Side-Scan Sonar Interpretation: A Practical Guide to Reading the Seabed

Side-Scan Sonar Interpretation: A Practical Guide to Reading the Seabed

Side-scan sonar is one of the most effective tools for imaging the seabed. It is widely used for route surveys, debris searches, pipeline and cable inspections, wreck investigations, environmental mapping and offshore construction support.

However, a side-scan image is not a photograph. It is a record of acoustic backscatter arranged by time and range. Brightness does not directly represent colour or depth, and a dark area is not automatically a depression. Correct interpretation requires an understanding of sonar geometry, seabed material, target orientation, acoustic shadows, towfish behaviour and acquisition quality.

This article explains how side-scan images are formed, how to recognise common seabed features and targets, which artefacts can lead to incorrect conclusions and which checks should be performed before reporting an interpretation.

How side-scan sonar creates an image

A side-scan sonar transmits short acoustic pulses to port and starboard. The pulses travel through the water, strike the seabed or an object, and return energy to the sonar receiver.

The display normally places the sonar track or nadir near the centre. Successive pings are added line by line to create a scrolling image called a waterfall. The horizontal direction represents slant range away from the towfish, while the along-track direction represents the sequence of pings as the platform moves.

The strength of each echo is converted into image intensity:

Strong returns are displayed as bright tones.

Weak returns appear darker.

Areas receiving no direct acoustic energy form shadows.

The final appearance is influenced by transmitted frequency, pulse length, gain, range, towfish altitude, grazing angle, seabed roughness, material, target shape and the selected display palette.

The main parts of a sonar record

Water column

The area between the towfish and the first seabed return appears near the centre of an uncorrected waterfall. It is often called the water column.

Its width changes with towfish altitude. A sudden increase may indicate that the towfish has risen, while a decrease may show that it is closer to the seabed. Changes can also result from bottom-tracking errors or steep seabed slopes.

First bottom return

The first bottom return marks the nearest seabed insonified on each side. It often appears as a relatively strong boundary between the water column and seabed data.

Seabed backscatter

Outside the first bottom return, the image shows acoustic energy returned from the seabed. Its tone and texture may indicate changes in roughness, sediment, bedforms or human-made disturbance.

Acoustic shadow

An object raised above the seabed blocks acoustic energy from reaching the area behind it. This creates a dark shadow extending away from the sonar.

Shadows are extremely valuable because they reveal shape, orientation and approximate height. In many cases, the shadow is easier to interpret than the bright target return.

Brightness is not the same as hardness

Hard or rough seabed commonly returns more energy than soft, smooth sediment, but image brightness is controlled by more than material.

A bright patch may result from:

A rough surface

A hard material

A slope facing the sonar

A target oriented favourably to the acoustic beam

Increased gain

Reduced towfish altitude

A change in incidence angle

A dark patch may represent soft sediment, a smooth surface, a slope facing away from the sonar, an acoustic shadow or reduced sensitivity at long range.

For this reason, sediment classification should not be based on tone alone. Texture, boundaries, morphology, regional geology and supporting samples or other sensors should also be considered.

Understanding acoustic shadows

When an acoustic pulse strikes a raised target, the facing surface can return a bright echo. The area behind the target is not insonified and appears dark.

The shadow direction should extend away from the nadir or sonar track. If the apparent shadow points in an impossible direction, the interpreter should check image orientation, channel reversal or whether the feature is actually a shadow.

Shadow length depends on:

Target height

Towfish altitude

Horizontal range from the sonar

Seabed slope

Target geometry

Under suitable geometry, target height can be estimated using similar triangles:

Target height ≈ Towfish altitude × Shadow length ÷ (Ground range to target + Shadow length)

The exact implementation depends on the software and whether measurements use slant range or ground range. Height estimates should therefore be treated as calculated interpretations, not direct observations. Confirm the software convention and use corrected geometry.

Slant range and ground range

The sonar directly measures slant range: the straight-line acoustic distance from the towfish to a target. The useful horizontal seabed distance is ground range.

If towfish altitude is known:

Ground range = √(Slant range² − Altitude²)

Without slant-range correction, objects near nadir are displaced and compressed in the across-track direction. Many processing packages remove the water column and remap samples into estimated ground-range positions. The result is easier to view as a map, but its quality depends on accurate altitude and bottom tracking.

Frequency, range and image detail

Higher frequencies generally provide finer resolution but shorter usable range. Lower frequencies travel farther but normally provide less detail.

The selected range affects both coverage and pixel representation. An unnecessarily long range can reduce effective target detail and increase the importance of outer-range attenuation. A short range improves detail but requires tighter line spacing and more survey time.

