My Favorite Bostonian Cetaceans; Dive into a World Different from Our Own

Educational Background on Whales:

Whales are one of the most amazing creatures that live in the planet. They inhabit all oceans of the world. Whales belong to the order cetacea, which means that they are mammals fully adapted to aquatic life. Like all cetaceans, including dolphins and porpoises, whales are descendants of land-living animals which returned to water after living millions of years in land. Most whales can grow to be extremely large. In fact, the Blue whale is considered to be the largest animal in the world. Whales are closely related to dolphins and porpoises. There are two types of whales that are identified by scientists, baleen whales and toothed whales, having each of these categories many sub species. You can easily identify which category a whale belongs based on its feeding and physical characteristics.

One startling fact: The most dangerous predators for whales are humans. They aren’t bothered by any other creatures in the water. However, the changes to their food source, hunting by humans, and even pollution to the waters that they live in can affect their abilities to survive which is why they continue to try to adapt to the environment that they are in.

Background Information on My Favorite Bostonian Cetaceans

NORTH ATLANTIC RIGHT WHALE (Eubalaena glacialis):

The North Atlantic right whale is currently one of the rarest large whales in the world, having been drastically reduced to critically low numbers by years of exploitation. Its robust, somewhat rotund, body is mostly black, with a large head that measures up to one third of the total body length. Some individuals may also bear white patches on the underside, while others have a more mottled appearance. More often, the only conspicuous feature of this great whale are the irregular patches of thickened tissue, called callosities, on the head. These callosities are inhabited by many small amphipods, known as cyamids or whale lice, and form a pattern unique to each individual whale, thus providing a means of identification. 

The North Atlantic right whale lacks a dorsal fin, but has large, paddle-like pectoral fins used for steering, and an enormous tail that provides propulsion with powerful vertical strokes. The downward-curved mouth of the North Atlantic right whale contains between 200 and 270 baleen plates on each side of the upper jaw. These plates, each measuring around two meters long and fringed with fine hairs, replace teeth in Balaenidae whales, and are central to their method of feeding. The North Atlantic right whale has two blowholes situated on top of its head through which it breathes, producing a distinctive, bushy, v-shaped cloud of spray when it exhales at the surface.

10 Fun Facts about North Atlantic right whales:

Did you know?

1. The right whale's scientific name, Eubalaena glacialis, means "good, or true, whale of the ice."

2. Distribution pattern – mostly found along the Atlantic Coast of North America. Now believed to only survive on the east coast of the North America and Canada.
 

3. Scientists recognize individual whales based on their pattern of callosities – the white patches of rough skin that give right whales their characteristic appearance.

4. The NARW usually do not fear boats and can be easily approached by boats in the water. In result, this is one of the main reasons the NARW is endangered because they become involved in ship collisions and entanglement in fishing gear. 

5. It is estimated that North Atlantic right whales live up to 67 years. 

  

6. They were named 'right' whale by fisherman, because they thought it was the right whale to hunt, as they swim close to the shore, float when killed and 40 % of their body is blubber. In other words, their slower pace, the fact that they come close to land, their tendency to float after being killed and their "productivity" in terms of oil made them lucrative animals to target.

7. Baleen whales feed with their baleen; they 'skim' the water with their mouth open. Water and prey come into the mouth of the whale, but only the water can pass through, leaving prey like zooplankton, krill, and little shrimps behind.  

8. A young / baby of a northern right whale is called a 'calf'. The females are called 'cow' and males 'bull'. A northern right whale group is called a 'gam, pod or herd'.

9. Average weight of adult – 50,600 lbs.

10. There are about 300 Atlantic Northern right whales living today, almost all in the western North Atlantic, and are endangered species. The North Atlantic right whale is one of the most endangered of all large whales, with a long history of human exploitation and no signs of recovery despite protection from whaling since the 1930s. Between 300 and 350 individuals still exist, but despite seven decades of protection efforts, no population growth has been observed.  

