Introduction to Side Scan Sonar
Side scan sonar is an acoustic imaging system designed to generate high-resolution two-dimensional representations of the seafloor. Depending on the imaging principle, side scan sonars are commonly categorized into conventional single-channel or dual-channel systems, multi-beam systems, and synthetic aperture sonar (SAS). According to the surveying method, they may be operated as towed systems, hull-mounted/vehicle-mounted systems, or electronically scanned systems.
Side scan sonar offers high resolution, wide-area coverage, operational efficiency, and cost-effectiveness. It plays a critical role in seafloor mapping, seabed morphology surveys, pipeline and cable route inspection, search for seabed obstructions, port and harbor construction surveys, wreck and missing-vessel search, underwater archaeology, and many other marine applications.
A typical side scan sonar system consists of the following components: the sonar transducer unit, tow cable or communication cable, topside electronics, control console, and GPS positioning system.
Working Principle of Conventional Towed Side Scan Sonar
In a conventional towed system, the towfish follows a stable trajectory behind the vessel while the port and starboard transducers continuously emit acoustical pulses toward the seabed. The reflected echoes are received by the corresponding channels and transmitted to the topside processor. By analyzing echo travel time, amplitude, and backscatter characteristics, the system reconstructs acoustic images of underwater targets and seafloor texture.
Since seabed morphology and geological composition vary, the intensity of the backscatter signal also varies. The side scan processor converts the echo intensity into grayscale imagery: stronger returns appear darker, while weaker returns appear lighter, forming high-contrast acoustic shadows that highlight seabed features.
Range and Frequency Characteristics
Side scan sonar range is determined by the acoustic slant range, which is strongly influenced by operating frequency: lower frequencies provide longer range, whereas higher frequencies yield shorter range but much higher image resolution. Sound propagation is further affected by water properties such as temperature, salinity, pressure, and turbidity. Target material and shape also influence detectable range, as different objects exhibit different acoustic reflectivity.
High-power, low-frequency systems support long-range coverage but with reduced resolution, while high-frequency systems offer fine seafloor detail at limited range.
Resolution Characteristics
Resolution refers to the sonar’s ability to distinguish small objects and is defined in the along-track and across-track directions. These parameters are interrelated and depend heavily on operating frequency, beamwidth, and pulse characteristics.
Higher frequencies provide finer resolution and clearer target definition but shorter propagation range. Beamwidth affects both along-track and across-track resolution: narrow beams improve image sharpness but reduce swath width.
Environmental conditions—such as depth, temperature, salinity, and pressure—alter sound speed and refraction paths, influencing both range and resolution.
Platform Influence
Side scan sonar can be deployed on surface vessels, submarines, towfish, autonomous underwater vehicles (AUVs), and remotely operated vehicles (ROVs). Vessel speed significantly affects along-track sampling density for conventional side scan systems: higher speeds increase pulse spacing and reduce along-track resolution.
Advancements in Modern Side Scan Systems
Modern side scan sonars feature dual-display interfaces, multi-mode operation, CHIRP wideband processing, and advanced variable-bandwidth telemetry.
CW Mode (Continuous Wave)
CW mode transmits narrowband pulses with a constant frequency. This stable frequency structure helps maintain consistent beam characteristics and supports solid along-track performance under specific conditions.
CHIRP Mode
CHIRP mode transmits frequency-modulated pulses whose frequency varies over time. As a wideband signal, CHIRP enables pulse compression processing, providing significantly higher resolution, improved target separation, and enhanced signal-to-noise ratio. By analyzing the frequency structure of the returned signal, the sonar accurately estimates target range and generates high-clarity acoustic imagery.
Variable-Bandwidth Telemetry
Variable-bandwidth telemetry enables dynamic adjustment of transmitted bandwidth and provides real-time telemetry of system status and performance data. This technology increases operational flexibility, supports remote monitoring and control, and enhances system reliability in challenging acoustic environments.
For more information on our side scan sonar solutions and related products, please visit our Side Scan Sonar Product Page: https://china-sonar.com/underwater-acoustic-camera .
Side Scan Sonar: Imaging Principles and Operational Capabilities