Good interpretation begins with suitable acquisition settings. Processing cannot recover target detail that was never resolved during acquisition.

Recognising common seabed textures

Smooth sediment

Fine, relatively flat sediment often appears uniform and low in texture. Its tone can vary with grazing angle and gain. Bioturbation, small depressions or scattered shells may introduce a mottled pattern.

Coarse or rough seabed

Gravel, cobbles and rock can produce strong, irregular returns with many small shadows. The image may appear speckled or highly textured.

Sand ripples and dunes

Regular bedforms often appear as repeating bright-and-dark bands. The bright side faces the sonar, while the opposite slope is darker. Pattern direction, wavelength and continuity help distinguish natural bedforms from acquisition artefacts.

Rock outcrop

Rock may show strong returns, irregular boundaries and complex shadows. Exposed ridges can create elongated patterns that change significantly when insonified from the opposite direction.

Scour

Scour around a structure, pipeline or debris may appear as a combination of slope returns, dark areas and displaced sediment. Interpretation should consider sonar direction because the appearance can reverse between opposing lines.

Trawl marks and drag scars

These often appear as narrow, linear or curving features that may occur in parallel groups. Continuity and regional pattern help separate them from cables or pipelines.

Pockmarks and depressions

A depression can produce a dark near-facing slope and a brighter far-facing slope—the reverse of a raised object. This relationship is essential when distinguishing holes from targets.

Interpreting common targets

Pipelines and cables

An exposed pipeline may appear as a bright linear return with a continuous shadow. A partially buried line may alternate between visible and low-contrast sections. A fully buried cable may not be detected directly, although burial-related disturbance can sometimes be visible.

Interpretation should record continuity, orientation, apparent exposure, shadow, crossings and associated scour. Side-scan sonar alone should not be used to determine reliable burial depth.

Debris

Debris may produce an isolated bright return and shadow. Shape, dimensions, context and repeated observation from another direction help determine whether it is human-made or natural.

Wrecks

A wreck commonly produces complex, high-intensity returns and multiple shadows. Orientation and overall dimensions can often be identified, but structure interpretation may change greatly with sonar aspect.

Anchors, chains and wire

An anchor may generate a compact return with a characteristic shadow, while chain or wire may appear as a thin linear feature. Detection depends strongly on resolution, seabed contrast and orientation.

Subsea structures

Manifolds, mattresses, frames and protective structures can generate strong returns with geometric shadows. Compare the observed orientation and dimensions with approved drawings, while recognising that drawings and installed coordinates also contain uncertainty.

Biological and natural features

Vegetation, shell beds, coral or other biological communities can create distinctive textures. Interpretation should remain conservative unless supported by ground truth such as video, sampling or environmental data.

Why viewing direction matters

A target can look very different when insonified from the opposite side. One direction may produce a strong return and long, clear shadow, while the reciprocal direction may hide important geometry.

Whenever a target is important, use at least two useful viewing aspects where practical. A second line can help confirm:

Whether the feature is raised or depressed

Target orientation

Shadow geometry

Dimensions

Whether the first detection was an artefact

Position consistency

One attractive image is not enough for a confident interpretation.

Common side-scan artefacts

Artefact or problem

Typical appearance

Main checks

Towfish roll

Alternating port/starboard brightness or striping

Review sea state, roll data and towing stability

Towfish yaw

Distorted or curved targets and uneven along-track sampling

Check tow stability, speed and cable behaviour

Heave or altitude change

Changing water-column width and image scale

Review altitude, depth and seabed profile

Cable strum

Repeating bands or noise patterns

Inspect tow speed, cable tension and sea conditions

Acoustic interference

Regular lines, spikes or repeated noise

Compare with other acoustic equipment and ping timing

Multipath

Duplicate or displaced returns

Review geometry, water depth and surface/bottom reflections

Poor bottom tracking

Incorrect water-column removal or stretched seabed

Inspect raw data and manually correct bottom picks

Gain saturation

Large areas clipped bright with lost texture

Reduce gain or apply controlled processing

Low gain

Weak targets and missing seabed detail

Check range-dependent gain and raw amplitude

Navigation jumps

Mosaicked features displaced or broken

Review GNSS, layback and time synchronization

Incorrect layback

Targets shifted along the survey direction

Verify towfish model, cable out and sensor offsets

Channel reversal

Port and starboard displayed on wrong sides

Confirm system configuration using known geometry

Towfish altitude and towing geometry

Towfish altitude controls grazing angle, shadow development and useful coverage. If the fish is too high, small targets may be poorly resolved. If it is too low, the usable swath narrows and collision risk increases.

A commonly used starting point is to fly the towfish at roughly 10–20% of the selected range, but this is not a universal rule. The correct altitude depends on target size, frequency, seabed relief, current, vessel control, towfish design and project requirements.