I had been eagerly awaiting the presence of the North Atlantic right whale for 4 months during my time in the Massachusetts Bay - "The Humpback Whales in the Sunset; a Mission for Marine Mammal Preservation." My last month for the project was in December and I still had yet to see this magnificent creature. I heard of one other boat working on the prospect that had briefly seen the North Atlantic right whale. Our rotation was soon coming to an end, but I was determined to not give hope of seeing the critically endangered species. I will never forget the afternoon when I finally laid eyes on the whale that I was searching for the entire last half of the year. It was your typical hot afternoon with a splash of strong sun glare rested on the water. I was talking to Kyle and Whitney (more about them in next post) right after lunch outside on the port bridge wing. All of a sudden the three glance at the water immediately because something had caught each of our eye’s. To our surprise we observe a large splash! The girls and I quickly skimmed the water and discovered a peculiar shape in the water. Distinctive characteristic that we perceived were the lack of dorsal fin on this whale and the “V” shaped blow. We had begun scratching our heads. What was this whale and why is it so unique?  Almost immediately it hit us, we were staring at the North Atlantic right whale! Unfortunately, it was a quick sighting and there were no photographs taken. Definitely a remarkable day in the beginning of my long-term career!

Cool fact: In 2007 and 2009, the US government changed shipping routes out of Boston in an attempt to reduce collision. NOAA estimates that implementing an "Area to Be Avoided" and narrowing the "Traffic Separation Scheme" by one nautical mile will reduce the relative risk of right whale ship strikes by 74% from April to July! While NOAA was changing their shipping routes, I was studying the North Atlantic right whale’s migration route. Some of our data was used to determine the effectiveness of the newer implementation of the shipping route. As a result of the population dynamics of the North Atlantic Right Whale, I would say that it was a success!

 

HUMPBACK WHALE (Megaptera novaeangliae):

Humpback whales are known for their magical songs, which travel for great distances through the world's oceans. These sequences of moans, howls, cries, and other noises are quite complex and often continue for hours on end. Scientists are studying these sounds to decipher their meaning. It is most likely that humpbacks sing to communicate with others and to attract potential mates. These whales are found near coastlines, feeding on tiny shrimp-like krill, plankton, and small fish. Humpbacks migrate annually from summer feeding grounds near the poles to warmer winter breeding waters closer to the Equator. Mothers and their young swim close together, often touching one another with their flippers with what appear to be gestures of affection. Females nurse their calves for almost a year, though it takes far longer than that for a humpback whale to reach full adulthood. Calves do not stop growing until they are ten years old.

Humpbacks are powerful swimmers, and they use their massive tail fin, called a fluke, to propel themselves through the water and sometimes completely out of it. These whales, like others, regularly leap from the water, landing with a tremendous splash. Scientists aren't sure if this breaching behavior serves some purpose, such as cleaning pests from the whale's skin, or whether whales simply do it for fun.

What is a humpback whale? The Humpback whale (Megaptera novaeangliae) is the fifth largest of the great whales. Their scientific name comes from the Greek word ‘mega' meaning ‘great' and ‘pteron' meaning ‘a wing', because of their large front flippers that can reach a length of 5 meters, about one-third of their entire body length! They are named humpbacks because of the distinct ‘hump' that shows as the whale arches its back when it dives.

What do they look like? Humpbacks are ‘rorquals', whales which have distinctive throat grooves. They also have knobs on their heads known as ‘tubercles', each of which has a long coarse hair growing from its centre which is believed to act as a sensor. They have very long flippers (more correctly known as 'pectoral fins') with knobs on the front edge, and a humped dorsal fin. They are blackish, with white undersides and sides. The underside of the tail fluke is usually white with black patterning, which is unique to each humpback, like a fingerprint, so can be used to identify individual whales! Males average 14.6 meters and females 15.2 meters long. The maximum length is 18 meters and a mature adult may weigh up to 45 tons. Humpback whales have a life expectancy of 45 to 50 years.

