Comparisons of Surface and Borehole Broadband Ambient Seismic Noise at IRIS Station RAR: Raratonga, Cook Islands

Abstract

Abstract : This report addresses reduction of ambient broadband (.01 - 10 Hz) seismic noise achieved by a 100 m deep borehole deployment on a small oceanic island, Raratonga in the Cook Islands, relative to simultaneously recorded surface levels. Between .5 - 5 Hz, no difference was observed between surface and borehole noise levels. Significant noise reduction is achieved by the borehole seismometer for horizontal components at frequencies below .5 Hz, but these vary with time of day. A noise disturbance is observed during the day at RAR that can raise long period horizontal surface noise levels 20 dB above quiet periods. During this daily noisy period, borehole horizontal noise in the borehole is about 20 dB less than surface levels. Away from this noise disturbance, horizontal long-period noise reduction varies from 0 to about 12 dB between .01 - .05 Hz. On the average there is no reduction in long-period noise on the vertical component. Above about 5 Hz, a moderate noise reduction (3-6 dB) is observed on the average for both vertical and horizontal components. These results are similar to comparisons of surface and borehole noise in a continental setting. While a borehole deployment is helpful in reducing seismic noise in some frequency bands, it does not mitigate the inherently noisy conditions associated with a small oceanic island.