The COSZO project will add seafloor science junction boxes connected by 200 meter cables to the OOI RCA nodes on the continental slope (PN1B, PN1C) and the continental shelf (PN1D), which will provide power and internet connectivity to ocean bottom seismic (OBS) packages that house a broadband seismometer, strong-motion accelerometer, and a digital recorder. A low-frequency (LF) hydrophone and differential pressure gauge (DPG) will be connected directly to each OBS package via 5 meter cables. An OBS package with a LF hydrophone and DPG will also be added to a preexisting science junction box at the Oregon Shelf site (MJ01C). A seafloor pressure gauge, ocean current meter, and a Geodetic and Seismic Sensor Module will also be installed at the three continental sites and a Self-Calibrating Pressure Recorder will be installed at PN1B and PN1C. Each new science junction box will also have 2-3 unused ports that will be available for additional instruments through PI-driven initiatives.

Science Junction Box

Science junction boxes connect to node sites along the RCA via 200 meter long cables. They are equipped with wet-mateable ports to provide low-voltage power and real-time internet connectivity to scientific instruments.

Instrument specifics: The science junction boxes are designed and manufactured by the University of Washington (UW) Applied Physics Laboratory Electronic & Photonic Systems Department. A description of existing science junction boxes on the RCA is available here.

Ocean Bottom Seismic Package

The ocean bottom seismic (OBS) package houses a broadband seismometer, strong-motion accelerometer, and a digital recorder that collects and streams three-component seismic velocity and strong-motion acceleration time series in real-time. It can be used to search for and localize shallow tremor signals and very low frequency earthquakes, accurately localize earthquakes below the continental shelf, measure temporal changes in the velocity structure of the accretionary prism, and facilitate offshore earthquake early warning research.

Instrument specifics: The OBS package is a Nanometrics Atlantis Cabled Observatory (Model T360-COBST2) that includes a Trillium 360 ocean bottom broadband seismometer, a Class A Titan accelerometer, and a Centaur Gen5 datalogger within a titanium pressure vessel proven for deployment depths up to 6000 m. It has a dynamic range of 220 dB at 1 Hz, a 4 g acceleration clip level, and a noise floor below the Peterson Low Noise Model (Peterson, 1993) from 200 s to 10 Hz. Each OBS package is placed within a caisson, buried in the sediment, and covered in silica beads.

Low-Frequency Hydrophone & Differential Pressure Gauge

Low-frequency (LF) hydrophone and Differential Pressure Gauge (DPG) data, when collocated with a broadband seismometer, can be used to better distinguish between seismic signals, such as tremor and acoustic arrivals such as the tertiary phases of T waves of oceanic earthquakes, and enable optimal corrections for tilt noise on vertical seismometer channels.

LF hydrophone instrument specifics: The HTI-90-U LF hydrophone has a frequency response ranging from 2 Hz to 20 kHz, a maximum operating depth of 6096 m, and a self-noise of 63 dB re 1 µPa. It is mounted on a tripod ~1 m above the seafloor and connected to the Atlantis with a 5 m cable. 

DPG instrument specifics: The DPG (Cox et al., 1984) is depth rated for 6000 m and has a passband of 2 Hz to 500 s. The noise is 5 x 10-3 Pa**2/ Hz at 100 s. It is mounted on a tripod ~1 m above the seafloor and connected to the Atlantis with a 5 m cable.

Seafloor Pressure Gauge

High-resolution, uncalibrated absolute pressure gauge time series can be used to search for and remove artifacts of pressure calibrations that might be confused with slow slip and to support measurements of compliance that are sensitive to changes in the seismic velocity structure.

Instrument specifics: The Paroscientific Series 8000 Submersible Depth Sensor incorporates Digiquartz Transducers into submersible housings as depth sensors. The pressure range of each instrument is calibrated to its deployment depth to maximize performance.