Where do they live? Humpback whales are found in all of the world's oceans, although they generally prefer near-shore and near-island habitats for both feeding and breeding. From June to July the Australian humpback whales migrate northwards to their tropical calving grounds in the Kimberley and between September and November they begin travelling south to their feeding grounds in Antarctic.  During their southerly migration you can sometimes see these beautiful animals from the shore, as they pass along the WA coast including through the proposed Dampier Archipelago Marine Park, the Montebello Islands Marine Park, Ningaloo Marine Park, Shark Bay Marine Park, Jurien Bay Marine Park, Marmion Marine Park and the proposed Ngari Capes Marine Park (which will protect the waters between Busselton and Augusta). As they are often accompanied by their calves on the southern migration, they tend to stay much closer to shore than they do when heading north.

How are they identified? Humpbacks are solely identified by the markings on the flukes - Researchers have discovered that individual Humpbacks can be identified by the black and white patterns on the underside of their flukes, similar to fingerprints for humans. Researchers take pictures called fluke ID's and compare them to ones taken before; this helps them determine the Humpback’s distribution pattern.  Humpbacks have five basic types of whale flukes: Mostly white, 25% white, half white and half black, 25% black, and last mostly black. Often one can eliminate three types, and look through only two types. Sometimes one has to look through three types, especially if not 100% of the fluke can be seen. Sounds like a difficult process!

What do they eat and how? Humpbacks are ‘baleen' whales, so instead of teeth they have 270-400 baleen plates which hang from the top jaw. They feed by taking big gulps of water and filtering shrimp-like krill and small fish between these plates. An average-sized Humpback Whale will eat 4,400-5,500 pounds (2000-2500 kg) of plankton, krill and small, schooling fish each day during the feeding season in cold waters (about 120 days). They eat twice a day. Humpbacks cooperate in hunting and have developed a method of rounding up highly concentrated masses of prey that is called “bubble netting.” The hunting members of a pod form a circle 10-100 feet (3.1-31 m) across and about 50 feet (15 m) under the water. Then the humpbacks blow a wall of bubbles as they swim to the surface in a spiral path. The cylindrical wall of bubbles makes the trapped krill, plankton, and/or small fish move to the surface of the water in a giant, concentrated mass. 

Do they sleep? All whales sleep and they stay at the top of the water, with their blowhole above the surface. In order to make sure that whales perform the basic functions to breathe, only one half of their brain will sleep at a time. This is the only way that they are able to get the amount of rest that they need and still take care of this function that is necessary for their bodies to survive.

Besides humans, what are their other threats? Humpbacks can live up to 45-50 years.  In the past, Humpback Whales were heavily exploited by commercial whalers all around the world, hunted for their oil, meat and whalebone. Hunting was banned in 1963 after the species became nearly extinct. In 2007, the Japanese proposed to resume killing the humpback for so-called ‘scientific purposes', but gave them a last-minute reprieve. Other threats to humpback whales include them ingesting plastic, which accumulates in their gut and leads to a slow death, and becoming entangled in crayfish lines and pots. Supported by the Department of Environment and Conservation, the Western Rock Lobster Council has introduced a Code of Practice to help reduce whale entanglements. Natural predators include killer whales which prey on the young humpback calves.

What is their behavior? Humpbacks travel in groups known as “pods”. When in a playful mood, they may put on spectacular displays of breaching, rolling, spyhopping, slapping their pectoral fins and generally having a ‘whale' of a time. During the breeding season, the humpback males are known for singing the longest and most complex songs in the animal kingdom. An adult humpback's two lungs, each the size of a small car, are emptied and refilled in less than two seconds. As it surfaces, the humpback exhales through two blowholes on the top of its head. The air is expelled and cooled so rapidly that it forms a distinctive cloud, which is often mistaken for water! Why do Humpbacks breach? For humpback the purpose for this exciting display of power is not completely understood, but has been suggested to be a way to lose parasites, communicate long distances, or simply play.  Orcas tend to use This behavior has been suggested to be a form of long range communication with other whales, a display of dominance, a way to observe above the water, and just for fun - herding and concentrating feed.

