Contents
of January, 1998 - Vol. XIX No.1
A RE-LOOK INTO GRAVITY FIELD OF NORTHEAST INDIA
J.V. Rama Rao and R.M.C. Prasad
SEISMICITY OF NORTHEAST INDIA AND SURROUNDINGS
- DEVELOPMENT OVER THE PAST 100 YEARS
J.R. Kayal
GROUND MOTION ESTIMATION CRITERIA FOR
LARGE EARTHQUAKES IN THE URBAN AREAS OF NORTHEAST INDIA
Chandan K. Saikia and Paul G. Somerville
STOCHASTIC CHARACTERISATION OF COPPER MINERALISATION - A CASE
STUDY
D.D. Sharma
MEHSANA HORST - TECTONICS AND ITS IMPORTANCE
IN HYDROCARBON EXPLORATION, CAMBAY BASIN
M.C. Kandpal, N.K. Mahajan and B. Ghosh
FINITE DIFFERENCE EM MODELLING OF TWO-DIMENSIONAL
BURIED CONDUCTORS IN AUDIO-FREQUENCY RANGE
K. Prabhakara Rao, K. Mallick and P.D. Saraf
HYDROGEOLOGICAL SURVEYS IN DROUGHT
PRONE BARGARH DISTRICT, ORISSA, AIDED BY REMOTE SENSING STUDIES, GROUNDWATER
EXPLORATION AND APPLICATION OF GIS TO DECIPHER GROUNDWATER POTENTIALS
S. Das, S.C. Behera, D.P. Pati and G. Prasad
A RE-LOOK INTO GRAVITY
FIELD OF NORTHEAST INDIA
J.V. Rama Rao* and R.M.C. Prasad**
* Geophysics division, Geological Survey of
India, Shillong
**
Geophysics Division, Geological Survey of India, Hyderabad
Abstract
The gravity field of Northeast India is
analysed with objectives of correlating with geology, inferring the crustal
structure and tectonics and its relationship on seismicity and also to prepare
crustal thickness map for the entire region.
Historically, this region is devastated
several times by major earthquakes in recent past. By studying the areal
distribution of the earthquakes with respect to inferred lineaments, four zones
could be identified within the study area where the chances of major
earthquakes are high. Twenty two lineaments have been identified with NW-SE,
NE-SW, E-W and N-S trends. The gravity responses of different geological
formations and role of lineaments in the disposition of lithology are studied.
The relative variations of thickness of the crust are computed using Qureshy’s
empirical relation. The variations are of the order of 33 Km over Shillong
massif and Bengal Basin to more than 50 Km in the Himalayan region.Awarded the
MBR Rao Gold Medal for best paper presented in the Seminar
Paper
presented at the 23rd Annual Convention
and Seminar on Exploration Geophysics during 18-21 November, 1997 and
was adjudged as the best paper and was awarded the M.B.R. Rao Gold Medal
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SEISMICITY OF NORTHEAST
INDIA AND SURROUNDINGS - DEVELOPMENT OVER THE PAST 100 YEARS
J.R. Kayal
Geological Survey of India, 4 Chowringhee
Lane, Calcutta-16
Abstract
Northeast India is lying at the
juncture of Himalayan Arc to the north and Burmese Arc to the east, and is one
of the most active regions of the world. 18 large earthquakes (M 7.0) including
two great earthquakes (M > 8.0) occurred in this region during the last 100
years. The 1897 great Shillong earthquake is the first instrumentally recorded
earthquake in the country. Since 1964, with the inception of the WWSSN and
increase of local networks, location quality of the epicentres are much
improved. In the recent years, since early 1980s, several temporary
microearthquake networks are run along with earthquake precursor investigations
in selected areas of the region. Regional earthquake data (ISC and USGS),
temporary close-spaced microearthquake (MEQ) network data and geophysical
precursors of the felt/large earthquakes have given us some understanding of
generating processes of the earthquakes in this region.
The seismicity in the main Himalayan
seismic belt believed to be due to the collision tectonics between the Indian
Plate of Eurasian Plate, and the earthquakes are generally correlated with the
known regional thrusts, the Main Boundary Thrust (MBT) and Main Central Thrust
(MCT). Distribution of epicentres is sparse, and the earthquakes are confined
within a depth range 0-70 km. Thrust faulting of the earthquakes are reported.
MEQ surveys in Arunachal Pradesh, in the Eastern Himalaya, however, revealed
that the earthquakes are not correlatable with the regional thrusts. The
earthquakes are rather generated by strike-slip mechanism; transverse tectonics
are suggested in this part of the Himalaya. The earthquakes in the Burmese arc,
on the other hand, are typical of subduction-tectonics. Intense activity is
observed along the Indo-Burma ranges, and the earthquakes are as deep as 200
km. Normal, thrust and strike-slip faulting of earthquakes are observed. The
seismological data reveal a subduction structure. The syntaxis zone, the
meeting zone of the Himalayan Arc and Burmese Arc, is seismically active and
produced the great Assam-Tibet earthquake (M:8.7) of 1950. The Shillong Plateau
earthquakes are referred to the plate-boundary earthquakes which are produced
by the stress accumulation due to the collision tectonics to the north and
subduction tectonics to the east. The MEQ surveys in the Shillong Plateau and
adjoining areas have revealed seismogenic faults and spatial variation of
tectonic stress. Seismic tomography study, using the MEQ data, further confirms
the fault zones at depth, and lateral variation in velocity structure. The
great 1897 earthquake (M:8.7) occurred beneath this Plateau, and the high
microearthquake activity indicates that the area is seismically very active.
