ABSTRACT from IRS poster on OSN-1
Under what circumstances should a broadband borehole seismometer be
incorporated into a real-time seafloor observatory? The IRIS Glob al Seismographic Network
(GSN), in conjunction with the international Federation of Digital Seismographic Networks
(FDSN) has the objective of distributing 128 state-of-the-art seismic stations uniformly over
the surface of the globe. This will require about twenty stations in the deep oceans.
Wherever a real-time seafloor observatory location coincides with a gap in the global seismic
coverage there should clearly be a seismic station, but does it need to be in a borehole? The
Ocean Seismic Network Pilot Experiment (OSNPE) completed in June 1998 demonstrated clearly that
a combination of shallow buried and borehole broadband sensors could provide comparable quality
data to similar installations on islands and continents. Short period ambient noise (0.1-10Hz)
was high on the seafloor and shallow buried sensors because of shear modes in the sediments.
The broadband borehole installation was essential to reduce short period ambient noise to
island and continental levels. Above 2Hz the OSNPE borehole sensor was over 20dB quieter than
the nearest GSN station (KIP) over the same time interval. This quiet ambient noise level
translates into improved detection thresholds for earthquakes. On the OSNPE borehole sensor, a
short period teleseismic arrival in the band 0.8-3.0Hz was detected for a 4.7mb earthquake at
45° epicentral distance. This is almost 1 magnitude unit lower than previous estimates of
detection threshold for Pacific sites. The improved short period performance of a borehole
sensor over a seafloor or shallow buried sensor indicates that a broadband borehole seismic
station should be included in any suitably located real-time seafloor observatory.