Figure F15. Spectra of the instrument self noise for the horizontal components of the ocean borehole seismometers (OBHs) deployed during Leg 191. Because the spectra were estimated from records of ~3 min, the results at <0.05-Hz frequency have low reliability. The noise curves are compared with the observed short-period noise level for the OBH deployed in Hole 1150D during Leg 186 (dashed line). The OBH self-noise curves were estimated by taking the incoherent power spectrum for ground-noise records of two adjacent OBHs deployed at Nokogiriyama vault in Chiba, Japan, on 14 July 2000. The increase of the ~2-s period noise level is due to insufficient reduction of the large ground noise. The legend (lower right) shows curves representing the comparison of different pairs of sensors (sensors 5, 6, and 7).