Flexible multimethod approach for seismic site characterization

Journal of Seismology - We describe the flexible multimethod seismic site characterization technique for obtaining shear-wave velocity (VS) profiles and derivative information, such as the...     

Multiwavelength study of Cygnus A – I. Precession and slow jet speeds from radio observations

We study the jet and counterjet of the powerful classical double Fanaroff–Riley type II (FR II) radio galaxy Cygnus A as seen in the 5-, 8- and 15-GHz radio bands using the highest spatial resolution and signal-to-noise ratio archival data available. We demonstrate that the trace of the radio knots that delineate the jet and counterjet deviates from a straight line and that the inner parts can be satisfactorily fitted with the precession model of Hjellming & Johnston. The parameter values of the precession model fits are all plausible although the jet speed is rather low (≲ 0.5 c ) but, on investigation, found to be consistent with a number of other independent estimates of the jet speed in Cygnus A. We compare the masses and precession periods for sources with known precession and find that for the small number of active galactic nuclei with precessing jets the precession periods are significantly longer than those of microquasars.     

Ore Group Classification - A Case Study from Ingaldhal Sulphide Deposit, Karnataka Craton, India

All varieties of base metal sulphide deposits may be classified using base metal ratios viz. Copper Ratio (CR=100Cu/{Cu+Zn}), Zinc Ratio (ZR=100Zn/{Zn+Pb}) and Lead Ratio (PR=100Pb/{Pb+Cu}). Based on this ore group classification an attempt has been made to identify ore groups present in the late Archaean volcanic associated Ingaldhal Sulphide Deposit (ISD) of Karnataka Craton. Using base metal ratios (CR, ZR and PR) five distinct fields are suggested: Cu, Cu-Zn, Cu-Zn-Pb, Zn-Cu and Zn-Cu-Pb/Zn-Pb-Cu with their associated geological and geochemical characteristics. A transitional change in the ore groups is observed from the North Block (Cu rich groups; CR>50) to South Block (Zn and Pb dominated groups; CR<50) and the Main Block is represented by all the five ore groups.This classification may be useful as a pointer in the exploration of polymetallic base metal deposits. Copper rich groups (Cu, Cu-Zn and Cu-Zn-Pb) exhibit high concentration of Bi and Co, and low CaO and Cd whereas the Zn rich groups (Zn-Cu and Zn-Cu-Pb/Zn-Pb-Cu) show high CaO and Cd, and low Bi and Co. The CR of most of the ore groups exhibit a sympathetic relation with Co and Ag. A preliminary attempt has been made for better understanding of ore group genesis of ISD by integrating ore group characteristics with deduced geothermometry.     

Observations of bedforms on a dissipative macrotidal beach

Field measurements of wave ripples and megaripples were made with a Sand Ripple Profiler in the surf and shoaling zones of a sandy macrotidal dissipative beach at Perranporth, UK in depths 1–6 m and significant wave heights up to 2.2 m. A frequency domain partitioning approach allowed quantification of height (η), length (λ) and migration rate of ripples and megaripples. Wave ripples with heights up to 2 cm and wavelengths ～20 cm developed in low orbital velocity conditions (u _m?<?0.65 m/s) with mobility number ψ?<?25. Wave ripple heights decreased with increasing orbital velocity and were flattened when mean currents were >0.1 m/s. Wave ripples were superimposed on top of megaripples (η?=?10 cm, λ?=?1 m) and contributed up to 35 % of the total bed roughness. Large megaripples with heights up to 30 cm and lengths 1–1.8 m developed when the orbital velocity was 0.5–0.8 m/s, corresponding to mobility numbers 25–50. Megaripple heights and wavelengths increased with orbital velocity but reduced when mean current strengths were >0.15 m/s. Wave ripple and megaripple migrations were generally onshore directed in the shoaling and surf zones. Onshore ripple migration rates increased with onshore-directed (+ve) incident wave skewness. The onshore migration rate reduced as offshore-directed mean flows (undertow) increased in strength and reached zero when the offshore-directed mean flow was >0.15 m/s. The migration pattern was therefore linked to cross-shore position relative to the surf zone, controlled by competition between onshore-directed velocity skewness and offshore-directed mean flow.     