The survey team should monitor:

Towfish depth

Altitude

Cable out

Vessel speed

Towfish position or layback

Cable tension

Water depth

Seabed clearance

Any operational limit and safe towing procedure supplied by the equipment owner and project should take priority.

Positioning the towfish

The vessel GNSS position is not the same as the towfish position. A layback solution is required for a towed system.

Methods may include:

Simple cable-out and depth calculation

Catenary-based layback

Acoustic tracking

Combined navigation and cable model

Position uncertainty generally increases with cable length, current, vessel turns and uncertain towing geometry. Target coordinates should be reported with accuracy appropriate to the positioning method.

During turns, the towfish does not immediately follow the vessel track. Data collected during unstable turns should normally be treated with caution or excluded according to the survey procedure.

A systematic interpretation workflow

1. Verify data quality

Before interpreting targets, confirm that navigation, altitude, bottom tracking, range, gain and towfish stability are acceptable. Identify periods affected by turns, dropouts or interference.

2. Review raw waterfall data

Processed mosaics are useful for regional context, but raw waterfalls preserve important shadows and original geometry. Review both.

3. Understand orientation

Confirm the direction of travel, port and starboard channels, nadir location and whether the image has been reversed or slant-range corrected.

4. Inspect at several scales

Use a wide view to recognise geological patterns and a closer view for targets. Excessive zoom can remove context and encourage false identification.

5. Identify return-and-shadow relationships

Determine which bright return belongs to which shadow. Check that the geometry is consistent with sonar direction.

6. Measure carefully

Measure target length, width, shadow and orientation using corrected data where possible. Record whether each dimension is observed directly or calculated.

7. Compare adjacent and reciprocal lines

Confirm that the feature repeats in the expected location. Differences may reveal a positioning problem, processing error or direction-dependent appearance.

8. Correlate other sensors

Compare side-scan findings with multibeam bathymetry, magnetometer data, sub-bottom profiles, ROV video, previous surveys and asset drawings.

9. Assign interpretation confidence

Use clear categories such as:

Confirmed: Supported by multiple observations or independent evidence.

Probable: Strong acoustic evidence but no independent confirmation.

Possible: A valid anomaly with limited defining characteristics.

Uncertain: Insufficient quality or conflicting evidence.

10. Record an audit trail

Save the source line, time, channel, range, screenshot, coordinates, dimensions, confidence and interpreter comments for every reported target.

Checks before reporting a target

Ask the following questions:

Is the feature present in the raw data?

Is the port/starboard orientation correct?

Does it have a physically consistent shadow?

Could it be a geological or biological feature?

Could it be an acquisition or processing artefact?

Is it visible on an adjacent or reciprocal line?

Is its position consistent between passes?

Are its measured dimensions based on corrected geometry?

Does another sensor support the interpretation?

Is the assigned confidence level justified?

Common interpretation mistakes

Treating the sonar image as a photograph.

Assuming every bright return is a hard object.

Assuming every dark area is a depression.

Ignoring the direction of insonification.

Measuring targets on uncorrected slant-range imagery without accounting for distortion.

Interpreting a mosaic without reviewing raw waterfall data.

Using excessive gain that removes texture and shape.

Reporting vessel coordinates instead of towfish-corrected coordinates.

Giving false precision to poorly positioned targets.

Classifying seabed type without ground truth.

Deleting weak anomalies before completing a structured review.

Reporting an important target from only one viewing direction.

Reporting recommendations

A clear target record should include:

Unique target identification

Easting and northing

Datum and projection

Source survey line and time

Port or starboard channel

Target length, width and estimated height

Orientation

Description of return and shadow

Interpretation

Confidence level

Supporting datasets

Image extract with scale and orientation

Positioning and measurement limitations

The report should separate observation from interpretation. For example, “isolated high-amplitude return with a 3 m shadow” is an observation; “probable metallic debris” is an interpretation.

Conclusion

Successful side-scan sonar interpretation depends on understanding how the image was formed. Brightness, texture and shadow must be considered together with towfish altitude, sonar range, viewing direction, seabed slope and positioning quality.

The best interpreters do not rely on pattern recognition alone. They verify acquisition quality, work from raw data, examine targets from more than one direction, correlate other sensors and state uncertainty honestly.

When these checks are followed, side-scan sonar becomes more than an attractive seabed image. It becomes a defensible dataset for identifying hazards, mapping seabed conditions and supporting safe offshore decisions.

Suggested excerpt: Side-scan sonar imagery is not a photograph. Learn how to interpret backscatter, seabed texture, target highlights and acoustic shadows while avoiding common acquisition and processing artefacts.