Field Notes: Deploying the Mariana Array at 6,000 m
- Published
- Reading time
- 2 min
- Author
- Aiko Tanaka
Eleven days, fourteen moorings and one pressure hull that did not come back the way it left. The operational record of the deepest Bathys deployment to date.
The Mariana array is the deepest Bathys deployment we have attempted: fourteen instrumented moorings between 4,800 and 6,050 m, spread over a 40 km transect along the Challenger Deep approach. This is the operational record, written mostly on the transit home.
Day 0 to 3: staging in Guam
We staged out of Apra Harbor with two containers of hardware and a vessel we had not chartered before. The first two days were spent re-terminating acoustic cables that had been stored in a humidity we did not expect, and re-running every pressure hull through the vacuum test bench. Two hulls failed the second pass. Both were replaced from spares; both failures were later traced to a single batch of O-rings.
Lesson: the vacuum bench is not optional in the field, even for hardware that passed in Lisbon.
Day 4 to 9: deployment
Deployment ran on a 36-hour cycle per pair of moorings: acoustic release check, ballast trim, lowering at 40 m per minute, and a 90-minute settle before the first telemetry handshake. Thirteen of fourteen moorings came online inside the 12 ms acoustic latency budget on the first attempt.
Mooring M-09 did not. It settled 220 m shallower than planned on a slope we had mapped at 2° and found to be closer to 7°. The bathymetry model was six months old. It has since been re-synced from the multibeam data we collected on the transit; see Bathymetry Model Sync for what changed in the process.
Day 10 to 11: the hull
On recovery testing of a sacrificial test mooring, one pressure hull came back with a hairline deformation on the end cap. It held pressure. It also held 0.4 ml of seawater, which is 0.4 ml too many. The hull is now in Lisbon being sectioned, and the ballast release policy that would have brought M-09 up early has been tightened.
What the array is telling us
Three weeks in, the Mariana array is reporting a stable bottom temperature of 1.6 °C, a persistent along-slope current of 4 cm/s and two sediment turbidity events that the anomaly console flagged eleven minutes before they would have been visible in the raw pressure trace.
Fourteen moorings. One hull we will not reuse. Zero lost instruments. We will take that.