Surrounding Wildlife and…Whoa, How Fast is that Sound and Cream Pie?

September 19, 2008 · Filed Under Journey · Comment

Jessica Robertson, U.S. Geological Survey Public Affairs Specialist

The weather has been better these last couple days, so we have had several people traveling back and forth between Louis and Healy. Visitors are given briefings and tours to learn about ship operations and experience life onboard. We have also started a series of evening presentations by scientists and crew members so we can understand each other’s responsibilities and work on Healy. Yesterday, the Canadian Coast Guard won second place in our pie eating contest, too!

After our polar bear sighting the other day, I have become increasingly curious of what other wildlife we might come across. We have two people onboard Healy who record the presence and proximity of species we encounter. So far we have only seen two polar bears (parent and cub), a few seals, and a glaucous gull, but we could see whales, walruses, and several other bird species as we near Barrow, Alaska. Both observers inform U.S. Coast Guard personnel when an animal is nearby so we can alter our path and ensure we do not disturb them in their native habitat. Our marine mammal observer is Justin Pudenz, who is contracted by NOAA, and our community observer is George Neakok, who works for the Barrow Arctic Science Consortium and reports information to the Alaska Eskimo Whaling Commission and the North Slope Borough government. George also educates the crew about the local communities’ lifestyle and use of the Arctic. In addition, he is helping to ensure that when close to land, we do not bother nearby hunters.

Next, I want to explain an essential factor that must be measured to accurately determine ocean depth and map the seafloor. That factor is the speed of sound through seawater. I hope this isn’t too much detail, but without this calculation, our representations of the seafloor would be inaccurate.

In previous journal entries, I explained how scientists are using echo sounders to map the Arctic seafloor. Sound signals are sent into the ocean and the total time it takes for that energy wave to hit the seafloor and bounce back is recorded. That timing, however, does not directly tell you ocean depth. The speed of sound through the water needs to be considered in the equation. There are several ways to determine this speed, which is calculated using temperature, pressure (which increases with depth), and salinity data.

The speed of sound varies continuously with depth beneath the ship. Once you know the salinity, temperature, and pressure at multiple locations under the ship, you can calculate how the speed of sound varies at those different depths. In this case, the speed is measured in meters per second. When you combine that information with the total time it takes for sound to bounce off the seafloor and return to the surface, you can calculate how deep the ocean is.

One way to calculate this speed is using a hand-held launcher that sends a probe into the water to measure depth and temperature variation below the ship. The instrument is called an Expendable Bathythermograph (XBT) and is used on Healy. “Bathy” means depth and “thermo” means temperature. A salinity value based on preexisting research is used in the computation since the XBT does not measure salinity. Healy also has a Conductivity Temperature Depth Profiler (CTD), which I briefly discussed in a previous blog post. Conductivity is used to determine salinity, so this instrument provides a more accurate sound velocity profile than the XBT calculates.

Louis also obtains sound speed data using a Sound Velocity Probe (SVP), which measures speed directly. It is not expendable like the XBT and takes up to 2 hours round trip to lower to the ocean floor at 3,800 meters. It can only be used when the ship is stopped and not surrounded by thick ice. Louis is also using an Expendable Conductivity Temperature Depth Profiler (XCTD), which uses similar sensors to Healy’s CTD but it is expendable. Louis and Healy take turns collecting these data and share findings with each other so efforts aren’t duplicated.

Don’t forget to check out my new audio files, including the sounds of ice breaking and a conversation between Healy’s and Louis’s captains, in the posts below!

Until next time,

Jessica Robertson

: USGS oceanographer Ellyn Montgomery and Shigeto Nishino with the Japan Agency for Marine Earth Science & Technology (JAMSTEC) discuss measurements in a lab on Healy. Shigeto has spent three months on Louis this summer using XCTD’s to characterize the physical properties of the top 1000 meters of the Canada Basin and adjacent regions of the Arctic.

: U.S. and Canadian Coast Guard members discuss their duties and services.

: Marine Mammal Observer Justin Pudenz, who is contracted by NOAA, peers through his binoculars. Community Observer George Neakok, who works for the Barrow Arctic Science Consortium, joins him to see what’s going on.

: Canadian Coast Guard member takes a turn driving Healy.

: Group of Canadian and U.S. Coast Guard members gathered in the Aloft Conn.

