Gravity and Magnetic Methods

Lateral variations in the density and magnetization of the rocks within the crust give rise to "anomalies" in the Earth's gravity and magnetic fields. These anomalies can be measured and interpreted in terms of the geology both in a qualitative sense, by mapping out trends and changes in anomaly style, and quantitatively, by creating models of the subsurface which reproduce the observed fields. Such interpretations are generally less definitive in themselves than the results from seismic surveys (see chapter 12), but the data are widely available and can provide information in areas where other methods are ineffective or have not been applied. As the different geophysical techniques respond to specific rock properties such as density, magnetization, and acoustic velocity, the results are complementary, and a fully integrated approach to data collection and interpretation is generally more effective than the sum of its parts assessed on an individual basis. Gravity and magnetic data have been acquired, at least to a reconnaissance scale, over most of the world. In particular, the release into the public domain of satellite altimetry information (combined with improved methods of data processing) means that there is gravity coverage to a similar standard for most of the offshore region to within about 50 km of the coast. Magnetic anomalies recorded from satellites provide global coverage, but the high altitude of the observations means that only large-scale features extending over many 10s of kilometers are delineated. Reconnaissance aeromagnetic surveys with flight lines 10-20 km apart provide a lateral anomaly resolution similar to that of the satellite gravity data. Oceanographic surveys undertaken by a variety of academic and research institutions are another valuable source of data in remote regions offshore which supplement and extend the more detailed coverage obtained over the continental shelves, for example, by oil companies in areas of hydrocarbon interest. Surveys over land vary widely in terms of acquisition parameters and quality, but some form of national compilation is available from many countries. A number of possible applications of the potential field (i.e., gravity and magnetic) data follow from the terms set out by UNCLOS. Paragraph 4(b) of article 76 states, "In the absence of evidence to the contrary, the foot of the continental slope is to be determined as the point of maximum change in the gradient at its base" (italics added).

Present-Day Methods of Depth Measurement

Bathymetric data are needed to derive the morphological criteria that define the extent of the juridical continental shelf. Two features in particular, the '"foot of slope" and the 2500-m contour, must be defined. The previous chapter considered historical methods of determining bathymetry. This chapter will cover the present day methods that can be used to better meet the need for accurate bathymetry. In order to satisfy the demands of UNCLOS, bathymetric data are required in depths ranging from about 200 m to more than 5000 m. Shallower depths, while useful for demonstrating the morphology of the physical continental shelf, do not bear any relevance to the delineation of juridical continental shelf boundaries, other than where they are required to establish the baseline. Alternate methods to derive bathymetry other than using sound are available. Those involving airborne electromagnetic methods (e.g., electromagnetic induction, red-green lasers, and inversion of sea surface radar images) are not capable of determining depths much in excess of 40 m. The only other method potentially useful for deriving deeper water bathymetry is through inversion of sea surface altimetry obtained from satellites. This will be discussed at the end of this chapter. The optimal method thus remains acoustic. The traditional approach has been to use single-beam echo sounders (see previous chapter). This chapter discusses the more modern '"swath" sonar techniques, which are becoming widely used. The great majority of historic bathymetry has been collected using the single-beam sounding approach. As discussed in chapter 9, this method has a number of limitations, three of the most critical of which are i. incomplete coverage; ii. uncertainty about the exact location of the first arrival of the acoustic pulse; and iii. distortion of short-wavelength topography. In order to achieve more complete coverage, better echo location, and higher spatial resolution, methods were devised to project acoustic energy both within narrower solid angles (figure 10.1) and while deriving this information over angular sectors extending further out from the side of the survey vessel. All the methods commonly applied involved scanning the seabed orthogonal to the ship heading. Sequential scans, accumulated as the ship progresses, form a corridor (or swath) of seabed information (figure 10.2).

