COSZO_OBSTech_Spec_1-00.docx.pdf

Cascadia Offshore Subduction Zone Observatory (COSZO)

Ocean Bottom Seismometer Technical Specifications

Version 1-00 04/22/2024

Cascadia Offshore Subductions Zone Observatory Ocean Bottom Seismometer Technical Specifications

Introduction

The Ocean Observatories Initiative (OOI) Regional Cabled Array (RCA) has been delivering real-time oceanographic data to the public since 2014. Through a Mid-scale Research Infrastructure-1 award from the US National Science Foundation (NSF), a set of seafloor junction boxes and geophysical instrumentation will be added to the RCA along the Cascadia Subduction Zone in summer 2026 at water depths from 80m to 1300m to better characterize seismic activity and plate boundary deformation. Along with existing geophysical instrumentation on the RCA, the added infrastructure will create the Cascadia Offshore Subduction Zone Observatory (COSZO), which is designed to operate for at least 15 years.

This document contains technical specifications for the Ocean Bottom Seismometer (OBS) instrument. This instrument shall comprise several integrated and matched components: a Broadband Seismometer, a Strong Motion Accelerometer, and a Digitizer, all to be installed in titanium pressure housing(s) that will be buried in the seafloor, and a Low Frequency Hydrophone and optionally, a Differential Pressure Gauge (DPG), that will be located in the water column near the OBS. The system shall have a cabled interface to a seafloor junction box, which will provide GPS-synchronized one Pulse-Per-Second (1 PPS) timing pulses, an NTP server, and optional time-of-day message strings. The integrated instrument will be capable of being deployed by ROV within a caisson buried in sediment at specific locations. The caisson will be backfilled with silica beads to reduce flow-induced noise at the seafloor.

Contractual terms and other non-technical information are not part of this document.

Threshold and Objective Requirements

Threshold Value – The limiting acceptable value of a technical parameter, denoted by a “shall” requirement statement. If the instrument does not meet the performance as specified by the threshold value, it may not be sufficient for inclusion on COSZO.

Objective Value – The desired value of a technical parameter, denoted by a “should” requirement statement. This value, if provided, may be more challenging to achieve than the Threshold value. It is a goal, not a requirement, for the instrument.

Technical Specifications

1. Seismometer (Velocity) Measurement Specifications

1.1. The Broadband Seismometer (hereafter ‘seismometer’) shall produce 3 orthogonal outputs representing ground motion.

1.2. The seismometer shall have an analog output, (e.g. ±20 V), differential, one output for each channel, and have a flat response to earth velocity.

1.3. The seismometer shall have a sensitivity at differential output of no less than 1,200 V/m/s.

1.4. The seismometer and its associated leveling system shall be free of parasitic resonances below 100 Hz

1.5. The seismometer and associated leveling system shall make broadband measurements of ground velocity at instrument noise levels that are no more than the USGS NLNM within a frequency band extending over the range from ≤8.3 mHz (120 s) to 10 Hz (0.1 s).

1.6. The seismometer shall support a dynamic range of ≥ 150 dB in the frequency band 8.33 mHz to 10 Hz.

1.7. The seismometer shall have a frequency pass band with a flat velocity response within 3 dB from the -3 dB points of 2.8 mHz (360 s) to ≥ 20 Hz (0.02 s).

1.8. The seismometer shall have a clip level that is greater than the output voltage.

1.9. The seismometer shall measure and deliver mass position outputs for all 3 components.

1.10. The Broadband Seismometer shall be linear from the noise floor to 100 dB above the noise floor.

1.11. The Broadband Seismometer shall have capability of calibration via remotely injected signals while deployed.

1.12. The seismometer shall provide a means of leveling axes when the OBS package is deployed at an angle of up to 20 degrees from horizontal.

1.13. The seismometer masses shall be capable of being auto-leveled and auto-centered or re-leveled upon application of a remote mass-centering command.

1.14. The seismometer shall have the capability to remotely command the sensor to lock or unlock masses, if mass-locking is necessary.

