bathymetric survey TongaOne of the most surprising and interesting finds our bathymetric survey company during our recent major survey of the Niue region, was a shipwreck.  One that was on the move!

To be fair the sunken ship had originally been discovered and charted long before we all arrived.

The ‘Nicky Lou’, an Albacore fishing vessel, ran aground in March 1992, when the ship was transiting Niue’s EEZ by dead reckoning due to broken navigation equipment, en route to

Pago Pago.  The boat was abandoned and has been deconstructing over the last twenty-seven years!

Initially, a Satellite-Derived Bathymetric Survey (SDB) of Beveridge reef was conducted in December 2017.   Five months later, when the Airborne Laser Bathymetry (ALB) survey was undertaken for Stage 2, the wreck didn’t appear in the same area.  It had moved some considerable distance.

In the image above, ALB data shows the location of wreck in May 2018, and the black circle shows the location in the SDB data from Dec 2017.  Note: scour marks present from where ship was dragged presumably during Cyclone Gita.

Why did the wreck move?

As the wreck was in both datasets but in different locations, we concluded that it had been dragged to a new location, most likely during the powerful Cyclone, Gita, that hit Tonga in February 2018.

Gita was the most powerful storm to hit the region in 60 years and caused massive destruction to the islands.  Gita strengthened to a Category 5 as it passed over Tonga, and the Tongan government declared a state of emergency.

Cyclone Gita Tonga

Figure 1.  Path of Cyclone Gita over Tonga in February 2018.

Many structures on land were damaged, as were the shallow reefs.

Several shallow reef locations were ground-truthed with video during our survey, and it was interesting to see coral rubble with new growth emerging.  The marine area appears to be very resilient, and storms are part of the natural ebb and flow of undersea life.

The survey technology that provided accuracy, safety and cost savings

The project

As part of the LINZ Pacific Regional Navigation Initiative (PRNI) project recently awarded to iXblue, a regional Satellite Derived Bathymetry (SDB) survey from SDB experts – EOMAP was commissioned.  This would be one of the largest SDB surveys ever undertaken by EOMAP, and certainly the largest of its kind ever undertaken in the South Pacific. The data was rendered to the project in early 2018.  In addition to SDB, the project acquired Airborne Laser Bathymetry (ALB) across specified shipping routes between the three main Tonga subregions in May of 2018.  These additional data products were used to assist in the planning of a Multibeam bathymetry survey and to fill-in shallow areas near reefs in hard-to-reach zones.

What exactly is SDB?

Satellite-Derived Bathymetry (SDB) is derived from electromagnetic sensor data from satellites orbiting the earth.

Specific ranges of wavelengths of light reflections, known as “bands” are recorded from land and sea.  By running these data through an algorithm, one can extract shallow depth information if the water and atmospheric conditions are relatively clear.  We generally are interested in bands such as Red, Green, Blue (RGB) as these are in the visible spectrum for the human eye.  However, we can also gather the reflectance for non-visible light from the near and far infrared ranges.

satellite derived bathymetry light spectrum

Figure 2. Light spectrum with zoom to visible spectrum between 380nm and 780nm.

SDB is a very cost-effective way to map elevations and shallow depths over large regions, especially in the tropical waters of the South Pacific.  Being a satellite-based sensor, it is amazing to think that the resolution of the data from this sensor is in the order of 2m.

Whilst this is not quite good enough to meet the feature detection requirements of a high order bathymetric survey, it is certainly very useful to aid the surveyor in the planning of higher resolution surveys and also to assist the cartographer compile new charts or update existing charts.

As part of the PRNI project, EOMAP used this technology to accurately position all the islands around Tonga in our area of interest.

SDB RGB data Beveridge Reef Tonga SDB elevation Beveridge Reef Tonga

Figure 3. Example SDB data (RGB bands-LEFT, Elevation - RIGHT) of Beveridge Reef, Niue December 2017 EOMAP

What is ALB?

ALB also uses light to measure elevation, however it uses a laser pulse emitted from the LiDAR unit mounted in an airplane.  The laser uses four bands, including green wavelength (532nm) to penetrate the water column and get accurate depth readings down to 2-3 secchi depth.

Since ALB also measures topography, it is an ideal system to map shallow reefs, where survey vessels cannot safely navigate.  Whilst more expensive than SDB data, the ALB data is the only cost-effective solution to acquire survey grade data with a feature detection requirement in shallow water environments.

US firm - Geomatics Data Solutions (GDS) acquired ALB data for PRNI along the shallow reefs and coastal areas within the project.  The data was most useful for our crews of the Multibeam survey vessels as it allowed them to safely and efficiently navigate around the numerous reefs and atolls without fear of grounding.

shipwreck Beveridge Reef Tonga

Figure 4.  ALB elevation data over site of shipwreck in Beveridge Reef.

Beveridge Reef shipwreck

Figure 5.  RGB data from ALB shows submerged wreck in lagoon of Beveridge Reef

What have we learned from our combined approach to survey

  • SBD is a cost-effective solution to plan and refine an ALB survey. When planning such large, ‘regional’ area surveys such as PRNI, SDB is very useful to provide background information regarding the “shallow spots” to create the airborne survey plan.  It saves time as the reconnaissance is done from the desktop using the satellite information, reducing time spent in the field.
  • ALB helps the MBES survey in a similar fashion. The ALB survey is able to quickly and efficiently survey all shallow areas that would be very dangerous and very costly to survey with vessel mounted multibeam.  The combined end product is a “seamless” dataset between land and sea.
  • Data collected at different times (with a cyclone in between!) may not necessarily be the same in dynamic seafloor environments. Bathymetry, especially shallow areas impacted by waves, are constantly shifting and change is to be expected.  Areas, not anchored by rocks or reef, may have mobile sediments that shift on a variable scale depending on the oceanographic conditions.  This must be taken into account when surveying!

Elizabeth Johnstone

Dr. Elizabeth Johnstone is a marine geologist specializing in bathymetric surveying and sediment dynamics. She has over 15 years of experience using geophysical tools to conduct seafloor mapping around the world. She develops novel approaches to produce four-dimensional data products for integration into marine geological studies.

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