Introduction

As pointed out in the Foreword, the United Nations Convention on the Law of the Sea (the Convention) is, by any measure, a remarkable document, which for the first time provides a comprehensive framework of governance for a large part of the world ocean. It covers such issues as delimitation, environmental impact and management, scientific research, economic and commercial issues, and technology transfer and provides a regime for the peaceful settlement of disputes. The resolution of disputes is especially important, given that there are 151 coastal States, all with sovereign rights to the adjacent seas and shelf. Under the Convention, those rights cover a total area of about 60 million km2 or around 20% of the world ocean within the 200-nauticalmile (M) limit. But there is perhaps an additional 5% (15 million km2) which lies beyond the 200-M limit, to which sovereign rights may also extend under the terms of the Convention. Up to 54 coastal States may be able to claim extensions of their continental shelf beyond 200 M (figure 1.1). What is intended is that over the next 10 years or so, nations will document and lay claim to an area of around 75 million km2, equal to more than half the Earth's land surface. Viewed against the background of human history and land conquest extending over thousands of years, the magnitude of the undertaking is extraordinary. What is also remarkable is the key role that science and technology will play. Science and technology have always played a role in exploration and documentation of the oceans in the past. The development of an accurate chronometer by Harrison in the 18th century was critical to developing an accurate means of establishing longitude (Sobel, 1995). This in turn made it possible to accurately chart the oceans for the first time, which then enabled nations to lay claim to newly explored areas, establish trade routes, document marine hazards, and exploit ocean resources. Parts of the world's territorial sea baselines are and will continue to be based on 19th-century data. As will be evident from this book, such data are sufficiently important in some areas that we have felt it necessary to document just how those "historical data" were gathered so that we can establish their reliability.

Geological Techniques

In the previous chapters, the use of geophysical data for delineating the continental shelf has been discussed in some detail. But the determination of the case for any extension of the legal continental shelf beyond 200 nautical miles (M) from the territorial sea baseline may in some circumstances require a geological survey to confirm that a topographic or geophysical feature comprising what appears to be a natural prolongation of land territory is of continental or oceanic origin. A geological survey may also be necessary to determine the occurrence, thickness, and extent of sediments beyond the foot of the slope. Continental margins represent regions of transition from the landmass to the ocean basin and may be present-day areas of sediment erosion or deposition. Sediment supply to the continental shelf and slope, or the extent of erosion on the continental shelf and upper slope, is influenced by tectonic activity, sea-level fluctuations, climate change, variation in the wave or current regime, and various other processes. Bottom currents or gravity transport (turbidity) processes combine to varying degrees with pelagic sedimentation (the accumulation of the remains of marine organisms) to extend the supply of sediment well beyond the shelf and slope to the continental rise, ocean trench, or abyssal plain (Evans et al., 1998). In order to understand the geology of such areas, it is necessary to determine the structural setting, the tectonic and sedimentary evolution, the chrono-and lithostratigraphy, and the volcanic history. Understanding the ocean floor is a prerequisite for the determination of the extent of the continental shelf under UNCLOS. It is also highly relevant to the identification and delineation of mineral and energy resources, for determining the waste disposal potential of parts of the seafloor, and for undertaking an assessment of the risk of slope failure. None of these are directly relevant to establishing the new limits of the continental shelf, but they are highly relevant to its long-term exploitation. In order to achieve the necessary level of knowledge, the seafloor morphology and seabed character derived from bathymetric and sonar surveys (described in chapters 9 and 10) and the three-dimensional geology determined by geophysical surveys using seismic profiling, magnetometer, and gravity meter (discussed in chapters 12 and 13) need to be calibrated or "ground truthed" by sampling and coring (figure 14.1; Stoker et al., 1994).

Satellite Positioning Methods

In the previous chapter, positioning was examined from a historical perspective, recognizing that in many parts of the world, such data are not just useful, they are frequently the only data available. But in many areas, the case for extending the limits of the continental shelf will be dependent on the acquisition of new data, and for the most part, this will mean the use of satellite navigation systems. Therefore, this chapter deals in some detail with current and future satellite navigation and positioning systems. The first generation of satellite navigation systems used the principle of the Doppler shift of transmissions from satellites to provide measurements of a user's position. The Doppler shift of an emitted signal is related to the relative velocity between the source of the signal and the point at which it is received. The apparent frequency of the received signal is increased when the emitter is moving toward the receiver, and decreased when it is moving away. This phenomenon can often be observed in everyday situations, such as when a vehicle drives past a pedestrian. The pitch of the sound from the vehicle appears to drop as it passes the pedestrian, due to the transition from increased to decreased frequency of the sound. In satellite Doppler systems, measurements of the Doppler shift of signals from the satellites are combined with knowledge of the satellite's position and velocity (its ephemeris), to give an indication of the receiver's position. TRANSIT was the first operational satellite navigation system (see chapter 7). Data-processing techniques were developed which allowed a receiver to be located with respect to another at a known location, to an accuracy of the order of 1 m. TRANSIT ceased operation as a position and timing system at the end of 1996. A similar system to TRANSIT was developed by the Soviet Navy in 1965. The system, known as TSIKADA, is still operational today (2000). Since satellite Doppler systems rely on the accumulation of measurements over a period of time to provide a useful measure of a receiver's position, they could not be used as true real-time satellite navigation systems (see chapter 7).

