ESCI 426:

Geological Interpretation of 2D Seismic Data

Fall 2012

Introduction to Seismic Data

Nankai

Seismic Waves

• Body waves (P and S) and surface waves

generated by earthquakes

• Oil exploration uses active source seismology

2D Seismic Acquisition

Basic Equations

• Acoustic energy is reflected when there is a change in

acoustic impedance between adjacent rock layers

• Acoustic impedance:

AI = 𝑣

• For normally incident raypaths, the following equation

defines how much energy is reflected

• Reflection coefficient:

Reflection coefficient

R = 2𝑣2 −1𝑣1

2𝑣2+1𝑣1

Seismic Fundamentals

What is a seismic trace?

Seismic trace is a convolution

• Convolution

For a single reflector, the seismogram (or trace) we record is simply the seismic wavelet scaled by the reflection coefficient, with some noise added

Seismic trace is a convolution

• Convolution

What is a seismic trace?

Impedance is directly related to properties of the Earth – this is what we want

Reflectivity is convolution of RC series and wavelet – this is what we get

Impedance

Seismic Resolution

Averaging geology

Resolution

• Vertical

– Vertical resolution can be thought of as the minimum

resolvable bed thickness

– Tuning thickness λ/4

• Horizontal

– Fresnel zone describes area along a reflector from

which most energy is reflected

– Features smaller than Fresnel zone cannot be

resolved (typically ~100 feet)

Resolution

• λ = v/f

• Assuming velocity of 10,000 ft/s and a dominant

frequency of 20 Hz, what is the vertical

resolution limit?

• 125 ft

Resolution

Resolution

Time versus Depth

After Hart, 2005

Vertical Exaggeration

• Comparison between horizontal and vertical

scales

• Take what a unit, such as an inch, represents on

the horizontal scale and divide it by what that

unit represents on the vertical scale

• VE = 1 no distortion

Vertical Exaggeration

Vertical Exaggeration

Horizontal scale 5 km Vertical scale 3.3 km

5/3.3 = 1.5 VE = 1.5X

Interpretation

• What is the approximate vertical resolution?

– λ = v/f, λ/4

• Are the data in time or depth?

– Speed x time = depth

• What is the vertical exaggeration?

– Horizontal/vertical scales

0

4.5 s

280 KM

Chukchi Sea, Alaska

Zoom in – noisy data

1.2 s

4.0 s

10 KM

0 s

1.6 s 5 KM

Zoom in – noisy data

Zoom in - Faults and Layers

0

2.5 s

2 KM

Zoom in - Faults and Layers

0

2.5 s

2 KM

10 KM

1 KM

Iberia Margin

Gulf of Mexico – low resolution

1 s

8.5 s

55 KM

Mapping Subtle Features

2.5 s

3.0 s

Eagle Ford

Thicker Eagle Ford expressed as

doublet before spectral enhancement

Spectral enhancement reveals

additional reflector

On the Use of Seismic Data

• Consider everything to be geology until proved

otherwise.

– Milo Backus

• Where oil is first found is in the minds of men.

– Wallace Pratt