What is their conservation status? Humpback whales are threatened and they are specially protected under the Wildlife Conservation Act. It is estimated there are over 10,000-15,000 Humpbacks world-wide; however, they are indicated as an endangered species.

How you can protect the Humpback Whale? Follow the whale watchers code. Boats should not approach closer than 100 meters to a whale, a vessel should not separate a group or mother and calf and, if you are in the water and a whale approaches, you must stay at least 30 meters from the whale. If you should spot a whale entangled in rope or fishing gear call the Department of Environment and Conservation so the department's special whale and dolphin disentanglement team can help the animal.

The Ultimate Experience with a Humpback Whale and her Calf:

Once the end of the Boston project approached, I successfully identified and witnessed 375 Humpback whales. I had a lot of great moments with Humpbacks, but my ultimate favorite happened on an early evening in December 2007. I had just finished eating dinner and was walking directly outside to watch the sunset over Boston. I had begun skimming the water for any signs of whales and/or dolphins in the immediate area. I must have been looking about 1 kilometer from the boat off the port side, when all of a sudden I felt a cold gush of water splash me in the face! Immediately shocked and startled, I glanced down to see a whale and its eyes looking right at me! Not just any whale, this was a Humpback whale with her calf! I must have stared at her right in the eye for a solid 5 minutes; she was only a few feet from the bottom of the boat! The massive size of her flippers, her white patched fluke, and the size of her dorsal fin all caught my eye! I remember thinking how gorgeous this mammal was and how amazing it was to see her and her calf so up-close and within my personal view!  I could never put this moment into words (I tried dozens of times), but the adrenaline that overcame my body and the sheer excitement that I felt when first saw the Humpback’s eye looking at me was absolutely INCREDIBLE! Not to mention the goose-bumps that rose all over my body, my heart pounding violently out of my chest, and my eyes in awe of such a majestic creature will forever live on in my memory of that fantastic night!

*All North Atlantic right whale photographs are taken by various National Geographic Biologists. In addition, all Humpback photographs are provided by a respectable colleague, friend, and biologist Juliet Shrimpton.

The Humpback Whales at Sunset; a Mission for Marine Mammal Preservation

Two Humpback Whales; Atlantic Ocean: Texas Horizon 08.2007

Location of Northeast Gateway Pipeline Project: 

My first offshore assignment was the construction and implementation of the Northeast Gateway Pipeline Project (NEG). Northeast Gateway Energy Bridge, L.L.C. acquired a license to construct, own, and operate the Northeast Gateway Deepwater Port (NEG Port or Port). This NEG was located approximately 13 miles southeast of Gloucester, Massachusetts. The Port, which will be located in Massachusetts Bay, will consist of a submerged buoy system to moor specially designed Liquefied Natural Gas (LNG) carriers approximately 13 miles offshore of Massachusetts in Federal waters approximately 270 to 290 feet in depth. The facility will deliver regasified LNG to onshore markets via new and existing pipeline facilities owned and operated by Algonquin Gas Transmission, LLC (Algonquin).

Two Humpback Whales Blowing and Tail Slapping; Atlantic Ocean: Texas Horizon 09.2007

Main Objective of Northeast Gateway Pipeline Project:

