Kate pictured with large mooring float.

Recovering a mooring of any significant size essentially amounts to getting it to the surface and picking it up with a ship using a crane. Recall that one of the key items contained on the mooring is the acoustic release. The release is essentially a chain link that opens up when it receives a particular acoustic waveform placed into the water.

An acoustic release with integrated beacon.

Many, such as the one Kate is using, also send out a beacon or “ping” when acoustically interrogated. So, to recover a mooring, one steers the ship to the GPS location closest to where the mooring was deployed, places the appropriate set of tones in the water which will induce the release to start sending beacon tones and release the mooring from the anchor, and then home in on the mooring’s location using the beacon. At this point, the mooring can be reached at the surface, and only the anchor remains at the bottom.

To physically perform the recovery on a big ship such as Healy, a rigid inflatable boat (RIB) is dispatched from the recovering ship to find the top float(s). Of course, if the top float(s) ascended under a blanket of sea ice, there are more details to handle, but we didn’t have to deal with this particular problem on this cruise. We avoided it by electing not to recover a particular mooring on the first day and coming back later to recover it (when the ice had moved on).

Rigid inflatable boat (RIB) used to recover moorings.

The small boat will locate the mooring by looking for the float(s). Floats are typically painted in colors or contained within painted containers that are relatively easy to see, even in poor weather conditions (e.g., optic yellow or orange). One of the more common float arrangements is to strap together a collection of glass spheres each of which provides a certain amount of buoyancy and can resist the high pressures of submergence at depth. Such spheres can also be used to house instruments. Floats for less extreme depths, made of plastic or foam materials, can be used for applications involving less pressure (foam compresses at high pressure and loses its buoyancy; glass does not). In some applications the float is allowed to bob around on the water surface (called a “surface expression”), but in cold areas such as the arctic this can be a problem because floating ice can damage a float or, worse, carry off the entire mooring.

Once the small boat operators get a hold of the recovery line attached to the float, they maneuver to the stern of the recovering ship and connect the recovery line to the ship’s crane line. At this point, the small boat gets out of the way so the ship’s mooring crew on deck can recover the mooring. This is accomplished by winching in the line and moving the A-frame, which is a large metal inverted-U-shaped hoist attached to the ship. It is operated by hydraulic lifters and is used to position moorings or other equipment beyond the stern of the ship over the water.

Crane and A-frame used to hoist mooring (floats and audio recorder are visible).

Deploying a mooring, or re-deploying a mooring, involves placing the mooring into the water top-to-bottom (there are other ways, but all the moorings deployed on this cruise were done top-to-bottom), with the recovery line first and anchor last.

Mooring with acoustic release, audio recorder, CTD and float ready for deployment; the recovery line and its floats are already in the water.

If the mooring is sufficiently small (like Kate’s) it is possible to lay the entire mooring out on deck ahead of time.

Kate awaits the ship to be positioned for the next deployment. The full mooring is visible on deck.

First, the top float(s) are placed in the water, then the instrument(s), then any additional floats and finally the anchor. The anchor is placed in the water by careful use of the ship’s crane. It may be partially lowered into the water and released at just the right time so as to land on the ocean floor at the desired location. After the anchor is released, it sinks to the bottom pulling the rest of the mooring along with it. It is thus possible to see the primary float(s) rapidly approaching the stern of the ship until they are pulled below the surface.

Anchor weights are deployed by the winch. John holds a line that when pulled releases the weights from the crane.

Although the recovery and deployment of moorings may seem conceptually simple, there are many details to get just right. Errors in mooring design or deployment execution could mean a costly waste of ship time, a loss of data or equipment, or possible injury to the scientists or crew. This is true of most heavy work on ships, but is exacerbated when working in the Arctic due to the hostile conditions. For this set of scientists and crew, mooring recovery and deployment is conducted by the Woods Hole Oceanographic Institution’s Mooring Operations Engineering and Field Support Group, led here by John Kemp, a legend in the “mooring community.”

Emblem of WHOI Mooring Ops.