The activity in the Tripura folded belt is also referred to the plate-boundary
earthquakes, the stress is transmitted mainly from the Burmese Arc. The low
seismic activity in the Bengal Basin is interpreted to be due to a locked
portion of the Indian plate below this basin. The E-W Dauki fault is possibly
the surface expression of lateral segmentation of the Indian Plate, the
Shillong Plateau of the north and the Bengal Basin to the South.
Continuous monitoring of deep
resistivity, repeat microgravity measurements and microearthquakes surveys by
temporary networks were useful to monitor precursor anomalies for felt/large
earthquakes in this region. The deep resistivity survey shows that resistivity
values may increase or decrease (40%-50%) or may show no change at all
depending on the measurement direction, stress pattern, fracture/fault
orientation and rock formation. The
repeat microgravity surveys indicate change in microgravity values (200-250
gal) before a felt/large earthquake in the region. The temporary MEQ network
data indicates change in microseismicity level and velocity-ratio (V/V) before
a felt/damaging earthquake in the area. Continuous monitoring of seismicity,
deep resistivity and microgravity may be fruitful for prediction research of an
impending large earthquake in this region.
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GROUND MOTION ESTIMATION
CRITERIA FOR LARGE EARTHQUAKES IN THE URBAN AREAS OF NORTHEAST INDIA
Chandan K. Saikia and Paul G. Somerville
Woodward-Clyde Federal Services, 566 El Dorado
St., Pasadena, CA 91101, USA
Abstract
Historical records show that the
northeast region of India experienced several large earthquakes (M>6.0)
during last two hundred years, including the June 12, 1897 and August 15, 1950
earthquakes (M³8.5) which caused widespread damage in the region. Considering
the population growth and recent urban development in northeast India, the loss
of lives, critical infrastructure, dwellings, dams, bridges and life-lines
would be significant and perhaps more severe, if any of these earthquakes were
to occur to-day. To reduce these losses, performance analysis of these
structures under severe ground motions from large earthquakes becomes
essential. Strong-motion recordings of large earthquakes in other geographical
regions demonstrate that ground motions that are recorded at a site are
frequently influenced by specific aspects of earthquake source, seismic wave
propagation and local site conditions. Therefore, the choice of input motions
to perform such analysis becomes crucial as conclusions can be biased when
selected motions do not include the characteristics of the relevant earthquake
source and site conditions. Ground motion time histories used for these
analyses would ideally be recorded at sites from nearby earthquakes. Presently,
there are no such recordings available for use. We need to operate
strong-motion and broadband networks in the region to continually collect data
for quantifying seismic parameters for seismic hazard estimation in the future.
To compensate for the lack of nearby strong motion recordings from large
earthquakes in northeast India, it is important that modern seismological
methods be used to generate ground motion time histories for such events to
include these effects. In this paper, we discuss methods for generating
reliable ground motion time histories to develop design spectra for large
earthquakes useful for performance analysis. We discuss their usefulness in the
stress and deformation analysis for dams since dams fall into the category of
critical resources for northeast India and are located close to the seismic
sources.
Paper
presented at the Workshop on The Great
Shillong Earthquake - 1897 : A Centennial Retrospective during 18 November,
1997
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STOCHASTIC CHARACTERISATION
OF COPPER MINERALISATION - A CASE STUDY
D.D. Sharma
National Geophysical
Research Institute, Hyderabad
Abstract
Stochastic charachterisation of copper
mineralisation including modelling aspects are discussed in this paper with
reference to the mineralisation at Ingladhal, Karnataka. The exposed lode
consisted of eleven horizontal levels (drives) driven from MRL 862 m to MRL 563
m in depth. Based on the auto-correlation function (acf), the partial
auto-correlation function (pacf) and the Akaike criterion it is inferred that
AR (1) model could be used to forecast copper prospects with a reasonable
degree of confidence. As spectral representation may reveal useful information
on hidden periodicities, the same has been carried out employing maximum
entropy method. Significant spikes in the spectra in some of the levels
indicated possible periodicity in mineralisation ranging approximately from 24m
to 111m. The suggested model(s) would be highly useful to understand the
charachteristics of copper mineralisation.
