Feature

Mapping a Muddy Swath

Thousands of square kilometres of our seafloor were mapped in weeks by a high-tech French ship -- tens of thousands remain.

Keith Lewis

In the last few months, New Zealand and French scientists have begun poring painstakingly over maps of large areas of New Zealand's offshore 200-mile Exclusive Economic Zone or EEZ. These maps show a detail and complexity that nobody had previously imagined. They are the result of a new technology, called swath mapping, that is rapidly changing the way we see the seabed.

We all take maps for granted. We have had detailed maps of onshore New Zealand for well over 100 years. They are a prerequisite for developing our land and its resources, for engineering projects, for managing diminishing natural environments or just for planning a holiday trip. They show, not only the shapes of valleys and mountains, but also the extent of sand-dunes, swamp and forest -- the textures of the land.

Yet these maps cover only 7% of the area for which New Zealand has economic jurisdiction. The other 93% is our EEZ and most of that is less clearly mapped than the face of the Moon. But this vast marine area is our responsibility and we can exploit it wisely or, in ignorance, let its value be destroyed. Since the starting point for almost any job is a good map, how do we get good maps of such an enormous area of seabed at reasonable cost?

Up until 1930, charts of the deep oceans around New Zealand were based on a few lead line soundings from a ship. Depths at a single point were measured by allowing a weight on a fine wire to fall to the seabed and measuring the length of wire paid out. This required the ship to stop, making it time consuming and therefore costly, particularly in deep water areas. It also required experienced mariners who could recognise when the lead hit the seabed, and positioning was obtained from the stars, providing there was no cloud.

The coverage of most of what is now New Zealand's EEZ was equivalent to two survey measurements for the whole of the North Island and three for all of the South Island. Since World War II, echo-sounders have produced lines of close-spaced sounding from directly beneath ships. But even so, after 50 years, maps of most deep water areas are based on lines of depth soundings tens of kilometres apart. What depths occur between sounding lines is anyone's guess. On land, such a density would reveal the main ranges and plains, but that is all -- hardly an adequate basis for planning anything of importance.

This lack of knowledge is being eliminated by the new technology of swath mapping, which produces intricate maps of depth and seabed texture for a wide strip, or swath, of seabed on either side of a ship, leaving no unexplored peaks or valley between tracks.

Sideways Sound

All swath mapping systems use echoes of sound projected sidewards, instead of downwards like a conventional echosounder. The concept of projecting sound outwards to "illuminate" the underwater world has been used by whales for 40 million years. It has also been the basis of anti-submarine and fish finding asdic or sonar for decades. In swath mapping, multiple sound beams are highly focused to intersect the seabed perpendicular to the ship's track and at a precise angle to the vertical.

In its simplest form, a recorder displays an image of the varying intensity of sound that is reflected from the seabed out from the ship -- a concept somewhat similar to ultrasound baby scanners. At sea, this builds up a remarkable side-scan sonograph or swath-image that resembles a strip aerial photograph of the seabed's texture. The images show the extent of mud, sand, shell and gravel and any protrusions or depressions such as rocky reefs, scour-marks, sand dunes, trawl marks, submarine cables and the gamut of wrecks and rubbish that litters the seabed.

A short-range side-scan, which images a swath of seabed about 300 metres wide, has been used in New Zealand for nearly 20 years. It has been deployed on site surveys for oil platforms and harbour developments, on route surveys for gas pipelines, power cables and telephone cables, and to search for downed aircraft and sunken ships. A very much larger British version named GLORIA (Geological LOng Range Inclined Asdic), which images a swath of seabed up to 30 kilometres wide, mapped over 23,000 square kilometres north of the Bay of Plenty in only four days in 1987. It discovered dozens of new volcanoes, massive lava fields and major faults. However, despite their usefulness, side-scan images do not measure depth.

In a parallel development, sideways-projected sound has been focused into almost pencil-like beams at discrete angles to the vertical, so that, with simple geometry, it is possible to calculate the depth where each beam strikes the seabed. Instead of a single line of echo-soundings it produces parallel lines from each beam. Early versions used in our EEZ had up to 15 beams and so simultaneously produced 15 lines of echo-soundings. These systems gave a swath of depth measurements (swath-bathymetry) but no aerial photo-type swath imagery.

It is only when both swath bathymetry and swath imagery of the same patch of seabed can be compared that it becomes possible to interpret the geological processes, biological habitats and oceanographic effects on the seabed. Such a breakthrough came only recently with the development of dual sonars that simultaneously produce both contour maps and texture maps of swaths of seabed tens of kilometres wide. This coincided with the availability of satellite-based Global Positioning Systems (GPS), which accurately position the vessel and its data anywhere on Earth 24 hours a day. Thus, swath-mapping was born.

