Sustainable alternatives to sonobuoys?

Question

Sonobuoys are expendable temporary hydrophones typically used by the military, but they can and are used for real-time tracking and localization of vocalizing marine animals. There are obvious benefits to using sonobuoys (e.g., McDonald & Moore, 2002; McDonald, 2004; Wade et al., 2006; Oleson et al., 2007; Soldevilla et al., 2022), and specialized software has even been developed for analyzing the data (e.g., PAMGuard and Ishmael). But they last only up to 8 hours and ultimately contribute to additional trash and plastic to the ocean. Some cruises regularly deploy them by the dozens. Is the data worth the environmental cost? Are there sustainable alternatives to sonobuoys for identifying and localizing vocalizing marine animals?

Answer 1

Sonobuoys drift with current, transmit data as long as battery lasts or until deployment time is exceeded, then they sink to the bottom.

The reason of this approach is that originally sonobuoys were deployed from aircraft and data were also recorded by aircraft. So, there was no opportunity to recover. But this is not the case when used from ships and boats for civilian research, where buoys are hand deployed and left alone. Data are recorded via radio link on board the ship.

Alternatives are surface drifting buoys with suspended hydrophones. Such systems are indeed used by the bioacoustic community. One issue I see is the tendency of developers to add functionality, so system can become more expensive.

If you have access to sonobuoys, it would even be possible to take them apart and add the receiver and transmit system to a buoy system. IMO, this would constitute the cheapest maritime PAM system possible.

Edit: IIRC, when setting up sonobuoys, there are three operations: setting up radio frequency, depth, and duration. This means they will sink after the pre-programmed duration. When deployed, the sonobuoy first sinks to the programmed depth, then releases a small radio antenna that is in a small air-filled Portuguese man o' wars (Physalia physalis) type of cover returning back to surface. So, by construction you cannot recover the buoys (at least easily). Therefore, the need for a different type of construction: Surface float with submerged hydrophone. And yes, you must plan your survey to come back to the buoys to recover them.

Answer 2

There have been numerous attempts to rebuild sonobuoys such that they can be reused, and I have been involved in at least 3 of these, each of these were unsuccessful to varying degrees. I've also spent a bit of time trying to decide if sonobuoys would be useful for population-level studies on our surveys.

For what it is worth... Overall, my assessment was that for our surveys, they did not bring enough value to serve as a tool for population level monitoring (or, not enough to justify the negative environmental impact). For our purposes (NOAA surveys in offshore waters of the US West Coast), they still remain valuable for collecting recordings in the presence of sighted baleen whales (and especially rarely encountered whales such as sei whales).

First, regarding recovery of sonobuoys at sea: This is not advised. The batteries are meant to be in seawater and you do not want these on your boat if you retrieve them. If you want to retrieve the parts, then you would be wise to extract the battery and dispose of that in the water (see this video or this publication for more info on batteries). Beyond the batteries, all the materials are highly engineered to be (relatively) good at one job, and to do it for a maximum of 8 hours. The wires, bungee, drogue-- none of these will last for more than a few deployments.

Previous Versions of Rebuilt Sonobuoys: Most attempts to rebuild try to reuse the electronics and the 'DIFAR Ball', but in a different housing, and with rechargeable or replaceable batteries.

The 'Donobuoy' was a reusable sonobuoy developed by Don Ljungblad (very briefly explained in this overview of IWC-SOWER Surveys in Antarctica). Reception range was VERY short, so these would be tethered to the ship for overnight recording and were therefore dominated by ship noise. The batteries could be recharged, but as I recall, ship noise dominated the DIFAR signal. If target species were detected, sonobuoys would be deployed. In 2013 I worked with Don to build several-- but these never worked out (they are still in my lab, waiting for their next life?).

I have also worked with others to try to rebuild buoys-- but they were also unsuccessful. I also have a large PVC version built by Jay Barlow (never worked). I have heard rumors that an 'RMS' (Real Smart Cookie) will be making a directed effort in the upcoming years, and I eagerly await this effort (as well as offload more of my aging sonobuoy parts).

Using sonobuoys successfully: As the OP has mentioned, there has been some great improvements in use of sonobuoys & DIFAR for marine mammals. PAMGuard makes DIFAR analysis very easy (yay!).

I put together this report that outlines some considerations that should be taken when using sonobuoys for marine mammal research. An important lesson learned (from a Navy Sonobuoy Technician) is that the common method of removing 'extra' plastic bits may be problematic. These highly engineered devices include a pin that is held in a precarious position and can be easily knocked out of place if the parachute & plastic holding the parachute are removed. This pin plays a critical role in depth of hydrophone deployment. Basically, if you remove the plastic, you cannot be guaranteed that you will get the intended depth of hydrophone deployment. The way I see it is: If you are going to throw this garbage in the ocean, you surely want to be sure you are getting the best data possible!

I would highly recommend consideration of recent work by Aaron Thode using 'azigrams' for directional information from DIFAR buoys (see link here) and Brian Millers' work on Estimating Drift of Sonobuoys from Acoustic Bearings (see here) to get the most out of your sonobuoys. Keep an eye on these two for new & exciting advances in software (and potentially hardware).

