McMurdo Days 23-25: Test Plot 1

It was extremely foggy on Wednesday, so we took a break from digging at the Castle Rock test site. Instead, we set up the datalogger and modem for the new system in the Crary lab to make sure that we could telemeter data back to PASSCAL in New Mexico.

We got someone from PASSCAL to confirm that data was being transmitted, so the equipment is all good to go for Castle Rock!

Centaur datalogger (left, white) and XI-100b modem (center, yellow) set up in our Crary office.

The grad students and mountaineer on the POLENET team were supposed to fly out on Wednesday, but the weather was so bad that their plane got delayed. They have officially departed as of Thursday, and made it safe and sound to New Zealand.

It was extremely foggy on Wednesday.

We spent a few hours on Thursday and Friday setting up the first of three systems we'd like to test before next year. Finally, setting up equipment instead of digging it out!

It was still a little foggy on the ice sheet in the distance on Thursday.

These three systems are going to be deployed on Mt. Erebus next year. We are testing them at Castle Rock to make sure the equipment works together.

One of the main things we are testing is the ability to telemeter seismic data back to PASSCAL. The experiment on Erebus would like us to telemeter some of the seismic data back. Pushing our existing equipment to its limits, we can transmit 20Hz data (that is, 20sps or 20 samples-per-second) at a 5 hour latency (the data arrives in New Mexico 5 hours after it was collected). The seismic data is collected by the Centaur at 100Hz. Seismologiest prefer higher sample rate data to do any real science -- 20Hz is not a lot by seismologist standards.

Even though it's still limited, this is pretty exciting for us because it's the first time we are trying it out. For most experiments, it is way too power-intensive to transmit that much data back. The stations on Erebus are intended to be more permanent, so we can haul up more batteries on our helicopter flights up next year.

Large orange box (center) contains Centaur and modem. The antennae are mounted on the solar panels (you can't really see them at this angle). The seismometer is connected to the black cable running towards the camera (off screen).

The system we installed this week (which we are calling Test Plot 1) includes:

• Centaur datalogger, which digitizes the analog signal from the two sensors connected to it, stores data to an SD card, and connects to a GPS antenna to ensure accurate timing of the data. The Centaur lives inside the large orange box, and the GPS antenna attaches to the solar panel pole outside.
  
• XI-100B modem and Iridium antenna, which telemeter small amounts of state-of-health (SOH) and seismic data back to PASSCAL over the Iridium satellite network (which has good coverage at the poles). The modem lives in the orange box, and the Iridium antenna attaches to the solar panel pole as well.
  

The Centaur, XI-100b, and assorted cables inside the orange enclosure. The blue box (junction box) breaks out the power coming into the enclosure.

• Horizon V2 seismometer. This is a direct-burial seismometer, meaning we just place it directly in the snow and bury it. Other, older seismometers require you to level the it on a foam tile and cover it in a large barrel or dome. Direct-bury are much easier to install. Seismometers are installed under the snow to reduce wind noise in the data.

When you install a seismometer, you also have to make sure it is level and oriented the correct direction so that the data can be properly analyzed. Seismologists can use the data to determine the directions that seismic waves come from -- so the sensor must be installed facing north. I used a compass to make sure it was facing true (not magnetic) north. Before we went out, I looked up the magnetic declination (the angle the mag. north pole is from my current location) so that I could adjust the compass and know which way true north is. Since we are so close to the magnetic south pole, this number was huge -- the compass told me mag. north was 140° away from where true north really was.

Another fun fact about orienting seismometers at the poles: since we are so close to the mag. south pole, the compass needle doesn't want to point horizontally; it actually tries to point more vertically upwards towards the pole (following the Earth's magnetic field lines). We use special polar-weighted compasses that weight the south arrow so it doesn't try to point upwards and get stuck.

Horizon V2 buried in the snow. I added a little more snow and put a piece of plywood on top.

Here I am orienting the seismometer with a Brunton compass.

Here is the seismometer now buried and covered in plywood. The bamboo mark the corners so we know where to dig it out next year. I added more snow on top of the plywood after this picture.

• Chaparral 64Vx2 infrasound sensor, in a custom wooden box Avi made. Infrasound sensors measure pressure created by low-frequency sound waves (less than 20Hz). The actual sensor is a small black disc (about 4" in diameter) with 3 ports. We put the sensor in a nice wooden box to protect it. There are rubber hoses running from the ports on the sensor to the walls of the box. Next year on Erebus, the sensors will be placed in sturdier fiberglass boxes that I am designing.
  

Infrasound sensor, contained in wooden box. You can see the cable off to the right (which connects to the Centaur through the enclosure). A small hole in the side facing the camera connects to the infrasound sensor with a rubber hose.

• Windscreen covering the infrasound sensor. We can't bury the infrasound sensor like we can the seismometer because it detects changes in air pressure (above the surface!). So we cover it with a windscreen designed to reduce wind noise. These wind screens are nicknamed "tin hats".
  

Windscreen covering the infrasound housing and sensor.

• Custom charge controller with a board designed by PASSCAL's very own Engineering department. There is an extra board connected to our normal charge controller that will send back more detailed state-of health info about the battery bank and solar panels. The charge controller will live in the plywood enclosure with the 36 Li batteries. If the temperature of the batteries dips too low, the charge controller will disconnect and stop charging to avoid damaging the batteries. Since the temperature sensor is on the charge controller, it needs to be placed in the same enclosure as the batteries.
  

Custom charge controller. Doesn't it look sufficiently complicated?? The numbers on the LCD screen tell me that there are no Lithium batteries (Vp = 0), the AGM lead-acid batteries are at 14.9V, the solar panels are at 19.3V, and the solar panel current is 6.91A.

• XI-202 modem and another Iridium antenna, which are connected to the custom charge controller. The charge controller data will be telemetered back to PASSCAL. Hopefully, this will give us more information if the power system at a site goes out unexpectedly.
  
• Plus, the large orange enclosure for some of the equipment (see above), the plywood enclosure filled with 36 AGM batteries, and three 80W solar panels connected in series (that we previously excavated -- two still to go!).
  

Avi installing the charge controller with beautiful Mt. Erebus in the background!

There are a few smaller sub-systems we want to test at Castle Rock, but this is the main system. It feels great to finally have it set up!!

PistenBully 314 looking out at the fog.