John Kemp, WHOI’s expert at mooring design/recovery/deployment.

Although I personally lack the level of sea experience this crew has by many orders of magnitude, I have been around ships and small boats for most of my life, and I can appreciate a virtuoso at getting work done at sea, and John is definitely one.

(Thanks to Dale Chayes (LDEO) and Kate Stafford (APL) for helpful comments on this article).

Placing a Mooring

Kate’s moorings, like most others, are constructed to be placed on the sea floor at a particular location and depth. Locating nearly anything on the surface of the Earth has become much easier with the widespread use of the Global Positioning System (GPS) and its relatives (assisted GPS, differential GPS, carrier-phase GPS, Real-Time Kinematic (RTK) GPS, etc). Knowing the depth (and associated bottom contours) at a point in the ocean is less straightforward, yet these details are important for designing and manufacturing moorings, as these details affect certain aspects of a mooring’s design such as the amount and strength of cable required, the weight of the anchor to be used, the amount of buoyance required, etc.

Ocean depths are typically measured with an echosounder (see below) and then recorded on bathymetric maps. Contour lines of the same depth are called isobaths. The map below indicates our ship’s cruise track, mooring deployment/recovery locations, geopolitical data, plus a number of isobaths (black lines) and bathymetric data (salmon-colored tracks).

Cruise track, made available to the science party underway, had a number of optional overlays. This shows the track, geopolitical map, bathymetric data, and mooring deployments/recoveries.

The bathymetric data used to create such maps are measured using either airborne platforms (e.g. satellites) or ships’ sonar systems. Healy has a multibeam sonar system. Using multiple sound beams allows the ship to map out more sea floor information per unit time than would a single beam.

In Healy’s multibeam, sound is emitted through hull-mounted projectors in the form of short acoustic “pings” (which sound a bit like bird chirps when you are on one of the lower decks– bring earplugs if you have to sleep there!). The pings are 12KHz acoustic waveforms that bounce off the sea floor and are picked up by an array of hydrophones (also mounted on the hull). The amount of time between when a ping is sent and when it is received gives an estimate of how long it takes sound to travel to the bottom and back. If the speed of sound in sea water is assumed to be 1500m/s, then the depth (in meters) can be estimated as (1500)(t)/2, where t is the time (in seconds) between the sending and receiving of the ping. However, this calculation really isn’t good enough. The reason is because the speed of sound in water varies based on a number of factors, including temperature, salinity and depth. So, data collected from onboard instruments: an expendable probe (see XBT, below), a CTD that is repeatedly cast into the water (see my previous dispatch that describes the CTD), and archived data, are used to form a sound speed profile which is then used as input to the ship’s SeaBeam 2112 control computer to make corrections to the simple formula above. Using all this information, while also taking into account motion of the ship itself when estimating the time between sent and received pings, the computer can form an accurate bottom map that is made available to anyone on the ship and also shared with others back on shore through one of the various archives at LDEO.

WHOI Mooring Ops. Engineering and Field Support Group logo.

As you can probably see, there is a considerable amount of subtlety in how this works inside the computer. For example, the ship may be experiencing rolling, pitching, or yawing. Movements such as these (especially roll and pitch changes) affect how the beams are reflected back to the hydrophones, and all this must be considered when creating an estimate of far away the bottom is and what features it has. In addition, the temperature of the water varies as a function of depth and can change over time. To determine the temperature/depth relationship at a moment in time, a device called an expendable bathythermograph (XBT) and/or a CTD (described last in my dispatch) is used (e.g., once daily or when the ship moves into an area with different ocean characteristics) to measure the temperature/salinity/depth profile.

An expendable bathythermograph sitting on its launcher.

An XBT looks like a small rocket that is launched by a fixed or hand-held launcher off the ship; the ship can be still or moving. The XBT itself remains attached to the launcher by means of a very fine 2-conductor copper wire that unspools as the XBT descends toward the sea floor. As it descends, it measures temperature while its depth is estimated using an assumed falling rate. The resulting information is displayed to the user and used in forming the sound speed profile for the multibeam echosounder (and for a few other things). To learn more about the finer points of estimating sound speed in water, this link provides a number of more detailed references as a starting point.

