Identifying and removing tilt noise from low-frequency (< 0.1 Hz) seafloorvertical seismic data

Low-frequency (<0.1 Hz) vertical-component seismic noise can be reduced by 25 dB or more at seafloor seismic stations by subtracting the coherent sign als derived from (1) horizontal seismic observations associated with tilt n oise, and (2) pressure measurements related to infragravity waves. The redu ction in effective noise levels is largest for the poorest stations: sites with soft sediments, high currents, shallow water, or a poorly leveled seis mometer. The importance of precise leveling is evident in our measurements: low-frequency background vertical seismic spectra measured on a seafloor s eismometer leveled to within 1 x 10(-4) radians (0.006 degrees) are up to 2 0 dB quieter than on a nearby seismometer leveled to within 3 x 10(-3) radi ans (0.2 degrees). The noise on the less precisely leveled sensor increases with decreasing frequency and is correlated with ocean tides, indicating t hat it is caused by tilting due to seafloor currents flowing across the ins trument. At low frequencies, this tilting generates a seismic signal by cha nging the gravitational attraction on the geophones as they rotate with res pect to the earth's gravitational field. The effect is much stronger on the horizontal components than on the vertical, allowing significant reduction in vertical-component noise by subtracting the coherent horizontal compone nt noise. This technique reduces the low-frequency vertical noise on the le ss-precisely leveled seismometer to below the noise level on the precisely leveled seismometer. The same technique can also be used to remove "backgro und" noise due to the seafloor pressure field (up to 25 dB noise reduction near 0.02 Hz) and possibly due to other parameters such as temperature vari ations.