The Relationship Between Geology and Geophysics

1. The Relationship Between Geology
and Geophysics. For Idiots
…why rocks and wiggles are related
image courtesy of Auden Groth
Dr Christopher Jackson and Dr Rebecca Bell
office: 1.46A
email: c.jackson@imperial.ac.uk

2. Lecture context
Tuesday 18th Nov (AM): Course introduction and geology-geophysics linkage
Tuesday 18th Nov (PM): Seismic stratigraphy
Wednesday 19th Nov (AM): Seismic facies analysis
Wednesday 19th Nov (PM): Interpreting structures in seismic data
Thursday 20th Nov (AM): 2D vs. 3D seismic data
Thursday 20th Nov (PM): Seismic artefacts and interpretation pitfalls
Wednesday 26th Nov (AM): Seismic attribute analysis
Wednesday 26th Nov (PM): Seismic interpretation deliverables
Geologists Geophysicist

3. Lecture Objectives
• This lecture aims to provide a brief refresher on
the relationship between geology and
geophysics
• Examples will be provided that highlight the
benefits and limitations of using seismic
reflection data for imaging geology
• The practical exercise will highlight the
relationship between ‘time’ (geophysics) and
‘depth’ (geology), and will introduce you to the
concept of seismic facies analysis

4. Seismic Reflection Theory
• The amount of energy reflected back from each interface is dependent on
the Acoustic Impedance (Z) contrast.
• The acoustic impedance for a medium is defined as the product of density
and P-wave velocity:

Z


v
where: ρ=density and v=P-wave velocity
• The reflection coefficient is the ratio of the amplitude ar of the reflected ray
to the amplitude ao of the incident ray
• The reflection coefficient can be written in terms of the acoustic
impedance CONTRAST across a boundary
OR

5. Seismic Reflection Theory
P-wave velocity
Kearey, Brooks & Hill. An introduction to
Geophysical Exploration

6. P-wave Velocity
• Velocity and density
closely correlated
• Nafe-Drake curve
• Velocity usually
increases with depth due
to compaction. Fractures
close and porosity
decreases
• Sediments usually have
lower velocities than
metamorphic and
igneous rocks (see
previous slide)

7. Reflection Coefficients

8. Who Wants to be a Seismic Interpreter?
1) Water-saturated sandstone
2) Oil-saturated sandstone
3) Gas-saturated sandstone
Which material would have the slowest P-
Wave velocity?

9. A geologist’s Chris’ Perspective…
image courtesy of Gavin Elliott

10. Synthetic Seismograms
source: http://www.glossary.oilfield.slb.com/DisplayImage.cfm?ID=276
• Synthetic seismogram are
generated by convolving
the sonic and density logs
with a seismic wavelet
• By comparing geological
markers picked on logs,
such as the top of the
chalk in this display on the
left, or the top of the thick
sandstone on the display
to the right, with major
reflections on the seismic
section, seismic data can
be used to map physical
properties between wells

11. • Seismic reflection data can provide relatively accurate
images of the subsurface of the earth
• http://www.youtube.com/watch?v=7Zf1dpJngXo
• http://www.youtube.com/watch?v=v9ghjrq5pV4&feature=related
(Boom clap)

12. A Worked Example...
Schwab et al. (2007)
• Top – Line drawing of outcrop cross-section. Nine sandstone-filled
deepwater channels encased in mudstones are observed
• Bottom – Photo of part of the outcrop cross-section showing
channels 1-4

13. A Worked Example...
Schwab et al. (2007)
• (a) Impedance model of
outcrop shown on previous
slide (yellow=sandstone;
grey=mudstone)
• (b-d) Synthetic seismic
sections of outcrop at 50Hz,
40Hz and 30Hz frequencies,
respectively
• Note the decreasing ability to
identify discrete channels at
decreasing seismic
frequencies...

14. A Worked Example...
• Top – Line drawing of outcrop cross-section. Five sandstone-filled
deepwater channels encased in mudstones are observed. Note dipping
stratal surfaces
• Bottom – Photo of part of the outcrop cross-section showing channels
2, 4 and 5
Schwab et al. (2007)

15. A Worked Example...
• (a) Impedance model of outcrop
shown on previous slide
(yellow=sandstone; grey=mudstone)
• (b-d) Synthetic seismic sections of
outcrop at 50Hz, 40Hz and 30Hz
frequencies, respectively
• Note the decreasing ability to identify
discrete channels at decreasing
seismic frequencies...
• What implications do these synthetic
models have for prognosing
reservoir presence and connectivity?

16. Lecture Summary
1) Understand how rock physical properties impact rock reflectivity (AI)
2) Understand how and why density and P-wave velocity are related
3) Appreciate the link between rocks and their density and P-wave velocity
4) Consider the utility of synthetic seismograms
5) Be mindful of the ‘smearing’ effect of seismic reflection imaging on
complex subsurface geology
You should now be able to…

17. Practical Exercise 1
Seismic-well tie, Williston Basin, USA
Johan (Han) Claringbould Hollie Taylor Idrus Puasa