altimetry and unclos altimetry and the law of the sea definition of the continental shelf dave...

Altimetry and UNCLOS

Altimetry and the Law of the Sea definition of the Continental Shelf

• Dave Monahan

• Canadian Hydrographic Service and

• Ocean Mapping Group, University of New Brunswick

• Global Bathymetry for Oceanography, Geophysics, and Climatology


The United Nations Convention on the Law of the

Sea (UNCLOS) • The biggest single boundary-making event in human history. • Within a short period of time, two-thirds of the earth’s surface will be

divided into zones fringing the coasts and over which individual Coastal States exercise varying degrees of sovereignty, with the residual “Area” being administered by the UN.

• Reasonably straightforward measurements of 12, 24 and 200 M establish the Territorial Sea, the Contiguous Zone and the Exclusive Economic Zone

• The Outer Limit of a Coastal State’s Continental Shelf, or its Exclusive Economic Zone where there is no Shelf, forms the boundary of the “Area”.

• A Coastal State must establishing the existence of, and delineate the limits to, a juridical Continental Shelf, as part of a claiming process


The idea behind the definition in Article 76

• Broad-margin states wanted the definition to permit an extensive shelf, in opposition to those with no physiographic shelves who would benefit from the Area being as large as possible.

• Eventually settled on a principal that would see “the natural prolongation” of the continent available to the Coastal State, with the true oceanic ocean floors being included in the Area.

• According to prevalent knowledge, this granted hydrocarbons to the Coastal State and minerals like “manganese nodules” to the International Seabed Authority.


The model that influenced the wording of Article 76

• Foot of the Slope was a sea floor feature that was seized on to mark the division between continental and oceanic crust.

• Article 76 definition: ‘In the absence of evidence to the contrary, the foot of the continental slope shall be determined as the point of maximum change in the gradient at its base.’


The Foot of the Slope and Beyond

• Since the Continental Rise is composed of sediment, and hydrocarbons occur only in sediment, it was argued that the shelf should include portions of the Rise.

• Options: either a measurement of 60 M seawards or the point where the thickness of sedimentary rocks is at least 1 per cent of the shortest distance to the Foot of the Slope.


Constraints to unbounded expansion

• The disadvantaged states forced the inclusion of a finite Outer Constraint

• Continental Shelf can not extend beyond the most seaward of a line 350 M from Baselines, or a line 100 M seaward of the 2500 m isobath.

• Outer Constraint restricted to 350 M over “submarine ridges”


Where are the Juridical Continental Shelves likely to be?


The first claim, Russia, December 2001

• Wakey, Wakey!


The Boundary Making Process

• 1. Boundary is defined through words of the Convention (Treaty)

• 2. Boundary is delineated on maps and diagrams

• 3. Boundary is demarcated - on land by constructing markers, at sea by depositing coordinates with the Secretary-General of the UN.

• Continental Shelf boundaries must be submitted to the Commission on the Limits of the Continental Shelf (CLCS) for review as part of Step 2

• The CLCS has issued Guidelines on the quantity and type of data they consider acceptable for delineation, (but in places may overstepped their mandate and entered the realm of definition)

• Coastal States have to steer a course between the Convention and the Guidelines


Elements of the delineation process that altimetry might contribute to

• Determining the 2500 m isobath for use in the 2500 m plus 100 nm outer constraint constraint

• Locating the “Foot of the Slope” if it exists as a physiographic feature, or, where it does not, locating the continent/ocean boundary

• Determining the geological nature of submarine “ridges”


World (excluding Polar Regions) 2500 m contour.

Extracted from GEBCO Digital Atlas


How well are continental slopes mapped worldwide?

• Relatively fewer investigations concentrated on continental slopes

• Much of the data over them was not collected by systematic surveys but by transit tracks.

• Water properties over Slopes often complicated and poorly known, leading to higher uncertainty in acoustic measurements


Sources of data • Instruments

– Single beam echo sounder, variable sounding line spacing – Sidescan, most notably GLORIA. – Multibeam coverage is only increasing at a slow rate.

• Data bases and maps– Contour maps, grids and raw sounding data bases exist,

based on the sporadic acoustic data.

• Limitation to acoustic methods– Vessel speed and availability. GoMAP project estimates 200

ship years to survey the world ocean (excluding the Arctic)

from the 500 m contour out


2500 m isobath specified by the Convention

• Definition “the 2,500 metre isobath, which is a line connecting the depth of 2,500 metres.”

• Implies an absolute 2500m isobath without specifying surface that depth is measured from

• Refers to “the 2500 m isobath” as if there were only one fringing a Continental Shelf, while allowing for “submarine elevations that are natural components of the continental margin” which might mean that there is more than one 2500 m isobath off a margin


Requirements specified by Guidelines • Uncertainty assessment based on International

Hydrographic Organization standard S44• “ no need to submit ... a full 2,500 m isobath…” but

“the full bathymetric database will be regarded as an essential component of the supporting scientific and technical data.”

