29 July, 2000

July 29, 2000

Matanuska Glacier, Alaska

Today the REU group went to Anchorage to drop off Greg Baker and to restock on some groceries. Greg has a very early morning flight tomorrow which necessitated this trip. I decided to stay behind and analyze all the samples that were collected the night of the 27th and morning of the 28th. We were hoping to determine if the M-4 vent had somehow gotten shut off from the moulin or if it was possibly a reflection of flow differences between morning and evening. In both the evening and morning samples there was no trace of dye. Either it has iced shut or the flow in both the evening and morning is too low right now to spill over into the right conduit. Hopefully we can get a series of warm sunny days to pick up the flow so that we can address that question more thoroughly.

We also wanted to confirm that M-1 and Mega Vent were still connected to the moulin. Unfortunately we got no evening samples from M-1 due to the battery problem. However, we got great results out of both M-1 and Mega Vent morning samples in spite of missing the first five samples at both locations. It does appear that connections between Mega Vent and M-1 exist at the current flow rates. We will continue to monitor all three vents in future tests.

I really enjoyed Greg Baker’s company while he was here. I mentioned July 19 that he was a geophysicist from SUNY-Buffalo who would be doing some seismic studies on the glacier and the surrounding area. Being a physics teacher I was very interested in what he was doing and the physics behind it all. He was always more than willing to talk and answer any questions that I had and for that I am most grateful. Surprisingly I found that nearly everything he does could be addressed in my classes and I’m looking forward to incorporating his work into my curriculum.

What he was doing here was called Active Source P-wave Seismology. P waves are longitudinal waves that transmit energy through materials. A longitudinal wave is one in which the vibration or disturbance is back and forth along the line of travel. Sound is a good example of a longitudinal wave and in fact he was using sound waves in the ice. By using a sledge hammer or a gun-like device a disturbance or vibration is sent through the ice that travels out in all directions from the strike just like sound would from a speaker. As the wave passes down through the ice it is reflected by boundaries that exist between different materials, such as that between ice and rock. The reflected wave returns to the surface where it can be picked up with sensors called geophones. A series of these geophones was laid out in a straight line across the ice. At every three meters of distance a strike of the hammer would produce signals that were processed by an instrument which basically recorded the size of the wave and the time it arrived at the geophone. The geophones pick up the direct waves that travel straight across the ice from the strike in addition to the reflected waves. They also pick up direct waves traveling through the air and also waves that are refracted at the ice/rock boundary but that explanation requires more detail than can be provided here.

Quite simply the location of (or distance to) the boundaries is determined by applying the concepts of velocity, distance and time. A good analogy is to measure the time between seeing lightning and hearing it. By knowing the velocity of sound and the time it takes the sound to arrive one can calculate the distance away of the storm. Perhaps you’ve been told that if you count the seconds between lightning and thunder and divide by five you know the distance of the storm in miles. Because sound travels roughly one mile in five seconds through air you can figure that if three seconds of time passes between lightning and thunder that it must be six tenths of a mile away. Greg’s data analysis is a bit more complicated than this but uses similar concepts.

Marvin Giesting

Greg Baker prepares his instrument for a seismic test on the ice. Geophones stretch 90 meters in two directions to "listen" for signals.

REU students Jessica Allewalt and Kevin Pemberton help in setting out the geophone lines.

Icy water sprays out each time a load is set off with the seismic gun. Holes are drilled a half meter into the ice. The seismic "gun" is set into the hole and a shotgun blank is set off with a strike of a mallet. Kevin looks on as I get a brief cold shower.