A time-varying parameter model of a body oscillating in pitch

Among the assumptions upon which linear time-invariant models of floating bodies are based is that the body motions are so small that any change in the body’s angular position can be disregarded. However, it is often a major design requirement of a wave energy conversion device that the response amplitude is large, thereby invalidating one of the assumptions of the linear model. In particular, the immersed geometry of a body undergoes considerable variation when it is moved in pitch. With regard to this we investigate the difference in performance between a quasi-linear model in which the change of immersed surface is modelled by time-varying parameters and a basic linear model in which the immersed surface is time-invariant. The time-varying parameter model is realized by interpolation between the appropriate parameter values of a set of linear time-invariant (LTI) models derived for the different immersed surfaces that occur at discrete body displacements. It is shown that the responses predicted using the time-varying parameter model are closer to those measured experimentally than those of a standard frequency-domain model. Particular improvement occurs when the responses are large, such as at or near the resonance frequency. A problem which may limit the general use of the model is also discussed.     

Population structure of Haliotis rubra from South Australia inferred from nuclear and mtDNA analyses

1Introduction ThemajorityofAustralia’sabalonefisheryex ports(5.135kt,worth＄216millionin2002～2003,ABARE2004)consistofblacklipabalone(HaliotisrubraLeach,1814).AssuchH.rubrais consideredasanimportantmarineresourcewithin Australia.Likemanyabalonespecieswor…     

Geology of the Continental Margin of Enderby and Mac. Robertson Lands, East Antarctica: Insights from a Regional Data Set

In 2001 and 2002, Australia acquired an integrated geophysical data set over the deep-water continental margin of East Antarctica from west of Enderby Land to offshore from Prydz Bay. The data include approximately 7700 km of high-quality, deep-seismic data with coincident gravity, magnetic and bathymetry data, and 37 non-reversed refraction stations using expendable sonobuoys. Integration of these data with similar quality data recorded by Japan in 1999 allows a new regional interpretation of this sector of the Antarctic margin. This part of the Antarctic continental margin formed during the breakup of the eastern margin of India and East Antarctica, which culminated with the onset of seafloor spreading in the Valanginian. The geology of the Antarctic margin and the adjacent oceanic crust can be divided into distinct east and west sectors by an interpreted crustal boundary at approximately 58° E. Across this boundary, the continent–ocean boundary (COB), defined as the inboard edge of unequivocal oceanic crust, steps outboard from west to east by about 100 km. Structure in the sector west of 58° E is largely controlled by the mixed rift-transform setting. The edge of the onshore Archaean–Proterozoic Napier Complex is downfaulted oceanwards near the shelf edge by at least 6 km and these rocks are interpreted to underlie a rift basin beneath the continental slope. The thickness of rift and pre-rift rocks cannot be accurately determined with the available data, but they appear to be relatively thin. The margin is overlain by a blanket of post-rift sedimentary rocks that are up to 6 km thick beneath the lower continental slope. The COB in this sector is interpreted from the seismic reflection data and potential field modelling to coincide with the base of a basement depression at 8.0–8.5 s two-way time, approximately 170 km oceanwards of the shelf-edge bounding fault system. Oceanic crust in this sector is highly variable in character, from rugged with a relief of more than 1 km over distances of 10–20 km, to rugose with low-amplitude relief set on a long-wavelength undulating basement. The crustal velocity profile appears unusual, with velocities of 7.6–7.95 km s⁻¹ being recorded at several stations at a depth that gives a thickness of crust of only 4 km. If these velocities are from mantle, then the thin crust may be due to the presence of fracture zones. Alternatively, the velocities may be coming from a lower crust that has been heavily altered by the intrusion of mantle rocks. The sector east of 58° E has formed in a normal rifted margin setting, with complexities in the east from the underlying structure of the N–S trending Palaeozoic Lambert Graben. The Napier Complex is downfaulted to depths of 8–10 km beneath the upper continental slope, and the margin rift basin is more than 300 km wide. As in the western sector, the rift-stage rocks are probably relatively thin. This part of the margin is blanketed by post-rift sediments that are up to about 8 km thick. The interpreted COB in the eastern sector is the most prominent boundary in deep water, and typically coincides with a prominent oceanwards step-up in the basement level of up to 1 km. As in the west, the interpretation of this boundary is supported by potential field modelling. The oceanic crust adjacent to the COB in this sector has a highly distinctive character, commonly with (1) a smooth upper surface underlain by short, seaward-dipping flows; (2) a transparent upper crustal layer; (3) a lower crust dominated by dipping high-amplitude reflections that probably reflect intruded or altered shears; (4) a strong reflection Moho, confirmed by seismic refraction modelling; and (5) prominent landward-dipping upper mantle reflections on several adjacent lines. A similar style of oceanic crust is also found in contemporaneous ocean basins that developed between Greater India and Australia–Antarctica west of Bruce Rise on the Antarctic margin, and along the Cuvier margin of northwest Australia.     

