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28 July, 2001

Visitor from Barrow and Some Technical Information

4:30 p.m.

A light rain started falling around 3:30 p.m. Because rain may cause a fuse to blow in the antennae, we ended our data collection several hours early. I am starting my journal early tonight because (a) Greg is leaving early tomorrow and (b) I have a visitor coming to camp!!

Life on ice ... almost

As we were packing our gear into the car this afternoon to head back to camp, Chris Donavon and her daughter arrived. Chris, a fellow TEA, has just finished her seven-week research stint in Barrow, Alaska, measuring how much carbon dioxide is absorbed into the tundra. Check out her journals: Click here to check them out!

She and Megan are spending time touring Alaska before heading back to Arizona, land of paved roads, flush toilets and dark nights.

It is amazing the things the body can get used to: the CRREL guys in camp have been here since May and typically wear t-shirts and jeans around camp and on the glacier, while I wear long underwear and my vest from the moment I get up to the moment I go to sleep. Two weeks ago, I couldn't sleep for the constant light; now, it's no problem. Indeed, Chris and I discussed how tricky it will be for our bodies to readjust to dark nights.

Other news: Greg and Kendra head to Ft. Wainwright (outside Fairbanks) tomorrow to complete a short seismic study up there. They are trying to map more accurately the bedrock, permafrost and pollution near the runways there. Jet fuel has spilled and the army is trying to determine if their efforts to clean it up were successful. Pretty cool to see geophysics in action!

Science at work

In yesterday's webcast conference, a question was received via e-mail as the conference was concluding: "Where is the physics in geophysics?"

The answer that leaps to mind is "At the end"; somehow, I think the person asking the question (Jeff Dunham, my mentor and physics professor at Middlebury) had something else in mind.

A more thoughtful response then.

A thorough understanding of physics is necessary to successfully use geophysics because physics (and parcticularly wave theory) is used to ... a) understand the potential applications and limitations of the equipment.< /i>

For example, in yesterday's journal I mentioned that the speed of seismic waves in ice at the surface and at the bottom of the ice are different. Physics tells us how and why.

Another example: radar measures the electrical differences of matter underground. GPR does not work in clay. Physics helps us understand why. Another example: we place geophones a certain distance apart; we take samples of GPR data at certain intervals. Physics tells us what those intervals should be.

b) process and interpret data

After all the data has been collected, it must be processed and interpreted. The objective of processing is to improve the "signal" and reduce the "noise". Doing so requires knowledge of the frequencies of the seismic and GPR waves and how to manipulate them. Physics teaches us about waves. Comparing GPR and seismic data requires us to convert time into distance. Physics shows us how.

Interpreting the data is the application of wave theory. Scientists looking for reflections in geophysical data must look for and avoid noise that indicate diffractions and interference of seismic and GPR waves. Physics is gives us clues about what this "noise" should look like.

c) develop models and hypotheses

By using physics, scientists are able to develop new models and run simulations to show how different earth materials would react in different situations.

Since we're on the subject, let's talk about interpreting data (hang on to your seats; the ship to really cool thinking is about to take off)

Check out the pictures... Think and look carefully.



This morning, Greg gave me a copy of the picture above and the following assignment: Figure out what this pictures is showing us. gulp Start with the obvious: The left hand picture shows processed data from ground penetrating radar, the right hand pictures shows processed data from seismic reflections. Both have been converted to show depth. Thinking a little harder: Each of the lines represents "signal" (good) or "noise" (bad). For this part of the project, we are mostly interested the thickness of the basal (dirty or sediment-rich) ice and the depth of bedrock. That means, we can probably ignore the stuff at the top. Question: Where are the reflections? Remember that GPR and seismic reflections can reflect off more than the top layer of sediment. Hmm... Thinking....

Still in the happy phase of thinking and working in the cave of science

Are these the reflections? I drew lines on the picture of the processed data, trying to locate the top of the basal ice and the top of the bedrock and any other especially interesting features. Even as I was working, it seemed arbitrary and quite make-believe.

Because I was so dissatisfied with my first try, I tried again... and again... and again. I was squinted my eyes and stepped back from the computer, looking for larger patterns in the data that my eyes may not pick up while staring at the computer screen at close range. I scribbled dozens of patterns on scratch paper and traced ones that looked most likely to me over the processed data.

This afternoon, Greg and I sat down together to review the work I had done. He pointed out some common features in geophysical data that can easily be MISinterpreted as "signal" by careless or less knowledgeable interpreters. D: represents diffraction, which occurs because signals reflecting off objects in the subsurface take longer to reach more distant receivers. On the data, diffraction of waves around boulders and other underground objects shows up as hyperbolas. S: represents signal processing artifacts or noise that results from the limitations of the equipment or processing software. R: represents repetitions of signals. When two signals are parallel and have similar character, they are usually from the same object underground, and only one of the signals should be considered "real". Noise can also repeat, in which case both signals can be ignored for the sake of interpretation. I: represents interference. When waves interact, some times the waves add together and end up larger and sometimes they end up smaller. A "pearl necklace" effect is usually a pretty good indication of interference and not an object.

With those things in mind, here is Greg's first-draft interpretation of the data we collected on July 18. The top of basal ice is probably represented by the top line (in both seismic and GPR data), and the bedrock is probably represented by the bottom line in the seismic data. The crack in the middle of the top line may represent a crack and uplift in the ice on the left hand side. How cool is that? That's right, VERY COOL.

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