Electrical conductivity of ultramafic rocks to 1820 kelvin

Electrical conductivity, σ, of six ultramafic rocks (garnet-bearing peridotites and an eclogite) has been investigated in the temperature range 670–1820 K under known environment. Between 670 and ～1320 K σ increases with 1/T monotonically, first slowly, by 1–1.5 orders of magnitude. Above 1320 K σ increases sharply; an increase of 3–4 orders of magnitude is observed between 1320 and 1670 K. In the same temperature range the σ values for all the six rocks fall within 1.5 orders of magnitude, the lowest conductivity being for a spinel lherzolite. The range of σ values is narrowed at higher temperatures. The differences in σ values above 1470 K may be explained on the basis of varying degrees of partial melting in the rocks. Over the entire range of temperature, the σ values for the ultramafic rocks are lower than those reported for basalts but higher (by 1–2 orders of magnitude) than those for single-crystal olivines.     

Mesh area vis-à-vis blast performance in a limestonequarry of the Philippines

The present research paper is a case study in a limestone quarry in the Philippines where major problems in terms of improper fragmentation (large boulder count and poor pay-loader performance), poor wall control, and poor heave characteristics of the muck pile were witnessed. The paper highlights the role of modifying the firing pattern and, also, the application of stemming plug without incorporating any change in the stemming column length due to which incremental expansion in the mesh area could be implemented to determine the optimum mesh area. The optimum mesh area, due to the said changes in the blast design, was considerably larger in comparison to the pre-existing mesh area. The study has also highlighted the importance of indirect methods for characterizing and precisely adjudging the blast performance.     

Identification of groundwater prospective zones by using remote sensing and geoelectrical methods in Jharia and Raniganj coalfields, Dhanbad district, Jharkhand state

The Dhanbad district in Jharkhand faces acute water scarcity and is chronically drought-prone. The groundwater resources in the area have not been fully exploited. The present study was undertaken to evaluate the groundwater prospective zones. Landsat-5 Multi Spectral Scanner (MSS) data of band 2 and band 4 and false colour composite (FCC) of band 2, 3, 4 were interpreted visually to differentiate different hydromorphogeological units and to delineate the major trends of lineaments. The different geomorphic features identified are linear ridges, residual hills, and pediplain, buried pediment and dissected pediplain, besides lineaments. The study shows that the pediplain and buried pediments are promising zones for groundwater prospecting. The occurrence and movement of groundwater is restricted to the unconsolidated material, weathered and fractured rocks. For the selection of tube well sites, geoelectrical resistivity investigations have been carried out at the sites, which were found suitable based on hydrogeomorphological and hydrogeological studies. Twenty-six Vertical Electrical Soundings (VES) have been carried out by using Schlumberger electrode configuration, which have brought out 3 to 7 layered sub-surface layers. The resistivity of water-bearing weathered/fractured rocks varies from 120–150 ohm m. The integrated studies have revealed that the blue colour zones are most promising for groundwater exploration and dug wells may be dug up to depths of 30±5m.     

Web-based service of a visualization package “Amira” for the geosciences

Amira is a powerful three-dimensional visualization package that has been employed recently by the science and engineering communities to gain insight into their data. We discuss a new paradigm for the use of Amira in the Earth sciences that relies on the client-server paradigm. We have developed a module called WEB-IS2, which provides web-based access to Amira. This tool allows Earth scientists to manipulate Amira controls remotely and to analyze, render and view large datasets through the Internet without regard for time or location. This could have important ramifications for GRID computing.     

The attenuation mechanism of seismic waves in northwestern Himalayas

We analysed local earthquake waveforms recorded on a broad-band seismic network in northwestern Himalayas to compute the intrinsic and scattered attenuation parameters from coda waves. Similar to other tectonically active and heterogeneous regions, attenuation-frequency relation for western Himalaya is   Q ⁻¹ _c = (113 ± 7)  f ^(1.01±0.05)  where   Q_c   is the coda Q parameter. Intrinsic  ( Q ⁻¹ _i )  and scattering  ( Q ⁻¹ _s )  attenuations was separated using   Q_c   and direct S -wave Q data  ( Q_d )  . It is observed that estimated   Q ⁻¹ _c   is close to   Q ⁻¹ _i   and both of them are much larger than   Q ⁻¹ _s   suggesting that coda decay is predominantly caused by intrinsic attenuation. At higher frequencies, both the attenuation parameters   Q_c   and,   Q_d   are similar indicating that coda is predominantly composed of back-scattered S waves at these frequencies.     

New Perspectives on Mantle Dynamics from High-resolution Seismic Tomographic Model P1200