During this time, Marine Mammal Scientists or also recognized as Marine Mammal Observers (MMOs) were placed on the project to document all marine mammal sightings, correspond with Cornell University and Woods Hole Oceanographic Institution, and enforce specific guidelines into the project’s structure. The guidelines applied by the Marine Mammal Scientists were to shut down operations in severe environmental conditions (thick fog), a Humpback whale within 100 meters of the ship (within exclusion zone), and if there was a sighting of the North Atlantic right whale (NARW) close to the ship. In accordance with enforcing these guidelines, the United States Coast Guard (USCG), the National Oceanographic and Atmospheric Administration (NOAA), the Commonwealth of Massachusetts and other Federal and State agencies has established a program for preventing, monitoring, and mitigating environmental impacts (Prevention, Monitoring, and Mitigation Plan [PMMP]). The PMMP integrated environmental permits, certificates, licenses, and approved monitoring and mitigation plans obtained by the NEG and Algonquin – this supported the collective pre-construction, construction, post-construction, and operation of the NEG Port and Pipeline Lateral (an extended piece of the pipeline). Without getting to technical, the PMMP served as a purpose to support the requirements that helped minimize adverse impacts to marine mammals. This objective mixes into the overall composition of working with the Endangered Species Act (ESA) Biological Opinion (BO), the Marine Mammal Protection Act (MMPA), Incidental Harassment Authorization (IHA), and Incidental Take Statement (ITS) as amended, and the National Marine Sanctuary Act (NMSA) Section.

Close-up of Humpback Whale; Atlantic Ocean: Texas Horizon 09.2007

In other words, this project had to be executed with careful consideration towards the marine mammals, especially the critically endangered North Atlantic right whale (Eubalaena glacialis) (NARW) that were subjected to the operation of NEG. The NARW was a rare occurrence in these waters, but the fact of the matter, was that they did exist – this created an effective approach in the NEG project to help conserve them and their environment. NOAA/National Marine Fisheries Service (NMFS), has determined that serious injury or mortality of even a single individual of the critically endangered North Atlantic right whale could jeopardize this species’ continued existence. 

Humpback Fluke in Strong Sun Glare; Atlantic Ocean: Texas Horizon 09.2007

In addition, serious injury or mortality to other large whale species that frequent greater Massachusetts Bay waters, including North Atlantic fin whale (Balaenoptera physalus), Humpback whale (Megaptera novaeangliae), Sei whale (Balaenoptera borealis), and Blue whale (Balaenoptera musculus), is also prohibited due to their endangered status. In addition, the Minke whale (Balaenoptera acutorostrata), Common dolphin (Delphinus delphis), Long-finned pilot whale (Globicephala melas) Harbor porpoise, (Phocoena phocoena), Harbor seal (Phocac vitulina), and Gray seal (Halichoerus grypus) also were taken in account for. Therefore, Federal actions that could lead to even a very small increased risk of serious injury or mortality must contain plans to mitigate the potential impact of those actions to these species. Specifically, Federal agencies whose actions may affect endangered and/or threatened species must consult with NMFS as specified under the implementing regulations for the ESA. Any harassment to any marine mammal species due to the licensed activity must also be permitted by NMFS as specified under the MMPA. 

Humpback Whale in Exclusion Zone; Atlantic Ocean: Texas Horizon 08.2007

In other words, these federal agencies were taken effective measures and implementing procedures into the design of the NEG project to protect the environment and its inhabitants. Honestly, this is why I was presently there – to help oversee our guidelines enforced and carefully monitor the area for marine mammals. Our project consisted of a dive bell, which concealed commercial divers who were constructing the pipeline below the ocean’s surface – the guidelines were also established to reassure their safety while they were working at critical depths.

North Atlantic White-Sided Dolphin; Atlantic Ocean: Texas Horizon 10.2007

The project plan emphasized that NMFS did not expect any whales to be injured or killed by these activities. However, planned monitoring and mitigation measures were designed to avoid sudden onsets of potentially disturbing noise, detection of marine mammals occurring near the activities, and avoided exposing them to sound sources that may have caused hearing impairment. NMFS had established guidelines for what constituted harassment and acoustic takes on marine mammals under the Marine Mammal Protection Act (MMPA) and the Endangered Species Act (ESA). Two levels of harassment had been defined in the MMPA: Level A harassment with the potential to injure a marine mammal in the wild, and Level B harassment with the potential to disturb a marine mammal in the wild by causing disruption to behavioral patterns such as migration, breeding, feeding, and sheltering. The current thresholds were 180 dBL for Level A harassment, and 160 dBL (impulse) and 120 dBL (continuous) for Level B harassment. If you are interested in learning about Cornell University’s and Woods Hole Oceanographic Institution acoustic concepts, additional information is listed below.

