Seismic Receiver and Source Array

Rajiv Kumar & Dileep Kumar

Department of Earth Science

IIT Bombay

IIT Bombay

Over View

Introduction

What is an Array?

Why we need Array?

Assumptions.

Array Design.

Different type of Arrays

IIT Bombay

Introduction

IIT Bombay

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.

IIT Bombay

Need of Array

To enhance the signal level and reduce the coherent

noise level.

IIT Bombay

Noise

Coherent Noise If noise has a consistent, repeatable

form.

Incoherent Noise Noise that does not occur in repeatable

form.

IIT Bombay

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.

IIT Bombay

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 .

IIT Bombay

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.

IIT Bombay

Field Layouts ......

Spread Types-

IIT Bombay

IIT Bombay

IIT Bombay

Common Midpoint Method

IIT Bombay

Single Fold Recording

Common Midpoint Method

Single Fold Recording

IIT Bombay

Vertical Section CMP Recording

IIT Bombay

Surface Stacking chart

IIT Bombay

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.

IIT Bombay

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.

IIT Bombay

Consider simpler 1-D Problem

Geophone Array ResponseIIT Bombay

A graph of the responses, using a linear output response

IIT Bombay

Amplitude response of a six geophone array with 5-m spacing

IIT Bombay

Amplitude response

IIT Bombay

Array Length………..

IIT Bombay

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

IIT Bombay

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

IIT Bombay

Weighted (1, 2, 1) array is same as the composite of

two simpler unweighted identical sub array.

IIT Bombay

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.

IIT Bombay

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)

IIT Bombay

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

IIT Bombay

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.

IIT Bombay

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.

IIT Bombay

Different type of Areal Array

3×3 diamond

IIT Bombay

X-array

IIT Bombay

Rectangular array

IIT Bombay

Crow’s foot array

IIT Bombay

Odd arm star array

IIT Bombay

Herring-bone array

IIT Bombay

Wind mill array

IIT Bombay

Thank you for your patience!

IIT Bombay