The State of Sonars & “Seeing” Subterranean 🔊
By: Anthony Dimare
Tags: Technology, Oceans, Sonar, Exploration, Sensors

Types of Sonars

Since it’s all just sound, why can’t every sonar “do it all”? This is by far one of the most misunderstood parts of sonars and reasonably so. It’s all about the beam that is emitted out of the transducers, the properties of the beam, and the processing done after the receiver picks up the reflected sound signal.

Beam properties are generally: beam shape, # of beams, orientation, frequency(s), phase of the signal, and processing technique. In the interest of being breif-ish, we’ll dig into the main strengths and weaknesses of the main types of sonars used for hydrographic, and geophysical surveying.

Side-scan sonar (SSS)

Dataset: Imaging

Beam pattern: 2, very wide fan-like beams that are directed out at max angles approaching 120°- 160°.

Characteristics: SSS is the premier large swath imaging sonar. It’s measuring the intensity of the return pulse over an amount of time. It can take very high-resolution images over extremely large areas. While you can’t see color, it’s incredibly useful for seeing objects because of the characteristic shadows that appear in the images it produces (as demonstrated above with the shadow behind the rock, and in the brown ship image from further above).

A downside is the lack of imaging information you get directly below the device holding the 2 side scan transducers. This information is usually filled in with another downward-facing sonar like a MBES.

Resolution is generally a function of altitude of the SSS above the seafloor. The higher above, the worse the resolution, but the larger the swath imaging area. The closer to the seafloor, the better the resolution, but small imaged area. As you can imagine this becomes challenging to balance in varying terrain.

Single-beam echo sounders (SBES)

Datasets: Ranging, Bathymetry

Beam pattern: A single focused small-angle beam (1–0.5°). (see right side of the picture below)

Characteristics: Because the ensonified area (the area hit with sound by a sonar) by a single narrow beam is very small, it’s used in a way to cheaply identify general under-vehicle clearance. It can also be used for obstacle avoidance

Scanning sonar

Dataset: Ranging, Imaging

Beam pattern: A single beam that has a very narrow horizontal beam (2–0.5°), but wide vertical beam (20–60+°). This creates a fan-like pattern. It’s then rotated along a vertical axis.

Characteristics: We are all familiar with this type of sonar — we know the concept well from the stereotypical submarine control room shots in Navy war films when a torpedo inevitably is shot toward at the vehicle of the main character. It creates a 360° map of hard objects around the axis of the rotating sonar. It can also be used as an imaging sonar like a side-scan sonar beam that rotated. The end data here depends on the processing and type of beam (fan or laser-like).

Multibeam echo sounders (MBES)

Datasets: Bathymetry, Backscatter, Imaging (poor at imaging)

Beam pattern: Up to several hundred very narrow beams that are configured in a fan-like pattern to create a wide ensonified swath. This means that data from each beam can be distinguished from another which has several benefits.

Characteristics: Think of this like LIDAR with sound instead of lasers. This is the premier bathymetry sonar system. You get high-resolution point clouds of the bottom of the seafloor over a wide swath (120°). You can also measure the reflectivity of each individual beam to give you a backscatter image as well (amazing).

Interestingly, there’s a new field of post-processing for multibeam systems where the noise within the raw multibeam data is used to get water column profiling (measuring layers of different water temp which changes the speed of sound through water + biomass between the bottom of the sensor and the ocean-floor). As post-processing continues to get cheaper and trickles into the workflow of teams that use this data, I suspect a plethora of new information to emerge from good quality data that’s been collected already about the water column.

Sub-bottom profilers (SBP)

Dataset: Sub-Bottom Profiling (shocker)

Beam pattern: a very concentrated single narrow-beam (exactly the same as SBESs).

Characteristics: It creates a single layering line of under the seafloor. There is no swath (area) for SBPs without expanding the number of SBPs over a horizontal distance perpendicular to the direction of movement. These transducers are often called “boomers” because the sound is usually a very low frequency, with a lot of power behind it — almost seismic in nature. The downside, these are very power-hungry sonars. If you want deeper profiles you need more power behind the sound pulse. So if you want to have a SBP on a small AUV, you have to give up depth underneath you see and resolution or sampling rate.

Synthetic Aperture Sonar (SAS)

Datasets: Imaging, Bathymetry, Backscatter

Beam pattern: See the right image diagram. It’s multiple wide sonar beams emitted in pulses like a SSS, but then digitally sticked together to get a lot of information over each part of an ensonified grid.

Characteristics: This is the state of the art at the moment. Unfortunately, that makes it still exceedingly expensive. The benefits of this are clear. You get long-range, ultra-high-resolution images (cm level resolution). Additional post-processing is done to create a deeper and more-detailed understanding of any point from multiple different sonar pulses, all from different angles. Just look at the type of images it can create:

😱😱😱😱😱😱😱😱- image: Kracken Robotics — MINSAS

Because each processed point has multiple different sources of information on it, you can also use interferometry to also get accurate-ish bathymetric and backscatter data as well. now you’re seriously upping the amount of processing needed, but the early results are pretty amazing and it’s not surprising that this will be the future of the space.

The kicker — like side scan, you still get “shadows” in the bathymetric & imaging data.