Vibrations of the ground and motions of earth's crust can be major background sources of motion in LIGO's mirrors. The seismic isolation systems for the LIGO-1 detector are designed to ensure that these vibrations do not affect LIGO sensitivity at frequencies above 35 Hz, the lower end of LIGO-1's gravitational-wave signal band. However, even vibrations below 35 Hz place strains on LIGO's control systems, which must stabilize the interferometer arms to a precision of 10-13 meters (1/10,000,000,000 of a mm) to prevent non-linear effects from upconverting low frequency earth motions to higher frequencies within the LIGO signal band.
Ground Noise From Man-Made Sources Project: In the band from 0.5 Hz to 35 Hz, earth's natural vibrations tend to be about 10-8 meters (1/100,000 of a mm) but large increases in vibration can result from "cultural noise," that is, vibrations caused by man-made sources both on the LIGO site and for many miles surrounding LIGO. Various land uses can have significant impacts on cultural noise measured at the LIGO site (see landmemo.pdf). A further complication is that many of the mechanical resonances of LIGO's support structures and vibration isolation systems occur in this band and these resonances act to amplify ground vibrations. As part of LIGO's SST program, Mr. Norm Graham of Kamiakin High School in Kennewick, Washington, will lead a research team of students working with LIGO scientists to monitor and characterize ground noise sources due to human activities surrounding LIGO Hanford Observatory.
LIGO Armlength Stabilization Project: In the frequency band below 0.5 Hz, earth's natural vibrations are dominated by earth tides and ocean-wave activity. The earth tides are distortions of earth's crust as the relative orientation of the moon and sun in the sky changes. This causes LIGO's end-station buildings to move, relative to the corner station, by as much as 0.4 mm with periods of approximately 12-hour and 24-hour motion. An estimate of the maximal values for earth tides at LIGO sites was made to specify requirements for LIGO seismic control actuators (see earth_tides.pdf). A LIGO SURF-99 student, Eric Morgonson of Caltech, will development a parametrized earth-tide model that can be used to evaluate the effects of local inhomogeneities in earth's surface structure on the earth tide effects. Rattlesnake Mountain is an example of such a surface inhomogeneity. The microseism, driven by ocean-wave activity, has a typical period of 6 seconds and an amplitude of order 1 micron (0.001 mm). The microseism is highly variable since ocean-wave activity is significantly affected by storms at sea. An evaluation was done of the effects of microseimic activity on LIGO detectors (see museism.pdf). Additionally there are potential low-frequency motions of LIGO's buildings due to seasonal and diurnal temperature changes, effects of moisture in soils, non-uniform heating of soils and structures by sunlight, etc. A LIGO SST research team, led by Mr. Dale Ingram of Gladstone High School in Oregon, will monitor and characterize these low-frequency motions and their effect on LIGO detectors and will evaluate plans to mitigate these effects using feedforward compensation in LIGO's seismic actuators.
Last modified September 28, 2004
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