Low-velocity Section Research Articles

Distribution of cyprinids in the stream during their spring upstream migration

The researches were carry out by stationary and mobile hydroacoustic computerized complexes in the floodplain-riverbed complex of the Irtysh River (Western Siberia, Russian Federation). It was established that during the migration period during the spring flood, patterns of distribution of fish in the stream are observe due to the size-taxonomic characteristics of the fish population and the physiological capabilities of the organism of fish. With an increase in the flow velocity characteristic from the near bank part of the river to its midstream, the proportion of cyprinids in the watercourse decreases, and the proportion of fish with body sizes> 15 cm increases. The number of fish smaller at high-speed sections of the river increases with an increase in water temperature, which in turn indicates the increasing physiological capabilities of representatives of Cyprinidae family. Absolute numerical dominance of migratory fish and the largest proportion of cyprinids were observed in the low velocity section of the river, located closer to the shore. The indicator of the proportion of cyprinids have a direct strong correlative relationship with the temperature factor (0.70-0.73, P<0.05).

Longitudinal bunch size measurement using an RF deflector

It is extremely important to diagnose beams in accelerators to improve accelerator operation. In the low velocity section of a proton or heavy ion linac, the diagnostic method for longitudinal beam properties is less established compared to that for transverse properties. We have developed a new diagnostic method for the longitudinal bunch size by utilizing an RF deflector. We evaluated the uncertainty in bunch size measurement through simulation, and it was obtained as 0.5°. In addition, we measured longitudinal beam emittance through bunch size measurements at several RF amplitudes of an upstream buncher. The measured emittance was 0.13±0.01π deg.MeV, which was consistent with the simulation result.

CRUSTAL VELOCITY STRUCTURE OF THE RUKWA RIFT IN THE WESTERN BRANCH OF THE EAST AFRICAN RIFT SYSTEM

Research Article| December 01, 2009 CRUSTAL VELOCITY STRUCTURE OF THE RUKWA RIFT IN THE WESTERN BRANCH OF THE EAST AFRICAN RIFT SYSTEM S. KIM; S. KIM School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea, e-mail: srk@snu.ac.kr Search for other works by this author on: GSW Google Scholar A.A. NYBLADE; A.A. NYBLADE Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, United States of America, e-mail: andy@geosc.psu.edu Search for other works by this author on: GSW Google Scholar C-E. BAAG C-E. BAAG School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea, e-mail: baagce@snu.ac.kr Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2009) 112 (3-4): 251–260. https://doi.org/10.2113/gssajg.112.3-4.251 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation S. KIM, A.A. NYBLADE, C-E. BAAG; CRUSTAL VELOCITY STRUCTURE OF THE RUKWA RIFT IN THE WESTERN BRANCH OF THE EAST AFRICAN RIFT SYSTEM. South African Journal of Geology 2009;; 112 (3-4): 251–260. doi: https://doi.org/10.2113/gssajg.112.3-4.251 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 SocietySouth African Journal of Geology Search Advanced Search Abstract We investigate the thickness and seismic velocity structure of the Rukwa Rift crust by modeling seismograms from the 1994 Mw 5.9 Rukwa earthquake and teleseismic receiver functions recorded on a broadband seismic station at the southeastern end of the rift. Two methods have been used to model receiver functions, H-κ stacking and waveform inversion, yielding Moho depth estimates of 34 ± 2 km (H-κ stacking) and 38 ± 2 km (waveform inversion), and a crustal Poisson’s ratio of 0.27 ± 0.01 (H-κ stacking). A 1D average velocity model for the rifted crust has been obtained by trial-and-error modeling of the Rukwa earthquake seismograms. The best fitting velocity model obtained has a 4.5 km thick near-surface low-velocity section over a 33 km thick middle-to-lower crustal section. The velocities and thickness of the near-surface layers are consistent with the known thickness and composition of the sedimentary basin fill. The middle-to-lower crustal section is characterised by a linear velocity gradient with shear wave velocities of 3.9 km/s or higher at depths > 20 km. Crustal Poisson’s ratio in the model is 0.27. The Moho depth in the model (37.5 km) is similar to the Moho depth obtained from analysing receiver functions, suggesting that there has been little, if any, crustal thinning beneath the rift compared to the southeastern edge of the rift. The Poisson’s ratio of 0.27, in combination with the high velocities in the middle and lower crust, indicate that a significant component of the rifted crust has a mafic composition. This finding is important for understanding the occurrence of lower crustal earthquakes in east Africa, because it suggests that at least some of the crust in east Africa could be sufficiently strong at mid- and lower crustal depths to support brittle deformation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Erosion–corrosion of mild steel in hot caustic. Part II: The effect of acid cleaning

