Heavy lifting: Notes from the field
Sean Griffin, EARS team leader.
“June 22-23, 2015: The EARS team started the “ship load out” (loading gear onto the ship) in port early Monday and finished on Tuesday long after the R/V Pelican left port. Like all load-outs, this one involved a lot of heavy lifting and squeezing a large amount of gear into a small amount of space. Working on the ship’s metal deck in 95 degree heat only added to the fun. All gear was completely secured: in high sea conditions, equipment can slide causing damage to the ship, personnel and the equipment. Once loaded, the mooring assembly process was started. The R/V Pelican shoved off at approximately 15:00 on Tuesday and mooring assembly continued until completed, just after dinner time. “
EARS (Environmental Acoustic Recording System) is a passive acoustic monitoring (PAM) system used to record sounds made by marine mammals. EARS is generally deployed on fixed moorings approximately 300 meters to 550 meters long and deployed in water depths between 1000 and 2000 meters. EARS electronics and hydrophone are deployed between a 1000 lb anchor and 10 to 12 glass ball floats. This configuration allows for positioning the recording system at water depths where marine mammals occur. The recording system continuously records sounds for approximately 100 days at which time the moorings are recovered and the data processed. The recovery uses acoustic releases that detach the anchor weight when a special acoustic message is received. The recording package will then float to the surface for recovery.
“Given the length and size of each mooring, everything had been shipped in sections and assembled on the deck. There was only enough deck space on the R/V Pelican to manage two assembled moorings at a time. Mooring assembly was tedious and the heat remained challenging to work in. We were extremely careful during the assembly phase because it is so critical: we made sure every shackle was properly secured, every cotter pin in place and taped and then checked and double checked: a single error could result in the loss of the entire array.”
We planned the mooring deployment very carefully to avoid any chance of personnel getting caught up in any of the gear as it went overboard. Each mooring was meticulously ‘snaked’ on deck, in the order in which the mooring components were to be deployed. To insure an orderly and controlled deployment, D rings were placed in strategic locations along the length of wires and Kevlar line that make up the majority of the mooring length, for ‘tag’ lines to keep everything under close control as the mooring goes over the stern of the ship. Safety First!
“June 24, 2015: The ship was scheduled to arrive on station at 11:00. We started our day at 06:30 with another visual inspection of each mooring (safety, safety, safety!) and then performance tests of the electronic packages: EARS buoys have specific diagnostic tests as do the acoustic releases, and all were checked for proper operation.”
“The plan for the day was to first send off the ASVs on their programmed routes, then release the glider, and the EARS systems would be last. When our turn arrived, we deployed the mooring floats first, then EARS, more floats, the acoustic releases and finally the anchor.”
“Once the first floats were in the water, the ship moved forward to insure the mooring lines did not get caught in the ship’s propeller. In order to allow time to deploy all the mooring components safely, the ship started 1000 meters downwind from the desired mooring position which allowed the ship to hold course and speed more easily. The ship sailed at 2 knots as the mooring was released. Once the entire mooring was in the water (except for the anchor), the mooring was towed until the anchor was 200 meters past the desired target point. The bridge called the release and the heavy weight barreled to the seafloor. The floats on the surface were pulled forward at first then gained speed towards the release point, creating a bow wave. After about 60 seconds, the anchor reached a point directly below the floats and dragged them beneath the sea.”
“After the mooring was deployed, the team went out in the RIB and surveyed its precise location, using the ranging systems built into the acoustic releases; these were deployed in sleep mode to conserve energy. We woke them later so that we could locate them by listening for their response to the short acoustic signal we sent into the water. The releases were ‘pinged’ from four positions, each approximately 1000 meters around the target location. Triangulation using the range and GPS location was used to obtain a precise mooring position.”
“A CTD was acquired which gave us a the required precise sound speed for the ranging calculations.
A CTD is an oceanographic instrument used to determine the conductivity, temperature, and depth of the ocean. It may be incorporated into an array of sampling bottles referred to as a carousel or rosette. The sampling bottles close at predefined depths, triggered either manually or by a computer, and the water samples may subsequently be analyzed further for biological and chemical parameters. The CTD instrument is a cluster of sensors which measure conductivity, temperature, and pressure. Depth is derived from measurement of hydrostatic pressure and salinity is measured from electrical conductivity. Sensors are arranged inside a metal housing. Titanium housings allow sampling to depths in excess of 10,000 meters (33,000 ft). Other sensors may be added to the cluster, including some that measure chemical or biological parameters. Most of the time a conducting wire cable is attached to the CTD frame connecting the CTD to an onboard computer, and allows instantaneous uploading and real time visualization of the collected data on the computer screen. The rosette will be stopped on its way back to the surface to collect the water samples using the attached bottles. CLICK HERE FOR MORE DETAILS. If you don’t see the video of the CTD deployment, hit the ‘refresh’ button on the bottom left of the Youtube video player.
“The ship was then ready to move to the next location. The plan was to deploy four moorings over the next two days. This required building the next array mooring immediately. We had an early morning and late afternoon deployment scheduled so we worked late into the evening to insure everything was ready to go in the morning.”
“June 15, 2015: First thing in the morning, we started the acquisition of acoustic data on EARS (the ‘GO’ command) and deployed the second EARS system. On the first mooring deployment, the mooring landed 100 meters past the desired position but considering that the target position was 1000 meters away and the mooring acts as a long pendulum swinging into position, we were reasonably pleased. This time, we again missed the target position by 100 meters, so decided to adjust the timing of the weight release for the next mooring deployment. After surveying the mooring position, taking a CTD and running search tracks with the ASVs, the ship transited to the third mooring site. We arrived around 17:00.”
“The third mooring was deployed with minor tweaks to the release position. This time we were within 40 meters of the desired position. The next two moorings were scheduled for deployment the next morning, so we had a long evening assembling both moorings.”
“The last of the EARS systems had to be deployed within 1000 meters of each other in order to use them as an array to determine precise locations of recorded marine mammal sounds. This is a tricky operation since the 500 meter long moorings cannot cross each other and tangle. With careful planning and coordination with the ship’s crew, the last two moorings were successfully deployed in a triangular arrangement 700 meters (+/- 25 meters) apart!”
“The heavy work is done and now we let the EARS collect acoustic data until October, when we come back to recover all the EARS moorings.”