SEISMIC CHARACTERIZATION

Characterization of local and off-site sources of seismic vibrations is 
important for present and especially future versions of LIGO. We have begun 
to characterize local sources by setting up a seismometer in the corner 
station and attempting to identify the main peaks in the 1 - 100 Hz range. 
To do this we placed an accelerometer on most major pieces of equipment in 
order to identify their characteristic frequencies. A number of the seismic 
peaks seemed to be coming from the small office area air handler so we shut 
it off and found that, of the equipment that will not be shut off during data 
runs, it is by far the largest source of vibrations in the LVEA. The much 
larger air handler for the LVEA is vibration isolated and comparably quiet. 
For future versions of LIGO, we may want to isolate the office area air 
handler. Our labeled seismic spectrum as well as our tabulation of equipment, 
specified frequency and measured frequency have been added to the LIGO web 
site at http://blue.ligo-wa.caltech.edu/PEM/lveaNoiseSources.html 
and lveaNoisSrcsSpect.ps.

We have also begun a program of identifying ambient or off-site noise 
sources, expanding on Alan Rohay's work by using two seismometers to 
identify source direction and setting up our electronics in a van so that 
we could move in the indicated direction. We shut off all large equipment 
at the Y-end station and found that the main remaining signal above 1 Hz 
was a 5-12 Hz signal coming from the direction of highway 240. Because 
the signal was so constant (even when cars were more than a km apart) we 
did not at first believe that it was coming from traffic. But time delays 
pointed towards the road from either side of the road and in the direction 
of the nearest traffic when we set up at the road. At X-end we found that 
the largest peak also came from the direction of the nearest road - the 
internal Hanford road which has traffic until well after swing shift. During 
the rare breaks in traffic, the peak was not evident. The traffic produced
a displacement noise of roughly a nm/sqrt Hz at the end stations. We did 
not repeat our measurements at the corner or mid-stations because we could 
not shut down all equipment there.

We also made measurements around the Wannawish damn, about 5 km from the
Y-end station and found that it produced vibrations comparable in magnitude 
to passing traffic at an equal distance. However, since it is considerably 
further from LIGO than the roads, it is an unlikely source of detectable 
vibrations in the 1 - 50 Hz range. 

GRAVITY-GRADIENT NOISE

Mass motions such as density fluctuations can cause varying gravitational 
fields which can shake test masses; this Newtonian noise is referred to 
as gravity-gradient noise. Gravity-gradient noise produced by seismic waves 
from uncontrollable sources may limit advanced versions of LIGO. Hughes 
and Thorne (Phys Rev D 58 122002, 1998) have shown that this may indeed be 
a limit in the 3 - 30 Hz range using a model of ambient seismic motions based 
on the theory of multimode Rayleigh and Love waves propagating in a multilayer 
medium. The magnitude of the seismic gravity-gradient noise depends on the 
proportions of the modes present; Love modes involve only horizontal 
displacements and no density fluctuations and so, in the idealized case, 
do not produce varying gravitational fields. The ratio of vertical to 
horizontal displacement measured previously by Alan Rohay at both LIGO
sites was used by Hughes and Thorne to constrain the possible mixtures of 
modes. Kip Thorne has asked us to help further constrain the possible 
mixtures of modes by using surface seismic arrays. The first step in this 
project is to obtain a dispersion relation for ambient seismic vibrations.

In consultation with Alan Rohay and Gabriella Gonzalez, we have set up a 
trial 3 seismometer array (L-shaped with 8 and 32 foot legs) in the desert 
near the Y-end and collected time series data. The traffic peak was evident 
in the data even though we made measurements from 8 to 11 pm. We are planning 
on analyzing the data using MatLab and SAC (seismic analysis code) and then 
designing a more ideal array.

In addition to characterizing the seismic background that may be important
for advanced versions of LIGO, we plan to help investigate potential 
non-seismic sources of gravity-gradient noise that we have identified at 
the out-lying stations such as vibrating roll-up doors and roof-access 
ladders as well as small flocks of cliff swallows and western king birds 
that we have seen passing near and alighting on the buildings.