—Recently a high-resolution tomographic model, the P1200, based on P-wave travel times was developed, which allowed for detailed imaging of the top 1200 km of the mantle. This model was used in diverse ways to study mantle viscosity structure and geodynamical processes. In the spatial domain there are lateral variations in the transition zone, suggesting interaction between the lower-mantle plumes and the region from 600 km to 1000 km. Some examples shown here include the continental region underneath Manchuria, Ukraine and South Africa, where horizontal structures lie above or below the 660 km discontinuity. The blockage of upwelling is observed under central Africa and the interaction between the upwelling and the transition zone under the slow Icelandic region appears to be complex. An expansion of the aspherical seismic velocities has been taken out to spherical harmonics of degree 60. For degrees exceeding around 10, the spectra at various depths decay with a power-law like dependence on the degree, with the logarithmic slopes in the asymptotic portion of the spectra containing values between 2 and 2.6. These spectral results may suggest the time-dependent nature of mantle convection. Details of the viscosity structure in the top 1200 km of the mantle have been inferred both from global and regional geoid data and from the high-resolution tomographic model. We have considered only the intermediate degrees (l = 12–25) in the nonlinear inversion with a genetic algorithm approach. Several families of acceptable viscosity profiles are found for both oceanic and global data. The families of solutions for the two data sets have different characteristics. Most of the solutions asociated with the global geoid data show the presence of asthenosphere below the lithosphere. In other families a low viscosity zone between 400 and 600 km depth is found to lie atop a viscosity jump. Other families evidence a viscosity decrease across the 660 km discontinuity. Solutions from oceanic geoid show basically two low viscosity zones one lying right below the lithosphere; the other right under 660-km depth. All of these results bespeak clearly the plausible existence of strong vertical viscosity stratification in the top 1000 km of the mantle. The presence of the second asthenosphere may have important dynamical ramifications on issues pertaining to layered mantle convection. Numerical modelling of mantle convection with two phase transitions and a realistic temperature- and pressure-dependent viscosity demonstrates that a low viscosity region under the endothermic phase transition can indeed be generated self-consistently in time-dependent situations involving a partially layered configuration in an axisymmetric spherical-shell model.     

Morphology of the subducted Indian plate in the Indo-Burmese convergence zone

Earthquake hypocenters and travel time residuals have been analysed to constrain the geometry and physical state of the subducted Indian plate in the Indo-Burmese convergence zone. A critical analysis of earthquake hypocenters reveals the existence of a non-uniform Benioff zone, progressively shortening from north to south. The deepest level of seismicity is observed beneath the Naga hills (160 km) followed by that under the Chin hills (120 km) and Arakan-Yoma ranges (80 km). The region seems to be devoid of moderate sized shallow (< 40 km) earthquakes. Differential travel time residuals from pairs of shallow and intermediate depth earthquakes recorded at teleseismic distances show significantly faster travel time (up to l.2s) in the north-northeast and south-southwest azimuths, whilst slower arrivals (1.2 to 1.5 s) are recorded in the transverse direction. This observation points to the presence of a high velocity slab possibly linked to the subduction of the Indian oceanic lithosphere.     

A comparison of rock mass disturbance in TBM and drill and blast drivages at the Donkin Mine,Nova Scotia

Summary The initial phase of the Donkin-Morien project involves the driving of two parallel tunnels through an interbedded sequence of coal measures strata to intersect the Harbour Seam which lies approximately 3.5 km offshore. The No. 2 tunnel was driven a total distance of 3579 m using a 7.6 m diameter full face shielded Lovat TBM and supported by steel ring beams. The No. 3 tunnel was initially driven a distance of 1027 m using conventional drill and blast methods (7.6 m wide by 5.3 m high with a uniradial arch and supported by rock bolts and shotcrete). The TBM was subsequently used to complete the drivage of the No. 3 tunnel to the Harbour Seam.Both the No. 2 and No. 3 tunnels followed a thick sandstone unit at a grade of –20% for the first 900 m. This provided an opportunity to compare the rock mass disturbance resulting from two different excavation methods. A programme of field and laboratory measurements was therefore undertaken, which included: the use of a borehole dilatometer to determinein situ rock modulus,in situ gas permeability testing, seismic reflection surveys on the tunnel walls, and the laboratory testing of core samples.The paper examines the four different techniques used and compares the results obtained.     

The theory of the recoverable linear strain sensor and its application in geomechanics

Summary After introducing the new linear strain sensors and the theory behind their modus operandi, the paper discusses in general the performance characteristics as well as the potential value of their application in geomechanics. Application features and effects of application environments are then outlined and examples of practical applications are given.     

High velocity anomaly beneath the Deccan volcanic province: Evidence from seismic tomography

Analysis of teleseismicP-wave residuals observed at 15 seismograph stations operated in the Deccan volcanic province (DVP) in west central India points to the existence of a large, deep anomalous region in the upper mantle where the velocity is a few per cent higher than in the surrounding region. The seismic stations were operated in three deployments together with a reference station on precambrian granite at Hyderabad and another common station at Poona. The first group of stations lay along a west-northwesterly profile from Hyderabad through Poona to Bhatsa. The second group roughly formed an L-shaped profile from Poona to Hyderabad through Dharwar and Hospet. The third group of stations lay along a northwesterly profile from Hyderabad to Dhule through Aurangabad and Latur. Relative residuals computed with respect to Hyderabad at all the stations showed two basic features: a large almost linear variation from approximately +1s for teleseisms from the north to—1s for those from the southeast at the western stations, and persistance of the pattern with diminishing magnitudes towards the east. Preliminary ray-plotting and three-dimensional inversion of theP-wave residual data delineate the presence of a 600 km long approximately N−S trending anomalous region of high velocity (1–4% contrast) from a depth of about 100 km in the upper mantle encompassing almost the whole width of the DVP. Inversion ofP-wave relative residuals reveal the existence of two prominent features beneath the DVP. The first is a thick high velocity zone (1–4% faster) extending from a depth of about 100 km directly beneath most of the DVP. The second feature is a prominent low velocity region which coincides with the westernmost part of the DVP. A possible explanation for the observed coherent high velocity anomaly is that it forms the root of the lithosphere which coherently translates with the continents during plate motions, an architecture characteristic of precambrian shields. The low velocity zone appears to be related to the rift systems (anomaly 28, 65 Ma) which provided the channel for the outpouring of Deccan basalts at the close of the Cretaceous period.