PPT Array Design

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

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

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

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

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

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

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

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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