Plate Boundary Surveys avec la Différence

One such dual system was based on vastly increasing the number of beams making it practical to record, not only the depth for each beam, but also its intensity to give a grainy representation of seabed texture. The only problem with this multi-beam technology is that it so large you must virtually build a ship around it. The French marine research agency IFREMER (Institute Francais de Recherche pour l'Exploration de la Mer) recently built the flagship of the French oceanographic research fleet, L'Atalante. With Gallic flair, they first built a restaurant around their dual-purpose multi-beam and then a ship. IFREMER's research fleet is available to any French oceanographic team awarded the necessary, massive science funding. Several French teams have an interest in the boundaries between the enormous crustal plates that make up the Earth's surface. Like us, their interest may be partly prompted by their proximity to such boundaries, where giant earthquake (active) faults relieve the immense pressures that build up between the plates.

Two such groups are ORSTOM (Office de la Recherche Scientifique et Technique Outre-Mer), which has a base in Noumea on the same plate boundary as ourselves, and is incidentally the largest and closest marine research organisation in the Southwest Pacific; and the University of Nice, which sits close to the boundary that is forming the European Alps. While on sabbatical at the University of Nice, Dr Rick Herzer of New Zealand's Institute of Geological and Nuclear Sciences, worked on a proposal to build on the work of New Zealand marine geoscientists and explore the geodynamic effects of the plate boundary that slices through New Zealand's EEZ. The proposal, codenamed GeodyNZ, coordinated French and New Zealand research effort to use L'Atalante's magnificent swath-mapping capabilities and to use a New Zealand research vessel to "ground truth", or sample, the features that L'Atalante would inevitably discover in a frontier area. The project was several years in the planning, but in 1993 it was one of only two projects out of over 40 proposals funded by France's science funding agency for work in the Southwest Pacific.

New Zealand marine geologists had pieced together the general story of how the Pacific Plate, which goes all the way to California, and the Australian Plate, which goes most of the way to India, grind into and past one another along a boundary that slices southwestwards from Tonga through the New Zealand region. In the northeast, the Pacific Plate, which is moving like a conveyor belt, dives back into the Earth s interior under the edge of the Australian Plate, which carries Tonga, the Kermadec Islands and eastern North Island. The boundary comes onshore between Marlborough and Fiordland as the Alpine Fault. To the southwest there is a mirror image system with the Australian Plate diving beneath its Pacific neighbour.

There is wide interest in the offshore story for several reasons. Firstly, it is a microcosm of the many processes that now affect, or have affected, the edges of other continents worldwide. Secondly, many of the world's mountain ranges, including our own, are ancient products of the same processes that are occurring now on the seabed off Hawkes Bay and Wairarapa. Thirdly, the varied offshore processes have a profound effect on the adjacent land and are one of the primarily reasons for New Zealand's scenic diversity. Despite the interest, it had become increasingly obvious in recent years that we had gone as far as we reasonably could with the conventional echo-sounding technology available in New Zealand. The next stage was clearly to get the full three-dimensional picture with swath-mapping.

With the massive capital investment for this equipment out of reach of any single research organisation in New Zealand, there had been several attempts at joint ventures with overseas organisations to swath-map key segments of our margin. There was no shortage of interest, but always the weeks of expensive ship time required to steam to New Zealand from northern hemisphere research centres left proposals stillborn. The breakthrough came because of French commitment to exploration in the Southwest Pacific from its base in Noumea. So, after all the planning, everything came together in November 1993 when L'Atalante reached Auckland to begin the GeodyNZ survey.

The vessel itself was an eye-opener. Reportedly equipped with some NZ$150 million worth of electronic equipment -- more than the cost of the ship -- the bridge and laboratories resembled some fantasy from Star Wars, particularly in the night-time glow of the monitors. In only three weeks, we mapped the northeastern segments of the plate boundary beginning in the trench northeast of East Cape and finishing where the plate boundary comes ashore at Kaikoura. The area totalled about 86,000 km², which is about three quarters of the area of the North Island, and at a rate of 4,000 km² per day. As the ship steamed along, a new, colour contoured chart of a strip of seabed up to 22 kilometres wide was appearing on one large plotter, while an aerial-photo swath image was emerging on another. Instant maps!

At regular intervals, probes dropped from the ship relayed back the temperature and salt content of the water column so that the changing velocity of sound could be calculated accurately to calibrate the depth measurements. In adjacent laboratories, seismic profiles showed the layers beneath the seafloor and a magnetometer recorded the variations in Earth's magnetic field to differentiate magnetic volcanic cones from other lumps on the seafloor.

