October, 2007

October, 2007   ||  Volume 11 No.4

Demarcation of liquefaction zones by Bouguer Gravity and Electrical Resistivity method

A.B.Narayanpethkar, A.Vasanthi1 and K.Mallick1

SolapurUniversity, Solapur, Maharastra
1National Geophysical Research Institute, Hyderabad - 500 007
E-mail : mallik_ku@yahoo.com

Of all the disasters the mankind faces, perhaps those due to the earthquakes are most severe. The danger to life and other damages are enormous due to soil liquefaction triggered by earthquakes. Prediction and demarcation of liquefaction zones in high seismicity regions will be a great help to mitigate hazards. In the present communication, it has been demonstrated for 1934 Bihar-Nepal and 1993 Killari earthquakes that Bouguer and electrical resistivity methods (only for Killari) can be of help to delineate the liquefaction zones. With this backdrop, we believe that in conjunction with soil and sediment characteristics indicating high susceptibility to liquefaction, gravity and resistivity anomalies will provide vital information to predict and identify the liquefaction zones.


Chirp Sonar Survey for development of Jawaharlal Nehru Port
R.S.Wadhwa, C.K.Rani, M.S.Chaudhari, Raja Mukhopadhyay and N.Ghosh
Central Water and Power Research Station, Khadakwasla, Pune - 411 024
E.mail : wadhwa_rs@cwprs.gov.in

Underwater geoengineering survey deploying dual frequency echo-sounder and chirp sonar system (pulsed frequency modulation signal 500 Hz to 8 KHz) from a precisely positioned vessel was carried out along seven traverses at Jawaharlal Nehru Port near Mumbai where the existing harbour is to be dredged to -16.5m depth near the berths and to -12.5m depth elsewhere to accommodate larger size vessels. The lengths of the traverses varied between 175m and 415m. Isopach map of the sediment thickness drawn from the results of the chirp sonar system revealed that the sediment thickness from sea-floor varies from 0m to 11m. Rock at places was inferred to be exposed to sea-floor. The boundary of the rock outcropping patch was precisely deciphered and marked on the site plan. The quantity of rock to be dredged was worked out to be 1.95 Lakh cubic metres. The basalt rock for dredging will either require rock cutter dredgers or blasting. The rock levels evaluated using chirp sonar system matched remarkably well with rock levels inferred in few boreholes drilled in 1980. At some locations the superficial material with physical properties different from that of sea-floor was detected and was interpreted to be dumped material. The maximum lateral extent and thickness of the dumped material was 100m and 5m respectively.

A new approach to seismic diffraction tomography technique and its significance in ore-body prospecting

Mahasweta Mahapatra and Samiran Mahapatra
Department of Geology, Durgapur Government College, Durgapur - 713 214
E-mail: mahasweta_mahapatra@rediffmail.com

Diffraction, once considered noise rather than signal in seismic tomography, has recently been successfully used in ore-body prospecting, particularly in delineating smaller objects which otherwise cannot be properly investigated by ray tomography methods.
Most prevalent seismic diffraction tomography techniques use Born's approximation of weak scattering instead of considering total wave field. In the present study, using total scattered wave field, at first, the forward modeling for objects of different geometrical shapes and sizes, both separately and in combination is done and then very fast simulated annealing is employed for inverse modeling. Subsequently, annealing parameters are used to determine the model parameters such as size, contrast and distance of the object from measuring plane. This gives fairly good estimate for model parameters within acceptable error limit.
This procedure of seismic diffraction tomography technique can potentially be used for investigating ore-body of smaller dimension having not only weak but also strong scattering properties.

