Sankar Kumar Nath

Groundwater potential modelling in a soft rock area using a GIS

A Geographical Information System (GIS) integration tool is proposed to demarcate the groundwater potential zone in a soft rock area using seven hydrogeologic themes: lithology, geomorphology, soil, net recharge, drainage density, slope and surface water bodies. Except for net recharge and slope, the other five themes are derived from remote sensing data. IRS-1B LISS-II data was used for a 631 km2 area in Midnapur District, West Bengal, India. While slope was calculated using topographic sheets, net recharge was obtained from annual water table fluctuation data. Each feature of all the thematic maps was evaluated according to its relative importance in the prediction of groundwater potential. The evolved GIS-based model of the study area was found to be in strong agreement with available borehole and pumping test data.

Seismic Hazard Mapping and Microzonation in the Sikkim Himalaya through GIS Integration of Site Effects and Strong Ground Motion Attributes

The seismic ground motion hazard is mapped in the Sikkim Himalaya with local and regional site conditions incorporated through geographic information system. A strong motion network in Sikkim comprising of 9 digital accelerographs recorded more than 100 events during 1998–2002, of which 41 events are selected with signal-to-noise ratio ≥3 for the estimation of site response (SR), peak ground acceleration (PGA) and predominant frequency (PF) at all stations. With these and inputs from IRS-1C LISS III digital data, topo-sheets, geographical boundary of the State of Sikkim, surface geological maps, soil taxonomy map in 1:50,000 scale and seismic refraction profiles, the seismological and geological thematic maps, namely, SR, PGA, PF, lithology, soil class, %slope, drainage, and landslide layers are generated. The geological themes are united to form the basic site condition coverage of the region. The seismological themes are assigned normalized weights and feature ranks following a pair-wise comparison hierarchical approach and later integrated to evolve the seismic hazard map. When geological and seismological layers are integrated together through GIS, microzonation map is prepared. The overall site response, PGA and predominant frequency show an increasing trend in the NW–SE direction peaking at Singtam in the lesser Himalaya. As Main Boundary Thrust (MBT) is approached, the attribute value increases further. A quasi-probabilistic seismic hazard index has been proposed based on site response, peak ground acceleration and predominant frequency. Six seismic hazard zones are marked with percent probability <22%, 22–37%, 37–52%, 52–67%, 67–82%, >82% at 3 Hz and <20%, 20–34%, 34–48%, 48–61%, 61–75%, >75% at 9 Hz. In the microzonation vector layer of integrated seismological and geological themes also six major zones are mapped, with percent probability <15%, 15–31%, 31–47%, 47–63%, 63–78%, >78% at low frequency end. The maximum risk is attached to the probability greater than 78% in the Singtam and its adjoining area. These maps are generally better spatial representation of seismic hazard including site-specific analysis.

GIS integration of remote sensing and topographic data using fuzzy logic for ground water assessment in Midnapur District, India

Abstract A GIS based approach is proposed for the integration of three thematic maps viz. geomorphology, drainage density and slope using fuzzy logic for the assessment of ground water resource potential of a soft rock terrain of Midnapur District, West Bengal, India. The geomorphology and drainage density maps of the area are prepared from IRS‐1B LISS‐II data, and the slope map is obtained from the contours depicted on the topographic map of Survey of India. Each feature of all the thematic maps is assigned with individual fuzzy set values within a range between 0 to 1 according to their relative importance in the prediction of ground water occurrence. The maps are then integrated through fuzzy operation to model the ground water potential zone of the study area. The evolved model while verified with surface geophysical results is found to be in good agreement.

Estimation of S-wave site response in and around Delhi region from weak motion data

Site response in and around Delhi is studied using digital seismograms recorded by a thirteen-station VSAT-based 24-bit digital Delhi telemetry network of the India Meteorological Department. Nine local (Ml ≥ 2.3) and nine regional (Ml ≥ 3.9) earthquakes are selected for the estimation of site amplification factor using the classical standard spectral ratio for regional events (Ridge Delhi Observatory being the reference station), normalized standard spectral ratio for local events, horizontal-to-vertical spectral ratio or receiver function and the generalized inversion techniques in the frequency range of 0.5 to 7.5 Hz. Site response curves at all the thirteen stations exhibit station to station variation of the site amplification factor reflecting the changes in geologic/geotectonic/soil conditions. A comparison of the site response values obtained by the generalized inversion with those computed using receiver function technique shows a large scatter even though the pattern of the curves remain more or less similar. However, the site effects computed by generalized inversion and standard spectral ratio exhibit a good 1:1 correspondence. The peaks yielded by all the methods have been observed to occur at the same frequencies. It is evident that the softer fluvial deposits of the newer alluvium of the east Yamuna sector show steeper site amplification gradient at lower frequencies, while the greater Delhi experiences moderate site amplification. The variation of site response corroborates the abrupt changes in intensity from one location to another due to local site condition.

Determination of S-Wave Site Response in the Garhwal Himalaya from the Aftershock Sequence of the 1999 Chamoli Earthquake

Site response in the Garhwal Himalaya is studied using digital seismograms recorded by a five-station 24-bit digital microearthquake network established to monitor the aftershocks of the 28 March 1999 Chamoli earthquake ( m b 6.3). Fifteen aftershocks ( M d ≥2.0) are chosen for the site response estimation using horizontal-to-vertical spectral ratio and generalized inversion techniques. Site response curves at all the five sites show station-to-station variation of the site factor reflecting the changes in geologic/geotectonic/soil conditions. A comparison of the site response values obtained by the inversion with those computed using receiver function technique show a large scatter even though the pattern of the curves remains more or less similar. The peaks yielded by both the methods have been observed to occur at the same frequencies. The variation of site response corroborates the abrupt changes in intensity from one location to another due to local site condition. Manuscript received 22 September 2000.

