Braving the storms - constructing hydroacoustic station HA11 at Wake Island

Typhoon Ioke swept over Wake Island on 1 September 2006, causing considerable damage.

Ioke, the Hawaiian equivalent of the name Joyce, is the name of the strongest typhoon ever recorded in the Central Pacific. For three weeks in August and September 2006, it moved across the Pacific from an area south of Hawaii to the east of Kamtchatka. On 1 September 2006, Ioke swept almost directly over Wake Island, causing considerable damage on the island. By taking this course, Ioke became a matter of serious concern to the people who are building a unique global alarm system to monitor the earth for nuclear explosions.

Hydrophone nodes are ready for deployment into the ocean.

Although referred to as a single island, Wake Island is in fact an atoll comprised of three islands around a central lagoon in the North Pacific, just west of the International Date Line. The island is home to three monitoring stations of the International Monitoring System (IMS), a global network of 337 facilities being built by the CTBTO Preparatory Commission. Once the Treaty banning all nuclear test explosions - the Comprehensive Nuclear-Test-Ban Treaty or CTBT - enters into force, these listening posts covering all continents and all oceans will monitor the entire globe for adherence to the ban.

A hydrophone node is being deployed. The orange ball is the subsurface float underneath which the hydrophone is suspended.

The IMS stations on Wake Island represent three of the four technologies used by the monitoring system - hydroacoustic, radionuclide and infrasound. The installation of the hydroacoustic and radionuclide stations is complete, and the hydroacoustic station was certified on 8 June 2007 as meeting all necessary requirements. The radionuclide station is expected to be certified towards the end of 2007. Both stations are now sending data to the International Data Centre in Vienna.

With the Wake Island hydroacoustic station installed, the hydroacoustic network is nearing completion. Ten of the eleven stations in the worldwide hydroacoustic network are now in operation, listening to sound waves in the water to detect possible signatures of an underwater nuclear explosion.

The cables are brought to the island from the sea through an underground tube and secured in an anchor pit.

The road to this success was rocky, and Ioke was not the only storm to have had an impact on the installation work of hydroacoustic station HA11 on Wake Island.

Each station installation starts with a site survey to take stock of the specific environmental conditions at the envisaged location. A range of particular challenges was identified for Wake Island in the two site surveys that were conducted in 2003 and 2005.

The building of the shore facility of hydroacoustic station HA11 has been completed. A manhole cover allowing secure access was put on the anchor pit in front.

HA11 is a typical hydrophone station using underwater microphones. The usual configuration of this type of station is two sets of three hydrophones, each installed on opposing sides of an island and connected to an onshore facility. In the case of HA11, the hydrophones were installed above underwater sea mounts, moored at a depth of 750 m below sea level. Underwater cables needed to be laid across distances of about 100 km between the hydrophone triplets and the island. Depths of up to 5000 m along the cable routes had to be taken into account and equipment was specially designed to withstand 500 bars of pressure.

Shore facility construction and communication installations had already been built in April 2006.

Describing the enormous challenges of this project, CTBTO's hydroacoustic officer Andrew Forbes said: "The deep sea is one of the toughest environments on earth to work in." A station like HA11 will have to last for 20 to 25 years. The required longevity in extremely inhospitable environments demands high-quality materials. Underwater installations have to be capable of withstanding the near-freezing temperatures, huge pressures and saline corrosiveness of deep ocean waters. The deployment of hydrophone nodes and cables is a highly sophisticated effort, putting stress on material and personnel alike.

Wake Island is known as a bird sanctuary - here a colony of Arctic Terns - an aspect which had to be taken into account when building the station.

It therefore doesn't come as a surprise that hydrophone stations are the most expensive to build. In fact, with a total cost of US$ 18 million HA11 is the most expensive station ever built by the CTBTO Preparatory Commission. While the biggest share was funded by the Commission, it was also supported by the US Air Force Technical Applications Center or AFTAC, which contributed US$ 3 million to the project. AFTAC was not only a partner in securing the monetary basis of the project, but also became more directly involved at different stages by contributing technical assistance and knowledge.

This graph shows the location of the underwater sea mounts near Wake Island where the hydrophones are deployed. The area in blue is the area that was surveyed prior to station installation.

After all surveys had been done, the plan was to install the station in June/July 2006. However, Hurricane Katrina ravaged the coastlines in the Gulf of Mexico in August 2005 with adverse after-effects on the installation of HA11 more than a year later. The ship which had been counted on to install the underwater segment at Wake Island was still tied up in reconstruction work in the Gulf of Mexico.

Just when the team had managed to secure a new ship suitable for such installation work, another storm - this time Ioke - put a stop to the endeavour. Once again, Andrew Forbes had to deal with the consequences of a storm. "Typhoons and hurricanes dogged my life for two years", he said.

All of the island's infrastructure and buildings suffered some damage. Sand had been driven up by the storm, covering the end of the island's only airstrip. Some buildings were completely flattened, others had their roofs taken off or damaged. Large parts of the island were flooded. Due to clean-up efforts, access to Wake Island, which is administered by the United States of America and mostly used for military purposes, was not possible for months.

Then, sooner than expected, after clean-up teams had swiftly dealt with the legacy of typhoon Ioke, the island was declared accessible to the CTBTO again. In February 2007, the team assembled by Andrew Forbes started another attempt - the final one as it turned out - to put hydroacoustic station HA11 in place.

The Japanese ship MS Shin Chou Maru was chartered as the deployment vessel for the underwater hydroacoustic equipment. Before beginning the deployment, hydrophones, cables and all other equipment were checked once again on shore. In fact, such tests took place during all phases of production and transport. Even in the factory "we could go in the yard, tap on the hydrophone and see whether the signal was received in Vienna", Andrew Forbes said.

During the entire deployment phase, engineers maintained live connections with the shore station and with Vienna, in order to be able to deal immediately with any potential problems.

Anyone who has ever been on a beach holiday knows that crossing the surf is not always easy. Equally, for the deployment of underwater cables the last couple of hundred meters from the open sea to the shore station are the most challenging of the entire 100 km distance. Here the cable is exposed to the energetic waves and currents, the sharp edges of corals and the slow grinding power of sand.

While the cable for most of its length is just light weight telecom optical fibre cable, closer to shore it is externally armoured with tough steel wire. Crossing the shore line, the cable even sports double armour. And as if that were not enough, the cable is led through a long underground tube that has been drilled starting on the island and exiting on the seabed in sufficiently deep water. This method has already been employed at another hydroacoustic station, and Andrew Forbes has had good experiences with it: "This way the cable avoids the worst part of the reef and is not likely to be damaged by ships' anchors or near-shore fishing."

Works were completed on 15 February 2007. Since then, hydroacoustic station HA11 at Wake Island has been continuously connected to Vienna by a satellite data link. With this hydroacoustic station up and running, the sounds of all the planet's oceans are listened to in Vienna. The highly efficient hydroacoustic monitoring network will ensure that no underwater nuclear explosion goes unnoticed.