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LIGO Livingston Observatory NewsStudents Assist in Environmental Measurements at Livingston
Now that the LIGO Livingston Observatory buildings, beam tubes, and vacuum equipment are in place, we decided it was time to re-measure the environment in which the interferometer will operate. Our initial measurements were made nearly ten years ago by Bill Hamilton, Warren Johnson and Allen Sibley to assess the seismic environment of the site. Then more detailed measurements were made by Alan Rohay and Rick Savage in 1995.
Now we want to check again to find out how the equipment and buildings we installed have altered the initial environment. We want to know, for example, how much the 30-inch thick concrete slab underneath the seismic support system vibrates due to ambient environmental effects, such as the heating, ventilation, and air conditioning (HVAC) system. We also want to know how much acoustic noise is generated by these systems, as well as by such weather effects as wind and rain. The amplitude of the mechanical vibration present at the seismic isolation supports of the suspended test masses ultimately limits the low frequency sensitivity of the interferometer. Assumptions about this spectrum have been used as design criteria to establish the needed performance of the seismic isolation system and the dynamic range of the length and alignment control servos. So a "reality check" at this point is very helpful. We are interested also in the magnetic field oscillation spectrum, since it can drive the magnets on the suspended test masses as well.
Professor Rai Weiss of MIT visited the Livingston Laboratory on December 17 and 18 to get us started with these measurements. We made detailed measurements of the acoustic, magnetic, and mechanical spectra in the laser and vacuum equipment area (LVEA) using a calibrated sound meter, a seismometer, accelerometers, a tilt sensor, and induction coils. As part of these measurements, we switched off various components of the building's HVAC system and made "quiet" measurements during the late evening hours. Fortunately, the spectra we have measured seem generally to satisfy our criteria.
The Winter Holidays followed shortly after these initial measurements, allowing an excellent opportunity for further studies in the LVEA, and also in the end stations. We arranged with two high school students, Tomyka Crier and Millicent Schmidt, to participate in these further measurements during the week between Christmas and New Year's Day. We had a lot of fun taking the data during a very quiet period and observing such features as the fluctuation in the micro-seismic peak, and also in the discovery that the magnetic field environment in the LVEA is about an order of magnitude more intense there than in the south end station. It was also quite impressive to work with two very smart students who had never heard about Fourier Analysis on their first day, yet by the third day were expert users of a Hewlett-Packard Dynamic Signal Analyzer. How many high school students can say "I spent my Christmas vacation making power spectral density measurements" (and be able to explain it too!)?
Figure 1 at left above reveals the vertical displacement spectrum measured at the south end station on December 30. The red line is the "LIGO Standard Spectrum." The displacement spectrum measured exceeds the standard spectrum for frequencies below about 5 Hz, but is actually of somewhat less amplitude than measurements made at the same location by Rick Savage and Alan Rohay on October 30, 1995, before the building was built. (It is interesting to note that at frequencies above about 5 Hz, the effects of the HVAC system are readily observable. Below that frequency, there are some peculiarities to the data that are probably the result of other episodic contributions to the noise spectrum, i.e. the background is not very stable. Otherwise, it is difficult to understand why the building should be noisier when the air handlers were turned off.)
Next, Figure 2 at right shows the difference between the magnetic field environments in the LVEA and the south end station VEA. We were surprised to find that the magnetic field intensity is more than an order of magnitude larger in the LVEA than it is in the south end station VEA. The reason why is presently a mystery. We also noted quite a bit of variability in the magnetic field environment, as we did when measuring the seismic environment. The overall shape of the spectrum fluctuates somewhat.
The two students who assisted with these measurements, Tomyka and Millicent, both attend the Louisiana School for Math, Science, and the Arts (LSMSA) located in Natchitoches (which is rather amazingly pronounced "Nak-uh-tish" by the cognoscenti). The physics faculty of this high school visited us last October as part of our initial discussions on educational outreach. It was they who suggested that providing students with opportunities like this during school breaks would be a valuable enrichment activity for their school to offer.
No other school is quite like the Louisiana School. It is one of the nation's first and only public, residential high schools offering rigorous training in both academics and the arts to promising juniors and seniors. The Louisiana School for Math, Science, and the Arts belongs to an expanding group of state-supported, residential high schools founded to serve the academic and creative needs of gifted, high-achieving students in Louisiana. Sister schools now exist in ten states, with others in various stages of planning. Two Caltech undergraduate students who have worked on LIGO during previous summers, Yale Wang and Michael Hochberg, are also graduates of LSMSA.
All Louisiana School students must meet the State of Louisiana's requirements for graduation plus special requirements of the Louisiana School. These requirements can be stiff. For instance, all students must take a semester course in C++ or Problem Solving with Computer Programming, and participate in a Special Projects Week each year. Here at the LIGO Livingston Observatory, we look forward to hosting more students from the LSMSA as part of this Special Projects program.
We had hoped for a major winter storm to really rattle the place while Tomyka and Millicent were here, but unfortunately (fortunately?) it missed us. When I told Rai Weiss that we had come to work very early to be ready when the storm was forecast to hit, he said, "don't look a gift horse in the mouth." We won't. Nevertheless, everything is set up and ready for the next "big one" that does eventually arrive.