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Jumping to Seismic Conclusions

Jumping to Seismic Conclusions

- Contributed by David Shoemaker

One jumping Otto. A multi-purpose staging, laboratory, and office building is in the design phase at the LIGO Hanford Observatory. (Construction is underway on a similar building at LIGO Livingston.) To help estimate the seismic impact of the activity in and around the new building, as well as to help choose a location, we undertook a small experiment. The basic idea was to have some standardized excitation at a variety of distances from the interferometer equipment in the High-Bay, also known as the "LVEA," and then check on the seismometers installed therein to see if the excitation registered and, if so, by how much.

Otto Matherny (Hanford Site Manager) and Robert Schofield (LIGO/University of Oregon) took on the challenge. They chose a series of points progressively further from the LVEA. At each point, a standard person (played by Otto) jumped in sets of three. Otto was in walkie-talkie contact with Robert, who monitored the seismometer using Dataviewer in the control room. In the event that the signals were nowhere to be seen, a backup squad was ready to increase the impulse to 10 person-jumps, but this turned out to be unnecessary. That the squad was not needed, and Otto was sufficient, is solely to be understood as a reflection of the high sensitivity of the seismometers, Otto. Really.

At left: Otto Matherny provides a calibrated vertical impulse to the earth.

Given the short time until our decision-making Doomsday, it was necessary for the experiment to be performed and analyzed within 24 hours of its conception. Soil mechanics engineers were due to begin testing the site, and any delays in choosing a location might cause the heavy construction to interfere with the commissioning and measurement work planned for the coming months. Fortunately, the team came through with beautiful results, as shown in the figure below.

Data points from the seismic attenuation experiment.

Above: Data points from the seismic attenuation experiment. The signal falls a little more steeply than 1/(sqrt r) as anticipated. Note that the solid trace is not a fit of the jump data; it is based only on the displacement amplitude at a single point (40 feet), the Q obtained from data from passing trucks (see Robert Schofield's talk, "Source and Propagation of Predominant 1-50 HZ Seismic Signal from Off-Site at LIGO-Hanford" from the August 2000 LSC meeting), and a propagation velocity at 10 Hz (to determine wavelength) obtained from a dispersion relation for tamper signals (as described in the above-mentioned talk).

Thanks to this timely team triumph, we now know where to situate our building. We are also tuning the Hanford activities and layout to put as much of the day-to-day human activity into this new building, now that we our armed with the understanding that one really hopping mad Otto at 100 meters could possibly be confused with two 100 solar-mass black holes at one light-year.