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[SCG51-04] Seafloor fiber optic strainmeter performance with different cable laying condition.
Keywords:fiber optic strainmeter, seafloor observation, slow slip event, slow earthquakes
Measuring seafloor strain in long baseline, can be powerful means to observe dynamics occurring at depth, because local interference at the seafloor may be reduced by averaging over the long baseline, while the effect of strain source at depth does not. Aiming at capturing slow crustal deformation which may episodically occur in the subducting plate interface, we installed a seafloor fiber strainmeter, which measures length of 200 m long optical fiber laid in the seafloor, in the Nankai Trough seafloor in June, 2019 (Araki et al., 2019). The instrument is linked to the DONET (Dense Ocean-floor Network for Earthquake and Tsunamis) and data is continuously obtained through the network. Since the initial installation, we have conducted a series of seafloor experiments by remotely operated vehicle (ROV) to investigate how we can make observation of seafloor strain with low-noise enough to capture expected slow slip event.
In the initial installation, we aimed to surficial bury the fiber cable in the seafloor by a few cm by ROV towed sled. This was partially successful, but we noticed the laid fiber was slightly meandered and suspected if the measured strain really represents distances between two end points of the fiber cable. Therefore, we pulled up and retention the fiber in October 2019 to lay the cable straight but not buried in the seafloor. The modified lay resulted in more reasonable response of the fiber length over long-term, which showed more rapid, but decaying drift. We consider the decaying drift characteristics reflected that the tension acting on the fiber was distributed along the whole length unlike the initial installation. The drawback of the retention fiber in the seafloor, on the other hand, is that a part of the fiber is not in friction to the seafloor sediment, but suspended in the water due to small undulation of the seafloor topography. Behavior of such suspended fiber is difficult to predict, because slight change in the contact surface of the fiber cable could result in the shift of the cable, leading to the total length change. We considered that reasonable friction between the laid fiber cable and the seafloor sediment was necessary to suppress such non-linear behavior. So, we revisited the seafloor strainmeter site again in August 2020, to visually check the condition of laid fiber by ROV camera, and placed metallic plates to secure the cable in contact with the sediment where was suspended in the water. After the seafloor operation, unexpected shift of observed strain data did not occur, and the strain data became quiet enough for low frequency seismic observation. Long-period stability of the measured fiber strain data is approximately 200 nano-strain after reduction of long-term trend and effect of ocean tide, stable enough to expect observation of slow slip event of moderate size, if occurred at the plate interface beneath the instrument.
We are currently planning to install more fiber strainmeters in the Nankai Trough. In the future installations, care need to be taken to completely bury the sensing fiber in the sediment while the whole length is laid straight so that measured fiber strain truly represents strain of the seafloor sediment.
In the initial installation, we aimed to surficial bury the fiber cable in the seafloor by a few cm by ROV towed sled. This was partially successful, but we noticed the laid fiber was slightly meandered and suspected if the measured strain really represents distances between two end points of the fiber cable. Therefore, we pulled up and retention the fiber in October 2019 to lay the cable straight but not buried in the seafloor. The modified lay resulted in more reasonable response of the fiber length over long-term, which showed more rapid, but decaying drift. We consider the decaying drift characteristics reflected that the tension acting on the fiber was distributed along the whole length unlike the initial installation. The drawback of the retention fiber in the seafloor, on the other hand, is that a part of the fiber is not in friction to the seafloor sediment, but suspended in the water due to small undulation of the seafloor topography. Behavior of such suspended fiber is difficult to predict, because slight change in the contact surface of the fiber cable could result in the shift of the cable, leading to the total length change. We considered that reasonable friction between the laid fiber cable and the seafloor sediment was necessary to suppress such non-linear behavior. So, we revisited the seafloor strainmeter site again in August 2020, to visually check the condition of laid fiber by ROV camera, and placed metallic plates to secure the cable in contact with the sediment where was suspended in the water. After the seafloor operation, unexpected shift of observed strain data did not occur, and the strain data became quiet enough for low frequency seismic observation. Long-period stability of the measured fiber strain data is approximately 200 nano-strain after reduction of long-term trend and effect of ocean tide, stable enough to expect observation of slow slip event of moderate size, if occurred at the plate interface beneath the instrument.
We are currently planning to install more fiber strainmeters in the Nankai Trough. In the future installations, care need to be taken to completely bury the sensing fiber in the sediment while the whole length is laid straight so that measured fiber strain truly represents strain of the seafloor sediment.