My First Sighting of Megaptera novaeangliae:

I will never forget my first “sighting” of a Humpback whale (Megaptera novaeangliae); I waited patiently observing the water for 3 days until I had my first encounter with a Humpback whale. Of course, my lead Meghan seemed to sight the marine mammals right away without any hesitation or uncertainty in the identification of the marine mammal. I had confidence that I would also develop the trained eye to easily spot marine mammals within the close proximity of the ship; however, I was praying that I would begin spotting marine mammals very soon! That particular afternoon as I stood looking at the sun, I started skimming the horizon for any visible blows. To my surprise, I discovered 4 bushy blows that were gliding along the horizon. 

Humpback Fluke Lowers in Water; Atlantic Ocean: Texas Horizon 09.2007

As I darted across to the port side of the ship to get Meghan’s attention, I almost slipped on some water dripping off the bridge deck. I was literally out of breath and all I could gesture was to grab her binoculars and follow me to the starboard side, where the sighting had taken place. Meghan looked into her binoculars in the direction that I had pointed out and immediately noticed them as well. I remember her telling me that I did a fantastic job observing the marine mammals from such a far distance away. I walked into the bridge grabbed the necessary information for our logs and enjoyed the rest of the sighting with Meghan. Would you believe that the Humpback whales remained in the same area as sunset quickly approached? What a great sighting! I love sunsets and observing Humpback whales blowing and breaching as the sun set was an incredible moment that I will never forget!

“Getting my Feet Wet”:

Once I was able to “get my feet wet” (literally), then I was finally able to understand the project and its primarily mission. I was happy to see that many precautions were taken in the overall design of this pipeline. The marine mammals were being observed by me and several other colleagues. Our working hours were increments of 12 to 12; meaning, 2 people would work midnight to noon, while the other group would work noon to midnight. There was always someone on watch, regardless of the time. Fortunately, Meghan and I were able to work noon to midnight and left the boys observing midnight to noon. 

Minke Whale Detected on IR Scanner; Atlantic Ocean: Texas Horizon 10.2007

During the night time monitoring we had to introduce an Infra Red (IR) scanner into our daily routine, which detected the marine mammals’ presence on a tiny screen. The idea of the IR scanner was to identify main differences in water temperatures. Most of the time the screen displayed blue, which was a representation of cold water; however, only a few times there was a red color shown - this was evidence that a marine mammal was under the surface at that exact spot where you have your IR scanner pointed. I had only one nighttime sighting with the IR scanner, but it was still a pretty cool method to effectively and efficiently detect marine mammals in nighttime conditions. 

Single Fin Whale; Atlantic Ocean: Texas Horizon 09.2007

After this initial sighting, I gained a great deal of confidence and was ready to play, “Where’s the cetaceans (whales and dolphins)?” I seemed to catch on to the identification of species pretty quickly; I soon was detecting marine mammals quite often. For my second week on the NEG project, we had noted 50 Humpback whales, 20 Fin whales, and 2 North Atlantic white-sided dolphins (Lagenorhynchus acutus). Needless to say, I was able to photograph many marine mammals, sunrises, sunsets, and any other wildlife in the immediate area. What I really enjoyed on the NEG project was the enthusiasm for marine mammals and their conservation status. I had the opportunity to chat with some biologists from Cornell University’s Bioacoustics Research Program and the Woods Hole Ocean Institution about the buoy-system that they had placed next to our ship for the sole detection of the NARW. The buoy coordinates were as followed - NEG Port was located at 42º 23’ 38.46” N/70º 35’ 31.02” W for Buoy A and 42º 23’ 56.40 N/70º 37’0.36” W for Buoy B in Massachusetts Bay.

