## LIGO diary 2002--report #1

by Wm. Robert Johnston
last updated 6 August 2002

Wednesday, 10 July 2002: Greetings from LIGO. It is day three for me here. There's been a thunderstorm daily so far, although they keep telling us they're short of rain.

I have been tasked with a project concerning seismic noise. Two undergraduates (one from CalTech, one from Univ. of Virginia) have started already preparing to set up an array of 18-20 seismometers in an area about the size of a football field. This area is adjacent to the southward LIGO arm, about 2-1/2 km (1-1/2 miles) from the corner station (where we work usually).

Sound traveling though the ground from (mostly human activities) can cause the ground to ripple by up to maybe 0.003 mm (one-ten-thousandth of an inch). This much displacement of the ground close to the suspended mirrors can change the gravitation pull on the mirrors enough to affect readings.

This project is initially intended to get a handle on the problem, but in the long term there has been suggestions of monitoring this routinely to subtract the effect from the recorded signals.

Such a tiny effect, you say? One paper I was reading considered the gravitational gradient from people walking with 10 meters (30 feet) of the mirrors, and another note expressed concerns about small birds launching from the roofs of the Hanford, Washington, LIGO site!

My first assignment is to model these ground ripples primarily as Rayleigh waves (one of several types of earthquake/ground waves). By model I mean create a mathematical description on a computer.

The other two students from UTB seem to be continuing the project I briefly worked on last year. The electrical ground for LIGO is not sufficiently consistent for their purposes. The students will survey the electrical properties of the ground in an area inside the corner formed by the two arms. Their results will hopefully identify a spot to bury a large metal grid that can serve as an electrical ground.

Image credits: Wm. Robert Johnston, © 2002

© 2002 by Wm. Robert Johnston.