Estimation of crustal discontinuities from reflected seismic waves recorded at Shillong and Mikir Hills Plateau,Northeast India

In this study, an attempt is made to determine seismic velocity structure of the crust and upper mantle beneath the Shillong-Mikir Hills Plateau in northeast India region. The principle of the technique is to relate seismic travel times with crustal thickness above the Conrad and Moho discontinuities. Broadband digital waveforms of the local earthquakes make a precise detection of the seismic phases possible that are reflected at these discontinuities. The results show that the Conrad discontinuity is at 18–20 (±0.5) km beneath the Shillong-Mikir Hills Plateau and the Moho discontinuity is at 30 ± 1.0 km beneath the Shillong Plateau and at 35 ± 1.0 km beneath the Mikir Hills.     

Detection of sub-basaltic sediments by a multi-parametric joint inversion approach

In many parts of the world sedimentary horizons with potential for hydrocarbon are located below flood basalt provinces. However, the presence of high velocity basaltic overburden makes delineation of sediments difficult due to the low velocity layer problem. Electrical and electromagnetic methods have been used in such scenarios because of the good electrical conductivity contrast between basalts and underlying sediments. However, mapping of the target sediments becomes difficult when the layer is thin as the data errors due to inherent noise lead to equivalent solutions. To tackle such difficult situations, a joint inversion scheme incorporating seismic reflection and refraction, magnetotelluric and deep electrical resistivity datasets is presented. Efficacy of the scheme is tested for a model comprising a thin sedimentary layer sandwiched between a thick basalt cover and a granitic basement. The results indicate that the parameters of the target sedimentary layer are either poorly resolved or equivalent solutions are obtained by the inversion of individual datasets. Joint inversions of seismic reflection (RFLS) and refraction (RFRS), or DC and MT dataset pairs provide improved results and the range of equivalent solutions is narrowed down. Combination of any three of the above datasets leads to further narrowing of this range and improvements in mean model estimates. Joint inversion incorporating all the datasets is found to yield good estimates of the structure. Resolution analysis is carried out to appraise estimates of various model parameters obtained by jointly inverting different combinations of datasets.     

Laboratory Investigation of Hollow and Solid Shaft Helical Soil Nail by Displacement Controlled Pullout Testing

Geotechnical and Geological Engineering - A sequence of laboratory pullout tests was conducted to examine the installation and pullout behavior of hollow and solid shaft helical soil nails. The...     

Shear Wave Velocity Estimates through Combined Use of Passive Techniques in a Tectonically Active Area

We made an attempt to assess the shear wave velocity values V _S and, to a lesser extent, the V _P values from ambient noise recordings in an array configuration. Five array sites were situated in the close proximity to borehole sites. Shear wave velocity profiles were modeled at these five array sites with the aid of two computational techniques, viz. spatial autocorrelation (SPAC) and H/V ellipticity. Out of these five array sites, velocity estimates could be reliably inferred at three locations. The shear wave velocities estimated by these methods are found to be quite consistent with each other. The computed V _S values up to 30 m depth are in the range from 275 to 375 m/s in most of the sites, which implies prevalence of a low velocity zone at some pocket areas. The results were corroborated by evidence of site geology as well as geotechnical information.     

Moment Magnitude (M W) and Local Magnitude (M L) Relationship for Earthquakes in Northeast India

An attempt has been made to examine an empirical relationship between moment magnitude (M _W) and local magnitude (M _L) for the earthquakes in the northeast Indian region. Some 364 earthquakes that were recorded during 1950–2009 are used in this study. Focal mechanism solutions of these earthquakes include 189 Harvard-CMT solutions (M _W?≥?4.0) for the period 1976–2009, 61 published solutions and 114 solutions obtained for the local earthquakes (2.0?≤?M _L?≤?5.0) recorded by a 27-station permanent broadband network during 2001–2009 in the region. The M _W–M _L relationships in seven selected zones of the region are determined by linear regression analysis. A significant variation in the M _W–M _L relationship and its zone specific dependence are reported here. It is found that M _W is equivalent to M _L with an average uncertainty of about 0.13 magnitude units. A single relationship is, however, not adequate to scale the entire northeast Indian region because of heterogeneous geologic and geotectonic environments where earthquakes occur due to collisions, subduction and complex intra-plate tectonics.     