Day and night, 11 French and five New Zealand scientists processed and discussed the incoming data. Preliminary interpretations were used to plan the mid-year ground-truthing cruise by NIWA (National Institute of Water and Atmospheric Research), when samples were collected from newly discovered peaks and canyons. L'Atalante continued its study of the plate boundary where it re-emerges off Fiordland, aided by a complementary cruise with IGNS.

Volcano Impact
Scars and Canyons

The results have been spectacular. Even the draft maps, with contours at 50-metre intervals, are a quantum leap from the existing published maps. A good example is off the East Cape, where published maps showed rounded hills and smooth valleys. L'Atalante's maps showed irregular ancient volcanic peaks protruding from a slurry with large rocky blocks that is reminiscent of an East Coast hill-side after a month of heavy rain -- except that it covers an area larger than either Coromandel or Banks Peninsula.

Samples collected during the recent cruise will show the origins of volcanic peaks and of slide blocks. The old volcanoes, carried on the conveyor belt of the Pacific Plate produce massive scars in the soft papa mudstones of the east coast margin as they collide with it. The East Cape debris may result primarily from such a collision but the clearest example may be off Mahia. The effect is rather like a bullet hitting cheese, except that the "speeding" volcano is travelling at 50 millimetres per year and takes several million years to penetrate the mudstone margin.

In the latest cruise we tried to find the magnetic signature of the volcano that may now be pushing up the eastern edge of Mahia Peninsula as it is carried down into the Earth. To the south, old volcano scars may have been partly healed by mud that has been scraped off the conveyor belt and plastered onto the scarred margin to form long ridges. As the heap of plastered mud thickens, gas is formed and is squeezed out at vents marked by bizarre faunas that have recently been discovered by exploring orange roughy fishermen.

The L'Atalante survey continued right to the head of the Kaikoura Canyon, where the plate boundary comes ashore and where the northward-moving beach is funnelled into the deep. The survey continued onto the northwest corner of the Chatham Rise where scours and mud drifts, looking like those of an enormous river estuary, were formed by currents funnelled through the gap between Banks Peninsula and the Chatham Rise during ice age lowerings of sealevel. The work off Fiordland was hampered by atrocious weather, but even so showed a new level of detail of fault-sheared rocky ridges and deep troughs that mark the colliding crustal plates.

Swath Mapping
in the Future

Later this year, when all processing for navigation and sound velocity through the water column is finished, we will be able to begin producing maps that rival the detail of the New Zealand National Topographic Series with closely spaced contours and the extent of textures such as mud, rock, current scour, sediment drift, slumping, faulting and lava flows all precisely defined. When analysis of the sample data is also complete, we will be able to reinterpret the evolution of the North Island margin that has implications for our own onshore geology, and for modern plate boundaries and ancient fold belts throughout the world. With only a few weeks use of very sophisticated equipment, we have been able to produce maps and geological understanding that might have taken decades to produce by traditional methods.

There has been one other major result of the L'Atalante survey. It has whet everyone's appetite for more. Whatever your interest in the seabed around New Zealand, be it fisheries, cable routes, minerals, defence or science, it is becoming clear that you can't afford to be without a good map. The L'Atalante's system, although producing the best quality contour maps, is inextricably linked to a large, expensive and generally inaccessible ship. But there are other ways of achieving similar, but not identical, results.

Scientists in Hawaii and in Britain have produced a much more portable system based on out-of-phase beams, which can be towed behind any vessel of convenience. Although depth measurements are not quite as accurate as those from hull-mounted systems, the sidescan imagery is more detailed. Variations on this system have already been used in New Zealand for submarine telecommunications cable-route surveys out of Auckland and very recently for a deep-sea fisheries survey of seamounts on the Chatham Rise. Right now, a joint venture between science and the fishing industry is exploring parts of the east coast and the volcanic ridges north of the Bay of Plenty.

On the horizon, is a new version of the very long range GLORIA sidescan, which will collect depth data and which has been whimsically christened GLORI-B. This offers the potential to map vast areas of our EEZ in a few years, a task hitherto thought to be infeasible. Although the results to date are impressive and the 180,000 km² that has so far been swath-mapped seems large, this is still only about 5% of our EEZ. There are still over 3,000,000 km² that qualify for the old map notation "unexplored".

Dr Keith Lewis is with NIWA, the National Institute of Water and Atmospheric Research Ltd.

Keith Lewis is a marine geologist with the New Zealand Ocean-ographic Institute.