Identifying and Removing Tilt Noise from Low-Frequency (0.1
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Bulletin of the Seismological Society of America, 90, 4, pp. 952–963, August 2000 Identifying and Removing Tilt Noise from Low-Frequency (Ͻ0.1 Hz) Seafloor Vertical Seismic Data by Wayne C. Crawford and Spahr C. Webb Abstract Low-frequency (Ͻ0.1 Hz) vertical-component seismic noise can be re- duced by 25 dB or more at seafloor seismic stations by subtracting the coherent signals derived from (1) horizontal seismic observations associated with tilt noise, and (2) pressure measurements related to infragravity waves. The reduction in ef- fective noise levels is largest for the poorest stations: sites with soft sediments, high currents, shallow water, or a poorly leveled seismometer. The importance of precise leveling is evident in our measurements: low-frequency background vertical seismic radians 4מ10 ן spectra measured on a seafloor seismometer leveled to within 1 (0.006 degrees) are up to 20 dB quieter than on a nearby seismometer leveled to radians (0.2 degrees). The noise on the less precisely leveled sensor 3מ10 ן within 3 increases with decreasing frequency and is correlated with ocean tides, indicating that it is caused by tilting due to seafloor currents flowing across the instrument. At low frequencies, this tilting generates a seismic signal by changing the gravitational attraction on the geophones as they rotate with respect 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 coher- ent horizontal component noise. This technique reduces the low-frequency vertical noise on the less-precisely leveled seismometer to below the noise level on the pre- cisely leveled seismometer. The same technique can also be used to remove “back- ground” 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 variations. Introduction Broadband seismology is moving into the oceans. As Collins et al., 1998) and as part of local seismic networks the recording capacities of ocean floor instrumentation im- (Romanowicz et al., 1998). A permanent broadband seismic prove and as broadband seismometers become smaller and station was installed on an underwater cable between Hawaii lower power, seismologists have begun measuring broad- and California as part of the Hawaii-2 seafloor global seismic band (0.001–50 Hz) seismic signals at the seafloor to answer observatory (Duennebier et al., 1998). Low-frequency seis- questions that cannot be addressed with land-based seis- mic measurements can also be combined with pressure mea- mometers. For example, researchers used a 320-km-wide ar- surements to study the oceanic crustal melt distribution ray of ocean-bottom seismometers to study the structure under spreading centers using the compliance method beneath the East Pacific Rise to 600-km depth using (Crawford et al., 1999). low-frequency teleseismic arrivals (the MELT experiment, Unfortunately, the typical background seismic-noise Forsyth et al., 1998). In another recent experiment, Laske et level is much higher at the seafloor than on land, especially al., (1998) measured Raleigh wave arrivals in the frequency at frequencies below 1 Hz. Although the noise levels ob- band between 0.014 and 0.07 Hz across an array of seafloor served over one week at the French OFM pilot seismic station differential pressure gauges to study lithosphere and upper (in a borehole beneath the Atlantic Ocean) approach the aesthenosphere structure beneath the Hawaiian Swell. Per- noise levels of good continental sites (Beauduin and Mon- manent broadband seafloor seismic stations are needed to fill tagner, 1996), most seafloor seismic measurements have in the gaps in global seismic networks (Montagner et al., much higher noise levels (Sutton and Barstow, 1990; Webb 1998), and several researchers have studied noise and the et al., 1994; Romanwicz et al., 1998). Some of the highest effects of seismometer emplacement for proposed perma- noise has a tidal signature (Romanwicz et al., 1998), indi- nent global seismic network stations (Montagner et al., cating that it is tied to currents. The noise may be diminished 1994; Webb et al., 1994; Beauduin and Montagner, 1996; by burying the sensor beneath the seafloor or placing it in a 952 Identifying and Removing Tilt Noise from Low-Frequency (Ͻ0.1 Hz) Seafloor Vertical Seismic Data 953 borehole (Montagner et al., 1994; Bradley et al., 1997; Col- stable if they tilt too far off of the long-axis center. On the radians 5מ10 ן lins et al., 1998), but this expensive option is probably only seafloor, the gravimeter levels to within 5 worthwhile for long-term or permanent stations. of a laboratory-determined center value. We calculate this In this article, we show how to reduce the vertical seis- center value by searching for the maximum apparent gravity mic noise level at frequencies below 0.1 Hz after the data as a function of the instrument cross level. The apparent are