Iceland and mid-oceanic ridges

Magnetic anomalies over Iceland, measured by Serson et al. (1968), are similar in shape and amplitude to those found over mid-oceanic ridges in general and over Reykjanes Ridge in particular. However, the geology of Iceland does not favour the simple model of sea floor spreading as formulated by Vine and Matthews. The Brunhes period volcanism can neither in place nor in time be related to an opening process of the Central Graben, which actually is a downthrown block and not an opening rift. Furthermore, the structure of Iceland is not symmetric with respect to the Central Graben. The geology of the Central Graben of Iceland does support a model proposed by Thorleifur Einarsson in 1967. In this model elongate ridges of pillow lavas are thought to have piled up on top of parallel volcanic fissures. The actual spreading is negligible. The fissures have been opening at random over a width of about 120 km, and no definite time scale can be set up for the associated magnetic anomalies. This conflict between Icelandic geology and the current views on sea floor spreading, can be evaded by supposing that the mere circumstance that Iceland is an island obscures a spreading process underneath. One might also postulate that Iceland nevertheless should stand as an example of a mid-oceanic ridge which implies that our ideas on sea floor spreading should be thoroughly revised.     

Nature and composition of shore-zone sediments between Jeddah and Yanbu, eastern Red Sea

The shore-zone sediments between Jeddah and Yanbu, west coast of Saudi Arabia, are composed mostly of skeletal carbonate sands. The nearshore sediments containing benthic foraminifera, algal fragments and molluscs are multimodal, the mean grain size varying between 0.76 and 2.35 ø. The beach sediments, except samples dominated by cerithid gastropods in some localities, are relatively finer than the nearshore sediments. Although the beaches to a great extent comprise sand-sized material, fine lime muds and coarse clastic gravels occur in certain areas. The dune sediments comprising mostly algal grains and ooids are very fine with 50 percent of the material in the 0.25–0.18 mm size grade. Except a general northward decrease in mean grain size, regional trends in the textural parameters of the sediments between Jeddah and Yanbu are not quite apparent. Lateral variations in the textural characteristics suggest a landward migration of the sediments in the shore zone under the influence of northerly and northwesterly winds. The carbon and oxygen (δ ¹³C + 4.80 to 4.84‰ PDB) (δ ¹³O − 0.04 to + 0.53‰ PDB) isotopic ratios of the lime muds occurring in certain shallow margins in the shore-zone, which are much higher than those of the green algae, indicate that the fine carbonates are at least in part inorganic in origin.

Like the shallow-water carbonates in tropical seas, aragonite and high Mg-calcite are the dominant carbonate minerals in the shore-zone sediments. There is a landward increase in aragonite contents caused by the landward migration of fine material from the nearshore. The dominant clay mineral in the nearshore sediments is kaolinite with subordinate swelling chlorite and little illite. Kaolinite is contributed by the coastal regions under the sub-tropical humid climate. Swelling chlorite is considered to have been formed in the nearshore by mechanical mixture of chlorite and montmorillonite derived from the metamorphic and igneous terrains of the Tertiary mountains bordering the coastal plain.