Rambhatla G. Sastry

Gravitational attraction of a vertical pyramid model of flat top-and-bottom with depth-wise parabolic density variation

In 3D gravity modelling, right rectangular vertical prism model with linear and nonlinear density and polyhedral bodies with linear density variation exist in geophysical literature. Here, we propose a vertical pyramid model with depth-wise parabolic density contrast variation. Initially, we validate our analytic expression against the gravity effect of a right rectangular parallelepiped of constant density contrast. We provide two synthetic examples and a case study for illustrating the effectiveness of our pyramid model in gravity modelling. The included case study of Los Angeles basin, California demonstrates the comparative advantages of our pyramid model over a conventional right rectangular vertical prism model. Our pyramid model could be quite effective as a building block for evaluating the gravity effect of an arbitrarily-shaped 3D or 2.5-D source(s).

Geo-electrical Imaging Based In-Situ and Site-Specific Transforms for Geotechnical Characterization of a Civil Construction Site

Major civil engineering structures require a good quality geotechnical soil testing to assess the bearing capacity for planning foundation structures. While geotechnical tests provide geo-mechanical information on a very refined depth scales, their advantage is offset by their inherent point-wise information attended by drilling, which is costly. Further, in a complex geological setting, the problems get compounded due to excessive drilling budget to fulfill the geotechnical site investigation needs. So, to meet this need, civil engineers include a very high factor of safety in their 1-D model parameters. While geophysical imaging being non-invasive and cost-effective with proven spatial resolutions of different scales in subsurface exploration, yet their results can’t be directly translated to geotechnical knowledge due to absence of site-specific suitable transforms. So, a leveraged approach is needed to yield better quality subsurface information at a much lesser cost. It concerns prediction of different formation and geotechnical parameter images of subsurface on the basis of few geotechnical investigations and ample number of geo-electric image results. Here, 2-D resistivity and IP image profile data along with projected geotechnical data (Standard Penetration Test, SPT / Dynamic Cone Penetration Test, DCPT / Static Cone Penetration Test, SCPT) from nearby boreholes have been used in a case study for predicting different 2-D formation and geotechnical parameter sections along the same profile. This prediction method is based on site-specific validated regression equations describing actual correlations of geo-electrical and geotechnical data and site-independent well established empirical relations of SPT ‘N’ with different formation and geotechnical parameters. Even though the eported rresults are SPT based, the outlined methodology is quite general enough to deal with any other relevant geotechnical data sets for a comprehensive geotechnical assessment of a site.

Combined Gravity and Electrical Imaging in Landslide Investigations at Narayan Bagar, Garhwal, Himalaya, India

In N-W Himalaya, landslides lead to frequent disruption in communication network, which requires a lot of resources for the maintenance and up keep. One of such important transportation networks is the motor road between Karnaprayag and Gwaldam of Chamoli district in the central part of the state connecting Garhwal and Kumaun physiographic division of Uttarakhand Himalaya, India. Among many, the landslide affecting this road is located at Narayan Bagar (30o08΄27˝ N & 79o22΄27˝ E) on the left bank of Pinder river. The upper reaches of the slope is covered with cultivated land of terrace farming. Here, periodically during every rainy season, the soil mass moves down leading to repeated failure of controlling measures. This study area forms a part of Garhwal lesser Himalaya comprised of medium to low grade metamorphic rocks, which occur as Nappe / Klippen over metamorphosed to un-metamorphosed sedimentary sequences. The geophysical investigations (three Electrical Resistivity Tomography (ERT), Induced Polarization Imaging (IPI) and gravity profiles) at the landslide site were carried out during July, 2007.The three ERT and IPI profiles, overlain on the map of gravity inferred faults parallel to the existing regional thrusts in the vicinity of the study region, and their lateral extensions helped us to infer the sources of failure mechanism. The crushed zone in the subsurface, as a consequence of a fault zone, serves as a conduit for water to flow from the upper reaches of the slope, leading to periodic instability during the rainy season.

Statistical analysis of geo-electric imaging and geotechnical test results – a case study

For conjunctive use of geoelectric imaging and geotechnical site investigations in geotechnical characterization of major civil engineering construction sites, an objective assessment of influencing factors is important. Here, we present multiple regression analyses of both geoelectric (Electrical Resistivity Tomography, ERT; Induced Polarization Imaging, IPI) and geotechnical site investigations (Standard Penetration Test, SPT) for two profiles at a construction site for CGEWHO Complex in Greater Noida region, Delhi to assess the role of influencing formation factors like sand, fines and water content. Achieved results show that SPT ‘N’ and IPI are well predicted by a linear multiple regression. On an average, the nonlinear regression has improved predicted SPT ‘N’, resistivity and chargeability by 28.55%, 22.45% and 9.58%, respectively. The influence of sand and fines content is more than that of water content in the prediction of chargeability and SPT ‘N’. RMS error is less in prediction of IPI chargeability (average error of 1.96%) in comparison to SPT ‘N’ value (average error of 11.35%). As factors affecting chargeability (IPI) and SPT ‘N’ are similar, non-invasive IPI can be used along with few geotechnical site investigations for detailed geotechnical site investigations.

