18 July, 2001
View from top of the Lion's Head & Seismic reflections
6:30 a.m. (July 19)
We played and worked hard yesterday. By the time 9:30 p.m. rolled around, virtually everyone in the camp was asleep... Thus, the late journal.
Life on the ice.. .. (almost)
The sun woke me by shining in my eyes; the clouds had disappeared and the surroundings were spectacular. Pat related that he is sometimes very pleasantly surprised that people pay him to travel and work in places like this.
Yesterday afternoon, Kendra, a student of Greg's at SUNY-Buffalo, joined us as we were leaving the glacier. She will be working (at least partly) with us as we collect data.
Shortly after we arrived back in camp after collecting the day's data, we piled in the van and drove the base of Lion's Head, a nearby mountain. We hiked 1,500 feet up and had our breath taken away, not just from the steep climb, but also the extraordinary view of the Matanuska Glacier. I've included a couple of pictures from the top of Lion's Head at the end of this entry.
It turns out that Lion's Head is made of intrusive igneous rock, molten rock that squeezed through existing sedimentary rock about 15 million years ago. The sedimentary rock that surrounded Lion's Head eroded more quickly than the igneous rock, leaving a steep mountain rising 1,500 feet above the valley floor. Question for especially clever readers: Where else in the United States might you find a good example of the same thing?
Science at work
For those readers who are still a bit confused about what seismic reflection is, I offer a couple of different ways to think about it:
(1) sonograms... When doctors take a sonogram, they direct low-power sound waves into a woman's uterus. The "echo" they receive back gives them many clues about the baby's health and gender.
(2) echoes... If you wanted to find the distance to a far building, all you would need is a stop watch. By yelling quickly and starting the stop watch at the same time, and then stopping the stop watch when you heard the echo, you could calculate how far away the building was. "How?" you may ask. If we know ...
(a) the speed sound travels in the air (about 330 m/s) and
(b) how long it took the sound to travel to the building and back (measured from stop watch),
then, by multiplying the speed by the time, we can find the distance the sound traveled.
Seismic reflections are echoes within the earth. It is more complicated than measuring echoes in the air because the velocity of sound through the earth can vary greatly (it travels at about 300 m/s in dry sand and 6,000 m/s in limestone). To collect seismic data, we need a couple of different tools: reliable way to create seismic waves, some way to measure the seismic waves and some way to collect the data.
A sledge hammer acts as our seismic wave creator, geophones (see below) measure the vibrations or echoes, and a seismograph collects the data.
Geophones are very cool little gadgets. A magnet is connected to spike, and is surrounded by a coil of wires mounted on very sensitive springs. When the ground trembles, the spike (and magnet) move, but the springs resist movement (Newton's first law strikes again!). The result is the magnet moving up and down relative to the wires. If you move a magnet through a coil of wires, you create electricity (indeed, the electricity to power your computer is generated this way... though, of course, the magnet and wires are much bigger). The more movement, the more electricity. The seismograph measures the amount of electricity from each geophone, and allows us to listen to echoes from all along the line.
I've got to wrap up... We're headed to the glacier soon!
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