Story: Engineering on the sea floor
Page 2 – Submarine cables
The first Cook Strait cable
In 1866, only a few decades after the arrival of the first European colonists, a simple copper telegraph cable was laid across the stormy and tide-scoured seabed of Cook Strait. It was not reliable. Further attempts were made in the late 19th century, but it was not until the 1920s that a reliable telephone cable link was established between the North and South islands.
Early cables were laid with little understanding of the extreme conditions of Cook Strait’s seabed. In the late 1950s plans were underway for a submarine cable linking South Island hydroelectric power generators with North Island towns, and only then were serious attempts made to understand how such cables were affected by the underwater environment.
Testing the Cook Strait seabed
In 1959 Henry Pantin was appointed New Zealand’s first marine geologist. His initial job was to determine whether Cook Strait’s well-known tidal rips would affect the seabed along the proposed route of the power cable. Scientists in other parts of the world had traced sand movement, but the seabed that Pantin found was made of pebbles.
He devised a unique way to measure pebble movement. Magnetic ironsand from the North Island’s west coast beaches was made into concrete blocks, then crushed into pebbles. These were dumped in heaps on the seabed at marked locations. Samples of the sea floor were taken at regular intervals in a grid around the dump sites, and tipped into a chute, where magnets sorted out the ironsand pebbles. The pebbles had moved far from where they had been dumped, indicating that the sea floor was swept by strong currents. Pantin’s study showed that the proposed cable would have to withstand constant battering by shifting pebbles. Re-routing the cable was considered too expensive at the time.
Despite being heavily armoured, the power cables laid in 1964 had a troubled history. They were replaced on the same route, along with new telecommunications cables, in 1991.
To avoid hazards in the strait’s narrows, in 2000 a new fibre-optics telecommunications cable was laid along the comparatively benign (but longer) route between Levin and Nelson.
Since 1876, when the first telegraph cable from Sydney came ashore at Cable Bay, near Nelson, New Zealand has been linked to the world by a web of fragile cables. These are laid across oceans that are typically 5 kilometres deep and studded with rocky seamounts.
Since the 1980s fibre-optic cables capable of carrying huge amounts of data have been laid along the ocean floor. They stretch from near Auckland to Sydney, and onto South-East Asia and North America, with links to the Pacific Islands. Underwater volcanic vents, rocky ridges and abyssal trenches, as well as avalanche-prone areas and deep-ocean channel systems, are all hazards that must be avoided by trans-oceanic cables.