Expendable Sonobuoys Research Articles

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.

Multiple episodes of volcanism in the Southern Austral Islands: Flexural constraints from bathymetry, seismic reflection, and gravity data

At the southeastern end of the Cook‐Austral Island chain, in the vicinity of the currently active Macdonald Seamount, multiple episodes of volcanism have left a diverse population of seamounts. Multichannel seismic reflection and shipboard gravity data provide observational constraints on the magnitude and wavelength of flexure, and multibeam bathymetry obtained by R/V Ewing and F/S Sonne within the study area provides full spatial coverage of all volcanic loads. Three‐dimensional flexural modeling of closely spaced loads emplaced at discrete times, performed both with analytical models of idealized loads and with fourier domain solutions of the observed volcanic loads partitioned into the younger and older seamounts, is compared with seismic and gravity data. This modeling provides a more complete view of the volcanic history than radiometric dating and geochemical analysis of sparse dredge samples alone. Wide‐angle seismic refraction data from ocean bottom hydrophones (OBHs) and expendable sonobuoys are also consistent with the flexural modeling results. Volume estimates of the different volcanic episodes show that one‐half to two‐thirds of the material added to the abyssal seafloor by midplate volcanic processes is due to older volcanism that erupted on young lithosphere, while the younger, higher seamounts contributed the remainder.

Calibration circuit for expendable sonobuoys

Calibration of expendable sonobuoys can be made in-situ to assure the transmission of representative data. A pair of integrated circuit timers are coupled together to provide on-off duty cycles during which a spectral distribution of calibration signals is fed to the sonobuoy. Since the spectral distribution and amplitude of the calibration signals are known, a remote monitoring station is continuously apprised of the sonobuoy's calibration.

A new undersea geological survey tool: ANGUS

A towed, near-bottom survey system termed ANGUS (Acoustically-Navigated Geological Undersea Surveyor) has been developed for geological and geophysical studies in the median rift valley of the Mid-Atlantic Ridge. System communication is solely acoustic and incorporates a relay transponder attached to either a camera fish, rock dredge, corer, or heat probe device with a shipboard transducer and a seafloor transponder array. The devices can be located with a precision of 5 to 10 m along a simultaneously collected high-resolution bathymtric profile. Automatic digital computer processing of the acoustic navigational information permits real-time monitoring of the path of the device on shipboard graphic display units. Approximately 60,000 high-quality photographs, 70 rock dredges, and 100 heat-flow observations have been collected and related to seafloor features with relief as small as 20 to 30 m. The basic camera fish allows for the addition of other geophysical instruments. Also, expendable sonobuoys have been positioned precisely to locate microearthquakes.

Display and processing of seismic wide angle reflection data from expendable sonobuoys

A method for the display of expendable sonobuoy data using a variable area recorder has been developed to improve the effectiveness and interpretation of the data. Each signal trace is digitised, processed and delayed using a mini-computer capable of shipboard operation. The final display flattens the reflection hyperbolae and enables improved trace correlation and reflector digitisation. The method has particular application to the interpretation of poor quality records.

Geophysical study of the easternmost Walvis Ridge, South Atlantic: Deep structure

Research Article| December 01, 1975 Geophysical study of the easternmost Walvis Ridge, South Atlantic: Deep structure J. GOSLIN; J. GOSLIN 1Institut de Physique du Globe de Paris, Laboratoire de Géophysique Marine, 4 Avenue de Neptune, 94100, St. Maurdes-Fossés, France Search for other works by this author on: GSW Google Scholar J. C. SIBUET J. C. SIBUET 2Centre Océanologique de Bretagne, B.P. 337, 29273, Brest, France Search for other works by this author on: GSW Google Scholar Author and Article Information J. GOSLIN 1Institut de Physique du Globe de Paris, Laboratoire de Géophysique Marine, 4 Avenue de Neptune, 94100, St. Maurdes-Fossés, France J. C. SIBUET 2Centre Océanologique de Bretagne, B.P. 337, 29273, Brest, France Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1975) 86 (12): 1713–1724. https://doi.org/10.1130/0016-7606(1975)86<1713:GSOTEW>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. GOSLIN, J. C. SIBUET; Geophysical study of the easternmost Walvis Ridge, South Atlantic: Deep structure. GSA Bulletin 1975;; 86 (12): 1713–1724. doi: https://doi.org/10.1130/0016-7606(1975)86<1713:GSOTEW>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A seismic-refraction study of the sedimentary structure of the South West African continental shelf was carried out between lat 17°S and 24°S using expendable sonobuoys. Striking differences exist both in the topography and sedimentary structure between the shelf north and south of the Walvis Ridge. South of the ridge, as far as lat 23°S, the shelf consists of a prograded series, whereas north of the ridge, at least as far as lat 17°S, east-trending canyons cut the shelf sedimentary cover. The steep northern scarp of Walvis Ridge can be traced eastward under the sediment of the continental margin. The southern flank of the ridge is buried under a thicker sedimentary cover and could only be traced eastward to long. 10°E on seismic-reflection records. This flank probably parallels the northern scarp under the continental margin. Two-dimensional structural models, built with the help of seismic-reflection and seismic-refraction results and based on the hypothesis of local isostatic equilibrium, account for the observed gravity profiles. A compensating root consists of light material (density 2.95 g/cm3) and reaches a depth of about 25 km. Gravity results also suggest that the Walvis Ridge does not constitute a superimposed load on the lithosphere; rather, the ridge and its underlying compensating mass were created at approximately the same time as the adjacent ocean basins.The creation of the two aseismic ridges of the South Atlantic — the Rio Grande Rise and Walvis Ridge — by a mantle hot spot and plume is accepted; this theory seems to explain most of the peculiar features of the Walvis Ridge. However, it is probable that the surface expression of the mantle hot spot was controlled by the presence of weak zones in the lithosphere such as transform faults. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.