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MEHSANA HORST - TECTONICS
AND ITS IMPORTANCE IN HYDROCARBON EXPLORATION, CAMBAY BASIN
M.C. Kandpal, N.K. Mahajan and B. Ghosh
RERIG, EBG, WRBC, Oil and Natural Gas
Corporation Limited, Baroda
Abstract
The existence of Mehsana horst has
played an important role in the exploration and exploitation of hydrocarbons in
the northern part of the Cambay basin. The eastern flank of the Mehsana horst
has proved to be rich in hydrocarbons with good reservoir facies, whereas the
western flank has sporadic hydrocarbon occurrences (mainly in Bechrajee field)
and is yet to be explored and exploited fully. The tectonic activity associated
with the Mehsana horst, the global/local eustatic changes and the environment
under which deposition took place during Late Eocene to Mid Oligocene are the
main factors responsible for hydrocarbon accumulation in this part of the basin
. Mehsana horst, trending in the NNW-SSE direction, had been an active and
upcoming high till Late Eocene-Mid Oligocene period. Study of latest seismic and
other Geo-scientific data have clearly revealed that the highest part of the
horst at the technical basement level (Trappean level) was quite shallow (i.e.
upto about 950 mts.). Due to the tectonic activity and global/local sea level
changes, during Middle to Late Eocene, the horst got exposed and erosion had
set in. Seismic lines in this part have clear evidences for the presence of
erosional cycles, the carving of technical basement at places in the eastern
side and the flattening of the top of the horst. The dumping of the weathered
and eroded material from top of the horst in the eastern and western flanks is
evident from the seismic data which might have created reservoir facies within
Older Cambay shale / Olpad formations. These facies are represented by high
amplitude seismic events.
During the deposition of Late Eocene
& Mid Oligocene period, Mehsana Horst was exposed and the north-south
flowing channels have skirted the exposed horst through lows (flanks) to create
excellent reservoir facies in the eastern and western flanks of the horst.
Together with the strati-structural entrapment conditions such as fault
closures, wedgeout/ pinchout prospects and the existence of reservoir facies
had created excellent ground for hydrocarbon accumulation in this part of the
basin. This geological model has already been proved in the eastern flank of
Mehsana horst, where number of oil fields like Lanwa, Balol, Santhal, Jotana
etc. have been discovered. The western flank, which is analogous to the eastern
flank, should also be prospective from hydrocarbon point of view which is
proved by limited exploratory drilling.
Subsequently, the bald Mehsana horst,
thus created was exposed, got weathered, and secondary porosity got generated
within this product. This weathered top become relatively good reservoir with
sporadic hydrocarbon occurrences. This thin weathered zone, about 30-40 m.
thick, named as conglomerate zone or North Balol Pay, is known to have the
hydrocarbon potential which is yet to be explored fully.
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FINITE DIFFERENCE EM
MODELLING OF TWO-DIMENSIONAL BURIED CONDUCTORS IN AUDIO-FREQUENCY RANGE
K. Prabhakara Rao, K. Mallick and P.D. Saraf
National Geophysical Research Institute, Uppal
Road, Hyderabad - 500 007
Abstract
The electromagnetic method. is an
effective tool in search of conducting ore bodies at shallower depths. However,
the presence of host rocks and the overburden often modify or even mask the
response of the target bodies.
Here an attempt is made (i) to study
the response pattern of buried conductors and (ii) to analyse the effect of the
host rocks. The finite difference technique is used to compute the
H-polarisation apparent resistivities of two-dimensional geological structures.
It is observed that in the presence of relatively conducting host rocks the
detection of conductors at greater depths is not easy. An interesting
phenomenon of resistivity inversion for deep-seated conductor is observed.
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HYDROGEOLOGICAL SURVEYS
IN DROUGHT PRONE BARGARH DISTRICT, ORISSA, AIDED BY REMOTE SENSING STUDIES,
GROUNDWATER EXPLORATION AND APPLICATION OF GIS TO DECIPHER GROUNDWATER POTENTIALS
S. Das, S.C. Behera, D.P. Pati and G. Prasad
Central Ground Water Board, Bhubaneswar
Abstract
The district of Bargarh,Orissa
chronically suffers from droughts. An integrated approach of remote sensing
studies, hydrogeological surveys and exploration was adopted to delineate both
shallow and deep aquifers in the district and to assess their productivity
characteristics, groundwater quality and groundwater potentiality. The district
is underlain generally by the consolidated formations, consisting of Granite
Gneisses,Khondalites, Charnockites with intrusives of Dolerites,
Quartzites,Phyllite, Shale, Limestone and Dolomite. Buried Pediments,
Intermotane valleys and fracture lineaments provide favouable locales for
ground water storage. Ground water occurs under water table conditions in
weathered residuum and under confined conditions in deeper fractured zones. The
depth to water table values range from less than 1 m to 10 m or more below
ground level.Specific capacity of dugwells varied from 0.56 to 142.1 LPM/M. The
yield of exploratory wells was recorded upto 22 LPS depending upon its location
under favourable topographic set up, closeness to fracture lineaments,
thickness weathered residuum and number of fracturencountered in the borehole.
The permeability of the weathered zone varies from negligible to 21 m/day which
reduces sharply with depth.
GIS approach was adopted in ILWIS to integrate
the collateral data like geomorphology,depth to water level, permeability
of phreatic aquifers, soil characteristic in a relational data base to bring
out an integrated thematic map delineating priority areas for groundwater
development.