap determine the area completed per day.
Multibeam coverage expands with water depth. NOAA gives a typical swath of two to four times water depth, but this should be treated as an operational guideline rather than a guaranteed usable width. A survey that needs complete, high-confidence coverage may use overlapping lines narrower than the theoretical maximum.
For context, NOAA’s Office of Coast Survey reports approximately 4,000 square nautical miles surveyed each year on its hydrographic-survey information page accessed in 2026. That figure describes the agency’s annual survey activity, not the area a particular vessel or sonar configuration will cover during a wreck search.
Choosing a system by project objective
| Your objective | Recommended plan | Why |
|---|---|---|
| Find a reported wreck whose position is uncertain | Wide-area side scan, followed by higher-detail side scan on contacts. | Maximizes the chance of detecting and recognizing the target before dense mapping. |
| Confirm whether a contact is a navigation hazard | Side scan for recognition, then multibeam for least depth and clearance. | Combines target evidence with a depth measurement. |
| Map a known wreck for an archaeological site plan | Multibeam for 3-D geometry plus high-detail side scan and optical inspection. | Provides measured relief, image texture and visual ground truth. |
| Test whether a buried steel object is present | Magnetometer with sonar, followed by targeted inspection. | Magnetic response addresses ferrous content that acoustic imagery alone cannot prove. |
| Document change over time | Repeat georeferenced multibeam, supplemented by repeat side scan when image comparison is useful. | Bathymetry supports quantitative elevation and scour comparisons. |
What equipment a serious survey requires
A professional side-scan installation consists of the towfish, transmission cable and topside processing unit, plus vessel navigation and a method for estimating the fish’s underwater position. A multibeam installation requires a hull-mounted transducer array integrated with vessel positioning, motion sensing, sound-speed measurements and data processing.
Equipment selection should follow the survey question rather than the label on a recreational fish finder. Wreck detection, least-depth measurement and chart-quality mapping require specialist sensors, acquisition settings, positioning and quality control. If you do not have a suitable survey vessel or experienced crew, a hydrographic contractor is the practical route; scope the work around target depth, search area, required resolution, vertical datum, deliverables and any ROV, diver or magnetometer inspection.
A field checklist before launching the survey
- Define whether the deliverable is a detection map, an identification assessment, a least-depth value, a 3-D model or all of these.
- Estimate target length, height, burial, expected seabed type and water depth.
- Choose side-scan frequency and range for the smallest feature that must be detected.
- Set towfish altitude and line spacing, including overlap and a plan for turns.
- Confirm navigation, towfish layback, vessel motion, sound-speed and vertical-reference procedures.
- Prepare a contact log with coordinates, image quality, shadow interpretation and confidence level.
- Reserve follow-up lines for alternate headings and higher-detail settings.
- Plan MBES coverage around each contact, including the surrounding seabed and sufficient overlap.
- Decide in advance whether magnetometer, ROV or diver observations are needed for identity, ferrous content or condition.
- Report uncertainty and limitations with every position, dimension and depth result.
Bottom line for wreck hunters
Side-scan sonar is usually the fastest and clearest way to locate and recognize a shipwreck. Multibeam sonar is the instrument to use when you must know exactly how deep it is, how high it stands above the seabed and where its measurable boundaries lie. The strongest survey design treats them as complementary: scan with side scan, verify and measure with multibeam, then use magnetometer or ROV/diver observations when acoustic data cannot establish identity or condition.
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