Corrosion rates of 1020 steel in 2.75 M NaOH solution at a temperature of 160 °C and velocities of 0.32 and 2.5 m/s were studied. The focus was on the effect of the acid cleaning which was performed by using strong, inhibited sulphuric acid in between the exposures to caustic. In situ electrochemical methods were used to measure the corrosion rate such as the potentiodynamic sweep and the polarization resistance method. Also used were the weight-loss method and scanning electron microscopy (SEM). Eight electrodes/coupons were used to monitor the metal loss rate, four were placed at the low velocity section, while the other four were placed in the high velocity section of a high temperature flow. The first three coupons in each section were placed within the disturbed flow region, while the fourth was placed in a fully developed flow region. During the exposure of mild steel to the inhibited acid, following the first caustic period, the corrosion rate increased significantly to between 3 and 10 mm/y with a few electrodes experiencing as high as 50 mm/y. The second caustic period following the acidic period typically started with very high corrosion rates (20–80 mm/y). The length of this corrosion period was typically 2–3 h with a few exceptions when the high corrosion period lasted 7–10 h. Following the very high corrosion rates experienced at the beginning of the second caustic period, the corrosion rates were reduced sharply (as the corrosion potential increased) to nearly the same levels as those observed during the passive part of the first caustic period.

Erosion–corrosion of mild steel in hot caustic. Part I: NaOH solution

A novel apparatus, high-pressure/high-temperature nickel flow loop, was constructed to study the effect of the flow on the rate of erosion–corrosion of mild steel in hot caustic. It has been successfully used to measure the corrosion rate of 1020 steel in 2.75 M NaOH solution at a temperature of 160 � C and velocities of 0.32 and 2.5 m/s. In situ electrochemical methods were used to measure the corrosion rate such as the potentiodynamic sweep, the polarization resistance method, and electrochemical impedance spectroscopy (EIS). Also used were the weight-loss method and scanning electron microscopy (SEM). Eight electrodes/coupons were used to monitor the metal loss rate, four were placed at the low velocity section, while the other four were placed in the high velocity section. The first three coupons in each section were placed within the disturbed flow region, while the fourth was placed in a fully developed flow region. The corrosion rate of the coupons in the high velocity section was generally higher than that of the coupons in the low velocity section. One coupon in the disturbed flow region had a significantly higher corrosion rate than the others. � 2005 Elsevier Ltd. All rights reserved.

Sediment velocity estimation using iterative 3-D migrations of short offset seismic reflection data in deep water

