T. Seshunarayana

GIS-based Evaluation of Water Quality Index of groundwater resources around Tuticorin coastal city, south India

Groundwater is a vital source of water for industrial, domestic and agricultural activities in Tuticorin city due to lack of surface water resources; groundwater quality and its suitability for drinking usage were evaluated. A total of 72 groundwater samples were collected from open wells and boreholes during pre and post-monsoon period. Samples were analyzed for physico-chemical properties, major cation and major anions in the laboratory using the standard methods given by the American Public Health Association. The geographic information system-based spatial distribution map of different major elements has been prepared using ArcGIS 9.2. These maps are classified as desirable, maximum permissible and the values that exceed maximum permissible limit are termed as not permissible prescribed by the WHO (2004). From the Piper plot, which gives the cation chemistry, the concentration of the alkalies (Na+ and K+) exceeds the alkaline earths (Ca2+ and Mg2+) and those of strong acids exceeds weak acids. In the case of anions, during the two periods strong acid shows dominance over weak acid and HCO3− and Cl− have influences almost equal to Na+, which indicate the saltwater intrusion into the freshwater aquifer of the study area. A Canadian Council of Ministers for the Environment Water Quality Index (CCME WQI) map was also generated with the GIS technique to better understand the water portability over space. The categories of water quality evaluated by CCME WQI values of major part of the study area fall under fair category during PRM and good category during the POM period. The CCME WQI is a very useful and an efficient tool to summarize and to report on the monitoring data to the decision makers in order to understand the status of the groundwater quality. Finally, the study concludes that the groundwater quality is impaired by man-made activities, and proper management plan is necessary to protect valuable groundwater resources in Tuticorin city.

Deciphering of groundwater potential zones in Tuticorin, Tamil Nadu, using remote sensing and GIS techniques

The present study was carried out to decipher the groundwater potential zones in the coastal terrain of Tuticorin using Indian remote sensing satellite IRS-1C, LlSS-III data on a 1:50,000 scale and Survey of India (SOI) toposheets. The thematic layers such as lithology, slope, land-use, lineament, drainage, soil and rainfall were generated and integrated to prepare the groundwater prospect and recharge map of the study area. These layers were transformed to raster data using feature to raster converter tool in Arc GIS 9.2 software. Subjective weights are assigned to the respective thematic layers and overlaid in GIS platform for the identification of potential groundwater zones within the area. These potential zones were categorized as ‘high’, ‘moderate’, and ‘low’ zones with respect to the assigned weightage of different thematic layers. The resultant map shows that 10% of the area has highest recharge potentials, this is due to the percolation of precipitated water into the sub-surface rocks, followed by 65% of the area with moderate groundwater recharge potentiality, and rest of the area has low recharge potential. The study highlights that the total average annual precipitated water (877 mm) is responsible for natural recharge of the aquifers in the Tuticorin area.

Determination of shear wave velocity and depth to basement using multichannel analysis of surface wave technique

The dispersive characteristics of Rayleigh type surface waves were utilized to estimate shear wave velocity (Vs) profile followed by imaging the shallow subsurface granitic layers in the heart of Hyderabad. The reliability of Multichannel Analysis of Surface Waves (MASW) depends on the accurate determination of phase velocities for horizontally traveling fundamental mode Rayleigh waves. Multichannel recording leads to effective identification and isolation of various factors of noise. Calculating the 1-D shear wave velocity (Vs) field from surface waves ensures high degree of accuracy irrespective of cultural noise. The main advantage of mapping the bed rock surface with shear wave velocity (Vs) is the insensitivity of MASW to velocity inversion besides being free from many constraints such as contrast in physical properties etc. Modeling of surface waves data results a shear wave velocity (Vs) of 250–750 m/s covering the top soil to weathering and up to bedrock corresponding to a depth range of 10–30 m. Further, the computed N values (which is an indicator of site characteristic) based on the harmonic shear wave velocity up to a depth of 5 m is found to be quite high (> 25–30) well above 5 indicating the site to be safe and strong enough and not prone to liquefaction. A pair of selected set of results over granites are presented here as a case study highlighting the salient features of MASW.

Estimation of shearwave velocity in drifts using multichannel analysis of surface wave (MASW) technique — A case study from Jammu & Kashmir, India

Multichannel analysis of surface waves (MASW) is a non-destructive seismic prospecting method utilizing Rayleigh waves for imaging and characterizing shallow sub-surface structure. Multichannel analysis of surface waves (MASW) studies were conducted in drift areas of two bridge sites in the hilly terrain of J&K for imaging and characterizing shallow sub-surface structure. The purpose of the present study is to estimate the shear wave velocity (VS) and subsurface structure in four drifts made in a hilly terrain for construction of two bridges. Rayleigh waves are having dispersive properties, travelling along or near the ground surface and are usually characterized by relatively low velocity, low frequency, and high amplitude. The study area comprises of Tertiary group of rocks which are underlain by Siwalik group. The main rock type in the study area is dolomite which has undergone various geological processes like weathering, jointing, fracturing and shearing. MASW data was collected inside four drifts in the mountainous terrain of J&K state which are located on either sides of Chenab river. The data was analyzed by relevant processing software using dispersion and inversion technique. Shear wave velocities were estimated up to 30 m depth. Average shear wave velocity (VS30) up to top 30m was also computed. It is observed that, VS in the range 400–800 m/s upto 10–15 m corresponding to weathered rock, followed by compact dolomite rock up to the depth of about 30 m with VS in the range 1200–1600 m/s. Some low velocity zones are also identified from these sections which represent shear zones.