Legal Aspects of the Continental Shelf

The 1982 United Nations Convention on the Law of the Sea (the Convention) establishes a comprehensive framework for addressing the issues implicated in the uses of ocean space. It represents both a codification of customary international law and the development of new rules of law. The Convention contains 17 Parts, 320 articles, 9 annexes, and a Final Act. The history of the development of the Convention and its subsequent implementation is outlined in chapter 2. The provisions of the Convention relating to the continental shelf are largely contained in articles 76-85, annex II (regarding the Commission on the Limits of the Continental Shelf), and annex II of the Final Act (which contains a statement of understanding concerning a specific method to be used in establishing the outer edge of the continental margin in very unusual circumstances). The provisions set forth the parameters of a regime in an area in which the coastal State exercises sovereign rights for the purposes of exploring and exploiting the natural resources of the seabed and seafloor. The first article of Part VI, article 76, defines the continental shelf in a manner which is scientifically based, legally defensible, and politically acceptable. The formula to define the outer limit of a coastal State's continental shelf is complex, but workable. At a minimum under the Convention, coastal States have sovereign rights regarding the seabed and subsoil of the continental shelf out to 200 M measured from its baseline, whether or not the physical continental shelf extends to that limit, subject to the delimitation of boundaries with neighboring adjacent or opposite States. Article 77 provides that the sovereign rights, both within and beyond 200 M regarding the continental shelf, are exclusive rights in the sense that if a coastal State does not explore the continental shelf or exploit its natural resources, no one may undertake these activities without the express consent of the coastal State. In addition, those sovereign rights "do not depend on occupation, effective or notional, or any express proclamation." The Convention allows a coastal State to claim an exclusive economic zone which shall not extend beyond 200 M from the baseline from which the territorial sea is measured.

Deep Sea Fan Issues

Deep sea fans occur along many continental margins. The Bengal Fan is the world's largest elongated submarine fan area, occupying over 3 x 106 km2 of seafloor in the Bay of Bengal. The Bay of Bengal is bordered by Sri Lanka, India, Bangladesh, Myanamar, the Andaman and Nicobar Islands, and Sumatra. The fan spans an area that is 2800-3000km in length and 830-1430 km in width. At the northern end of the Bay, the sediment cover is estimated to be more than 16 km in thickness (Curray and Moore, 1971, 1974, Moore et al., 1974). Recent drilling on the distal part of the fan just south of the equator during Ocean Drilling Program Leg 116 cored nearly 1 km of sediment without reaching hardrock basement (Cochran et al., 1990). The submarine feature of the Ninetyeast Ridge divides the fan into two major lobes, the main Bengal Fan and the eastern lobe, also known as the Nicobar Fan (Curray and Moore, 1974) (figure 19.1). The fan extends from 20°N latitude and, based on recent sedimentological and channel-system studies, to beyond 9°S latitude (Stow et al., 1990; Hübscher et al., 1997). The great size of the Bengal Fan is related to the history of the collision of the Indian tectonic plate with Eurasia and the subsequent uplift of the Himalayas. The first encounter of the northward-moving Indian Plate with the Asian mainland occurred around 50 million years (my) ago in the early Eocene Epoch (Haq, 1985). The first collision caused the initial uplift in the Himalayan region. Sedimentation in the bay is inferred to have started after this first collision, but extensive sedimentation probably did not begin until the early Miocene (ca. 17 my ago) after a major uplift in the Himalayas (Haq, 1985). Weathering and denudation of the Himalayas has furnished huge volumes of sediments that have built the Bengal Fan, supplied through the Ganges and Brahmaputra Rivers and their delta (figure 19.2). Sediments are transported largely by turbidity currents across the submerged continental terrace in the proximal part of the fan through a major delta-front canyon, also known as the Swatch-of-No-Ground. Currently, this canyon discharges its load into a single active channel that supplies sediment to the entire length of the fan.

