USV Multibeam and Side-Scan for Shallow-Water Survey

Shallow-water hydrographic survey has traditionally been constrained by vessel draft, mobilisation cost, and crew exposure. Uncrewed surface vessels (USVs) have changed that constraint. A small USV can operate in water depths of a few metres, transit narrow channels, and repeat the same survey lines frequently enough to detect change. When a USV carries both a multibeam echosounder and a side-scan sonar, the resulting dataset combines quantitative depth with high-resolution seabed imagery, and the two sensors compensate for each other's weaknesses.

Why Shallow Water Is Difficult

Shallow water is acoustically and operationally hostile. The acoustic footprint of a multibeam beam is small, but vessel motion, surface reflections, and rapid sound speed variation near the surface degrade bottom detection. In very shallow water, the swath width shrinks with depth, so line spacing must be tight and coverage rates fall. Tidal variation changes water level faster than the survey can be reduced to datum unless water level is logged continuously.

Shallow nearshore areas also contain the features that matter most to engineering: scour around structures, debris, rock outcrops, pipeline crossings, and dredged channel edges. A bathymetric surface alone may not resolve these features clearly. A side-scan sonar, operating at higher frequency and looking outward at a shallow grazing angle, produces a high-resolution acoustic image that reveals texture, objects, and seabed changes that a depth surface can only imply.

Multibeam echosounders provide quantitative depth and bathymetric surface geometry, while side-scan sonars provide high-resolution acoustic imagery of seabed texture and discrete objects. The two are complementary because they measure different physical quantities from the same acoustic returns.

USV Platform Considerations for Shallow Water

USVs used for shallow-water hydrography are typically small, lightweight, and either catamaran or monohull in configuration. Catamaran hulls offer greater stability and can carry a pole-mounted sonar between the hulls, clear of the bow wave. Monohull designs are simpler to transport and launch but roll more in surface chop.

Draft: Determines the minimum surveyable depth and the ability to work near shorelines and quay walls.
Payload capacity: Limits the sensor suite, battery or fuel load, and communication equipment carried.
Propulsion: Electric thrusters are quiet and reduce acoustic noise, which improves sonar performance in shallow water.
Stability: Affects motion sensor compensation and the quality of outer multibeam beams.
Autonomy level: Ranges from remote control to fully autonomous mission execution with predefined survey plans.
Communication: Radio line of sight, cellular, or satellite links determine range and supervision model.

Sensor mounting is critical on small platforms. The multibeam transducer should be mounted with a clear acoustic path and minimum bubble interference. Sound velocity and motion sensors must be positioned to represent the acoustic path, not simply wherever space allows.

Multibeam Integration on a USV

Multibeam systems for USV deployment typically operate at frequencies from 200 kHz to 700 kHz. Higher frequencies provide narrower beams and better resolution in very shallow water, at the cost of shorter range. Beam steering requires accurate attitude, heading, and sound velocity at the transducer face.

Because a USV is smaller and lighter than a crewed survey vessel, it responds more quickly to surface waves. Heave, pitch, and roll amplitudes may be larger relative to the platform size, so motion sensor quality and latency matter more, not less. The inertial measurement unit should be rigidly mounted and aligned with the multibeam reference frame through a patch test or equivalent calibration procedure.

Sound velocity management is the same in principle as on a crewed vessel but more logistically demanding. A USV cannot easily stop and lower a CTD cast at every change of water mass. Practical solutions include:

Using a surface sound velocity probe at the transducer and applying a bar-check or reference cast at the start of each mission.
Deploying a separate small vessel or a crewed support boat to take periodic profiles while the USV continues surveying.
Using an underway sound velocity profiler towed behind the USV or mounted on a drop keel.
Applying a seasonal or regional sound speed model only where it has been validated against local casts.

Sound velocity error is one of the largest systematic error sources in shallow-water multibeam survey. An incorrect profile bends outer beams and produces depth errors that grow with beam angle, often appearing as a smile or frown pattern across adjacent survey lines.

Side-Scan Sonar Integration

Side-scan sonar transmits two fan-shaped acoustic pulses outward from a towfish or hull-mounted unit and records the returning energy as a function of time. The result is an image of seabed reflectivity. Hard surfaces such as rock, metal, and compacted sediment appear strong; soft mud and sand appear weak. Objects such as pipelines, cables, debris, and boulders produce acoustic shadows that reveal their height and shape.

On a USV, side-scan sonar can be hull-mounted or towed. Hull mounting is simpler and avoids towfish handling, but places the sonar close to the surface and reduces the grazing angle. Towed configurations provide a more stable platform and better altitude control over uneven seabed, but require launch and recovery procedures that may complicate autonomous operation.