Site	Depth (m)	Absolute Pressure Range
PN1B	1241	0-1400 mH20 (2000 psia, 13.8 MPa)
PN1C	619	0-700 mH20 (1000 psia, 6.89 MPa
PN1D	115	0-130 mH20 (200 psia, 1.38 MPa)

 

Ocean Current Meter

Three-component ocean current meter near-bottom current measurements can constrain oceanographic pressure gradients and thus contribute to pressure geodesy and to an understanding of seismic noise that might be confused with tremor.

Instrument specifics: The Nortek Vector 4000 m single-point current meter acquires 3D measurements at rates up to 64 Hz and is suitable for deployment down to a depth of 4000 m. The instrument sits one meter above the seafloor.

Self-Calibrating Pressure Recorder & Geodetic and Seismic Sensor Module

The Self-Calibrating Pressure Recorder (SCPR) and Geodetic and Seismic Sensor Module (GSSM) collect time-series of calibrated pressure that can be used to measure secular vertical strain and detect slow slip events. Calibrated pressure measurements are duplicated to help ensure continuous very long-term measurements necessary to measure secular strain and have the SCPR provide a long-term check on the validity of the GSSM calibrations, which is difficult to do in the laboratory because of the challenges of maintaining stable, high pressures.

SCPR instrument specifics: The SCPR (Sasagawa and Zumberge, 2013) is designed and manufactured at the University of California San Diego Scripps Institution of Oceanography. It includes two redundant Paroscientific quartz pressure gauges (Model 42 K) that are periodically switched between the ocean pressure signal and a stable, reproducible reference pressure generated by a DH Instruments piston gauge (Model PC-7300-2) onboard the instrument using motor-driven Swagelok valves (Series 41G) turned by Hanbay motorized actuators (Model MDM1000). The sensors measure the shift in the resonant frequency of a quartz force transducer attached to a Bourdon tube as the applied pressure changes with a sensitivity of order 0.005 kPa. The system instruments and electronics are housed in a titanium housing, which also hosts a temperature sensor.

GSSM instrument specifics:  The GSSM (Wilcock et al., 2021) is an instrument designed and built by the UW Applied Physics Laboratory that utilizes a Paroscientific Seismic Oceanographic Sensor module (Models 7116-2K-2 (1400 m depth) and 7116-1K-2 (700 m depth)) comprised of two Paroscientific Digiquartz pressure transducers (Model 44.4K-302; maximum depth rating of 1400 m, range of 0-4400 psi, and accuracy of 0.01% of full scale with parts per billion resolution possible), a Paroscientific Digiquartz nano-resolution barometer (Model # 6000-16B), and a Paroscientific Quartz Seismic Sensor 1.5 g accelerometer (Model QA-15). It employs the A-0-A method where two pressure transducers are periodically switched using a VICI HPLC valve (Part # C52-3304IA-UWA) within the titanium pressure housing from measuring external water pressure to measuring the internal housing pressure for calibration. The housing also contains a temperature sensor to measure and correct for thermal effects.  

References

Charles Cox, Thomas Deaton, and Spahr Webb. A Deep-Sea Differential Pressure Gauge. Journal of Atmospheric and Oceanic Technology: Vol. 1, No. 3, pp. 237–246, 1984. (PDF, 688 KB)

Jon Peterson, Observations and Modeling of Seismic Background Noise, Open-File report 93-922 (Albuquerque, New Mexico: U.S. Department of Interior Geological Survey, 1993)

Glenn Sasagawa and Mark A. Zumberge. A Self-Calibrating Pressure Recorder for Detecting Seafloor Height Change. IEEE Journal of Oceanic Engineering: Vol. 38, pp. 447–454, 2013.

William S. D. Wilcock et al. A Thirty-Month Seafloor Test of the A-0-A Method for Calibrating Pressure Gauges. Frontiers in Earth Science: Vol. 8, 2021, doi: 10.3389/feart.2020.600671.