The First Data of the Autonomous BBOBS-NX (NX-2G) for New Era of Ocean
Bottom Broadband Seismology
Abstract
The ocean floor broadband seismology has been established based on
several practical observations by using broadband ocean bottom
seismometer (BBOBS) and its new generation system (BBOBS-NX) in Japan
since 1999. The data obtained by our BBOBS and BBOBS-NX is adequate for
broadband seismic analyses, especially the BBOBS-NX enables the quality
of the horizontal data comparable to land sites in longer periods (10 s
–). And, the BBOBS-NX with tilt measurement function, BBOBST-NX, is in
practical evaluation for the mobile tilt observation that may realize a
dense seafloor geodetic monitoring with low cost. The weak point of the
BBOBS-NX system lies in the intrinsic limitation of the submersible in
its operation. If this system can be operated alone like as the BBOBS,
it should be a true breakthrough of ocean bottom seismology. We call
this new autonomous BBOBS-NX as the NX-2G in short. Several problems to
realize the NX-2G have been almost cleared through test observations
since 2012. The function of the NX-2G system is based on 3 stage
operations as shown in the image. The glass float is added to obtain
enough buoyancy to extract the sensor unit from the seafloor and also to
suppress the oscillating tilt of the system in descending, not to exceed
the tilt allowance of the broadband seismic sensor. In Oct. 2016, the
first in-situ test of the NX-2G system was performed. The landing of the
NX-2G looked well and the maximum tilt in descending was about ±2.5°,
that ensured the effective suppression for the oscillating tilt by the
glass float. As the final step test of the NX-2G, the one-year-long
observation has been started in April 2017 with the BBOBS deployed
nearby, to obtain simultaneous data for the noise level evaluation. The
free-fall deployment and the transition from the landing stage to the
observation stage were completed, those were monitored through the
acoustic communication from the ship. This NX-2G was recovered in Oct.
2018 with the ROV, KAIKO Mk-IV, to watch the transition from the
observation stage to the recovery stage at the seafloor. All function of
the NX-2G at the seafloor was perfect with immediate extraction of the
sensor unit. Noise level comparison with the BBOBS shows about 10 dB
improvement that is not enough as expected, which may lie in a small
tension of the cable between the sensor unit and the recording unit.
And, scenes of the landing and the first transition were selfied by the
Deep-Sea CAM.