Forward modelling in cross-hole seismic tomography using reciprocity

Seismic transmission tomography provides means of direct estimation of compressional wave velocities using first arrival travel times from cross-hole data. Forward modelling in seismic tomography involves the computation of travel times at the receiver positions and the estimation of ray travel paths for a subsurface velocity model and source-receiver geometry. Conventional ray-tracing techniques face many difficulties when applied to real-life situations. These can be overcome to a great extent by using a ray-tracing technique based on the principle of reciprocity. In the present work, an algorithm is developed which makes use of the reciprocity principle and Fermats principle for the estimation of raypaths. In this technique seismic travel-time calculation is based on a two-dimensional dynamic programming approach, involving the systematic mapping of travel times over a grid of constant-velocity cells. The algorithm is seen to work even for most complicated velocity models. First arrival seismic energy can travel either as transmitted waves, diffracted waves or head waves, and this technique simulates all of them. The algorithm is tested for various subsurface velocity models, some of which are presented in this paper. The results are represented as ray diagrams, superimposed over the actual velocity model. The isotime lines depicting the wavefronts are also presented, which clearly reveal the effect of velocity contrast on seismic wave propagation.

Probabilistic Seismic Hazard Model of West Bengal, India

We deliver a next-generation Probabilistic Seismic Hazard model of West Bengal based on improved seismogenic source characterization considering both the Layered Polygonal sources & Tectonic sources in the hypocentral depth range of 0–25 km & 25–70 km, seismic local-specific site condition, and adaption of appropriate region specific ground motion prediction equations in a logic tree framework. The surface consistent Probabilistic seismic hazard distribution in terms of Peak Ground Acceleration (PGA) & 5% damped Pseudo Spectral Acceleration (PSA) at different time periods for 10% probability of exceedance in 50 years have been generated and the design response spectra computed.

A unified earthquake catalogue for South Asia covering the period 1900–2014

Seismicity analysis is very much pertinent for Indian subcontinent and its adjoining region which is seismically active including many great earthquakes associated with collision and subduction tectonics in the northern, north-eastern part of the subcontinent and in the Andaman and Nicobar Island. An earthquake catalogue has been generated for South Asia covering the period 1900–2014 by compiling the records of earthquake occurrences from International Seismological Center, Global Centroid Moment Tensor (GCMT), US Geological Survey, India Meteorological Department and published literature. The uniform magnitude scaling in moment magnitude MW,GCMT is achieved through connecting relationships between different magnitude types. These relationships are derived by orthogonal standard regression analysis on available data pairs. The derived relationships have been compared with the existing equations already reported by others. The catalogue is subsequently subjected to a seismicity declustering algorithm to identify the foreshocks, main-shocks and aftershocks. The catalogue thus compiled is envisaged to be a useful resource for seismotectonic and seismic hazard studies in the region.

An improved radial basis-pseudospectral method with hybrid Gaussian-cubic kernels

Abstract While pseudospectral (PS) methods can feature very high accuracy, they tend to be severely limited in terms of geometric flexibility. Application of global radial basis functions overcomes this, however at the expense of problematic conditioning (1) in their most accurate flat basis function regime, and (2) when problem sizes are scaled up to become of practical interest. The present study considers a strategy to improve on these two issues by means of using hybrid radial basis functions that combine cubic splines with Gaussian kernels. The parameters, controlling Gaussian and cubic kernels in the hybrid RBF, are selected using global particle swarm optimization. The proposed approach has been tested with radial basis-pseudospectral method for numerical approximation of Poisson, Helmholtz, and Transport equation. It was observed that the proposed approach significantly reduces the ill-conditioning problem in the RBF-PS method, at the same time, it preserves the stability and accuracy for very small shape parameters. The eigenvalue spectra of the coefficient matrices in the improved algorithm were found to be stable even at large degrees of freedom, which mimic those obtained in pseudospectral approach. Also, numerical experiments suggest that the hybrid kernel performs significantly better than both pure Gaussian and pure cubic kernels.

SEISPACK—an “HP-C” program for seismic refraction interpretation using ray-inversion technique

Abstract SEISPACK is an interactive interpretation software package on HP 9000/835 workstation for the systematic analysis of multifold seismic refraction data for engineering and groundwater problems. The algorithm has been developed by using the basic principle of Ray Inversion for Near-Surface Estimation (RINSE) and by introducing downward continuation of the refracted rays with the lowering of the observation plane and subsequent stripping of the overlying layer. The source code for the algorithm is written in “HP-C” and the supporting graphics modules have been developed using “Starbase” graphics libraries in an HP-UX environment. The listing of the source program implementing the algorithm is included. Because it is observed generally that raw field data yield useful information only after some preprocessing to remove the error involved in first-break picking, the present package also facilitates an option for an optimization of the raw data using a combinatorial optimization algorithm before proceeding to the actual computational stage. The package has been tested rigorously with the help of both synthetic data for 2-, 3-, and 4-layer horizontal and dipping interface Earth models and actual refraction data acquired from five profiles at the field site at Keshpal, Midnapore district, West Bengal, India. The results of the latter are tested by interpreting the data using the generalized reciprocal method. All the analyses carried out, some of which are presented in this paper, establish the applicability and usefulness of SEISPACK.