Moorings can be used for a variety of purposes, but for this cruise moorings are being used to keep acoustic and oceanographic sensors in place for some period of time (typically a year). Scientist Kate Stafford from the University of Washington’s Applied Physics Lab (APL) has a set of acoustic sensors forming a ‘hydrophone array’ that are to be recovered and (re) deployed later in the cruise.

Hydrophones are passive measuring devices– they do not emit any sound themselves, but instead convert acoustical energy in the water to an electrical signal. Kate analyzes the sounds or “vocalizations” of whales, bowhead and beluga whales in particular, using these instruments. Fixing a hydrophone array in place underwater for a relatively long period of time (a year) allows scientists to listen for the presence of cetaceans (whales, dolphins, and porpoises) from multiple locations, even in poor weather conditions.

Kate pictured with a large mooring float.

Detection Using Passive Hydrophone Arrays

Each hydrophone Kate uses sits inside an instrument that includes the hydrophone itself, a data recorder where the digitized audio samples are stored for later retrieval, and a pack of batteries to keep the whole instrument running for a year or more. These are the instruments that are attached to moorings that are deployed out at sea until they are recovered. When these types of instruments are placed sufficiently close together, more than one will pick up the same vocalizations or sound. When this happens it may be possible to locate and track a particular sound source (animal or otherwise) over time. When the instruments are used together in this way, they behave as an array, or group that is acting together cooperatively. Hydrophone arrays can also be used in estimating animal populations, although gaining high confidence using this approach is an ongoing challenge.

The hydrophone instrument.

When the data from Kate’s instruments is recovered, it can be processed in a number of ways. One way is to simply listen to the sounds on the recordings. A person with enough expertise in doing this can pick out some of the vocalizations and other sounds. As an example, you can listen to the recording of a beluga:

Beluga Whale Recording

A more quantitative approach involves taking the data and analyzing the frequency, timing and intensity (loudness) of particular sounds. A popular way of doing this is to visualize this information on a special kind of graph called a spectrogram. In this spectrogram of a beluga vocalization in the Beaufort sea, for example, the intensity of sound at a particular frequency at a particular time is indicated by the color (blue less intense; red most intense).

Spectrogram of beluga soundings in the Beaufort Sea.

By looking at the spectrogram or other types of graphs (or by using a computer program to do so), acoustic frequency patterns resulting from cetacean vocalizations (or boats, or the sounds of airguns used in seismic exploration, etc.) can be found.

For the interested reader, the analysis of hydrophone data is similar to both signal processing and image processing techniques. Common techniques include matched filters, band-limited energy summation and classification, image matching techniques on the spectrogram or time series, and other frequency-domain analysis techniques such as wavelet-based decomposition. A more detailed explanation of how this can be accomplished and why fixed passive arrays are useful for monitoring cetaceans is given in this paper.

The ultimate goal for much of the related science here is to estimate animal populations, which is important for both ecological understanding and policy-setting reasons. As suggested above, there are a number of factors posing challenges to doing this effectively using acoustic arrays. Animals may use different vocalizations at different points in time and these may vary in many ways (e.g., frequency, phase, amplitude, modulation, etc) based on its activity (e.g., feeding or seeking a mate). In addition, instruments are only capable of detecting a signal up to some distance (which depends on frequency), so forms of statistical inference must be employed in an attempt to estimate a population given a number of detections.

Necessarily, these are only estimates, but they are estimates based on measured data, and moored acoustic arrays offer some significant advantages for making such measurements versus the alternatives of visual observation from ships (or other platforms like airplanes, etc) or from towed hydrophones that only last a few days or weeks. Moorings last a long time and are relatively immune to poor weather so they can offer a richer data set on which to base estimates. Of course, they need to be designed, deployed, and recovered, which generally involves a ship such as HEALY…