Introduction to Geophysics

Ali Onceloncel@kfupm.edu.

saDepartment of Earth SciencesKFUPM

Seismic Reflection 3: Acquisiton, Processing, and Waveform Analysis

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Shaybah Field: Data AcquisitionIn

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Alfaraj et al., 1998, GeoArabia, Vol. 3, No 2.

Over 120 million seismic traces, covering an area of about 1,100 sq km, were recorded with over 100,000 shot points.Processing of the data was carried out in-house by Saudi Aramco and lasted about 18 months.

Example from Shaybah Field: Data Acquisition and Processing

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Alfaraj et al., 1998, GeoArabia, Vol. 3, No 2.

Previous Lecture

•Snell's Law •Reflection "Presentation" •Multifold Reflections •Receiver Arrays •CMP method •Processing •Normal Move out

Mid-Term Exam: May 3, Class Time

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Field-Trip: April 22, April 24 or April 26, 12.30 AM

Resulted from energy that undergoes only one reflection before returning to the surface

Primary ReflectionIn

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Multiples

Reflected energy back up again following a reflection back down at the surface.

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Comparison

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Gather

NMO – Normal Moveout Correction

X

H V

Function of X, H (To), V

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NMO – Normal Moveout Correction

= tf-t0

Where tf = two-way time of event at offset distance of x

t0= two-way time of event at zero-offset

tf

t0

X

NMOT

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Velocity Analysis

= tf-t0

Know x from geometry & t0

measured from seismic trace

Select a variety of V’s to get best T

210

2

2 Vt

XTNMO

NMOT

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Velocity Analysis

Overcorrected by too slow velocity

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Velocity Analysis

Under corrected by too fast velocity

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Velocity Analysis

NMO – Normal Moveout CorrectionNow Corrected

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correct VNMO is one which flattens the reflection hyperbolaIn

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Uncorrected

CMP gather

Correct velocity

(224 m/s)

Overcorrection:

velocity too low

(2000 m/s)

Undercorrection:

velocity too high

(2500 m/s)

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Gathers and NMO Corrected CMP

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NMO Corrections

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Mute

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Static Corrections

SC= (hS3 + h R5)VE

VE:Assumed velocity of the material above