Ultra Low Frequency (ULF) amplitude anomalies associated with the recent Pakistan earthquake of 8 October, 2005
Vinod Kushwah, Vikram Singh and Birbal Singh1
Department of Physics, R.B.S.College, Bichpuri, Agra – 283 105
1Department of Electronics and communication Engineering,
Faculty of Engineering & Technology, R.B.S.College, Bichpuri, Agra – 283 105
E-mail: birbal@sancharnet.in, bbsagra@yahoo.co.in

Employing a system of 3- component search coil magnetometer (f=0.01 -30 Hz), ULF magnetic field emissions associated with earthquakes have been monitored at Agra since September 2002. The bulk of the data shows that normally the amplitudes of the three components (Bx, By, Bz oriented toward north-south, east-west, and vertical directions respectively) are low between 0.01 and 0.3 nT. However, By and Bz components are found to be enhanced considerably in the range of 0.3 – 2 nT ( Bx being the smallest ) occasionally. Such enhancements are correlated mostly to occurrence of earthquakes where enhancements in By and Bz correspond to earthquakes mostly in North-East (or North- South) and Northwest direction from Agra respectively. Recently, one such correlation has been found between the enhancements in Bz components and large magnitude earthquake (M=7.7) that occurred in Muzaffarabad (Pakistan) about 900 km northwest from Agra on 8 October, 2005. We have examined the amplitude enhancements in the light of solar flares and magnetic storms during the whole month of October, 2005 and found a negative correlation with these events. An interesting result obtained was that the enhancements occurred as precursors, first about 10 days before between 27 and 30 September, and then 3 days before on 05 October, 2005. The precursory characteristics of the signal is confirmed by a statistical analysis of the data for a period of fifteen days before and fifteen days after from 17 Sept., 2005- 29 Oct.,2005 by employing mean (m) and mean around standard deviation (m±2s) approach. The enhancements in the intensity of the precursory signals are also observed from a wavelet analysis of the data. The frequency –time spectrogram and power spectrum analysis of the data show the enhancements around 2 Hz and 7-8 Hz and polarization analysis of the data shows that the signals propagated from below.



Probable storm motion in the Bay of Bengal in April and May
A.Muthuchami and P.Dhanavanthan1

Regional Meteorological Centre, Chennai
1Dept of Statistics, Pondicherry University, Pudhucherry

Using data of cyclonic storm tracks for the period 1891 to 2000 over Bay of Bengal an attempt is made to find out the probable storm track and probable latitude of crossing of storms in pre-monsoon months of April and May. It has been found that in pre monsoon season the predominant direction of motion of storm is north-northeast in April and north in the case of May. In the month of April initial formation of storm mostly determines the location of crossing whereas in the case of May such prediction is not reliable. The relation between longitudes and latitudes of storm positions is expressed in terms different mathematical expressions during April and May. In the month of April whether the storm is in the easterly or in the westerly regime, the storm track is mostly parabolic. But in the case of May when the storm is wholly under the control of westerly regime, the tracks are in the form of logarithmic curves whereas when they are in easterly regime and then caught under westerly regime they are parabolic.




An estimation of heat flux and its variability in the Southern Indian Ocean (SIO) using Ocean General Circulation Model

Anshu Prakash Mishra, S.Rai1 and A.C.Pandey2

Central Water Commission, Dibrugarh, Assam – 786 003
1K.Banerjee Center of Atmospheric and Ocean Studies, Nehru Science Center, Faculty of Science, University of Allahabad, Allahabad, India.
2Department of Physics, University of Allahabad, Allahabad, India.
E-mail: anshu_786@rediffmail.com, anshu_ms@yahoo.com

Evaluated annual mean and seasonal means net heat flux at the ocean surface for the period 1975-1998 from an ocean model results for Southern Indian Ocean (SIO) having spatial coverage 0.25°N-74.25°S & 15.75°E-120.75°E. It has been found that model annual mean and seasonal means net heat flux fields agree moderately with Southampton Oceanography Center (SOC) climatological heat fluxes and there are spatial inconsistencies in computation against a good agreement in-between model and SOC data sets in the region of interest. These discrepancies may be due to deficiency with model physics or sampling related bias in SOC data sets. Further, nature of variability of surface heat flux field in SIO was studied using Empirical Orthogonal Functions (EOF) techniques. Interannual variability represented in the first EOF of surface heat flux anomaly shows a strong trend in the Antarctic Circumpolar Current (ACC) region and significance variability around ten years