The NFSI will operate a pool of 120 state-of-the-art broadband ocean bottom seismometers (OBS). For a brief history of OBS development and use in Canada and globally, please see Nedimović (2019). This large number of receivers allows for dense data sampling and application of advanced analysis techniques to significantly increase the image and/or velocity model resolution of the subsurface, as well as improve the accuracy of earthquake location and characterization for passive seismic studies. The OBS will be able to record for up to 1 year at a time to enable longer-term earthquake recording. For studies where secondary deployments are needed, the OBS design and our deployment infrastructure will allow for quick data download and battery recharge of recovered instruments at sea.
In order to measure the full wave spectra from local and regional earthquakes, as well as from artificial controlled sources, and much of the wave spectra for teleseismic events, the instruments will include broadband 3-component seismometers and hydrophone sensors which record frequencies from 0.01 to 100 Hz (Figure 1). A large dynamic range (24 bit) and low instrumental noise level allow us to resolve the signal arriving from small earthquakes and artificial sources at larger distances without saturating on the high amplitude signals generated by larger earthquakes or controlled sources at close range.
To minimize the cost of field experiments in remote and hostile environments, the NFSI’s OBS will be able to be easily and reliably deployed and retrieved from smaller-sized, lower-cost vessels of opportunity which might be available close to the study areas. For deployment, the OBS recording package, enclosed in a pressure case and surrounded by syntactic foam, will be attached to a steel anchor so they sink together slowly through the water column and settle upright on the ocean floor. The OBS instruments will be rated to a maximum depth of 6000 m. Locations of the instruments at the seafloor will be determined using standard acoustic ranging or, where high accuracy is needed, using a Gyro USBL (ultra-short baseline) precalibrated acoustic transceiver. To recover the instruments, an acoustic release will be triggered from a nearby vessel causing the instrument to detach from the anchor weight and return to the surface under its own buoyancy provided by the syntactic foam cover. Multiple units in close proximity can be triggered together to minimize total recovery time. A satellite GPS beacon will alert NFSI staff if an instrument surfaces unexpectedly, and in general provide a precise location world-wide in near-real-time when the instrument is on the sea surface. For recovery purposes, instruments will also be equipped with a radio beacon and strobe flasher, which activate upon exposure of the unit to air, as well as a flag to aid in recovery.
To facilitate use of the OBS in the field, the NFSI will have a 20-foot container outfitted as a portable electronics laboratory space. This will allow for final calibration and testing of the tools immediately before deployment, as well as data offload and battery charging at sea for quick turnaround of recovered instruments.
For deployments from smaller ships, which may not have suitable equipment or personnel, the NFSI will have a portable knuckle crane to ensure the instruments can be deployed and recovered as efficiently as possible. The crane will be able to withstand dynamic forces of vessel motions at sea and capable of reaching large extensions over the water away from the vessel during deployment and recovery of the OBS.