1.15. The seismometer shall be capable of reporting case tilt with respect to the vertical.

1.16. The polarities of the seismometer’s output signals shall be positive for earth motion UP, NORTH, EAST (Z,N,E).

1.17. The seismometer’s magnetic susceptibility shall be less than 0.003 m/s^2 per T.

1.18. The temperature range over which no mass centering is required shall be no less than ±2°C.

1.19. Manufacturer’s name, model number, serial number, and factory calibration parameters sufficient to reconstruct the sensitivity and transfer function should be retrievable from the seismometer.

2. Strong Motion (Acceleration) Measurement Specifications

2.1. The Strong Motion Accelerometer shall produce 3 orthogonal outputs representing ground acceleration calibrated in units of V/m/s^2.

2.2. The accelerometer shall have an analog output, (e.g. ±20 V), differential, one output for each channel, and have a flat response to ground acceleration.

2.3. The accelerometer shall make broadband measurements of ground acceleration at instrument noise levels that are no more than -120 dB re. 1 (m/s^2)^2/Hz in the frequency band 0.1 Hz to 100 Hz when configured for a full-scale range of ±2g.

2.4. The accelerometer shall be capable of being centered via a control line.

2.5. The Strong Motion Accelerometer shall have a frequency pass band with a flat acceleration response extending from ≤ 0.075 Hz to ≥ 100 Hz.

2.6. The Strong Motion Accelerometer shall make measurements of ground acceleration with a dynamic range of ≥150 dB within the specified frequency band.

2.7. The accelerometer shall be capable of a full-scale range of at least ±2g.

2.8. The Strong Motion Accelerometer shall be capable of operating when the OBS package is deployed at an angle of up to 20 degrees from horizontal.

2.9. The accelerometer shall be aligned with the seismometer, i.e. north with north, etc.

2.10. Manufacturer’s name, model number, serial number, and factory calibration parameters sufficient to reconstruct the sensitivity and transfer function should be retrievable from the accelerometer.

2.11. The accelerometer shall be capable of outputting data in Z,N,E.

2.12. The accelerometer nonlinearity shall be <0.1 %.

2.13. Cross-axis sensitivity <1%.

3. Low Frequency Hydrophone Measurement Specifications

3.1. The Hydrophone shall measure the underwater sound pressure of scalar acoustic waves in the frequency range of 1 to 1000 Hz.

3.2. The Hydrophone shall have a minimum receive sensitivity of -186 dB re 1V/μPa without preamp (-160 dB with preamp).

3.3. The Hydrophone equivalent self-noise spectral levels shall be 10 dB below sea-state zero ocean noise sound pressure levels within the specified frequency range. Sea-state zero ocean noise sound pressure level is defined by NUSC TD7265 ambient noise curves.

3.4. The Hydrophone shall have a dynamic range of 80 dB above the noise floor

3.5. The hydrophone calibration shall have an accuracy of 1.0 dB//1 μPa.

3.6. The hydrophone calibration shall have a resolution of 0.1 dB//1 μPa.

3.7. The Hydrophone shall be omni-directional in response, within 3 dB, over the specified frequency range.

3.8. The Hydrophone shall be connected to the OBS package with a 5 m underwater-rated cable. (UW will provide a seafloor mount for the hydrophone.)

4. Optional Differential Pressure Gauge (DPG) Specifications

4.1. The DPG shall be the Cox/Deaton/Webb1 type

(https://marineemlab.ucsd.edu/instruments/dpgs.html).

4.2. The DPG shall have a pass band of 2 Hz to 500 seconds.

4.3. The DPG shall have a nominal gain of 1 mV/Pa at 1 Hz.

4.4. The DPG noise shall not exceed 5 x 10^-3 Pa^2 / Hz at 100 s.

4.5. The DPG preamp shall have a bandwidth that supports the full passband of the

DPG.

4.6. The DPG shall be connected to the OBS package with a 5 m underwater-rated cable. (UW will provide a seafloor mount for the hydrophone.)

