McMurdo Day 27: IS55 at Windless Bight

I technically had Sunday off, but I went out to Windless Bight to help another group take down their camp instead.

The group manages stations for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). The CTBT is a treaty that bans countries from testing nuclear weapons, and they have stations all over the world that monitor for signs of explosions to ensure that no country breaks the treaty.

The group down here (event number T-396) is based out of Fairbanks, AK. They manage several of the US-owned CTBT sites, including a few in Antarctica and the US. There are three types of CTBT sites monitoring for signs of nuclear testing: seismic, radionuclide, and infrasound.

The seismic sites detect seismic waves traveling through the earth, created by earthquakes or by weapons testing. Radionuclide sites monitor for radioactive particles in the air (created by nuclear explosions). The sites take in air samples, catch airborne particles in a filter, and use a gamma ray detector on the filters, which creates a spectrum that is sent out to be analyzed.

The infrasound sites detect low-frequency sound waves, generally 20Hz or below (just out of range of human hearing). Infrasound waves are created by large explosions (e.g. nuclear testing), but they are also created by earthquakes, weather events, and volcanoes. If you know me, you know I am all about using infrasound sensors to monitor volcanoes (and I helped deploy infrasound sensors on Villarrica, a volcano in Chile, a few years ago).

The T-396 group had been camped out at their infrasound site at Windless Bight for the past few weeks, repairing the site and digging it out from the snow. It was time to take down their camp and pack up for the season, so Avi and I tagged along to get a tour of their system and lend a hand.

The Windless Bight field camp. Yellow tents to the left are for sleeping, the large blue huts in the center are cooking/working spaces, and the wooden shelter on the right is the outhouse.

We all piled in a Mattrack and drove out on the Ross Ice Shelf to Windless Bight. It was about a 2 hour drive from McMurdo in the Mattrack (which averaged 20-30 mph on the bumpy snow road). Windless Bight is, in fact, pretty windless, making it an ideal location for an infrasound station, as wind noise can make it difficult to use the infrasound data.

Here's a map from Wikipedia. McMurdo station is down to the right near Hut Point. The field camp is about 26km NE of McMurdo (as the crow flies; the drive was longer).

The Windless Bight infrasound array (station IS55 in the CTBT system) consists of 8 elements deployed in a circle around the field camp and power station. Power cables run out from the central power station to each of the elements. Each element consists of an infrasound sensor, datalogger, radio, and backup battery. The equipment is stored in a large, yellow enclosure (not unlike PASSCAL's own) -- except you can see that the yellow has faded to an off-white color after 20 years in the Antarctic sun.

One element in the infrasound array. Equipment is contained in the yellow/white enclosure. The radio antenna is to the left of the enclosure.

The data is transmitted by the radio and antenna back to McMurdo, where it is then sent via satellite back to the data center in Fairbanks, and then on to Vienna. The datalogger they use embeds a digital signature in the data, so that when the data reaches Vienna, they can verify that it has not been tampered with.

Small grey radio (top left), yellow datalogger (top left), backup battery (top right), infrasound sensor (center), and assorted cabling (bottom left).

To minimize wind noise, permanent infrasound installations usually use a system of long hoses connected to the ports of the sensor. These hoses extend out 5-10m along the snow, and there are a few small holes along the length. The idea of this is to create a large filter that averages out the wind noise over a larger area. The hose system should be larger than the scale of turbulence, but smaller than the wavelength of the signal you are trying to detect (for 1-20Hz sound waves, this would be 340-17m).

At the Windless Bight array, each element has 16 hoses -- 4 hoses come out from the sensor, which then each break into 4 more with a manifold (built by one of the T-396 folks!). There are 6 holes (1.6mm) in diameter) along each hose. For the stations on Erebus that PASSCAL is installing next season, we will not have the time or space to install a large wind noise reduction system with hoses; this is why we are just using the windscreen, as shown in previous posts (but for stations on the volcano, this windscreen should be effective enough at blocking out high frequencies).

My crude powerpoint drawing of the hose configuration at each element. The yellow square represents the enclosure (containing the sensor), and the blue lines are the hoses. Note: this is definitely not to scale.

Here's a recent report from Sandia National Labs on infrasound wind noise reduction systems if you want to learn more.

The 4-to-1 manifold showing how the hoses connect to the enclosure. There's one of these on each side.

The Windless Bight station is a little more power-intensive than PASSCAL stations. As in, orders of magnitude more power-intensive. PASSCAL stations use 3-7 watts of power; the Windless Bight infrasound array uses kilowatts.

Part of this is because they have more equipment: 8 elements that each have a sensor and datalogger. But a good chunk of the power usage comes from real-time telemetry of data. All the CTBT sites are required to transmit data back to Vienna, and this data has to be there within 5 minutes of it being collected.

Fancy nameplate for the station. Not totally sure why they had to specify "2001 A.D."

The system that powers the Windless Bight array is contained in a huge orange structure called Bob.

Bob, the central power station for the array. Solar panels are also on the back side. About 1m of snow accumulated around Bob each winter, so there are skis along the bottom to slide it out and raise it (otherwise, that would be hours of digging).

Bob is topped by several solar panels and houses 2 diesel generators and a giant battery bank, in addition to equipment to turn the power from AC into 64V DC, monitor the state of each component, and turn systems on/off remotely if needed.

The two diesel generators and accompanying control boxes.

This system is way more complicated than any of PASSCAL's ever need to be.

Rectifiers and breaker box. Plus a computer that can be remotely accessed to control all systems.

There are also several heaters along the inside to maintain a toasty temperature so that the generators will be able to function. They don't work well below 0C, apparently. We heated up our lunch on one of them.

Mmm toasty pizza.

After getting a tour of the array, I helped the T-396 group pack up camp. Mostly this consisted of moving boxes into the Mattrack (boxes containing sleep kits, extra food, tables/chairs, etc.) and.... more shoveling snow. But this time, with a snowmobile!!!!

My first time riding a snowmobile! Wheeeeee!!

I helped shovel snow to surround the base of Bob. They do this every year to prevent the snow from melting out from under Bob. We attached the sled to the back of the snowmobile and drove it out to a fresh snow patch away from camp. Then got out our shovels and loaded up the sled, drove it back to Bob, and heaped it in a big pile. So I was shoveling to build up, not shoveling to dig down. Completely different work.

Me on the snowmobile.

Windless Bight is more on the south-east side of Erebus, so I got some cool views. Until now, I've only seen Erebus from the S/SW.

My daily picture of beautiful Erebus!

I really enjoy learning about the systems that other groups use -- a lot of the equipment and engineering constraints are similar, but the scale is totally different. PASSCAL systems are too small and temporary to ever need a diesel generator, much less two!