Side Scan Sonar: What It Is and How It Works

Side scan sonar is an active sonar system that detects and images objects on the seafloor. A transducer array, mounted on a ship hull or in a towed body, sends sound pulses out to both sides of the survey line and records the echoes to build an image of the bottom. Researchers use it to find and document shipwrecks and other submerged features before any diver or remotely operated vehicle goes down.

This page is for students, survey planners and archaeologists who need to understand what a side scan image shows, what it does not, and what to record when reporting a survey. It is a general explainer, not a product guide.

What is side scan sonar? Definition in one paragraph

NOAA Ocean Exploration describes side scan sonar as a category of active sonar system for detecting and imaging objects on the seafloor. The transducer array is mounted on the hull of a vessel or on a towfish, a streamlined body towed behind it. “Active” means the instrument emits its own sound and listens for the return, as opposed to passive systems that only listen.

DOSITS (Discovery of Sound in the Sea) adds that the towed instrument is called a towfish, that its beams pulse out at 90 degrees from the path along which it is towed, and that the echoes are received by hydrophones, amplified, processed and displayed as an image. DOSITS describes the result as almost photographic in quality and says features smaller than 10 cm can be detected.

How side scan sonar works

The vessel runs straight survey lines at a steady speed. On each ping the sonar sends a fan-shaped pulse to both sides, and the strength of the echo is recorded across time. As the vessel advances, successive pings are stacked into a continuous strip image of the seafloor on either side of the track. Because lines are straight and speed is steady, adjacent lines can be laid out to cover an area.

Frequency and resolution

Frequency is the main trade-off in a side scan survey. NOAA gives these bands:

Frequency band (NOAA)	Coverage	Detail
Lower, about 50 to 100 kHz	Large swaths	Lower resolution
Higher, about 500 kHz to 1 MHz	Smaller area	Finer detail

A reconnaissance survey that must cover a wide area quickly leans toward the lower band. Documenting a known target in detail leans toward the higher band. The two are often used in sequence: wide search first, close inspection second.

How to read a side scan sonar image

Image brightness reflects how much sound returns from each part of the seafloor. NOAA states that hard objects return a strong echo and appear dark, while soft sediment such as mud or sand returns a weak echo and appears light. Colour scales can be reversed in processing software, so confirm the convention used for a given image before interpreting it.

What you see	Reading	Source basis
Dark (strong return)	Hard surface, such as rock or a man-made object	NOAA Ocean Exploration
Light (weak return)	Soft sediment, such as mud or sand	NOAA Ocean Exploration
Isolated dark feature on a light background	Candidate object on the seabed, to be checked by another method	Editorial interpretation of the two rows above

What side scan sonar cannot do

It does not measure depth. NOAA notes that side scan sonar is typically paired with single-beam or multibeam sonar, which supply depth.
It is not proof of identity. An image shows a feature’s shape and reflectivity. Identifying it as a particular wreck requires other evidence, such as direct observation or historical records.

Side scan sonar in maritime archaeology

NOAA lists mapping of cultural heritage sites such as shipwrecks among the uses of side scan sonar, citing the schooner Typo and a World War II B-25 bomber in Papua New Guinea as examples, and notes it is less costly than deploying a remotely operated vehicle with an HD camera. DOSITS lists shipwrecks, downed aircraft, mines, pipeline and cable routes and seabed mapping as uses.

In practice, side scan sonar is a search and prioritisation tool. It narrows a large area to a short list of targets, which can then be inspected with higher-cost methods. For the wider discipline see what archaeology is; the underlying acoustic processing is introduced in what signal processing is. Fisheries researchers use related acoustic methods, covered in what fisheries science is.

How to report a side scan survey

A side scan report is only reproducible if the acquisition settings are stated. Reasonable minimum content, as editorial guidance consistent with the factors above, is:

Instrument type and whether hull-mounted or towed.
Operating frequency, since it controls coverage and detail.
Survey line layout and the paired depth data source (single-beam or multibeam).
The image convention used (for example, dark for strong return).
For each reported target, how it was or was not verified by a second method.