: Working together to get suited up before helicopter operations.

: Visitors from Louis learn about Healy bridge operations.

: Canadian helicopter preparing to land on Healy.

: Canadian helicopter circling Healy before landing.

: Preparing to launch the Expendable Bathythermograph (XBT) to help calculate the speed of sound in the ocean. The launcher sends a probe into the water to measure depth and temperature variation below the ship.

: Expendable Bathythermograph (XBT). There is a copper wire attached to both the launcher and the probe. When the probe is released, the wire unwinds from a spool as it descends. During this descent, the probe sends temperature data back to the launcher by means of an electrical signal through the wire. The launcher itself is connected to a computer that assigns each temperature value to a certain depth. The depth point is calculated using a predetermined rate at which the probe falls.

: Participants in our pie eating contest. How hungry are you?

Tags: canadian crew, measurements, probe, sound, xbt

Breaking Ice and Mapping New Ground

September 8, 2008 · Filed Under Journey · Comment

Jessica Robertson, U.S. Geological Survey Public Affairs Specialist

I had to catch my balance when climbing down my bunk ladder yesterday morning as our ship was swaying back and forth due to the wind and ocean waves. I peeked out my window to see what was going on and there was my first glimpse of bright blue sea ice. As the day progressed, we traveled through thicker ice, bringing colder weather and a bit more shaking. This made my run on the treadmill quite the adventure, but I was up for the challenge!

I ventured toward the labs yesterday afternoon and sat with some of our science team asking them for a rundown of each of the data collection systems. I was informed by USGS scientist Bill Danforth, who was on watch duty during my inquiring state, that we are using three major systems to collect data. So, here’s an overview of what I learned.

To map the seafloor, scientists are using the multibeam echo sounder. Some of you may be wondering, as I was too, what that means exactly. This system emits a sound pulse or “ping” into the water beneath the ship and onto the seafloor. That signal bounces off the seafloor and transmits an energy wave back, providing data on the seafloor depth. The deeper we are, the longer it takes for the signal to reach the floor and come back. Unlike other “single beam” echo sounders, the multibeam system’s sound pulse is received back at the system as a series of “beams” that are translated into depth data points across a wide but narrow swath of the ocean floor. This allows scientists to collect anywhere from 61 to 121 depth points at a time along a narrow line perpendicular to the ship’s direction of travel for each ping. This process repeats every 10 seconds in the depths we are working in (3500 – 4000 meters), and the result is compiled into a map that shows the depths of the ocean floor along the ship’s track in a swath up to 8 kilometers wide in some areas.

Another system is the sub-bottom seismic reflection profiler, which essentially measures the geologic structure of the sub-seafloor. To do this, a sound pulse is transmitted that penetrates through the seafloor. A strong return, or “reflector”, shows up as a black line in the data profile, and indicates a change in composition of the material beneath the seafloor as the sound travels through the sediment layers. Many of these reflectors can be present; the orientation of these reflectors to each other help the scientists to interpret the geologic history of the area are studying.

So, what do these pulses sound like? There are two sounds that can be heard by the human ear. The multibeam sends out a soft ping (12 kilohertz) and the sub-bottom profiler emits a chirp sound (3.5 kilohertz). While I can hear both these sounds faintly throughout the ship-whether in my room, the science lounge or the mess hall-some people can’t hear the ping at all.

My roommate USGS scientist Ellyn Montgomery and I stood in our room for awhile as she closed her eyes attempting to hear the sound. She was not successful, but I am sure she will keep trying!

The third thing we are measuring is the surrounding gravitational field using an instrument called a “gravimeter”. To us, the gravity field of the earth feels constant, but the composition, structure, and density of subsurface materials affect the gravitational field of the earth very subtly. When a change in the gravity field is detected by the instrument, it suggests a change in the geologic subsurface beneath us and provides insight on the type of rock and structure of the seafloor that may be present. A higher density material, such as volcanic lava or basalt, will have a higher gravitational force than a lower density material such as sedimentary rocks like shale or limestone. The gravitational field can also impact the ocean’s surface level. For example, a stronger gravity field will lower the water surface slightly, like a dimple on the sea.

So that’s the brief rundown. Thanks to everyone for your kind emails and messages on the blog site! I look forward to hearing more of your comments and updates of life back on land.