Effects Of Changing Surface Heat Flux On Atmospheric Boundary-Layer Flow Over Flat Terrain

We examine the unsteady response of a neutral atmospheric boundary layer (ABL) of depth h and friction velocity u _* when a uniform surface heat flux is applied abruptly or decreased rapidly over a time scale t<inf>θ</inf> less than about h /(10u _*). Standard Monin–Obukhov (MO) relationships are used for the perturbed eddy viscosity profile in terms of the changes to the heat flux and mean shear. Analytical solutions for changes in temperature, mean wind and shear stress profile are obtained for the surface layer, when there are small changes in h /|L_MO| over the time scale t_MO~|L _MO|/(10u_*) (where L _MO and t _MO are the length and time scales, respectively). They show that a maximum in the wind speed profile occurs at the top of the thermal boundary layer for weak surface cooling, i.e. a wind jet, whereas there is a flattening of the profile and no marked maximum for weak surface heating. The modelled profiles are approximately the same as those obtained from the U.K. Met Office Unified Model when operating as a mesoscale model at 12-km horizontal resolution. The theoretical model is modified when strong surface heating is suddenly applied, resulting in a large change in h /|L _MO| (>>1), over the time scale t _MO. The eddy structure is predicted to change significantly and the addition of convective turbulence increases the shear turbulence at the ground. A low-level wind jet can form, with convective turbulence adding to the mean momentum of the flow. This was verified by our laboratory experiment and direct numerical simulations. Additionally, it is shown that the effects of Coriolis acceleration diminish (rather than as suggested in the literature, amplify) the formation of the wind jets in the situations considered here. Hence, only when the surface heat flux changes over time scales greater than 1/f (where f is the Coriolis parameter) does the ABL adjust monotonically between its equilibrium states. These results are also applicable to the ABL passing over spatially varying surface heat fluxes.     

The luminous X-ray hotspot in 4C 74.26: synchrotron or inverse-Compton emission?

We report the discovery of an X-ray counterpart to the southern radio hotspot of the largest-known radio quasar 4C 74.26 (whose redshift is z = 0.104). Both XMM–Newton and Chandra images reveal the same significant (10 arcsec i.e. 19 kpc) offset between the X-ray hotspot and the radio hotspot imaged with MERLIN. The peak of the X-ray emission may be either due to synchrotron emission or due to inverse-Compton emission. If synchrotron emission, the hotspot represents the site of particle acceleration and the offset arises from either the jet exhibiting Scheuer's 'dentist's drill' effect or a fast spine having less momentum than the sheath surrounding it, which creates the radio hotspot. If the emission arises from the inverse-Compton process, it must be inverse-Compton scattering of the cosmic microwave background in a decelerating relativistic flow, implying that the jet is relativistic (Γ≥ 2) out to a distance of at least 800 kpc. Our analysis, including optical data from the Liverpool Telescope, rules out a background active galactic nucleus for the X-ray emission and confirms its nature as a hotspot, making it the most-X-ray-luminous hotspot detected at low redshift.     

Late Devensian and Holocene relative sealevel changes at Loch nan Eala,near Arisaig,northwest Scotland

Pollen, diatom, radiocarbon and lithostratigraphical data from isolation basins in northwest Scotland are used to quantify the reference water (tide) level, indicative range and age of differenty types of isolation and connection contacts. Tendencies of sea-level movement and relative sea-level changes from the mid-Lateglacial Interstadial (11.8 ka BP) to the late Holocene are constructed from these data. Relative sea-level fell continuously from + 17.8 m OD at 11.8 ka BP to ca. + 5.2 m OD at 10.1 ka BP. From an unkown minimum between 10 ka and 9 ka BP relative sea-level then rose to +6.3 m OD at 8.3 ka BP. The maximum Holocene sea-level occurred within the range +6.6 m OD to +9.3 m OD between 6.6 ka BP and 4.0 ka BP before falling to present. Isobase, age-altitude and quantitative rebound models for northwest Scotland are tested using these sealevel data, but none of the published models shows close agreement with the new results.