Deploying autonomous free-fall instruments for up to one year at full ocean depths is not easy. The deep seafloor is a difficult place for OBSs where the risk factors include high pressures, bottom trawling, local seafloor morphology, debris flows, and corrosion. The consequences are data or instrument loss that compromises experiments and reduces the ability of the community to conduct future experiments.
A certain level of risk is acceptable and necessary. Few other scientific fields are making such sensitive measurements in such extreme conditions, and some data or instrument loss is expected. However, the need to minimize risk is built into all OBSIP activities.
Risks can be broadly classified as engineering-related and environmental:
Engineering-related risks are relatively controllable and are mitigated using instrument design (e.g., redundant release systems) and operating procedures (e.g., simple standardized pre-deployment preparation to minimize human error). These risks are not trivial, however: complete instrument testing is not practicable and failure modes are being discovered as OBSs are used in deeper waters and for longer durations. Engineering risk reduction is primarily the responsibility of the OBSIP IICs.
Mitigating environmental risks is the joint responsibility of OBSIP and the PIs, and primarily involves avoiding OBS deployments in dangerous locations. These are described in the following table:
| Risk Table for OBS Deployments | |||
| Risk factor/description | Environment/circumstances | Consequences | Mitigation |
| Long duration | Any | Longer exposure to other risk factors. More chance of instrument degradation or failure | Deployment duration matched to scientific aims | Debris flow | Steep slopes and submarine canyon floors, esp. along continental margins | Instrument swept away and/or buried. | Locate on bathymetric highs. Obtain high-resolution bathymetry | Fishing | Anywhere shallower than 1000 m | Bottom trawling: OBS lost or washed up. | No deployments in < 1000 m. Research local fishing practices. | Ice | High latitudes, shallow water (< 500 m) | Damage by grounded ice. Non-recovery due to ice cover or bergs. | Volcanic hazards | Mid-ocean ridges/active seamounts or volcanoes | Damage by burial in lava. | Alternative instrument design ? | Deep water | Abyssal plains or trenches > 5500 m | Component implosion at high pressure. Usually destructive | No deployments > 5500 m. | Rough seafloor m-scale | Unsedimented seafloor | Instrument trapped in seafloor fissure. | Alternative instrument design ? | Rough seafloor: 100m scale | Ridges, continental margins | Poor instrument emplacement. Acoustics compromised | Avoid deployments on slopes. | Delayed recovery | Change in recovery cruise schedule. Recovery cruise too short | Batteries die. Timing information lost. | Contingency built into cruise planning. | Operator error | Instrument preparation mistakes | Data or instrument loss | Rested technical staff |
In general, passive-source broad-band experiments are riskier than short-term deployments. They have longer duration so OBSs are exposed to other environmental and engineering risks for longer times and the failure-mode learning cycle is slower. They are also more likely to be in deeper waters.