Monday, October 25, 2010


The first ever Abstract to a blog post:
We have taken CO2 concentration and 13C isotopic signature measurements across an area of approximately 2.5 km^2 with about 22 km of traverses.  CO2 signatures were as expected (360 ppm, d13C = -8 for the majority of the survey) We have 1 clear signal from a "hotspot" CO2 source, while other would-be observations were fouled by pulling water into the sampling tube.  Pulling water into the sampling tube presents a number of problems including fouling the measurements and potentially damaging the equipment and a new design for the sampling tube has been implemented for the next days.  In addition, the survey goal has been refined to produce a high resolution dataset for the area of known seepage activity.

We got in a couple of hours of surveying on saturday and a full day on yesterday and finally have some good data to analyze.  It's a really rainy day today so as Lin finds his way home, we are sitting in the Green River Coffee Company with everyone else in town, working on the data.

The green line in the map above shows our saturday path, and the burgundy line shows yesterday's. Despite the near perfect weather, saturday's survey was cut short when the mule carrying the instrument briefly bolted as we traversed the area of CO2 seepage. Nothing was damaged, but a tube that is used to keep a vacuum in the instrument snapped at its fitting, and it was a repair that could only be made back at the hotel.  In addition, we got a truck stuck in deep mud yet again which would have prevented us from receiving a relief battery if the tube issue hadn't come up anyway.  Logistical problems have been limiting our progress and Lin, Taku, and I had a big discussion on saturday night to strategize how to avoid these issues in the following days. More on that later.

Yesterday, however, was a different story. We managed to paint 2.5 km^2 with 16 km of traverses in about 5 hours of surveying.  The terrain is both soft ground and high relief for walking (see the elevation profile below) and this seemed to be about what we could do without driving the animals too hard.  For the final 2.5 miles, we got off and walked with the mules to give them a little boost at the end.
The above graph plots the CO2 concentration in ppm for the traverses (latitude and longitude). Note that the map is slightly different as it was taken with the coordinates of the path that the instrument actually took while it was logging data, whereas the first image shows the coordinates taken with the GPS I was holding.  There are a couple interesting observations.  The dark red area consists of a series of measurements of CO2 concentration above 1400 ppm (see below) which apparently started as the machine was walked over the large bubbling spring.  The red track stops when the mule bolted and measurements were effectively halted for the day, unfortunately.

The below graph shows the CO2 concentration profile and d13C profile as we moved over that location.     The initial drop in d13C signature is a typical startup profile for the machine, which makes the initial 3-4 minutes of d13C data throwaway.  It is interesting that after that, while the 12C concentration is very stable, the d13C signature is highly variable until 18:56 when we hit a hotspot.  I haven't found any particular source for this fluctuation and am looking into it further.  After that, the d13C signature stabilized at around -9 while the CO2 concentration seems to be slowly drifting downward.  My interpretation of this is that we hit the hot-spot, but because the tube was dragging on the ground, it pulled in some liquid water.  Thus, the initial spike in CO2 concentration was sustained because the plug of liquid water provided a reservoir of CO2 as it vaporized into the measurement chamber.

There is another interesting area of high CO2 concentration on the northern most traverse that crosses the -110.11 line.  The CO2/d13C profile for that is shown below.
That long light blue to green traverse is the result, again, of pulling water into sampling tube.  What is particularly frustrating about this one is that this took place right before we went over the area of the springs and didn't catch that the data was bad until too late. We will, however, be going back (see a subsequent post on plans moving forward) to the springs and raising the sampling tube on the leg of the mule proved for the rest of the day to have eliminated the problem of pulling water into the tube.

Before anyone gets too excited, the single red spot of high CO2 concentration is a location where I breathed into the tube to check that it was indeed sampling correctly after I had put it up a bit higher on the mule's leg.

In summary, we have taken a lot of good measurements covering a wide area around the location of known CO2 seepage.  We need to take another round of very detailed and non-fouled measurements over the known CO2 seepage itself to obtain a high-quality positive leakage signal.

Finally, the elevation profile below:

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