Ridge Issues

The Third United Nations Conference on the Law of the Sea sought to establish a definition of the continental shelf that would accommodate the interests of a number, albeit a minority, of coastal States. This included consideration of various submarine elevations, including ridges, and their relationship to the regime of the continental shelf. For a variety of reasons, submarine and oceanic ridges have proved to be contentious. Indeed, this chapter proved to be the most difficult of all the chapters in this book to obtain a text to which all the authors, scientists, and lawyers could agree. Therefore, rather than produce an anodyne chapter which might have summarized only those areas of agreement, we considered it best to also cover areas where agreement was lacking. This provides the reader with both sides of the argument and the opportunity to reach their own view on the basis of the evidence presented. Some of the contentious areas are . . . i. Whether or not article 76 should be interpreted in such a manner as to preclude a country situated on a ridge from having a continental shelf beyond 200 M. ii. Whether bathymetry (reflecting geomorphology) should be given more or less weight than, or the same weight as geology in any consideration of a continental shelf beyond 200 M, including extension along an oceanic ridge, iii. Whether the fact that article 76 refers to the continental shelf being a natural prolongation of the land territory "to the outer edge of the continental margin" means that it can (or cannot) be applied to an island sitting on top of an oceanic ridge, iv. Whether or not article 76 can be interpreted in such a way as to allow a coastal State to "jump" its claim from the margin onto an adjacent ridge. v. Whether or not article 76 limits the use of ridges so that coastal States do not unreasonably extend their continental shelf regime. . . . Ultimately, for the answers to these questions, the reader will need to look to the Commission on the Limits of the Continental Shelf (the Commission), together with the outcome of diplomacy.

The Practical Realization of the Continental Shelf Limit

The first steps in establishing the case for a possible continental shelf claim beyond 200 nautical miles (M) were covered in the previous chapter. The requirement for an initial assessment of existing data and information before proceeding with the practical stage of new data acquisition and assessment to determine the actual legal limit of the continental margin is clear. This chapter deals with the three possible cases: (case A) no extended continental shelf, (case B) foot of the slope plus 60 M, and (case C) limits based on the foot of the slope and sediment thickness. Finally, the chapter deals with the delimitation of the two possible outer limit lines that are required to be implemented by coastal States, provided their continental margins extend up to or beyond those two limiting lines. Figure 17.1 illustrates how those limits could be combined to form a coastal State's continental shelf limit. . . . The continental shelf of a coastal State comprises the sea-bed and subsoil of the submarine areas that extend beyond its territorial sea throughout the natural prolongation of its land territory to the outer edge of the continental margin, or to a distance of 200 nautical miles (M) from the baselines from which the breadth of the territorial sea is measured where the outer edge of the continental margin does not extend up to that distance. . . . This provision, in principle, provides every coastal State with a continental shelf extending at least 200 M seaward. In practice, there are a number of constraints which result in the situation where a coastal State's entitlement to a continental shelf does not extend beyond 200 M. Let us therefore consider the procedure that each coastal State should follow to enable it to reach this conclusion. If a coastal State's entitlement to a continental shelf does not extend beyond 200 M measured from the territorial sea baseline, it can be assumed that the maritime zone calculated out to that distance will be subsumed within an exclusive economic zone regime, provided the coastal State claims such a zone, under part V of UNCLOS.

Initial Assessment

The previous chapters have outlined the various techniques for acquiring data on the continental shelf and adjacent areas. We now need to consider how to most effectively draw those various data sets together. This chapter describes a generic procedure for determining whether a coastal State is likely to be entitled to establish a continental shelf limit beyond 200 nautical miles (M), in order to circumscribe an area where it may exercise sovereign rights over natural resources of the seabed and subsoil. In most cases, this procedure will begin with the assembly and analysis of existing information, with the objectives of determining provisionally the outer limit of the continental shelf and of assessing the long-term economic potential of seabed resources beyond 200 M. If the analysis of available information is satisfactory in all respects and justifies such action, the coastal State may proceed directly to the preparation of a claim for submission to the UN Commission on the Limits of the Continental Shelf. If, on the other hand, the result of the investigation is inconclusive or otherwise unsatisfactory on account of inaccurate or incomplete information, the coastal State may opt to acquire new information that enhances existing data holdings, and to repeat some or all of the analyses. The above steps are illustrated in the generic flow diagram of figure 16.1, outlined in table 16.1, and discussed in some detail in the remainder of this chapter. The essence of article 76 is to define a procedure whereby a coastal State with a wide continental margin may claim jurisdiction over certain resources of the seabed beyond the 200-M limit. It follows that the location of the 200-M limit should be known with a reasonable degree of reliability. It is portrayed on the official charts of many nations. However, not all of these charts are constructed at scales or projections that readily lend themselves to the visualization and analysis of information such as sounding profiles and seabed morphology that may need to be examined in conjunction with the 200-M limit. From time to time, therefore, it may be necessary to portray the 200-M limit on a chart that is custom-built, or which covers a more restricted area.