3-D GRACE gravity model for the 2011 Japan earthquake

The GRACE mission has contributed to the seismic characterization of major earthquakes in offshore regions of the world. Here, we isolate satellite gravity signal (μGal range) for the Japan Earthquake of 2011 using a difference method. Contrary to the existing gravity models, we propose a unit vertical pyramid based five-layer 3-D thrust fault model, which extends to the hypocenter and honors the ocean water layer and sea floor upheaval also. Our model partly uses existing seismological information (hypocenter depth of 32 km, rupture length of 300 km and vertical slip of 4 m), provides a snapshot of episodic subduction of the Pacific Plate below the Atlantic Plate and its gravity response closely matches the observed gravity (RMS error of 3.4012×10−13μGal), fully accounting for co-seismic mass redistribution including sea surface deformation. Our inferred rupture length, rupture velocity, average seismic moment magnitude and momentum, respectively, are 300 km, 4.49 km/s, 1.152×1021−1.8816×1021 N m and 2.319×106 GNs, which fairly agree with the literature. Further, our model inferred momentum at the sea floor corresponds to an area pulse that led to Tsunami generation.

Geophysics and Geologic Hazards

Sinkholes in Florida pose significant geotechnical, engineering, and hydrogeological challenges for using the land in constructive ways. In some instances, the sinkholes may prove unstable, thus limiting the overburden stress that can be applied. Additionally, the sinkholes may provide a conduit for accelerated contaminant transport from surface activities. In this case study, we use electrical resistivity tomography (ERT) to understand the scope of sinkhole activity under a planned landfill. As part of their application, the landfill permit applicant submitted a dense network of parallel, twodimensional electrical resistivity profiles as described in the following. We provided an alternative, three dimensional analysis of this data set to enhance detection of subsurface sinkhole targets. Eighty five parallel resistivity lines spaced 6m (20ft) apart were coalesced into a large three-dimensional resistivity model to map the 14 hectare (35 acre) site. The results revealed that resistive sand-filled sinkholes could extend at least 30m (100ft) below ground surface with a diameter that ranged from 30 to 100m (100-300ft). The host conductive limestone was shown to have a complex undulating topography with eroded pinnacles. Using cone penetrometer technology (CPT), the edge of the limestone pinnacles were also shown to have significant raveling, which coincided with a narrow range of resistivity values. The implications of the correlation between direct characterization using CPT and indirect characterization with ERT suggest that raveling could cover as much as 17% of the site. Based on these findings, the site was determined to be ill suited for landfill construction.

GEOELECTRIC IMAGING SCORES OVER MASW IN GEOTECHNICAL SITE CHARACTERIZATION

The routine point-based geotechnical site testing for assessing the bearing capacity of near-surface soil masses in major civil engineering construction sites succeeds in simple 1-D geological situations. However, the Multichannel Analysis of Surface Waves (MASW) method is routinely used in geotechnical engineering, which determines the shear wave velocity variation with depth. Earlier, our regression analysis based on the geoelectric imaging methods of electrical resistivity tomography (ERT) and induced polarization imaging (IPI) along with point geotechnical tests predicted 2-D geotechnical test results. Our prediction method is based on site-specific validated regression equations describing actual correlations of geo-electrical and geotechnical data and site-independent well established empirical relations of Standard Penetration Test, SPT ‘N’ with different formation and geotechnical parameters.

METHOD FOR ISOLATION OF GRAVITY SIGNATURES DUE TO MAJOR EARTHQUAKES FROM SATELLITE GRAVITY DATA

Often, satellite gravity (GRACE Satellite) is employed to infer near surface fluid exchanges and budgets on a global scale. However, devastating major earthquakes in offshore regions involving mass-wastage must also have a significant gravity signals in satellite gravity campaigns. But their isolation is masked by other time-varying fluid exchange signals near earth’s surface. Here, our differential data analysis (Spherical harmonics) attended by spectral filtering has successfully isolated weak gravity signals in micro-Gal range for three major earthquakes. For illustration purpose we include Sumatra earthquake (2004). Our method could infer the rupture zone in the source region. Presently, a combined seismological and gravity modeling of satellite gravity derived gravity signals are underway. These efforts if successful could be a huge step in predicting earthquakes.

Analysis of gravity and magnetic data along Mahe-Sumdo-Tso Morari

DOI = 10.3126/hjs.v5i7.1318 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.135-6

2D Stabilised analytic signal method in DC pole-pole potential data interpretation

Using analytic signal method, interpretation of pole-pole secondary electric potentials due to 2D conductive/resistive prisms is presented. The estimated parameters are the location, lateral extent or width and depth to top surface of the prism. Forward modelling is attempted by 2D-Finite Difference method. The proposed stabilised analytic signal algorithm (RES2AS) uses Tikhonov’s regularization scheme and FFT routines. The algorithm is tested on three theoretical examples and field data from the campus of Roorkee University. The stability of RES2AS is also tested on synthetic error prone secondary pole-pole potential data.