# Multibeam vs. Side-Scan Sonar: What Is the Difference?

> Multibeam sonar measures the depth and three-dimensional form of the seafloor across a wide swath. Side-scan sonar records detailed acoustic imagery that is especially useful for finding objects and changes in bottom texture. Both transmit sound sideways beneath a survey vessel, but...

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Byline: ARGO.net Editorial Team
Published: 2026-08-26T14:09:13+00:00
Categories: Explainer, Oceans

![Research_vessel_mapping_the_seafloor](https://www.argo.net/wp-content/uploads/2026/08/research_vessel_mapping_the_seafloor.jpg)

**Multibeam sonar** measures the depth and three-dimensional form of the seafloor across a wide swath. **Side-scan sonar** records detailed acoustic imagery that is especially useful for finding objects and changes in bottom texture. Both transmit sound sideways beneath a survey vessel, but their main products answer different questions.

A multibeam map tells a surveyor how high, low or steep the bottom is. A side-scan image shows the strength of echoes and the acoustic shadows behind raised objects. NOAA's overview of [hydrographic survey equipment](https://nauticalcharts.noaa.gov/learn/hydrographic-survey-equipment.html) notes that the two systems are often used together because depth and imagery complement each other.

The distinction is practical rather than absolute. Many multibeam systems record backscatter and some specialized side-scan systems can estimate bathymetry. Instrument design, mounting and processing determine what each survey can reliably deliver.

## Multibeam sonar measures a three-dimensional surface

A multibeam echo sounder sends a fan of narrow acoustic beams across the vessel's track. It measures the return time and direction of each echo. After corrections for [sound speed](https://www.argo.net/how-do-scientists-measure-ocean-currents/), vessel motion and water level, the observations become georeferenced depth points.

Repeated pings form a continuous swath. Researchers combine adjoining lines to create a digital terrain model with contours, slopes and shaded relief. NOAA Ocean Exploration provides a detailed account of how [multibeam sonar](https://oceanexplorer.noaa.gov/technology/sonar-multibeam/) collects both bathymetry and echo intensity.

Hull-mounted arrays can map large regions while a ship travels at survey speed. Systems placed on an autonomous vehicle operate closer to the bottom and resolve finer features, although the vehicle covers less area on each pass.

Survey deliverables should preserve both the processed map and evidence of quality control. A **bathymetry grid** needs uncertainty values and vertical-datum information. A side-scan mosaic needs range, frequency and towfish-position details. Without those records, later users may be unable to distinguish a real object from a processing artifact.

## Side-scan sonar produces an acoustic picture

A typical side-scan transducer sits in a towfish pulled behind a vessel. It emits thin fan-shaped pulses to port and starboard. The system records echo strength against travel time, building a strip of imagery line by line.

Hard or smooth surfaces can return strong echoes, while soft sediment often appears darker. An object protruding above the bottom produces a bright response followed by a dark acoustic shadow. The shadow helps an analyst infer its height and shape even when the object occupies only a few image pixels.

The raw image resembles an oblique view illuminated by sound rather than light. Most conventional side-scan products do not supply dependable depth values and geometric distortion increases away from the towfish track unless processing corrects it.

The systems also respond differently to a flat feature. A buried cable may produce little relief in multibeam depth yet remain visible through disturbed sediment or a small shadow in side-scan imagery. A broad smooth depression may dominate multibeam terrain while appearing nearly uniform in acoustic intensity.

## The best choice depends on the survey target

Hydrographers choose multibeam when they need least depths over shoals, terrain for a nautical chart or an accurate model of a channel. Geologists use it to measure fault scarps, volcanic cones and sediment pathways. Habitat studies derive slope and terrain complexity from the same depth grid.

Side-scan excels at detecting [wrecks](https://www.argo.net/ocean-floor-topography-explained/), debris fields, pipelines and subtle changes in seafloor texture. Its fine imagery can reveal an object's outline more clearly than a bathymetric grid. Search teams often cover a broad area with side-scan before inspecting a target with cameras.

