Multibeam Acquisition and Quality Control: A Practical Offshore Guide

# Multibeam Acquisition and Quality Control: A Practical Offshore Guide

A multibeam echo sounder can collect millions of depth measurements, but a large volume of data does not automatically mean reliable bathymetry.

Good multibeam data depends on the complete measurement chain: sonar configuration, vessel position, motion, heading, sound velocity, offsets, timing, water level and online quality control. A problem in any one of these inputs can introduce systematic errors across the entire survey.

The objective during acquisition is not simply to achieve full seabed coverage. It is to collect data that is accurate, repeatable, traceable and suitable for the required deliverable.

Understanding the Multibeam Measurement Chain

A typical multibeam bathymetric solution uses:

1. GNSS position 2. Vessel heading 3. Roll, pitch and heave 4. Multibeam transmit and receive timing 5. Transducer draft 6. Sensor lever-arm offsets 7. Installation and patch-test corrections 8. Surface sound velocity 9. Water-column sound velocity profile 10. Tide or vertical reference 11. Sonar detection settings 12. Acquisition and processing software

The final seabed position of every sounding depends on the correct combination of these inputs.

A sonar display may look clean while the derived positions are wrong. Online quality control must therefore examine both the acoustic data and all supporting sensors.

1. Survey Planning Before Acquisition

Reliable acquisition begins before the vessel starts the first line.

Review:

* Required survey standard * Expected water depth * Seabed type and slope * Required grid resolution * Minimum overlap * Maximum acceptable uncertainty * Vessel speed * Sonar frequency * Expected range scale * Line direction * Crossline plan * Sound velocity strategy * Tide or vertical referencing method * Data-storage requirements

The line plan should consider more than theoretical swath width.

A simplified estimate of total swath width is:

**Swath width ≈ 2 × water depth × tan(half-swath angle)**

This is only a geometric estimate. The usable swath may be smaller because of seabed slope, noise, vessel motion, refraction, weak outer-beam detections or project uncertainty limits.

Line spacing should be based on the expected **usable** swath—not the widest possible display setting.

2. Verify Sensor Offsets and Reference Points

Every sensor must be correctly related to a common vessel reference frame.

Confirm the measured position of:

* Primary GNSS antenna * Secondary GNSS antenna * Motion reference unit * Gyro or heading sensor * Multibeam transducer * Surface sound velocity sensor * Vessel reference point * Waterline reference

Check:

* X, Y and Z values * Axis directions * Positive and negative signs * Units * Reference origin * Sensor orientation * Whether an offset is already applied elsewhere

An offset entered with the wrong sign can create position and depth errors that change with vessel heading or attitude.

Compare the online configuration with the approved dimensional-control report or vessel-offset survey. Do not rely only on an old software configuration file.

3. Confirm Time Synchronization

Position, motion, heading and sonar data must refer to the correct time.

Check:

* GNSS time source * Pulse-per-second input * Network time synchronization * Motion-sensor latency * Heading latency * Sonar timestamps * Acquisition computer time * Input telegram delays

Latency errors can produce:

* Along-track displacement * Wavy seabed features * Target duplication * Misalignment between reciprocal lines * Errors that increase with vessel speed * Differences between port and starboard observations

If an object appears in different along-track positions when surveyed in opposite directions, timing and latency should be investigated.

4. Review the Patch Test

A patch test estimates the angular relationship between the multibeam transducer and the vessel reference sensors.

The main parameters are:

* Latency * Roll * Pitch * Heading or yaw

Review:

* Patch-test date * Test location * Sea conditions * Water depth * Seabed suitability * Vessel speed * Selected lines * Processing method * Calculated corrections * Final residuals

A previous patch test may no longer be valid if:

* The transducer was removed or reinstalled * The motion sensor moved * The GNSS antennas changed * The vessel structure was modified * The pole or gondola moved * A different navigation system is being used * Sensor offsets were updated

Incorrect patch-test values often create systematic differences between overlapping or reciprocal lines.

5. Check Surface Sound Velocity

The surface sound velocity sensor is normally used for beam steering at the transducer.

Its reading should be:

* Stable * Realistic * Free from air bubbles * Representative of the water around the transducer * Updated at the correct rate

A poor surface sound velocity value can distort beam angles and create errors across the swath.

Sudden changes may indicate:

* Air bubbles * Poor sensor location * Electrical noise * Contamination * Loss of water flow * Incorrect sensor communication

Compare the live reading with the upper part of the latest sound velocity profile. A significant unexplained difference should be investigated.