Ground penetrating radar for groundwater exploration in granitic terrains: A case study from Hyderabad

The Ground Penetrating Radar (GPR) is a newly developing geophysical tool for imaging the sub-surface and is potentially useful in groundwater exploration. We test its usefulness in characterizing a groundwater rich lineament near Gajularamaram in the Hyderabad granite terrain, where groundwater is limited to soil, weathering zone and lineaments. The lineament is 2 km long and 50–100 m wide, and oriented in WNW-ESE direction. It is characterized by many closely spaced sub-vertical fractures and faults, majority of which are parallel to the lineament. On either sides of the lineament, sub-horizontal sheet joints are abundant. The lineament is saturated with groundwater that discharge as springs at some places. About 450 m long, 400–100 MHz GPR data (∼5–30 m depth) were acquired along five profiles across the lineament. In the lineament, soil thickness varies from ∼0.5 m to 5 m, and is underlain by weathered granite. In the WNW part, a thick weathering zone (∼15 m) is present and a 10 m wide vertical anomaly zone (lineament) is also present. The presence of shallow reflectors at 1 m depth in the lineament is attributed to the groundwater surface. The GPR images reveal many sub horizontal to gently dipping reflectors, which are interpreted to be the sheet joints. The GPR data clearly reveal the saturated lineament, from which groundwater may migrate laterally to long distance through the sheet joints. We demonstrate the GPR as a rapid geophysical tool that can be used successfully to explore the nearsurface groundwater.

Horizontal to vertical spectral ratio in different geological formations

A study of horizontal-to-vertical (H/V) spectral ratios with the short duration data in different geological formations with varying soil thicknesses were carried out. The study has been implemented by using the ambient seismic noise and hammer impact to determine the fundamental frequency and the corresponding amplification of soils. In addition, the average shear wave velocity (Vs30) was calculated to infer the stiffness of soils. The results are correlating well with the local geology and therefore testify that the short duration data is adequate to estimate the H/V spectral ratio in site characterization and microzonation studies.

Shear Wave Velocities in the Estimation of Earthquake Hazard Over Alluvium in Seismically Active Region

It is well known that the potential hazard during an earthquake is mainly in alluvium or alluvium filled basins; shear wave velocity plays a significant role in estimating the possible hazard during an earth quake in such an area. This paper presents shear wave velocity profile from Jabalpur, Central India mainly dominated by alluvial soil that was moderately affected by an earthquake of magnitude 6.5 in May, 1997. The acquired shear wave velocity by Multichannel Analysis of Surface Wave (MASW) in as many as 36 sites over alluvial soil ranges from 200 m/sec to 400 m/sec and in a few sites marginally less than 200 m/sec corresponding to a sub-surface depth of 30–35 m. Further, the computed N values vary as low as near zero to less than 25. The study is substantiated by the estimation of P-wave velocity by refraction seismic method at the same locations of MASW which ranges from 350 m/sec to 2200 m/sec. The results suggest that the damage during an earth quake appears to be highly unlikely in view of the marginally high Vs up to depth of 30 m. This study on seismic hazard is substantiated by the estimation of frequency of the ground as well as amplification which is found to be a maximum of about 2.5 in the frequency band of 2–6 Hz in west and north western portion of the study area.

Magnetic Mapping of Banded Iron Formation of Sandur Schist Belt, Dharwar Craton, India

High ground magnetic anomalies are observed at western basin of the Sandur schist belt (SSB) near Muraripura village, Karnataka. Sources for these anomalies are inferred due to presence of exposed banded iron formations hosted by metabasalts, arenaous and argillaceous rocks associated with the Donimali Formation of the schist belt. Two varieties of BIF bands are observed in the study area viz. Band 1 and Band 2. The bands 1 and 2 are displaced by lateral strike-slip fault. Band 1 is dominantly composed of banded iron formation, rich in iron ore minerals and silica chert. Band 2 is largely composed of ferruginous quartzite and quartz arenites. Qualitative analysis of the magnetic data indicates that the study area can be divided into three lithologically important BIF/BQ formations. Spectral and quantitative analysis of the magnetic data indicates, the average depth of band 1 is 70 m and band 2 is 130 m. The magnetic data results are well correlated with chemical analysis of borehole and surface rock samples data.