In deep ocean settings where water depth greatly exceeds the source-to-receiver length, the geometry is insufficient for accurate determinations of velocity from reflection-moveout. However, velocities are crucial for estimates of physical properties and image processing. ‘Focusing analyses’ with conventional post-stack two-dimensional migration improves images, but does not produce geologically meaningful velocities except in the special case of a two-dimensional earth. For the more general case of the three-dimensional earth there is no a priori method to determine the degree of geometrical complexity. We present a technique using a short-offset three-dimensional (3-D) data set over the 5 km deep trench west of the Lesser Antilles. These data illustrate highly sensitive post-stack 3-D focusing analyses (± 20 m s−1 interval velocities), and the relationship of these seismically derived velocities to rock velocities. In our Barbados example we were able to establish the presence of a widespread 80-160 m thick low-velocity zone at and above the main low-angle fault. This observation suggests the water-rich decollement ‘leaks’ water into the overlying sections. Also evident is a low-velocity section associated with turbidite sands. These results are confirmed with sparse logging data and well samples. Deep-water short offset 3-D experiments provide a potentially effective approach for velocity estimation, replacing the operational complexity of long-offsets with simpler short-offset techniques. In areas of structural complications and abundant diffracted energy, it is a surprisingly accurate method, utilizing the high fidelity 3-D wavefield and the information carried in ‘zero-offset’ diffraction ellipsoids. The velocity used to properly collapse a diffraction ellipsoid is explicitly the velocity of propagation in the media since the travel path is known exactly. Thus, the derived velocities should closely represent rock velocities, unlike the 2-D case where the propagation geometry is not known.

Tectonic controls on magmatism in The Geysers–Clear Lake region: Evidence from new geophysical models

In order to study magmatism and geothermal systems in The Geysers–Clear Lake region, we developed a detailed three-dimensional tomographic velocity model based on local earthquakes. This high-resolution model resolves the velocity structure of the crust in the region to depths of approximately 12 km. The most significant velocity contrasts in The Geysers–Clear Lake region occur in the steam production area, where high velocities are associated with a Quaternary granitic pluton, and in the Mount Hannah region, where low velocities occur in a 5-km-thick section of Mesozoic argillites. In addition, a more regional tomographic model was developed using traveltimes from earthquakes covering most of northern California. This regional model sampled the whole crust, but at a lower resolution than the local model. The regional model outlines low velocities at depths of 8–12 km in The Geysers–Clear Lake area, which extend eastward to the Coast Range thrust. These low velocities are inferred to be related to unmetamorphosed Mesozoic sedimentary rocks. In addition, the regional velocity model indicates high velocities in the lower crust beneath the Clear Lake volcanic field, which we interpret to be associated with mafic underplating. No large silicic magma chamber is noted in either the local or regional tomographic models. A three-dimensional gravity model also has been developed in the area of the tomographic imaging. Our gravity model demonstrates that all density contrasts can be accounted for in the upper 5–7 km of the crust. Two-dimensional magnetotelluric models of data from a regional, east-west profile indicate high resistivities associated with the granitic pluton in The Geysers production area and low resistivities in the low-velocity section of Mesozoic argillites near Mount Hannah. No indication of midcrustal magma bodies is present in the magnetotelluric data. On the basis of heat flow and geologic evidence, Holocene intrusive activity is thought to have occurred near the Northwest Geysers, Mount Hannah, Sulphur Bank Mine, and perhaps other areas. The geophysical data provide no conclusive evidence for such activity, but the detailed velocity model is suggestive of intrusive activity near Mount Hannah similar to that in the “felsite” of The Geysers production area. The geophysical models, seismicity patterns, distribution of volcanic vents, heat flow, and other data indicate that small, young intrusive bodies that were injected along a northeast trend from The Geysers to Clear Lake probably control the thermal regime.

Marine geophysical survey in the southwestern Barents Sea

A marine geophysical survey in the southwestern Barents Sea reveals an over-all sequence of layered sediments. In the main part of this area relatively high seismic velocities (2.5–3.0 km/sec) occur near the sea floor, but a low-velocity (1.85–2.2 km/sec) sedimentary wedge exists near the shelf edge. The low-velocity section is believed to consist of material deposited during the Cenozoic when the Barents shelf was uplifted. However, the main portion of the sediments seem to be consolidated, of Mesozoic and Paleozoic age. Magnetic and gravity data suggest an irregular basement relief in the west and a smoother and probably deeper basement toward the east. The latter is associated with the Riphean structural complex, whereas the western area may reflect a continuation of the Caledonides.