Crozet Islands: installing our last hydroacoustic station

If you are not a marine biologist, the chances are that you have never heard of the Crozet Islands in the South Indian Ocean. These French administered islands, with a human population of only 40, are a unique nature reserve and one of the remaining 'untouched' places on earth. But it’s not only the unique flora and fauna that make these remote islands a very special place.

In February 2016, Jerry Stanley and Mario Zampolli, hydroacoustic experts from the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), went on a mission to add another important role for the archipelago; the installation of the CTBTO’s last hydroacoustic station, HA04.

The Crozet Islands are home to thousands of king penguins. Click to enlarge.

HA04 is one of eleven hydroacoustic stations that monitor the oceans for signs of nuclear explosions. Low frequency underwater sound, which can be produced by a nuclear test, propagates very efficiently through water. Consequently these underwater sounds can be detected at great distances, sometimes thousands of kilometers, from their source. This means that the International Monitoring System (IMS) requires only a few hydroacoustic stations to provide effective monitoring of the world’s oceans for signs of nuclear explosions.

The IMS also monitors the Earth’s crust and atmosphere: 170 seismic stations monitor the ground for shockwaves, 60 infrasound stations listen for atmospheric waves, while 80 radionuclide stations sniff the air for traces of radioactivity – see interactive map.

The hydrophones float in a channel where sounds travels very efficiently. This channel varies in depth around the world's oceans, but is typical found at a depth of several hundred meters at Crozet.

The Crozet Islands are the ideal observation point from the acoustic point of view. If you place receivers there you can hear sounds from the Southern Atlantic all the way to the Indian Ocean. It is just like being in the perfect spot - the perfect lookout.

The installation of hydrophones at one of the most remote places on earth is a challenging ocean engineering project and a complex logistical operation.

The station will use six hydrophones (underwater microphones) to monitor underwater sounds deep in the ocean. The HA04 hydrophones send their data via underwater cables, which are tens of kilometres long, to a receiving facility, the Central Recording Facility (CRF), on the island. From there the data is forwarded via satellite to the CTBTO in Vienna.

With no airstrip available on, or anywhere near, the Crozet Islands the HA04 installation team and its equipment was transported to the island on the Marion Dufresne II, a French oceanographic research and deep water survey vessel - a roundtrip journey of over 5,000 kilometres.

The experts’ first task was to install the upgraded CRF on the archipelago’s main island, Possession Island (Île de la Possession). The CRF includes communication equipment and equipment for processing the hydrophones' data. Within a few days of arrival of the team all the equipment was shipped to the island, installed and the dedicated satellite data connection to Vienna commissioned and tested.

The Marion Dufresne II, one of the world’s biggest and most sophisticated oceanographic research and deep water survey vessels, supported the mission. It is specifically designed to withstand the extremely rough weather conditions, which are often found around the Crozet Islands.

We have been installing with our contractor this hydroacoustic laboratory. It is ready to receive data from the underwater system. That comes into two computers where the data from the north and south underwater triplets is formatted and mixed with time stamps from GPS. This is then transferred to CTBTO computers which, via satellite link, forward everything in real time to Vienna.

The second task was to pave the way for safely landing, routing and laying of the underwater cables connecting the CRF to the hydrophones. Sea bed bottom surveys were conducted from a depth of 10 m to depths of over 2,000 m so the team could see the bottom topography and detect any underwater objects that could interfere with the safe installation of the cables. In past centuries, the island was used by the whaling industry so objects hazardous to the installation such as shipwrecks, machinery and anchors were to be expected; and indeed shipwrecks were identified.

Possible north and south cable routes (black lines) superimposed upon the bathymetric survey data.

We will have cables going out north of the island 40 to 50 kilometres to where we are going to install the hydrophones. These cables will be there for over 20 years. We need to understand the seabed profile to put the cables in places where the risk of abrasion or damage by the seafloor is minimized.

Thirdly, all objects that might compromise the installation were removed from the sea floor- this included over 50 m of antique anchor chain. These clearance operations were supported by divers and wherever the divers went they were accompanied by curious penguins that live in tens of thousands on the island. Together with rare migrating birds, sea elephants, orcas and endangered large whales, they are a part of Crozet’s rich protected fauna. All activities of the team followed strict environmental standards and operations were overseen by the nature reserve.

Once completely established, hydroacoustic station HA04 can help monitor, and thus protect, the local fauna. For example, hydroacoustic data can be used to track the migration of whales. Find here an example of a whale song recorded by a CTBTO hydroacoustic station.
The monitoring system that is put in place by CTBTO isn’t just applicable to the detection of nuclear testing. We have a lot of whales in the sub Antarctic region and the hydrophones will provide valuable materials that enable us to perform long term studies. The hydrophones also pick up seismic movements and, for tsunami alerts, the CTBTO network supplies a lot of information as well.

A follow-on mission in December 2016 will make use of the calmer Austral summer conditions and a specially designed cable laying ship to deploy and commission the underwater components of HA04. From early 2017 onwards the CTBTO's IMS network will benefit from the unique data that HA04 will provide. The Unites States and France are contributing to the project by way of voluntary contribution.