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4/18/00
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Location : 59.0582 degrees West 62.6468 degrees South at 16:00Z
hrs
Ship
We continued through the night picking up OBSs, until we needed to return and retrieve OBS #22 that didn't release yesterday. There is a backup release that will activate at 010:00 Z. We need to be at its location and retrieve it, then steam southwest quickly to retrieve the remaining OBSs at the southwestern end of line #2. Before we can transverse Nelson Strait safely, we'll have to wait for daylight. Until daylight, we'll proceed back to the northeast and use the Seabeam sonar to scan the ocean bottom along the southern boundary of the South Shetland Islands. At first light, we'll turn northwestward through Nelson Strait, and head offshore into the Drake Passage towards the northwestern end of line #3 to begin deploying OBSs.
At 09:57 Z, at 57.1333 W, 62.0171 S, we picked up OBS 22, 13,591 m on a 004 degree heading from where it was initially deployed . The conclusion is that OBS 22 must have released from the ocean bottom after we left this location yesterday. We're now heading back to pick up the remaining four OBSs. I've included a small map of the area. The purple circle depicts OBS #22's original deployment location at the intersection of line numbers # 1 and # 2, and the small red dot and line leading to it shows where we retrieved the OBS, circled in red. The blue line leading from the red circle to the southwestern end of line #2 is where we are presently headed. The green area on line #2 depicts the area where the remaining four OBSs are to be retrieved. Upon reaching the western end of line #2, we'll follow the blue line to the area of line #6 just South of King George Island, identified by a yellow line, and run the Seabeam sonar over the area until daylight. Next, while it's daylight, we'll move through the South Shetlands to the northern end of line #3, then deploy OBSs southward along the green line.
Party
There is a sense of relief after picking up OBS 22 and securing its data. Things seem to be going well for us. We should be passing through Nelson Strait tomorrow morning, so I can get a picture of the Islands.
Misc.
As I mentioned earlier, sound waves are used by geophysicists to assist them to
'see' and study Earth's structure. Seismology is the study of the passage of elastic waves through the earth. The velocity of a wave is equal to the product of its wavelength and frequency. The velocity of these waves change as they move through different densities of material. If a wave's velocity changes with its frequency kept constant, only the wavelength changes; stretching out or contracting, as may be the case, much like a rubber band. This is why the phrase "elastic waves" was used above.
Waves transfer energy from one location to another. Turning your stereo on may drive your Mom crazy as the energy moves from your Hi Tech speakers to your Mom's ears. One would think she would appreciate a little more energy, since she often complains about not having enough.
There are two general types of sound waves: Body waves and Surface waves.
The Body waves are the waves which travel through the body of the earth and obey Snell's Law. There are two types, P-waves and S-waves. The P stands for primary or pressure/ push-pull; the S stands for secondary or shear/shake. (Fowler, 1990)
A good example of a pressure wave is pushing the end of a "slinky"; the wire coils as we play. You can see the pressure wave move down the "slinky" from where we pushed it to the other end. Between two compressed phases is a rarefaction phase. P-waves change the volume of the material they pass through. Gases and liquids commonly transmit these types of waves. S-waves behave differently. They shake or shear and rotate the material they pass through. They don't change its volume. An example of an S-wave is when you shake a rope. Notice you can't push the rope and produce a P-wave. Yet, you can produce both types of waves with the "slinky." (Fowler, 1990)
Surface waves are guided along the surface or layers near the surface, yet not deep into the Earth's interior. There are three types of surface waves named after famous physicists: Rayleigh waves, sometimes called ground waves, Love waves, and Stonely waves. Love waves shake in the vertical plane, and Rayleigh waves shake in both a vertical and horizontal plane. Stonely waves propagate along the interface between two media and are sometimes referred as interface waves. (Fowler, 1990)
When a sound wave originates, it's form is dependent on the characteristics of the material its initially travels though. If it encounters a liquid or gas, it will take the form of a P-wave, because liquids and gases are unable to support a transverse or shaking motion. If it encounters a different material it may change its form, say from a P-wave to a S-wave. This reminds me of a science fiction movie where the alien changes form.
When geophysicists shoot airguns, in seawater the wave assumes a P-wave form; upon reaching the seafloor, some of the wave energy transforms into a shear wave and some continues as a pressure wave. Pressure waves travel faster, so they reach the OBSs first. Having a large variety of waves, each with unique characteristics, gives geophysics a larger
'toolbox' to assess the Earth's structure.
Stan Treanor
Fowler, C.M.R., 1990, The Solid Earth: An Introduction to Global
Geophysics, Cambridge University Press
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