Woods Hole Oceanographic Institution explains that [multibeam bathymetry](https://www.whoi.edu/ocean-learning-hub/ocean-topics/ocean-tech/acoustics/multibeam-bathymetry/) rapidly maps large areas. Side-scan deployed near the bottom may resolve smaller details, but towing a sensor adds operational risk around steep relief or obstructions.

**Acoustic frequency** determines range and detail, while sensor altitude controls footprint size. A high-resolution label means little unless those settings are known. Surveyors select them after considering target dimensions, depth and expected bottom type. **Line spacing** then provides enough overlap to expose gaps and test whether a feature appears consistently from neighboring passes.

## Both systems depend on careful calibration

**Sound speed** changes with temperature, salinity and pressure, bending the acoustic path through the water. Multibeam processing uses profiles measured during the survey. A poor profile can curve an otherwise flat seabed upward or downward at the swath edges.

Surveyors measure the exact position and orientation of each sensor. They also account for roll, pitch, heading and heave. A towfish is harder to position than a hull array because it trails behind the vessel and can move sideways in current.

Side-scan interpretation depends on range settings, tow height and the angle at which sound reaches the bottom. Multibeam uncertainty also varies by beam angle and depth. Neither method produces a photograph-like truth without correction and expert judgment.

## Combining the data removes ambiguity

A strong echo may indicate rock, a smooth hard surface or an object facing the sensor. Bathymetry shows whether the same patch forms a mound, ledge or level pavement. Conversely, a low-relief object may be obvious in side-scan imagery even when it disappears inside a coarse depth grid.

Co-registered datasets place depth and reflectivity in the same coordinate system. Analysts can compare a wreck's acoustic shadow with its measured height, or trace a rough lava flow across a ridge. Camera dives and sediment samples then test the geological interpretation.

Ocean instruments often work in such combinations. Argo's explanation of [ocean-current measurements](https://www.argo.net/how-do-scientists-measure-ocean-currents/) similarly shows how distinct sensors contribute different pieces of a physical picture.

## How to read sonar products

Colorful multibeam maps usually assign colors to depth, so the legend and vertical reference are essential. A side-scan mosaic normally maps echo intensity. Light and dark areas can reverse under different display conventions, making the legend and survey notes just as important.

Resolution should be judged against sensor altitude, acoustic frequency and line spacing. Higher frequencies can reveal smaller targets but attenuate more rapidly in water, limiting range. Lower frequencies travel farther and generally sacrifice fine detail.

Multibeam provides the strongest general answer to "how deep and what shape?" Side-scan provides the clearest answer to "what object or texture is there?" Using both yields a safer chart and a richer scientific interpretation than treating either instrument as a complete view of the seafloor.

## A survey plan often uses both in sequence

A reconnaissance team may begin with hull-mounted **multibeam coverage** to establish depth, slopes and safe routes. Areas containing unusual relief or uncertain targets can then receive closer side-scan passes. The terrain model helps pilots keep a towfish at a stable altitude and avoid a collision with steep ground.

Search patterns differ. Multibeam lines overlap enough to prevent holes at the edges of adjacent swaths. Side-scan lines also overlap, but operators often arrange them so a target is viewed from more than one direction. A wreck hidden in an acoustic shadow on one pass may be obvious when illuminated from the opposite side.

**Target confirmation** remains a separate step. Sonar can locate a promising outline and estimate dimensions, but a camera, diver or remotely operated vehicle identifies fine visual features. Magnetometers can add evidence for ferrous metal, while sub-bottom profilers show objects or geological layers buried beneath sediment.

Cost and sea conditions influence the combination. A hull array is protected and can operate during routine transits, whereas towing requires deck handling and careful turns. Autonomous vehicles remove the tether and map close to the bottom, but they require launch time, navigation planning and data recovery. Instrument choice follows the question rather than a simple ranking of which sonar is better.

Archived data also deserve caution. A multibeam grid may have excellent depth accuracy but coarse spacing, while a side-scan mosaic may show sharp textures with uncertain absolute position. Keeping original navigation, calibration and processing records allows later researchers to compare the products honestly.

Learn how the measurements become a depth map in Argo’s guide to [bathymetry](https://www.argo.net/what-is-bathymetry/), then examine what signal strength can reveal through [sonar backscatter](https://www.argo.net/what-is-sonar-backscatter/).