Ground noise studies using the TAMA300 gravitational-wave detector and related highly-sensitive instruments

In order to detect gravitational waves and establish a field of gravitational-wave astronomy, several gravitational-wave detectors are under construction. These laser-interferometric detectors have large baseline length, from 300 m to 4 km. It is because the gravitational-wave signals, expressed by a strain of spacetime, are expected to be very small and the gravitational wave detectors inevitably have long baseline to enhance signal amplitudes. This means that the gravitational wave detectors are subjected to earth strain variation due to their long baseline. We successfully observed a long-term change in the 300 m cavity in terms of the absolute length using the TAMA detector. The accuracy of the measurement was about 1 μm, which corresponds to 3·10-9 in strain. The cavity length was affected mainly by artificial ground strain change due to groundwater pumping. After removing these artificial spikes with simple groundwater model, we could obtain long-term variations, which seemed to be the ground-strain response of the barometric pressure, thermal drift, and earth tides. In Kamiokande Mine observation using 20-m detector, we have shown a stable data acquisition and high quality data including earth tides. With a km-baseline underground detector, we expect to find out some sources that have never been detected even with the superconducting gravimeters. (Y. Tanaka)