GROUND MOTION ESTIMATION CRITERIA
FOR LARGE EARTHQUAKES IN THE URBAN AREAS OF NORTHEAST INDIA
Chandan K. Saikia and Paul G. Somerville
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
The gravity field of
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
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
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.
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
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.
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.
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.
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.