Two Humpback Whales Fluke and Tail Slapping; Atlantic Ocean: Texas Horizon 09.2007

If the Marine Mammal Scientists spotted the NARW, then immediate attention would be given to the situation and the two research teams would be called. During this call the coordinates of our position, environmental conditions, observations, and any other relevant information would be provided.  Those two research teams were the main consultants for developing, implementing, collecting, and analyzing the acoustic data; reporting; and maintaining the acoustic monitoring system. It was nice to work together with a bunch of individuals that could talk about marine mammals all day to you. I could not have asked for a better project, especially for my first one.

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Underwater Acoustic Concepts:

The loudness of sound is dependent on the radiated sound power of the source and the propagation and attenuation characteristics of the medium through which the sound passes (sea water). The standard unit of sound is the decibel (dB), a logarithmic scale formed by taking 20 times the logarithm (base 10) of the ratio of two pressures: the measured sound pressure divided by a reference sound pressure. For underwater sound, this reference sound pressure is 1 micro-Pascal (μPa). The hearing capabilities and frequency (Hz) responses of marine mammals vary significantly. Therefore, underwater sound levels are typically expressed using unweighted or linear broadband levels (dBL) spanning the entire frequency spectrum under consideration. (For this study, the frequencies analyzed span 10 Hz to 20k Hz). The National Marine Fisheries Service (NMFS) criteria used to assess impact and determine the potential of acoustic take or harassment are also presented in dBL sound levels.

Sound sources are typically presented as sound pressure levels at a distance of 1 meter from an idealized point source, i.e. dB re 1 μPa at 1 meter. This standardized reference distance was developed to allow for direct comparison of different sound source levels. Received sound levels include the effects of propagation and attenuation that occurred between the source and receptor. Under standard propagation conditions and in non-shallow water environments, received underwater sound levels lower at a horizontal distance 100 meters away from a source will be approximately 40 dBL lower than the source level at a reference of 1 meter. However, because many man-made underwater sound sources have dimensions that are much larger than an idealized point source, the relationship between near-field and far-field sound levels is more complicated than this simple rule and must therefore be determined through field measurements. In the acoustic near field, propagation losses will be generally lower than expected. Conversely, received source levels extrapolated from far-field measurements will be higher when the acoustic energy from a large area source is back-calculated to characterize an idealized point source. To account for sound propagation resulting from a large area source such as the Energy Bridge Regasification Vessel (EBRV), the transition from the acoustic near to far field, as well as the site- specific characteristics, must be well understood.

The propagation and attenuation of sound waves under water is a complex phenomena influenced by gradients of temperature, water column depth, salinity, currents, sea surface turbulence and wake bubbles, scattering by seafloor and surface, etc. Within close range of the sound source, attenuation and propagation losses are primarily driven by geometric spreading, i.e. sound levels decreasing with increased distance from the sound source as the sound energy is gradually spread across increasingly larger and larger surfaces. In unbounded sea water, free field spherical wave spreading will occur at a decay rate of TL = 20 log R, where R is the horizontal propagation path between the source and receptor in meters and TL symbolizes sound energy transmission loss. 

Extensive research has demonstrated that spherical wave spreading, together with seawater absorption rates, provides a reasonable fit to measured underwater sound levels under a wide variety of conditions. Because the ocean is bounded by the surface above and the seafloor below, additional adjustments must be made. When the propagation path becomes greater than the water depth, free field spherical spreading can no longer continue. If perfectly reflective boundaries were assumed, the spherical wave spreading would transition to cylindrical spreading, represented by the decay rate of TL = 10 log R. However, to account for the fact that neither the surface or seabed floor are perfectly reflective, modified or transitional cylindrical spreading represented by decay rate of TL = 15 log R has been shown to have the best fit when compared to actual TL measurements made at sea. At horizontal propagation distances much greater than the depth, standard cylindrical spreading combined with a linear (dB per km) absorption and scattering rate provides conservative modeling results.