SUMMARY of John Collins' IRIS 1998 poster for OSN-1
The recently completed Ocean Seismic Network Pilot Experiemnt (OSNPE) at site OSN-1 225 km
southwest of Oahu, Hawaii showed that high-quality broadband seismic data can be acquired in
the deep oceans using portable seismographs. At the OSN-1 site, three broadband seismographs
were deployed within 300 m of each other. The seismometers were (i) a Teledyne GeoTech KS54000
deployed in a borehole 240 m beneath the seafloor; (ii) a Guralp CMG-3T deployed on the
seafloor; and (iii) a Guralp CMG-3T surficially buried in the seabed. We refer to the latter
two seismographs as BBOBS (Broadband Ocean Bottom Seismographs). Each BBOBS also carried a
long-period pressure sensor. Three short-period (1 Hz) seismographs were also deployed.
All three broadband seismographs acquired data continuously for at least 115 days, and each
recorded over 50 teleseisms. Frequency- and time-domain comparisons of the data recorded by
these seismographs allow the following conclusion to be drawn. At frequencies less than ~0.1
Hz, the surficially buried BBOBS is 20-40 dB quieter than the seafloor BBOBS. The difference
in noise levels is most pronounced on the horizontal components. The high background noise
levels of the seafloor BBOBS appear to be due to ocean-bottom currents pushing on the
seismometer, generating tilt-induced accelerations. Within the microseism band (0.1-5 Hz), the
background noise levels of the seafloor BBOBS, buried BBOBS, and short-period OBS are similar.
In contrast, the background noise levels recorded by the borehole seismograph are substantially
less, by as much as 20-30 dB. Earth noise in this band is a result of scattering of
low-frequency Rayleigh waves into short-wavelength Scholte/shear modes by the topography on the
sediment-basement boundary. The amplitude of these evanescent modes decays with depth away
from the seafloor and basement interfaces, and hence this energy is less prominent in the
borehole data. This difference in short-period noise levels means that the detection threshold
for short-period body waves is lower for the borehole seismograph than for the BBOBS.
The two major lessons of the OSNPEA are simple stated. (i) To acquire high-quality long-period
(< 0.1 Hz) data the seismometer should be buried. This is true for all components of ground
motion but is especially critical if horizontal component data are required. (ii) At those
locations where noise in the microseism band is high, installation of the seismometer well
below the seafloor results in a lower short-period detection threshold.