An illustrative methods sentence (placeholder values, not a real survey): Side scan data were collected with a towed 500 kHz system at about 10 m altitude on parallel lines; depth came from a separate multibeam survey; targets are shown with dark as strong return and were not verified by a second method. When citing the technique itself, cite the sources listed below in your journal’s style, for example in APA style: NOAA Ocean Exploration. (n.d.). Side-scan sonar. https://oceanexplorer.noaa.gov/technology/sonar/side-scan.html

Slant range, ground range and resolution: the formulas

Side scan sonar records time, not distance on the seabed. Two standard geometric and acoustic relationships turn recorded time into positions and tell you how fine the detail can be. These are textbook geometry and acoustics (editorial explanation, not a figure taken from the NOAA or DOSITS pages).

Slant range: R = c × t / 2
Ground range: G = √(R² − h²)
Range resolution (along the beam): ΔR = c × τ / 2

Symbols: R is slant range, the straight-line distance from the transducer to the reflecting point (m). c is the speed of sound in water (m/s); 1,500 m/s is a common nominal value, but real surveys use a measured sound speed profile. t is the two-way travel time from ping to echo (s); dividing by 2 accounts for the out-and-back path. h is the towfish altitude above the seabed (m). G is horizontal distance from the point directly below the towfish to the reflecting point (m). τ is the transmitted pulse length (s), and ΔR is the minimum separation along the beam at which two objects can be told apart. The ground-range formula assumes a flat seabed.

Worked example with real numbers

These are hypothetical survey settings chosen for easy arithmetic, not data from a real survey.

An echo returns 0.0667 s after the ping. With c = 1,500 m/s, R = 1,500 × 0.0667 / 2 = about 50 m.
The towfish is flying h = 10 m above the seabed. Ground range G = √(50² − 10²) = √(2,500 − 100) = √2,400 = about 48.99 m.
The pulse length is τ = 0.0001 s (0.1 ms). Range resolution ΔR = 1,500 × 0.0001 / 2 = 0.075 m, or 7.5 cm.

Treating slant range as ground range here would overstate the distance by about 1 m. The error grows as the echo comes from nearer the track, where slant and ground range differ most. Along-track detail is set separately by the beam width and range, so a short pulse alone does not guarantee fine detail in both directions.

Assumptions and common mistakes

Flat seabed. The ground-range formula assumes flat bottom; slopes and relief distort geometry.
Constant sound speed. Using 1,500 m/s when temperature and salinity differ gives a scaling error in every range.
Ignoring altitude. Reading distances straight off the slant-range axis without correcting for h.
Confusing the two resolutions. Pulse length governs resolution along the beam; beam width governs resolution along the track.
Reading brightness as a measurement. Brightness reflects echo strength and processing settings, not depth or size on its own.
Treating a target as identified. See the limits section above.

How to calculate it in Excel, R or Python

With slant range in cell A2 (m) and altitude in B2 (m), Excel ground range is:

=SQRT(A2^2 - B2^2)

slant <- 50; h <- 10
ground <- sqrt(slant^2 - h^2)   # 48.99
res <- 1500 * 0.0001 / 2        # 0.075 m

Python:

import math
slant, h = 50, 10
ground = math.sqrt(slant**2 - h**2)   # 48.99
res = 1500 * 0.0001 / 2               # 0.075 m

FAQ

What is side scan sonar used for?

Detecting and imaging objects on the seafloor, including shipwrecks, aircraft wreckage, mines, and routes for pipelines and cables (NOAA Ocean Exploration; DOSITS).

What is a towfish?

The towed body that carries the side scan transducers behind the survey vessel (DOSITS).

Which frequency is better, low or high?

Neither is better outright. About 50 to 100 kHz covers wide swaths at low resolution; about 500 kHz to 1 MHz covers less area in finer detail (NOAA Ocean Exploration).