acquired, by subtracting out the coherent signal from gravity is gcos(h), where g is the local gravity (approxi- other channels such as the horizontal geophones. On the or- mately 9.8 m/sec2), and h is the cross-axis tilt from the מ der of 25 dB of the vertical seismic noise in the band from vertical. The deviation in measured gravity equals g(1 0.002 to 0.1 Hz can be caused by seafloor “compliance” or cos(h)) ϵ 2gsin2(h/2). The centering precision depends on by tilt noise caused by seafloor currents. This noise can be the microseism noise that overlies the gravity signal. In the removed in the frequency or time domain, by generalizing laboratory, we can distinguish changes in apparent gravity m/sec2, corresponding to a center value 8מ10 ן the technique described by Webb and Crawford (1999) to as small as 8 .radians 4מ10 ן remove compliance noise. uncertainty of approximately 1 3מ10 ן Long-period vertical-component noise at the Pacific At the seafloor, the STS-2 levels to within 5 seafloor should in theory be dominated by the seafloor de- radians of its center value. We estimated the STS-2 center formation under the loading of very low-frequency ocean value in the laboratory using a manufacturer-installed bubble waves (infragravity waves). In the absence of other noise level mounted on the seismometer base. The STS-2 hori- sources, the pressure and vertical displacement are nearly zontal and vertical channels are electronically derived from perfectly coherent at frequencies below about 0.05 Hz (the measurements of three geophones aligned in a cube-corner frequency limit depends on the water depth), as the seafloor geometry, allowing the sensor to correct for slightly off-level deforms under pressure loading (Crawford et al., 1998). A emplacements. The sensor specifications suggest that the -radi 2מ10 ן differential seafloor pressure gauge is an indispensable part electronics correction is accurate to within 1 of a broadband seismic station, both to remove the defor- ans. We will show that this inaccuracy can introduce signifi- mation signal from the seismic background spectrum and to cant vertical channel noise at the seafloor. detect other noise sources. The coherence between the ver- tical seismic and pressure measurements indicates the qual- Seafloor Seismic Spectra ity of the low-frequency vertical-component data. Low co- herence in the frequency band 0.002–0.04 Hz suggests that We estimate the background seismic-noise levels on other noise sources, such as tilting due to ocean currents, both instruments using spectra calculated from finite Fourier dominate the “background” noise level. transforms (FFT) (Bendat and Piersol, 1986) of data win- dows visually inspected to avoid seismic events. The gra- The Experiment vimeter and STS-2 background vertical seismic spectra in- clude a microseism peak at frequencies above 0.1 Hz, a We deployed two pressure-acceleration sensors in 900- “compliance” peak centered at 0.02 Hz, and a red noise spec- m-deep water at 31Њ24ЈN, 118Њ42ЈW, at the outer edge of trum at lower frequencies (Fig. 1a, b). the California Continental borderlands. The sediments there The STS-2 vertical channel is significantly noisier than are about 1-km thick, and the seafloor is relatively flat. The the gravimeter at frequencies below 0.05 Hz. The STS-2 instruments, deployed 1-km apart, collected data from 10 horizontal channels are more than 45 dB noisier than the September to 24 September 1998. vertical channel at frequencies below 0.1 Hz, but the vertical Both instruments carry a differential pressure gauge spectrum slope below 0.05 Hz is similar to the horizontal (Cox et al., 1984)) and a seismometer. In one instrument, spectrum slopes (Fig. 1a). We will show that the STS-2 the seismometer is a Lacoste-Romberg gravimeter (Lacoste, vertical-component noise below 0.05 Hz is caused by “leak- 1967), in the other, a Streckeisen STS-2 three-component age” of the horizontal noise due to the geophone being tilted broadband seismometer. The gravimeter acts as a single- from the vertical. We show how to remove this noise from channel (vertical) long-period seismometer (Agnew et al., the vertical data using its coherence with the horizontal- 1976). Both instruments sample twice per second, giving an component noise. upper (Nyquist) frequency limit of 1 Hz. The important dif- ferences between the two instruments are that (1) the STS- The “Unavoidable” Noise Source: 2 records horizontal and vertical motions, whereas the gra- Seafloor Compliance vimeter measures only in the vertical, and (2) the gravimeter The spectral peak centered at 0.02 Hz on the vertical is much more precisely leveled than the STS-2. seismometer channels comes from the seafloor “compli- Both the gravimeter and the STS-2 are leveled using ance”, which is simply the seafloor deformation under pres- motorized gimbals to center positions determined in the lab- sure forcing from linear surface gravity waves.