Finishing Up

Today is my last day working at Virginia Tech; tomorrow I fly back home. In the last three weeks since getting back from the field, I've been helping to get the GPS data in order, working on my abstract and poster, and then looking at the actual seismic data from the experiment and doing some preliminary analysis. The first two weeks were a bit slow, especially once the GPS stuff was mostly done and I was still waiting for the seismic data to be ready. Then when the data did come in last week, we had some trouble getting everything working in Promax and whatnot. I finally got started picking a few points and doing some simple 1D calculations last Friday and have been working on that this week. Although I looked a little at the refractions too, I've mostly been concentrating on the reflections while Michelle is doing models based on refraction. Many of our data sets have pretty clear Moho reflections, but I still found it could be rather difficult to pick the exact point of their arrival, since they are mixed in with the noise of the refracted first arrivals. Nonetheless, I made some picks and then plotted the squares of the time and distance in order to fit to the formula T^2 = x^2/v0^2 + 4*h0^2/v0^2, where h0 is the depth to the moho and v0 is the crustal velocity. Obviously, this is a pretty simple model, assuming constant crustal velocity and a flat moho, but it was intended to give only a preliminary model and was something I could do with only a week to work on it and no previous experience. By doing this for several shots, I was able to plot the results versus distance to get a rough idea of how things change along the line. The results of that are plotted below. I also tried today dividing data from each shot into multiple parts and solving for these separately, but this gave me a mess that didn't leave me very confident in the quality or consistency of my results, though perhaps I could do more to figure it out with more time. Here, anyway, are the less messy plots:

Plot of moho depth versus distance. The horizontal bars show the range of distances picked to produce each point, while the vertical bars are error in the depth.

Plot of crustal velocity versus distance. Again the vertical bars are error in the velocity, while the horizontal ones are the distance covered by the picks for each point. The one point way up at the top corresponds with a similarly out there one on the moho depth plot, and I think it must be some sort of error.

As you can see, these plots show a general trend, though with a fair amount of variability. The shallow moho around 150 -175 km is surprising, since this is supposed to be under the batholith and should be pretty deep, so I'm inclined to think it's not real, although it would be interesting if it is. The trends in moho depth correlate pretty strongly with those in velocity, with shallower depths corresponding to lower velocities. Since the depth calculated depends partly on the velocity, as shown by the equation above, errors in velocity or effects of non-uniform velocity not modeled here may carry over to affect the moho depth calculations. These are very preliminary results, and I'm not all that confident in them, but it is nice to have something to look at as a product of the work this summer. 

Well, that's the end of my scheduled internship. I may be able to do a bit more with these data from home, and I'll definitely still be working on the abstact and poster and will post about those when they are finished, but I'll be leaving Virginia tomorrow. My internship has been far more field work focused than many, so I've gotten a good idea of what that's like and only a small taste of the data analysis side of things, but one can only fit so much in a summer, and I certainly wouldn't trade any of the great time I had in Canada for more time in front of a computer. Overall, it's been a good experience; I've learned a lot and had fun and traveled to some amazing places. I still haven't really figured out whether I want to go into geophysics or one of the other areas of geoscience yet, but I expect that the research experience of this summer will be helpful whatever I end up deciding on.