Ocean Floor Survey for the Interisland Cable

Photo of two men standing on either side of a boat. Blue ocean waves are in the background. The man on the right is holding onto a chain that is attached to a metal square apparatus. He has on denim shorts, a yellow shirt, red vest and red hard hat. The man on the left has on a black shirt and red vest and hardhat.

Using a towcam and magnetometer, a researcher from the University of Hawaii at Manoa School of Ocean and Earth Science and Technology (UHM-SOEST) collects ocean floor data needed determine the feasibility of laying an undersea cable between O‘ahu and neighboring islands.

To gauge the feasibility of laying a power transmission cable on the ocean floor between the islands of O‘ahu, Lana‘i, Moloka‘i, and Maui, the state of Hawai‘i Department of Business, Economic Development and Tourism (DBEDT) contracted with the University of Hawaii at Manoa School of Ocean and Earth Science and Technology (UHM-SOEST) to conduct an ocean floor survey and to recommend cable routes based on the results.

The project was broken into the following tasks:

  1. Compile existing data regarding bathymetry (measurement of ocean depth at various locations), critical habitats, and seafloor
  2. Collect and process bathymetric and seafloor data.

Task 1

Photo of a motorized gray catamaran with red hulls on glass-like water. Photo of a large white yacht next to a dock Colorful flags span the boat's length from bow to stern. The boat's name, Ka‘imikai-O-Kanaloa, is written on its side near the bow.

The 25-foot NOAA survey launch R/V Ahi and the R/V Ka‘imikai-O-Kanaloa.

As part of Task 1, UHM-SOEST made a digital compilation of publicly available bathymetry data gridded at 10 meters (m), for the seafloor area between O‘ahu, Moloka‘i, Lana‘i, and Maui. UHM-SOEST consulted with the State Office of Planning, which has an extensive Geographic Information System (GIS) database, including marine layers that include the location of existing submarine telecom cables, the Hawaiian Islands Humpback Whale National Marine Sanctuary, and other restricted/managed areas.

Photo of a spherical apparatus hanging under water. The entire photo is blue. Lights are shining up from an apparatus underneath the spherical shape.

The remotely operated vehicle (ROV) videocam UHM-SOEST used to survey the ocean floor.

UHM-SOEST reviewed and collected data from

  • The National Oceanic and Atmospheric Administration (NOAA)-commissioned study describing the near-shore benthic habitats of the main Hawaiian Islands (focusing on the intertidal or eulittoral zone, which begins at the shoreline and extends downward out to sea) (For additional information, see also the NOAA's National Centers for Coastal Ocean Science study, Shallow-Water Benthic Habitats of the Main Hawaiian Islands - 2007)
  • The United States Geologic Service studies of the benthic habitats off the south coast of Moloka‘i (for more information, visit the U.S. Pacific Coral Reefs Web site) 
  • 30 KHz sidescan sonar coverage offshore southeast O‘ahu and north of Maui.

Based on its desktop study and gap analysis, the technical cable requirements, available information on proposed sites for cable landings and converter stations, and its knowledge of marine geology, benthic biology, and oceanography, UHM-SOEST developed a set of initial recommended submarine cable routes and alternatives.

UHM-SOEST used the following guiding principles to identify recommended routes:

A map showing existing bathymetric data that existed as of April 30 for areas surrounding O‘ahu, Moloka‘i, Lana‘i and Maui, as well as gaps in the ocean floor data. The existing data is represented by a variety of colors, including gray, red, pink, yellow, light green, and blue. The gaps are shown in white in the areas east of Waimanalo on O‘ahu, south of Moloka‘i, north of Lana‘i, and north of Kahului Harbor on Maui. The islands are dark green. Enlarge image

Existing bathymetric ocean floor data as of April 30, 2009. As part of the survey project, DBEDT contracted with UHM-SOEST to fill in the gaps in this data, notably the areas east of Waimanalo on O‘ahu, south of Moloka‘i, and north of Kahului Harbor on Maui. 