5. Digitizer Specifications

5.1. The Digitizer shall convert analog signals from the broadband seismometer, strong motion seismometer and low frequency hydrophone to digital representations of Voltage.

5.2. The Digitizer shall collect broadband seismic measurements with at least 24-bit resolution, and with a dynamic range of ≥140 dB above the digitizer noise floor.

5.3. The Digitizer shall have an input voltage range of ±20 V, differential.

5.4. The Digitizer shall have multiple preamplifier gain settings remotely selectable.

5.5. The Digitizer shall be capable of acquiring broadband measurements with multiple sample rates at least one of which will be ≥200 Hz.

5.6. The Digitizer shall be capable of acquiring 3-axis analog broadband measurements, outputting at least 4 different sampling rates per axis (1, 8, 40, 200 samples per second).

5.7. The Digitizer shall be capable of acquiring 3-axis analog strong motion measurements, outputting at least 2 different sampling rates per axis (8, 200 samples per second).

5.8. The Digitizer shall be capable of acquiring 1 channel of analog hydrophone measurements, outputting at least 2 different sampling rates per axis (1, 200 samples per second).

5.9. The Digitizer shall be capable of acquiring 1 channel of analog DPG measurements, outputting at least 2 different sampling rates per axis (1, 8 samples per second).

5.10. Data samples on all channels running at the same sample rate shall be taken simultaneously to within 1% of the sample interval.

5.11. The Digitizer shall enable broadband measurements of ground velocity at instrumental noise levels that are no more than the USGS NLNM within a frequency band extending from ≤8.3 mHz (120 s) to 10 Hz (0.1 s).

5.12. The Digitizer shall have a linearity of -110 dB or better.

1 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.

https://urldefense.com/v3/__https://marineemlab.ucsd.edu/instruments/dpgs.html__;!!K-Hz7m0Vt54!hz4jWIg5tnsE-GLFIN5Srzm5Fv-zYBECWAO-ZhG8TwKk3Vgpsyf0sPcBp6KYH3yMOPFZH24BDsh1EhmONFDhFA$

5.13. The Digitizer time shall be accurate to 1 ms absolute with a free running oscillator with a maximum temperature sensitivity of 0.1 ppm/degree C and a maximum drift rate (at constant temperature) of 0.1 ppm.

5.14. The Digitizer shall be capable of generating sine, step, and random calibration signals, configurable to provide sensor outputs between 5% and 50% of full scale, and accompanied by calibration-enable signals for the appropriate set of 3 sensor channels.

5.15. All digitizer channels shall be derived using linear phase “brick- wall” filters with high frequency corner fc at least 80% of the Nyquist frequency and stop band amplitude (Nyquist to higher frequencies) at least 95 dB below the pass band with pass band (DC to fc) ripple of less than 5%.

5.16. Configuration information available through the instrument interface should allow reconstruction of the complete transfer function for all active data streams.

Configuration information should include: manufacturer name; digitizer type;

digitizer serial number, board serial numbers and version numbers, factory calibration parameters, including FIR filter coefficients, and A/D conversion factors for each channel. This is an objective.

6. Environmental Specifications

6.1. The Broadband Seismometer and Strong Motion sensors shall be co-located in the same housing.

6.2. The Broadband Seismometer and Strong Motion sensors shall be co-oriented.

6.3. The OBS instrument shall operate with full functionality at a water depth of 3500 meters.

6.4. All parts of the instrument in contact with sea water (housing, connectors, etc.)

shall be non-corrosive and constructed from titanium or non-metallic materials.

6.5. The Instruments shall be capable of operating in water salinities from 0 to 40 on the PSS.

6.6. The Instruments shall be capable of operating in water temperatures from -2 to

35°C.

6.7. Instruments shall be capable of being deployed, recovered, and redeployed by a scientific-class ROV (e.g. Jason, ROPOS) without impairment of the performance of any of their components.