• Want to use the most landward 2500 m isobath where multiple isobaths exist

• Want to judge whether submarine elevations are part of the continental margin on a “case-by-case basis”.


Which 2500 m isobath to use

• a) where 2500m contours surround “submarine elevations”

• b) where the 2500m contour doubles back on itself

• c) whether the contour is on a “ridge”


Vertical uncertainty at 2500 m from IHO S44

• Combines fixed (constant) and variable ( with depth) errors as Root Sum of Squares.

• Provides multipliers and constants for depth measurement and isobath


Horizontal uncertainty at 2500 m from IHO S44

HORIZONTAL UNCERTAINTY VERICAL 1st 3rd

ACCURACY Quartile Median QuartileGradient Gradient Gradient1.1 deg 2.09 deg 3.83 deg

SP 44 Table IDepth 57.51 2995 1576 859

SP 44 Table 3Contour 125.10 6515 3428 1869


Comparison of four different 2500 m isobaths

• Off New Jersey, plotted the 2500m contour from – ETOPO-5 data – GEBCO-97 Digital Atlas CD – NGDC’s Coastal Relief Model CD’s – Predicted Topography Smith and Sandwell

(1997 )


mbes


Horizontal differences between 2500m contours from ETOPO5, Predicted,

GEBCO and multibeam. Multibeam is assigned the value of zero.


Histogram of horizontal differences between 2500m contours from ETOPO5, Predicted, GEBCO and

multibeam survey. Multibeam is considered true and the displacement of the other three

measured seawards (+) or landward (-).


Histogram of horizontal differences between 2500m contours from ETOPO5, Predicted, GEBCO and

multibeam survey. Multibeam is considered true and the displacement of the other three

measured seawards (+) or landward (-).

S44 Uncertainty


Findings for 2500 m isobath

• Over the sample area, the 2500 m isobath based on altimetric data – a) is as accurate as those from other

world-scale maps and – b) meets the IHO spec S44.


Maximization of area claimed

• Outer Limit can be delineated by straight lines not exceeding 60 nautical miles in length, connecting fixed points.

• To maximise area must select the fixed points carefully


Optimal selection of points

• Long wavelength contours may miss localised seaward protrusions

• Strategy conduct large scale, short wavelength MBES surveys along the altimetric based 2500 m isobath

• Select most seawards points on which to base 100 nm arcs


Roles for predicted bathymetry in Continental

Shelf delineation• “desk –top study” to determine the likely area to be included

within a juridical Continental Shelf• Quality control device to assess the accuracy of existing

sounding data • In developing a plan for building the case data to substantiate it.• Infer morphologic trends between existing sounding lines

• Plan the orientation of any new sounding lines.


Such use of altimetric data is consistent with the view expressed by the Commission on the Limits of

the Continental Shelf

• “4.2.6. Other sources of evidence, such as satellite altimetry-derived bathymetric data or imaging side-scan sonar information, will not be regarded as admissible for the purpose of delineating the 2,500 m isobath. This information, however, might be useful as additional qualitative information in support of other parts of a submission but will not be considered during the determination of this or any other isobaths. However, this data will be considered admissible as

supporting information in a submission.”


Roles for predicted bathymetry in Continental

Shelf delineation• “desk –top study” to determine the likely area to be included

within a juridical Continental Shelf• Quality control device to assess the accuracy of existing

sounding data • In developing a plan for building the case data to substantiate it.• Infer morphologic trends between existing sounding lines

• Plan the orientation of any new sounding lines. • That’s what I thought before yesterday’s

presentations!


Locating the “Foot of the Slope” if it exists as a physiographic feature, or,

where it does not, locating the continent/ocean boundary

• Presence/absence of sediment

• Slope/Rise junction


Determining the geological nature of submarine “ridges”

• Outer Constraint at 350 nm along “ridges”

• Article 76 “...on submarine ridges, the outer limit of the continental shelf shall not exceed 350 nautical miles... This paragraph does not apply to submarine elevations that are natural components of the continental margin...”

• CLCS Guidelines 7.2.11. “As it is difficult to define the details concerning various conditions, the Commission feels it appropriate that the issue of ridges be examined on a case-by-case basis.”


Example from the Arctic

• Maximum area would terminate at 350 nautical miles if Alpha Ridge is ruled to be oceanic ie not a natural prolongation of land


Example from the Arctic 2

• Yellow line is median• Maximum area

would terminate at median if Alpha Ridge is ruled to be continental ie a natural prolongation of land


Take Home Message

• Delineating a juridical Continental Shelf under Article 76 of UNCLOS is complicated and contains many judgmental elements

• Altimetry can contribute to: – 2500 m isobath– Foot of the Slope or evidence to the contrary – nature of ridges

altimetry and unclos altimetry and the law of the sea definition of the continental shelf dave...

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