Figure 4. Noise spectra from the borehole seismometer at Site JT-1 (Leg 186) off Sanriku, Japan (from Suyehiro et al., 1999). The noise level is positioned at a satisfactory level between the high noise model and the low-noise model. The rise of noise around 0.01 Hz is known to be infragravity wave noise induced from a long-period surface wave in the ocean. At Site WP-2, the borehole seismometer will be installed in the basement so that such serious noise should be sufficiently suppressed. The seismometer in Hole 1150D (proposed Site JT-1C; Leg 186) has a vertical (V) and two horizontal components that are perpendicular to each other. The direction of the horizontal components could not be determined during installation of the instrument; therefore, H1 and H2 denote the noise spectra from records of two horizontal components of the seismometer. dB = decibels.