6.8. Instruments shall be capable of being stored without damage or degradation between -20 and 50°C for periods of up to twelve (12) months.

6.9. Instrument output shall be unaffected by reasonable changes in magnetic field.

7. Electrical Specifications

7.1. Instruments shall be powered by two-wire (+VDC, -VDC) voltages floating free of the housing.

7.2. Instruments shall operate from a supply voltage of 48 VDC +/- 5%.

7.3. The Instrument electronics should be capable of operating in the presence of a +/- 50 VDC potential between the power conductors and the pressure case.

7.4. The interface between the Broadband Seismometer and the digitizer shall be differential.

7.5. The interface between the Strong Motion Accelerometer and the digitizer shall be differential.

7.6. The OBS shall ground all circuitry internally with no electrical connection to the seawater.

7.7. All instrument electronics and electrical connections shall be isolated from seawater by greater than 100 MΩ.

7.8. The OBS shall be capable of utilizing a 2-wire EIA-485 1 PPS signal with 20 µs pulse length, synchronizing on the rising edge.

7.9. The OBS shall be compatible with an MHDXL-12-TI connector, to be provided by UW, in order to interface with the RCA system.

8. Operational Specifications

8.1. Instruments shall recover from sudden loss of power in a known state and with full functionality.

8.2. Instruments shall be designed for deployed operations of at least 15 years.

8.3. Instruments shall have external alignment heading markings visible to the ROV during deployment.

8.4. Instruments shall provide a user interface to remotely monitor and control all sensors and data telemetry.

8.5. Instrument calibrations shall be good to 1% and have a gain stability of 1% between calibrations.

8.6. The OBS shall be capable of providing state-of-health (SOH) monitoring. SOH parameters shall include the following as a minimum: mass position, internal temperature and voltages, external power sources, external power source history, and battery voltages. SOH parameters shall be available in continuous telemetry packets for ease of monitoring SOH without special requests.

9. Data and Communications

9.1. The instrument shall provide a 10/100 BaseT Ethernet Interface.

9.2. The instrument shall support error corrected standard IP communications protocol.

9.3. Instruments shall support user-settable static IP address or DHCP configurations.

9.4. Instruments shall be capable of being remotely accessed and controlled via the communication interface.

9.5. The OBS shall allow the user to remotely set all parameters necessary for full and robust control of the system, including sample rates, recording modes, sensor control signals (specified separately under “Control Functions”).

9.6. The OBS shall support real-time data output.

9.7. The OBS shall support miniSEED data output format.

9.8. All data stored on the instrument shall be accessible remotely over the communication interface.

9.9. The OBS system shall buffer data for at least 1 hour during a remote communications outage, and shall backfill data when reconnected.

10. Timing

10.1. The instrument shall time-stamp data with an accuracy of 1µs with respect to its internal clock.

10.2. The instrument shall time-stamp data with jitter of ≤ 1 µs.

10.3. The internal clock of the instrument shall be synchronized to a timing accuracy of

1µs with respect to supplied 1 PPS differential signal in combination with an external NTP server or NMEA time of day message strings.

10.4. The instrument shall be capable of transmitting data with a latency of 1 second or less from the time of the first measurement in the data packet.

10.5. The instrument shall be capable of detecting and reporting when clock synchronization has been lost and of automatically resynchronizing.

10.6. Data-logger shall provide timing info, such as state of locking to external time signal, etc.

11. Compliance

11.1. To the greatest extent practical, instruments should be compatible with applicable national and international standards, including those of the IEEE, ANSI, and IEC.

11.2. Instruments in their shipping containers shall be capable of transport without damage or degradation during shipping conditions defined by ASTM D4169 truck assurance level 1.

11.3. Instrument transportation cases shall have external labels specifying safe handling precautions.

11.4. Instruments shall be marked indelibly on an exterior surface. Marking shall include manufacturer’s part number, and unit serial number(s).

11.5. Instruments shall be capable of being marked indelibly by OOI without damage to the instrument or housing.