Alternatives to Sonobuoys: Vector sensors have great potential, but have limited use in marine mammal surveys to date. To date, the main vector sensor used is the DASAR by Greeneridge Sciences (see here), which is a moored seafloor vector sensor using DIFAR capabilities. These have been in use in the Arctic for decades. I expect the increased need in recent years, along with alternative monitoring platforms and companies working with marine acoustics, may increase the options for future research.

Answer 3

With the plastic rubbish from nearly a thousand sonobuoy deployments weighing heavily on my conscience, I wanted to ask a very similar question. However, I was encouraged by StackExchange to search first, and the original question here was similar enough to my own. So instead of asking my own question, I'll resurrect this year-old thread.

Building on Shannon's mention of vector sensors: I've become aware of a commercially available vector sensor available from GeoSpectrum Inc. The sensor is called the M20, and they offer 2-axis (similar to DIFAR) and 3-axis versions, and a couple of different depth ratings with a max of 1000 m. To me, these look very similar to DIFAR sensors, so wouldn't be surprised if they shared a pedigree or manufacturer. However, these are just the reusable sensors, so do not provide the floatation, or radio communications that a sonobuoy would provide. Also, M20 seem to provide multiple cables, with one (or maybe pairs) for each of the directional, omnidirectional, and compass sensors within the unit. Since they do not multiplex the signals like DIFAR sonobuoys do. So they might not be an exact drop-in replacement for DIFAR sonobuoys in terms of the software that is already available in Pamguard.

I have my own experiences with modifying DIFAR 53F and SSQ955 (HIDAR) sonobuoys for reuse. In my experience, the radio and the sensor can be made reusable with a relatively small amount of effort. The wiring diagrams for making the sensor and float of 53F buoys reusable can be found in the appendix of NK Naluai's PhD thesis linked here. However, the cabling, suspension, and drogue are essentially single-use only with a working life measured in dozens of hours. Unfortunately, the cabling, suspension, and drogue are essential for keeping the sensor stable in the water column, and without these important parts, the sensor will bounce up and down with waves, and wobble with the wind and currents, and eventually the coiled cable will stretch out and the sonobuoy will be at a different depth than expected. Because DIFAR sensors are very sensitive down to low frequencies, accelerations from such movement will show up as acoustic noise.

So, in my experience, the main issues of making sonobuoys reusable was lack of a suitably reusable cabling and suspension system to keep the drifting DIFAR sensor still. If anyone has a solution to this issue, I'm sure we'd all be keen to hear about it.

The other main issue with making sonobuoys reusable is that they would then need to be recovered. For many of us, the value in sonobuoys is that they can be deployed enroute without requiring the ship to alter course or keep station. Keeping station for 1-8 hours can be quite costly in terms of ship time, especially on large, expensive, ships that operate in remote regions with few alternatives (i.e. Antarctic icebreakers). However, uncrewed vehicles seem like they could provide a potential solution here as they continue to proliferate and reach higher latitudes with more autonomy. I've been working on some proposals towards this end, essentially, towards (a fleet or swarm of) autonomous robotic sailboats with dipping DIFAR sensors. Always keen to collaborate, so feel free to reach out if you're also keen to pursue this line of work.

In 2016 I had the pleasure of meeting with engineers and product managers from a major sonobuoy company to discuss my work. I mentioned the desire for a more environmentally friendly product, and pitched the idea of a low-cost eco-sonobuoy for deployment from ships instead of aircraft. The representatives from the sonobuoy company were quite surprised, as their general sentiment was that the existing sonobuoys didn't contain much plastic, and were already thought of as eco-friendly compared to many of the other military equipment that goes into the ocean. I mentioned the 'pristine' Antarctic environment where I conduct my research, and asked them to consider replacing the plastic with compostable materials. I then half-jokingly suggested that my dream was a $200 per unit green buoy powered 'MacGyver style' by potatoes, and with signals transmitted over a 'Gilligan's Island style' coconut radio. Needless to say, nothing came of it.

However, not all sonobuoys are created equal. For example, US and Australian militaries use Q53F/G sonobuoys with a large form-factor about a metre tall and weight of approx 11 kg each. These are powered by seawater activated batteries. In contrast, UK military uses SSQQ955 (HIDAR) sonobuoys which are about 1/3rd the size of the US sonobuoys, weigh 'only' 6 kg, and are powered by lithium metal batteries with a battery life of up to six hours. Variants of these smaller HIDAR buoys are also supplied to some European nations where the battery life can be extended up to 48 hours. So clearly the manufacturers have scope to modify the designs.

Also, years ago I remember seeing a grant awarded to develop even smaller sonobuoys. The goal was for the sonobuoy to fit into the form factor of an emergency flare. And the idea was that these tiny, ultra-lightweight sonobuoys could eventually be deployed from uncrewed surface vehicles. Would be interesting to know if anything ever came from that, but unfortunately bioacoustic applications of sonobuoys seem to lag 10-20 years behind military applications.

So with all that extra rambling, I think I've now covered the three R's (reduce, reuse, recycle) of waste management of sonobuoys.