Optical communication system expands CORK seafloor observatory's bandwidth

Abstract

An important aspect of deploying scientific sensors in the deep sea is reliable underwater communication. We have developed an optical communication system that complements and integrates with an acoustic system to provide underwater communications that is capable of high data-rates and low latency in clear water combined with long range and robustness in the presence of high turbidity. This combined optical/acoustic telemetry technology was recently tested at a CORK (Circulation Obviation Retrofit Kit) borehole observatory in the deep ocean of northeast Pacific. A CORK is a seafloor system to seal a borehole from the overlying ocean to allow the subseafloor hydrologic regime within the sediments and volcanic basement to retain its pre-drilling pressure state. CORKs are instrumented with downhole thermistor strings and pressure sensors and are typically visited on a semi-regular basis by submersible for downloading data and for collecting physical samples of subsurface fluids. We deployed the Optical Telemetry System (OTS) at the Hole 857D CORK in 2420 m water depth using the submersible ALVIN in July, 2010. The OTS was plugged into the existing underwater connector on the CORK to provide not only an optical and acoustic communication interface but also additional data storage and battery power for the CORK to sample at an increased 1 Hz data-rate. Using a CTD-mounted OTS similar to the seafloor unit we were able to establish an optical communication link at a range of 100 meters at rates of 1, 5 and 10 mega bits per second (Mbps) with no bit errors. Subsequent tests were done to establish the optical range of the various data rates and the optical power of the system. After approximately 1 week we repeated the CTD-OTS experiment and downloaded 20 Mb of data over a 5 Mbps link at a range of 80 m. The CORK-OTS will remain installed at the CORK for a year. Our Optical Telemetry System (OTS) enables faster data rates to be employed for in situ measurements that were previously limited by data download times from a submersible. The OTS also permits non submersible-equipped vessels to interrogate the CORK borehole observatory on a more frequent basis using a receiver lowered by wire from a ship of opportunity. In the future, autonomous vehicles could interrogate such seafloor observatories in a "data-mule" configuration and then dock at a seafloor cabled node to download data. While borehole observatories may ultimately be linked into undersea cables relaying real-time data back to shore they represent a superb opportunity to test free water optical communication methods. This application of seafloor optical communication could be used for a number of other types of seafloor sensors that may not be linked into a cabled network. The lessons learned from our CORK development efforts will go a long way towards establishing the viability of underwater optical communications for a host of autonomous seafloor sensor systems in the future.