The time dependence of ambient noise beneath the deepsea floor
R. A. Stephen, S. T. Bolmer, J. A. Collins, K. R. Peal,rstephen@whoi.edu

Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

J. A. Hildebrand, J. A. Orcutt, F. N. Spiess, and F. L. Vernon
Scripps Institution of Oceanography, La Jolla, CA 92093, USA
Between February and May 1998, the Ocean
Seismic Network Pilot Experiment (OSNPE)
acquired over 115 days of broadband
borehole seismic data at ODP Site 843B in
4407m water depth off Oahu (Collins et al.,
2001; Stephen, 1998; Stephen et al., 1999a;
Stephen et al., 1998; Stephen et al., 1999b).
The borehole seismometer was clamped in
casing within the upper 6m of igneous
basement beneath 242m of sediment. In
addition to over fifty earthquake events thatwere observed, ranging from a 4.5Mb event at
44° epicentral distance to the 7.9Mw Balleny
Islands earthquake at 91° epicentral distance,
this data set provides an opportunity to study
the time dependence of ambient noise in aborehole in the deep sea over a four monthduration. During the OSNPE we acquired
seafloor current measurements near the siteand sea state and wind data were obtained
from near-by NOAA weather buoys.
Estimates of the tides at the site are obtained
directly from the borehole sensor itself at very
long periods.
Figures 1 and 2 show true amplitude
spectrograms of the vertical and horizontal(X) components, respectively, in the band
0.6mHz to 7.5Hz for over 110days of thebroadband borehole seismometer deployment
at OSN-1. The diagonal events around
100mHz, with lower frequencies arrivingearlier, correspond to dispersed surface
gravity waves from distant storms. The steadybands between 350mHz and 2.0Hz
correspond to Scholte modes (or shear waveresonances) within the sediment column.
The arithmetic and geometric means of all of
the spectra in Figure 1 are shown in Figure 3.
Also shown are the largest and least spectral
values at each frequency. The very highlevels in bands B through F correspond to thelarge (Mw=7.9) Balleny Islands earthquakeon Julian Day 84. The USGS high and low
noise models (land models) are based on asynthesis of data from land and island
stations. In general seafloor noise levels arecomparable to land noise levels. The ambient
noise behavior can be divided into ten
frequency bands as shown: 0.6 to 3mHz (A),
3 to 8mHz (B), 8 to 55mHz (C), 55 to 70mHz(D), 70 to 90mHz (E), 90 to 120mHz (F), 120
to 350mHz (G), 350mHz to 2Hz (H), 2 to
4.5Hz (I), and 4.5 to 7.5Hz (J).
The mean noise level in the infra-gravity band(0.6-3mHz, band A) correlates extremely wellwith tides (Figure 4). The 'tidal' curve in thisfigure is just the time series of the KS54000
vertical component. In the infra-gravity band
the borehole sensor exhibited much higher
noise levels than a co-located buried sensor(Collins et al., 2001), suggesting that thesensor is suffering from instrument or
installation noise. The correlation with tides
indicates that the noise may be related tofluid-flow in the well or that it may be caused
by non-linearities in the sensor.

currents washing against the re-entry cone
(Figure 8). (Also see http://msg.whoi.edu.)

The largest amplitude changes occur in thenoise notch (C, 8-55mHz), where verticalcomponent noise varies from the quietestlevels observed world-wide [less than -190dB
re: 1(m/s^2)^2/Hz] to levels above –90dB re:1(m/s^2)^2/Hz after the Balleny Islands
earthquake (Figure 3). The noise notch
minimum rises dramatically after mostearthquakes observed on the IRIS DMC
catalog (Figure 5). All of the events above 160dB can be associated with quakes in theIRIS DMC catalog. The smaller peaks that donot occur at a known earthquake event may
be caused by smaller local and regional
earthquakes that are not observed on theglobal network. If we use -175dB as thethreshold for an 'event' there are about 34 ofthese local and regional events in the 30 day
interval shown.

Acknowledgement This work was funded by
the National Science Foundation and a
Mellon Grant from Woods Hole
Oceanographic Institution.

The micro-seism band (D, E, F, and G), 55350mHz, is characterized by three peaks.
Levels of the single frequency micro-seism
peak (D) at 60mHz can increase 80dB after alarge earthquake. The levels of the two
double frequency micro-seism peaks (G), oneeach from distant and local sources, are muchless variable (about 20dB). The highest levelsof the microseism peak (120-350mHz, bandG) correlate very well with sea state in theband 60-175mHz as predicted by the wavewave interaction mechanism (Figure 6).
The short period band (or HOLU spectrum,H), 350mHz to 2.0Hz, consists of a set ofpeaks that correspond to Scholte modes in theseafloor which are excited by local sea state.
Mean noise levels in the HOLU spectrum also
correlate with sea state at half the frequency(0.175-1.0Hz) (Figure 7).
Above 4.5Hz (J) there is a weak tidal
dependent effect, primarily on the verticalcomponent, which could be related to bottom

References
Collins, J.A., Vernon, F.L., Orcutt, J.A.,
Stephen, R.A., Peal, K.R., Wooding,F.B., Spiess, F.N., and Hildebrand, J.A.
(2001). "Broadband seismology in theoceans: Lessons from the Ocean
Seismic Network Pilot Experiment,"
Geophys. Res. Lett. 28, 49-52.
Stephen, R.A. (1998). "Ocean Seismic
Network Seafloor Observatories,"Oceanus 41, 33-37.
Stephen, R.A., Austin, G.L., Babcock, J.,
Bolmer, S.T., Collins, J.A., Gieskes,J.M., Gould, M.R., Hildebrand, J.A.,
Hollinshead, C.B., Jabson, D.M., Jonke,P., Orcutt, J.A., Peal, K.R., Price, D.V.,
Spiess, F.N., and Vernon, F.L., (1999a).
The Ocean Seismic Network Pilot
Experiment Recovery Cruise (WHOI03-99), WHOI Technical Memorandum,
Woods Hole, MA.
Stephen, R.A., Austin, G.L., Bolmer, S.T.,
Chadwell, C.D., Collins, J.A., Jabson,D.M., Jonke, P., Goldsborough, R.G.,
Gould, M.R., Hildebrand, J.A.,
Hollinshead, C.B., Offield, D.G., Orcutt,J.A., Peal, K.R., Price, D.V., Rosenblad,S.G., Spiess, F.N., Vernon, F.L.,
Willoughby, D.F., and Wooding, F.B.,(1998). The Ocean Seismic Network
Pilot Experiment Deployment Cruise(WHOI-02-98), WHOI Technical
Memorandum, Woods Hole
Oceanographic Institution.
Stephen, R.A., Collins, J.A., Peal, K.R.,
Hildebrand, J.A., Orcutt, J.A., Spiess,F.N., and Vernon, F.L. (1999b).
"Seafloor Seismic Stations PerformWell in Study," Eos, Transactions, AGU
80, 592.

Figure 1: OSN Vertical Spectrogram

Figure 2: OSN Horizontal Spectrogram

Figure 3: Summary of OSN power spectral densities which were computed every 1.82hrs for thewhole data set.

Figure 4: The top line is the geometric mean ofband A. The bottom line is tides.

Figure 5: The bottom line is the minimum levelin band C. +’s mark earthquake events.

Figure 6: The top line is wave height (60-175)mHz). The bottom line is the maximum in.
in band G.

Figure 7: The top line is wave height (0.175-1.0Hz). The bottom line is the geometric mean ofband H.

Figure 8: The top line is tides. The bottom line is the geometric mean of band J.