The Son-O-Mermaid: A Green Platform for the Blue Ocean


In the last few decades seismologists have mapped out elastic wavespeeds of the Earth's interior with often perplexing if not always uncontested detail. Earthquake sources used in seismic tomography lie mostly on plate boundaries; receivers mostly on dry land. The uneven coverage resulting from this fundamentally inadequate source-station distribution leaves large volumes inside the Earth entirely unsampled. Placing seismic stations on the ocean bottom is often touted as the only solution. Bud Vincent from the University of Rhode Island and I have proposed to develop and build a low-cost geophysical instrument with the same long-term goal of closing the coverage gap between continental and oceanic data collection, but one with a much expanded versatility, wider range, and longer life-span than MERMAID. In a nutshell, Son-O-Mermaid is a fully autonomous, and long-lived marine instrument. It is a freely drifting buoy that derives energy from wave action, enough to power a vertical array of hydrophones suspended from a compliant cable connected to a damping plate below the waves, a full-ocean-depth echo sounder, GPS for location and timing accuracy, an IRIDIUM satellite communication unit for near real-time data transfer, and an on-board digitizing and processing unit that, like its predecessor MERMAID, uses sophisticated wavelet detection and discrimination algorithms. The Son-O-Mermaid is to be deployed by untrained personnel from ships of opportunity, which gives it an extra advantage over conventional (e.g., ocean-bottom, tethered, moored) approaches. The hurdles that need to be overcome and the problems solved in order to make this instrument be of use for seismology, most specifically, set a very high bar in terms of energy efficiency, instrument accuracy, and longevity, and as a result, future generations of it should be easily adapted to less demanding data collection exercises --- be they physical, chemical, or biological.
Supported by the A. H. Phillips Instrument Fund at Princeton University.

Live update 8 | 11/30/2013 | Frederik Simons

After our encounter with Hurricane Sandy we regrouped and made some design modifications. Remember, Son-O-Mermaid survived, but we lost a piece of the cable and thus any recordings made. Luckily our first experiment was conducted with a cheap replaceable surrogate cable... but we couldn't afford to lose our nice and expensive fiber-optic one if such a thing were to happen again. Bud and his team at the Equipment Development Lab ultimately settles on an all-copper solution. The faired and terminated cable was delivered right before Thanksgiving. Thanks! Here are some pictures.

Live update 7 | 12/10/2012 | Bud Vincent

Well... just barely, that is. Son-O-Mermaid was deployed on October 9th, 2012 at 19:07:04 UTC. During the deployment period, it was hit by three storms: the first two were tropical storms and the third was Hurricane Sandy. Hurricane Sandy's path cut directly through the deployment site of Son-O-Mermaid. At this time, the Hurricane was officially a category 2 and registered wind speeds as high as 105 mph. We stopped getting position updates on October 25th, 2012 at 17:10:05 UTC and did not receive any updates from the device until October 27th at 23:50:59 UTC. When we received the first update after the hurricane, Son-O-Mermaid was approximately 47NM southeast of its last recorded position. The device remained deployed for an additional 11 days. On November 7th, our contact on Great Exuma flew to Ragged Island and hired a local to take him out on a boat to pick up Son-O-Mermaid. The surface buoy was successfully located but, the recording system and hydrophone array was detached from the end of the line. In the end, all that was left of Son-O-Mermaid is the surface buoy with the Iridium unit and about two thousand feet of line. This was brought back to Great Exuma.
Oh. And did I mention sharks.
The recording system was not recovered and therefore, the hydrophone data recorded during the final deployment were lost. Fortunately, on the second day of deployment when the instrument was checked up on, about 24 hours of acoustic data was copied onto a laptop hard drive.

Live update 6 | 10/29/2012 | Son-O-Mermaid

I met my first shark. Then I survived hurricane Sandy! More to come. All did not end well.

Live update 5 | 10/11/2012 | Bud Vincent

Some pictures from the maiden voyage first deployment (Phase I testing), which was, until further notice, a resounding success. The experiment is still going on!


Live update 4 | 09/21/2012 | Frederik Simons

We are getting ready for Phase I testing of the prototype! The prototype system is ready to go. It contains all but the actual communications cable and the wave-generator. Phase I of the test will be for a 30-day deployment to test the mechanical integrity and stability of the design (using a surrogate cable), and the noise floor of accelerometers and the hydrophone arrays. The recording package hangs at the end of the surrogate cable; in the final design this will be moved to the surface buoy once the communication/power cable is finished. Of course the goal is to detect some actual earthquakes in this time period! The test will be conducted in the Tongue of the Ocean, Bahamas... We received a very official looking permit for this test. Phase II testing (like Phase I but including the fiber-optic cable, remote communications control, GPS and Iridium teleconnections) can happen soon thereafter. And if that all goes well, Phase III testing (like Phase II but with the wave-energy scavenging module on it also, for a total target production price of $10000) shouldn't be that far off. Fail-safe and super-low cost scenario (target: $5000) is to build the final unit without the wave generator but with conventional batteries... but why build a Civic when you can get a Prius? Let's think big and be job-creators. Below are some pictures from the ballasting tests.


Live update 3 | 09/10/2012 | Bud Vincent

The custom cable that we had made (kevlar for strength, 2 copper conductors for power, 2 fibers for communications) is "finished", but needs to be "finished"... in a process called "termination" by "overmoulding" by a liquid polymer that hardens for strength and watertight integrity. Making the cable took about 9 weeks after final design approval. The company that made the cable is Falmat in San Marcos, CA. The termination is in the hands of Teledyne Impulse. This is an expensive, one-of-a-kind piece of equipment... costing about $8000 to make and loads more to finish. Will let you know when it makes its way from California to Rhode Island.

Live update 2 | 06/15/2012 | Bud Vincent

Things are proceeding well with Son-O-Mermaid. We have conducted bench top evaluation and power consumption testing of your Printed Circuit Board assemblies, and we will be conducting a test of the hydrophone array and data recording sub-system next week in Narragansett Bay (of course not to 2000 feet!). The surface GPS-Iridium unit underwent a great test this past week during a sea test I just did offshore in Hawaii. It worked flawlessly and I can send you a Google Earth KML file if you would like. (You can also see the messages at the Son-O-Mermaid Gmail account).

Live update 1 | 03/17/2012 | Bud Vincent

The hydrophone array is built and was installed in the acoustic tank for its first functional test. I have attached a couple MATLAB data files. The first data file is "HammerTest1_3CH.mat". There are four channels of acoustic data stored in a variable Data (matrix of size 250E3 x 4). The first 3 channels are SonOMermaid. The 4th is another hydrophone (ITC-1089D). The sound source was very light taps on the concrete wall of the tank with a hammer. The second file is "weightDrop1.mat". This has two channels of data (the first from mermaid the second from ITC-1089D). The source of the data is dropping a lead weight on the concrete floor a few meters away from the tank. You will notice that everything is working correctly but the noise floor is fairly high (still much better on mermaid than the 1089D hydrophone). This is due to a few reasons: we did not use batteries for the hydrophone pre-amp power source, did not use the embedded data acquisition system, and had the hydrophone signals brought out of the tank on long cables back to the test bench. All of these factors create a higher noise floor than we should see in the final configuration. Our next test is of course to repeat this with the at sea configuration of battery power and embedded data acquisition. As soon as we do this I will send some more data.

Frederik Simons
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Last modified: Thu May 8 16:28:41 EDT 2014