ppt array design
TRANSCRIPT
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Seismic Receiver and Source Array
Rajiv Kumar & Dileep Kumar
Department of Earth Science
IIT Bombay
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Over View
Introduction
What is an Array?
Why we need Array?
Assumptions.
Array Design.
Different type of Arrays
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Introduction
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Array
The term Array refers either to the pattern of a group of
geophones that feed a single channel or to a distribution
of sources that fired simultaneously.
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Need of Array
To enhance the signal level and reduce the coherent
noise level.
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Noise
Coherent Noise If noise has a consistent, repeatable
form.
Incoherent Noise Noise that does not occur in repeatable
form.
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Example
For example consider a simple horizontal wave with two
receivers close together. As the horizontal wave moves past the
two receivers, provided they are spaced a half wavelength apart.
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Synthetic Record Analysis
Schematic shot record IIT Bombay
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Assumptions
all geophones have identical coupling
with the earth.
phones are all positioned at the same
elevation.
There are no error in horizontal
positioning .
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Array Design
We can use the fact that the useful signal is arriving
almost vertical when compared with horizontally arriving
noise. The signal is enhance compared to noise, by
using many receivers in a geometric pattern that
attenuates energy traveling horizontally.
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Field Layouts ......
Spread Types-
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Common Midpoint Method
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Single Fold Recording
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Common Midpoint Method
Single Fold Recording
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Vertical Section CMP Recording
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Surface Stacking chart
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Subsurface Stacking chart IIT Bombay
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Array may fail to perform optimum attenuation
of ground roll because of………
Erratic element spacing because of terrain (land).Elevation variation (Land) or cable depth variation
(marine).Arrival time variation because of weathering thickness
variations.
Detector sensitivity and coupling variations.Change in surface-wave characteristics because of change
in weathering-layer lithology parameter.
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Uniform Array
In this type of array, spacing between each receiver of
array is constant along the seismic line. when the
elements are distributed over an area.
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Consider simpler 1-D Problem
Geophone Array ResponseIIT Bombay
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A graph of the responses, using a linear output response
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Amplitude response of a six geophone array with 5-m spacing
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Amplitude response
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Array Length………..
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feature of uniform array’s
Attenuation is maximum at notch points occurring at , .
……… .
The first alias peak occurs at wave number 1/d, has no
attenuation.
Attenuation lobes form “envelope” of attenuation.
The amplitude of array response at the envelope base is
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The Weighted Array
Array where different numbers of elements are located at the
successive positions are called weighted (tapered) array.
array is weighted at the center by a factor of 2.
(1,2,1) weighted array
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Weighted (1, 2, 1) array is same as the composite of
two simpler unweighted identical sub array.
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Weighted arrays also results from combination of
source and receiver arrays, where the effective array is
the result of convolving the source array with receiver
array.
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Convolution...
(1,1)*(1,1)=(1,2,1) (1,1)≈(1,1,0)
(0,1,1) (1,1,0)
1,1,0 0,1,1 1 (a)
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First (reversed) array Second array
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1,1,0 0,1,1 1 (a) 1 1 (b) 1,1,0 0,1,1 1 (a) 1 1 (b) 0 1 0 (c) IIT Bombay
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1,1,0 0,1,1 1 1 (a) 2 1 1 (b) 1 0 1 0 (c) 0 0 0 (d) SUM
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Final result
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Stack Array
The stack array is a uniform liner array involving the entire
common midpoint gathers .
This can be achieved in a number of way with geophones spread
uniformly over the entire geophone group interval
With a split spread having sources located midway between group centers and
source spacing equal to the geophone-group interval.
With end on spreading having sources spacing equal to half the geophone-
group interval.
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Areal Array
The principal application of linear arrays is in
discriminating against coherent noise traveling more or
less in a vertical plane through the array. Coherent noise
traveling outside this plane can be attenuated by an Areal
array.
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Different type of Areal Array
3×3 diamond
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X-array
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Rectangular array
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Crow’s foot array
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Odd arm star array
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Herring-bone array
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Wind mill array
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Thank you for your patience!
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