6. Collect Representative Sound Velocity Profiles

The sound velocity profile describes how the speed of sound changes through the water column.

An incorrect or outdated profile may create:

* Smiling swaths * Frowning swaths * Port/starboard mismatch * Outer-beam depth errors * Disagreement between adjacent lines * Errors that increase with swath angle

Profiles should be collected:

* Before starting the survey * When moving into a different water mass * After major weather or tidal changes * When overlap differences increase * When unexplained refraction patterns appear * At the frequency required by the project procedure

Record:

* Time * Position * Maximum profile depth * Instrument serial number * Calibration status * Raw profile * Edited profile * Profile applied to the data

Do not select a profile simply because it makes the data look better. The profile must be representative of actual environmental conditions.

7. Configure the Sonar for the Environment

Sonar settings should be selected according to water depth, seabed reflectivity, vessel speed and survey objectives.

Monitor:

* Frequency * Ping rate * Range scale * Pulse length * Transmit power * Receiver gain * Beam mode * Swath angle * Bottom-detection mode * Water-column settings * Absorption and spreading parameters

### Range scale

The range should cover the seabed comfortably without being unnecessarily large.

An excessive range may reduce ping rate and data density. A range that is too short may cause bottom loss.

### Swath angle

A wider swath increases coverage but may reduce outer-beam quality.

Outer beams travel farther through the water and strike the seabed at more oblique angles. They are therefore more sensitive to:

* Sound velocity errors * Vessel motion * Weak returns * Refraction * Multipath * Seabed slope

Use the widest swath that still meets the required quality—not simply the maximum available angle.

### Pulse length

Short pulses can improve range resolution in shallow water. Longer pulses may provide more energy at greater range but reduce resolution.

### Gain and power

Too little gain may cause bottom loss. Excessive gain may increase noise and false detections.

Automatic modes can be useful, but the operator must still monitor their behaviour.

8. Control Vessel Speed and Line Keeping

Vessel speed affects:

* Along-track sounding density * Ping spacing * Motion * Aeration * Acoustic noise * Ability to remain on line

The selected speed should provide adequate along-track coverage and allow the vessel to maintain stable heading.

Monitor:

* Speed over ground * Cross-track error * Heading deviation * Rate of turn * Roll, pitch and heave * Thruster use * Propeller noise * Bubble sweep

Aggressive steering can degrade data quality, particularly in the outer beams.

If possible, stabilize the vessel before entering the required acquisition line. Turns and run-ins should provide enough time for heading, motion and sonar settings to settle.

9. Perform Continuous Online Quality Control

Online QC should continue throughout every survey line.

### Position

Check:

* GNSS solution type * Correction status * Satellite count and geometry * Age of corrections * Primary and secondary position agreement * Position alarms * Data gaps

### Heading and motion

Check:

* Heading stability * Roll, pitch and heave status * Sensor warnings * Update rate * Delayed or frozen values * Unexpected spikes * Agreement with secondary sensors

### Sonar

Check:

* Bottom detection * Swath shape * Outer-beam quality * Noise * Data density * Ping rate * Range * Port/starboard balance * Water-column interference * Missing sectors

### Coverage

Check:

* Line keeping * Swath overlap * Holidays * Gaps near slopes * Coverage around structures * Required feature detection

### Environmental inputs

Check:

* Surface sound velocity * Applied sound velocity profile * Draft * Tide or vertical correction * Sea state * Squat or settlement where applicable

Important alarms should be visible and audible where practical.

10. Recognize Common Multibeam Problems

### Smile or frown across the swath

The outer beams are consistently shallower or deeper than the centre beams.

Possible causes:

* Incorrect sound velocity profile * Incorrect surface sound velocity * Refraction * Roll bias * Excessive swath angle

### Port and starboard mismatch

One side of the swath does not agree with the other.

Possible causes:

* Roll error * Sound velocity error * Transducer installation problem * Unequal noise or obstruction * Incorrect angular offsets

### Wavy seabed

A flat seabed appears to rise and fall.

Possible causes:

* Heave error * Delayed motion data * Poor motion-sensor performance * Vessel vibration * Timing problems * Incorrect filter settings

### Parallel-line disagreement

Adjacent lines do not match vertically or horizontally.

Possible causes:

* Tide error * Vertical-reference problem * Sound velocity error * Positioning error * Heading bias * Patch-test error * Incorrect draft

### Noisy outer beams

The outer part of the swath contains scattered or unreliable detections.