  • Minimize distance (given cost of cable).
  • Keep depth above 800 m (for better navigation and to reduce the weight of the cable as it is being layed).
  • Minimize length in the whale sanctuary and other marine protected/restricted areas.
  • Minimize crossing of steep slopes, hard grounds, important benthic habitats, precious corals, and dumped materials.
  • Minimize crossing of existing telecom cables.
  • Prefer crossings at high angles.

From existing data, UHM-SOEST identified numerous constraints, including the following:

  • There is no viable route north of Moloka‘i due to submarine canyons and landslides.
  • Connecting Moloka‘i, Lana‘i, and Maui will require cables in the whale sanctuary.
    A map of highlighting the near-shore benthic habitat off of Kāne‘ohe Point at the southeast shore of Oahu in various colors, including purple, pink, blue, green, and orange. Topographical detail of the island appears in black and white. Enlarge image

    Near-shore benthic habitat off of Kane‘ohe.

  • Offshore areas less than 120 m deep were subaerially exposed and eroded during the last glacial maximum 20,000 years ago.
  • During the subsequent sea level rise, modern coral reefs grew and now fringe all the islands.
  • Living (including precious) corals and extremely rugged seafloor dominate between east Lana‘i and west Maui–Kihei.
  • The approaches to Penguin Bank and O‘ahu are generally characterized by steep edges.
  • There is no viable route across southeast Penguin Bank (rugged and steep paleo-reef slopes).
  • Deep former reefs occur in some places between the islands.
  • There is material dumped south of Pearl and Honolulu Harbors. 

Using these criteria and the data collected previously, UHM-SOEST developed nine possible routes.

A map highlighting the benthic habitats offshore the south coast of Moloka‘i. The island is shown in brown. The benthic habitats offshore are shown in multiple colors, including orange, yellow, green, and various shades of blue. Enlarge image

Near-shore benthic habitat near south Moloka‘i.

A black and white sidescan sonar image of the southeast shore of O‘ahu mapping the topographical detail of the island as well as solid material below the surface of the ocean just offshore, which appears as white spots. A line points to a white area labeled as reef rocks. To the right of that line, three lines points to white areas labeled as dumped materials. At far right, two lines point to white areas labeled volcanic rocks. Enlarge image

30 KHz sidescan sonar coverage offshore southeast O‘ahu.


Task 2

For Task 2, UHM-SOEST collected new data that included

  • Shallow water multibeam bathymetry mapping (using the 25-foot NOAA survey launch R/V Ahi) to fill the existing shallow water (<200 m depth) data gaps around where the cables may be routed
  • Cruises of R/V Ka‘imikai-O-Kanaloa that surveyed along the proposed cable routes using deep-towed subbottom profiling and sidescan, camera/video transects, and seafloor sampling at various points. 
Map of the ocean floor showing possible interisland cable routes at 10 m of the seafloor area between O‘ahu, Moloka‘i, Lana‘i, and Maui. The island shapes are gray. The bathymetric data for the area surrounding the island shapes is represented by shades of white, brown, green, and blue. Enlarge image

UHM-SOEST developed nine initial cable route suggestions based on ocean floor data that existed at the time of its survey and characteristics of ocean floor cables.

Map of the ocean floor surrounding the islands of O‘ahu, Moloka‘i, Lana‘i, and Maui, showing filled-in data gaps at depths of less than 200 meters for possible interisland cable routes. Island shapes are shown in ivory. The bathymetric data for the area surrounding the islands is represented by orange, yellow, and shades of blue and green. Enlarge image

The routes that were videotaped during the survey of the ocean floor.


More Information

For more information on the feasibility study conducted by UHM-SOEST, read the full Hawai‘i Interisland Cable Project Ocean Floor Survey: Final Report (PDF 3 MB). Download Adobe Acrobat.

Department of Energy
State of Hawaii