Characterization of tropical precipitation using drop size distribution and rain rate-radar reflectivity relation

Characterization of precipitation is important for proper interpretation of rain information from remotely sensed data. Rain attenuation and radar reflectivity (Z) depend directly on the drop size distribution (DSD). The relation between radar reflectivity/rain attenuation and rain rate (R) varies widely depending upon the origin, topography, and drop evolution mechanism and needs further understanding of the precipitation characteristics. The present work utilizes 2 years of concurrent measurements of DSD using a ground-based disdrometer at five diverse climatic conditions in Indian subcontinent and explores the possibility of rain classification based on microphysical characteristics of precipitation. It is observed that both gamma and lognormal distributions are performing almost similar for Indian region with a marginally better performance by one model than other depending upon the locations. It has also been found that shape-slope relationship of gamma distribution can be a good indicator of rain type. The Z-R relation, Z = AR^b, is found to vary widely for different precipitation systems, with convective rain that has higher values of A than the stratiform rain for two locations, whereas the reverse is observed for the rest of the three locations. Further, the results indicate that the majority of rainfall (>50%) in Indian region is due to the convective rain although the occurrence time of convective rain is low (<10%).     

Generation and Validation of two Day Composite Wind Fields from Oceansat-2 Scatterometer

The Oceansat-2 scatterometer (OSCAT) of the Indian Space Research Organization (ISRO), provides surface wind speed and direction with a spatial resolution of 50 km × 50 km. With a revisit time of 2 days it had provided ocean surface wind vectors over the global oceans. In the present work, an attempt has been made to generate two day composite of OSCAT wind vectors using Data-Interpolating Variational Analysis (DIVA) and compare them with daily composite winds to check how better is the two day composites in comparison to daily composites. The daily and two days composite wind vectors of zonal (U) and meridional (V) components have been validated with wind measurements from in situ buoys and Advanced Scatterometer (ASCAT) for the year 2012 over the tropical Indian Ocean region. The statistical comparison with the in situ measurements and ASCAT has shown that the two-day OSCAT wind composites are slightly better than the daily composite winds. The improvement in the statistics can be attributed to the use of ascending and descending passes pertaining to two days which results in fewer gaps between passes, thereby reducing the interpolation errors.     

Recharge,upflux and water table response for shallow water table conditions in southwest Florida

A disproportionate increase or decrease in water table in response to minor water input or drainage is observed in shallow water table conditions inside drainage lysimeters. This increase happens because the capillary fringe of the shallow water table reaches up to or near the surface (Wieringermeer effect). The correlations between water table level changes and rainfall, seepage irrigation, drip irrigation, and drainage were analysed. Correlations with rainfall, seepage irrigation, and drainage were high (R² ranged from 0·46 to 0·97). Drip irrigation had low correlations due to the low rates of application (R² ranged from 0·26 to 0·44). Conventional methods of calculating recharge, such as multiplying the specific yield with the water table fluctuations, cannot be used for Wieringermeer effect situations. A method using water balance data and soil moisture at different depths in the lysimeters was developed to estimate recharge and upflux. The recharge results were used to develop the apparent specific yield S_ya, which could be used to calculate consequent recharge events from water table fluctuation data. Combining the water table fluctuation relationships developed with the S_ya value will allow the prediction of recharge from rainfall and irrigation events without the need for soil moisture equipment. Copyright © 2005 John Wiley & Sons, Ltd.     

Characterization of Wear Mechanisms in Distorted Conical Picks After Coal Cutting

The interest in understanding the wear mechanisms of cemented carbide (CC) is not a new development. For a long time, there have been studies on different wear mechanisms under different coal/rock cutting conditions. These studies have helped improving the quality of CC, thereby preventing such wearing of tools. Due to highly unpredictable character of coal/rock, the wearing phenomena cannot be attributed to one single domain of conditions. However, one conclusion that can be drawn in this context is that, under similar working conditions, similar types of CC undergo similar nature of wearing process. An optimum combination of high wear resistance, strength and hardness has facilitated widespread application of CC in the field of mining engineering. The abrasive parts of the mining tools are made of CC materials. The current study is focussed on a detailed characterization of the wear mechanisms of conical picks, which are used for coal mining. Conical picks consist of a steel body with an inserted cone-shaped CC tip. After being used for a certain period of time, both, the CC tip and the steel body get distorted. In this study, selection of appropriate samples was followed by critical observation of them through field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDS). In the previous study, we explained the distortion process of both, the tip as well as the body, using the SEM images. For the present study, two samples were taken from our previous investigation for further analysis. Three other samples were also included in the present study. Seven different types of wear mechanisms, such as, cracking and crushing, cavity formation, coal intermixing, chemical degradation along with abrasion, long and deep cracks, heating effect and body deformation were observed in the five tool samples.