Key operational parameters include:

Frequency: Higher frequencies (for example 900 kHz to 1.2 MHz) give finer resolution but shorter range; lower frequencies (for example 400 kHz to 600 kHz) give longer range but coarser resolution.
Range setting: Determined by the required along-track resolution and seabed conditions. Long ranges produce narrow grazing angles and stronger shadows but weaker returns.
Altitude: The height of the sonar above the seabed controls the across-track coverage and the quality of object detection.
Line spacing: Should ensure overlapping coverage, typically 30 to 50 percent, to avoid gaps and to allow cross-checking between adjacent lines.

Side-scan sonar does not measure depth directly. Its value is in object detection, seabed classification, and verification of features identified in the multibeam surface. Where both sensors see the same object, the multibeam provides position and depth, and the side-scan provides shape, extent, and material indication.

Positioning, Motion, and Timing on a USV

A USV requires the same positioning discipline as a crewed vessel, but with additional constraints. Real-time kinematic GNSS provides horizontal accuracy, but the antenna position must be corrected for lever arm offsets to the multibeam, side-scan, and motion sensor. Vertical accuracy from GNSS alone is generally insufficient for hydrographic survey, so water level must be applied from a tide gauge, a GNSS buoy, or an ellipsoidal-to-orthometric conversion using a geoid model.

Motion compensation is more demanding on a small platform. The motion reference unit should be mounted close to the sonar heads and calibrated for alignment. Any latency between motion data and sonar transmission must be measured and compensated, otherwise the outer beams are mis-positioned by an amount proportional to vessel speed and motion amplitude.

Timing synchronisation across GNSS, motion, multibeam, and side-scan is essential. A USV typically logs these streams through a single acquisition system so that all data share a common time base. Post-processing then applies trajectory smoothing and reduces the data to the project datum.

Data Integration and Deliverables

Multibeam and side-scan data are processed separately and then integrated. The multibeam dataset produces a bathymetric surface, which is cleaned, corrected for refraction, and reduced to datum. The side-scan dataset produces a geo-referenced mosaic, which is corrected for slant range, layback, and vessel track. The two products are then co-registered for interpretation.

Typical deliverables for a shallow-water USV survey include:

Bathymetric surface in the project coordinate reference frame and vertical datum.
Side-scan mosaic showing seabed texture and objects.
Contact list with positions, dimensions, and interpreted characteristics.
Survey line coverage and overlap maps.
Accuracy statement with vertical and horizontal uncertainty reported against the applicable survey order or project specification.
Processing and calibration record, including sound velocity profiles, patch test results, and tide corrections.

Optimization Strategies and Common Pitfalls

Good practice: Plan line spacing from the shallowest depth in the survey area, not the average depth, so that coverage remains complete across the entire site.
Good practice: Deploy the sound velocity probe at the transducer and take a reference cast at the beginning of each mission and after any major water mass change.
Good practice: Configure the side-scan range setting based on survey objectives: long range for reconnaissance, short range for high-resolution object detection.
Good practice: Overlap side-scan lines by 30 to 50 percent to eliminate gaps and enable cross-checking.
Good practice: Perform patch test and motion latency checks after any sensor mounting change.
Good practice: Reduce bathymetry to datum using measured water levels rather than assumed or historical tide values.
Poor practice: Mounting the multibeam transducer near thrusters, wake zones, or air entrainment paths.
Poor practice: Using a single sound velocity profile for the entire survey area in water with strong spatial variability.
Poor practice: Presenting side-scan imagery without slant range correction or correct layback, producing geometric distortion.
Poor practice: Relying on GNSS ellipsoidal height alone as a vertical reference for bathymetric reduction.
Poor practice: Assuming the USV is immune to motion effects simply because it is uncrewed. Small platforms react more, not less, to surface conditions.
Poor practice: Treating side-scan targets as confirmed without cross-verification from the multibeam surface, diver inspection, or ROV imagery where required.

Practical Implications for Geomak Projects

In the Arabian Gulf, shallow nearshore areas, dredged channels, and port approach zones require repeated, high-density survey that crewed vessels cannot always deliver efficiently. USV-based multibeam and side-scan surveying supports channel monitoring, pre-dredge and post-dredge surveys, pipeline route clearance, berth inspection, and pre-construction seabed mapping in areas where access is difficult.

Geomak applies USV hydrographic survey within a controlled geodetic framework, with sound velocity management, motion compensation, tidal reduction, and integrated multibeam and side-scan deliverables. The value of the approach lies not only in the platform, but in the integration of sensors and the documentation that allows the resulting data to support dredging control, engineering design, and regulatory reporting with confidence.