UNCLOS article 76 surveys: Claims for extended boundaries

Sea Technology, Aug 2002 by Wilson, Jerry C

Marine Geodesists, Geologists and Geophysicists Depend on a Suite of State-of-the-Art Systems for UNCLOS Surveying

Definition of seaward national boundaries under United Nations Convention Law of the Sea (UNCLOS) requires a team including marine geodesists, surveyors, geologists, geophysicists and supporting technical specialists. Boundary determination has always been the provenance of surveyors, and national or global-scale definitions require experienced geodesists. Geologists are called upon under UNCLOS to interpret and define seafloor conditions such as geomorphology, crustal types and limits, and sedimentary prisms. For this, the geologist utilizes geophysical measurements of seismic reflection/refraction and velocities, magnetics and gravity, as well as high-resolution bathymetry.

In order for the expert team to provide the client country with the best results and most advantageous outcome, the team must utilize a suite of state-of-the-art tools. These tools are best integrated on a single specialized vessel dedicated to UNCLOS surveying. An example of such a vessel is the R/V Thales Venturer.

This specialist vessel was originally designed for seismic survey, which is an important part of UNCLOS surveying. Therefore, a purpose-built seismic vessel is an excellent starting platform from which to build an integrated UNCLOS survey vessel. The additional essential capabilities include full ocean-depth swath bathymetric systems, gravity and magnetics, plus onboard charting and interpretive facilities. The latter provide for survey adjustments as discoveries are made.

The importance of marine surveyors is directly linked to the fact that none of these data types are of value unless precisely positioned. Thales' SkyFix system is a worldwide open-ocean navigation service for UNCLOS applications.

Technical Requirements

By its very definition, the Law of the Sea rules for determining jurisdiction beyond 200 nautical miles calls for integrated procedures.

All aspects depend upon surveying principles of both the land (e.g., metes and bounds) and hydrographic surveyors' practices. The basis of all UNCLOS references is the coast baseline. Since coastlines change with landmass movement (earthquakes, isostatic rebound, continental drift, etc.), erosion, deposition and other geological processes, it is possible that the very fundamental basis for UNCLOS surveying may need to be verified or reestablished. These baselines are a direct function of a tidal datum such as lowest astronomical tide, which may also be updated with a new series of tidal records.

But the more significant differences in the states' seaward boundaries will depend upon deepwater seafloor conditions. In terms of the Law of the Sea, these conditions are defined by two categories of seafloor survey data.

Bathymetry and Geomorphology.

Bathymetric data are required first to define the 2,500-metre isobath. This contour provides the reference for an offshore baseline, which contributes to the definition of the outermost cut-off line, or maximum possible extension of the states' seaward jurisdiction. Adding an offset line 100 nautical miles seaward of the 2,500-metre isobath develops this component of the cut-off line. The other component is a line 350 nautical miles from the coastal baseline; it is notable that bathymetry may allow jurisdiction even beyond the 350-nautical-mile maximum.

Geomorphology interpreted from the bathymetric database is used to define the location of the foot of the continental slope. To this offshore reference line is added a 60-nautical-mile offset line, which defines an extension of jurisdiction based on geomorphology

Geology. Seismic data can be used for definition of offshore points beyond the foot of the continental slope that mark the extent of the continental sedimentary wedge over oceanic crust where its thickness is one percent of the distance from the foot of the slope.

These interpreted data points must be separated by no more than 60 nautical miles.

Key UNCLOS data requirements include geodesy, geophysical, hydrography and geology. These are merged with knowledge of Law of the Sea, geomorphology, structural geology and geophysics to determine whether an extended boundary claim is justifiable. Other data types include seismic refraction using sonobuoys or ocean bottom seismometer (OBS) units, magnetics and gravity.

Equipment and Processes

From these UNCLOS requirements, the importance of detailed deepwater bathymetry and subsurface geological information is evident. These parameters interact to provide definition of the maximum jurisdiction for coastal states.

While the 2,500-mile contour and foot of slope may be derived from a single bathymetric profile, it is clear that multibeam echosounding offers powerful benefits to the applicant state. The deep sea terrain can be as complex as the land and this knowledge provides a better basis for definition of these two important offshore baselines.

Seismic data are the most reliable means for a country to assure itself that the best definition of the extent of its continental geology has been achieved. For UNCLOS, the style of marine seismic data acquisition is more similar to global crustal studies than to the more common exploration seismic configurations.