Possible causes:

* Excessive swath angle * Weak seabed return * High vessel speed * Rough sea state * Incorrect gain or power * Sound velocity error * Acoustic interference

### Gaps or holidays

Areas of seabed are not covered sufficiently.

Possible causes:

* Incorrect line spacing * Poor line keeping * Reduced usable swath * Unexpected depth change * Seabed slope * Bottom loss * Data dropout

11. Use Overlap as a Quality Indicator

Adjacent swaths provide one of the most useful checks during acquisition.

Compare:

* Port-to-starboard overlap * Reciprocal lines * Main lines and crosslines * Repeated features * Flat seabed areas * Sloping seabed areas

A consistent difference may indicate a systematic problem. Random differences may indicate noise or unstable sensors.

Do not look only at the colour display. Use:

* Difference surfaces * Cross-sections * Profiles * Statistics * Uncertainty information * Sounding density

A visually smooth surface can still contain systematic errors.

12. Run Crosslines

Crosslines provide an independent comparison with the main survey lines.

They should:

* Cross the main lines at useful angles * Cover representative depths and seabed types * Be distributed across the survey area * Be acquired under controlled conditions * Use the same validated configuration

Crossline analysis can reveal:

* Vertical bias * Positioning errors * Sound velocity problems * Tide errors * Patch-test problems * Localized sensor instability

Any disagreement should be investigated before demobilization.

13. Monitor Data Density and Feature Detection

Full coverage does not automatically mean adequate data density.

Sounding density depends on:

* Ping rate * Vessel speed * Number of beams * Water depth * Swath width * Beam distribution * Sector settings * Detection quality

Check whether the collected data can reliably identify the features required by the project.

Increasing vessel speed or swath width may improve productivity but reduce effective data quality.

14. Review Backscatter and Water-Column Data

When required, backscatter quality should be monitored during acquisition.

Check:

* Saturation * Gain changes * Uneven intensity * Striping * Nadir effects * Port/starboard imbalance * Missing data

For water-column data, confirm:

* Recording is enabled * File size is manageable * Storage capacity is sufficient * Required targets are visible * Time and position data are correct

A configuration that is suitable for bathymetry may not automatically be suitable for backscatter or water-column deliverables.

15. Protect and Document the Raw Data

Raw data is the primary survey record.

Confirm:

* Correct file naming * Survey line names * File rollover * Time continuity * Disk capacity * Backup location * File integrity * Event logging

Maintain at least one verified backup as required by the project procedure.

The survey log should record:

* Line start and end times * Configuration changes * Sound velocity casts * Sensor alarms * Data gaps * Rejected lines * Weather and sea state * Equipment problems * Re-runs * Operator comments

A change that is not documented may be difficult to explain during processing or final reporting.

Recommended Online QC Workflow

A practical sequence is:

1. Confirm all sensors are online and time-synchronized. 2. Verify offsets, draft and patch-test values. 3. Confirm the correct coordinate and vertical reference systems. 4. Review the latest sound velocity profile. 5. Select suitable range, swath, frequency and pulse settings. 6. Stabilize the vessel before the line. 7. Monitor position, heading, motion and surface sound velocity. 8. Check bottom detection and outer-beam quality. 9. Confirm overlap and coverage. 10. Compare adjacent lines and crosslines. 11. Record all changes and operational events. 12. Back up the raw data and confirm file integrity.

When Should a Line Be Repeated?

A line may require reacquisition when:

* Coverage requirements are not achieved * Significant data gaps are present * Position or motion data are invalid * Sound velocity corrections are unreliable * Outer-beam quality falls outside project requirements * The vessel deviates excessively from the planned line * Sensor timing is incorrect * Crossline or overlap checks reveal unexplained differences * Raw data is corrupted or incomplete

The decision should be based on project specifications and technical evidence—not only visual appearance.

Final Thoughts

Good multibeam acquisition is not achieved by maximizing swath width or collecting the greatest possible number of soundings.

It is achieved by understanding and controlling the full measurement chain.

Reliable online QC requires the surveyor to monitor:

* Position * Heading * Motion * Sound velocity * Timing * Offsets * Sonar performance * Coverage * Vertical referencing * Data integrity

The most important principle is simple:

**Do not wait until processing to discover a problem that could have been identified offshore.**

Data should be checked continuously, problems should be investigated systematically, and every important decision should be recorded.