Around 6:30 this morning we left the hotel. The ride to the boat dock took about 25
minutes because of its remote location. But once on the runner boat the ride to the
Strong took only 5 minutes. We have our routine down pat. Like clock work everybody moves to setup equipment and deploy the instrumentation.
Before you know it we are drifting slowly backwards, collecting data.
We were really close to an outcrop we saw from the day before. Guo Lei, Xenia and
Brian obtained passage on the runner boat over to the outcrop. A lot of clay and sand/gravel
was at the base, but knowing what to look for we headed up the steep banks to find
the source. Climbing up probably 70 ft high we spotted above us the source of the
sand and gravel (Quaternary) capping a silty-clay, possibly the Wilcox…MAYBE the Porter’s
Creek. Anywho, at our feet was a nice spread of Virginia Creeper and our favorite
three-leaf-with-a-red-dot-in-the-middle friend, Poison Ivy. Nice.
We expedited our return to the runner boat. Brian stepped into a near liquid face
of the cliff and made great strides to safer ground. Back on the Strong all washed
up in hopes that any Poison Ivy oil would be removed in time. Why does my hand itch
so right now?
Guo Lei and Xenia are oscillating days spent in the lab to help process data. They
are assisting in the data processing and running of the software by Steffen, Kirk
and Beatrice. Processing the data means that the collected raw data is filtered and
combined into a readable and hopefully interpretable form. When filtering we are attempting
to remove any noise in the data (e.g., sound reverberation off dikes and revetments,
multiple reflections by sound bouncing off the water surface thus acting as a second
sound source, sound bouncing between geologic layers, etc.) without adding artifacts
to the result. When moving backwards through the water, our energy source and multichannel
streamer are moving, so the sound waves generated by the source are reflected off
several points in the subsurface. These reflections are recorded by the hydrophones
(pressure sensors) along the streamer. Because we have many hydrophones, the same
point is hit multiple times. During processing we take advantage of this redundancy
and we sum (stack) together those traces that image the same point, so that the noise
(which is assumed not to follow any order) will be reduced, while the amplitude of
the signal (which has order) will increase. This process requires a combination of
science and experience that the students are learning. Performing a rough processing
of the data will allow us to assess the data quality (do we need to slow down, shoot
more often, rerun a line, etc.) and help us to see if something interesting is showing
up in the subsurface that we may want to resurvey for confirmation and/or determine
the strike of the feature.
We collected data for an extra two miles to make up for Day 1. We try to survey 12
miles a day and we were only able to survey 10 the first day. Debris on the river
is lessening. Every now and then a log passes by, but none has caused problems… thank
goodness. The students get tired of swimming out to the buoys to remove debris.