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VelocititesVelocitites

Basic Basic petrophysicspetrophysics: controls on : controls on

seismic velocityseismic velocity

Not constant! Typically about 1% per decade (I.e. x10) in frequency e.g. 20Hz-20kHz � 4%

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Some of the physical properties influencing Vp and Vs

sandstone limestone dolomite

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Unconsolidated material: Dry sand 0.2 - 1.0 km/sWet sand 1.5 - 2.0 km/s

Clay 1.0 - 2.5 km/s

sedimentary rocks: Tertiary sandstone 2.0 - 2.5 km/s

Carbon.

sandstone 4.0 - 4.5 km/sChalk 2.0 - 2.5 km/s

Limestone 3.4 - 7.0 km/sSalt 4.5 - 5.0 km/s

Igneous/metamorphic

rocks: Granite 5.5 - 6.0 km/sGabbro 6.5 - 7.0 km/s

Gneiss 3.5 - 7.5 km/s

Air: 0.33 km/sWater: 1.43-1.54 km/s

Petroleum: 1.3-1.4 km/s

Lithology variations?

(Vp only)

NB -unusually low

density

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Porosity effectsPorosity effects

Porosity: Pore volume per unit volume

Not the same as permeability. A porous rock without interconnected

pores is not permeable!

http://earthnet-geonet.ca/glossary/display_term_e.php?term=porosity

Wyllie’s Time Average Equation

( )mf VVV

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11φφ −+=

Total traveltime

=Travel time

in pores

(fluid)

+Travel time

in

matrix

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Seismic Impedance

Why is impedance important?

Reflectivity depends on impedance

⇒ important for reflection surveys

Gardners Formula:

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aV=ρ

a = 0.31

ρ in g/cm3

V in m/s

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VVP P / V/ VSS ratioratio

Poisson Ratio

xx

zz

xx

yy

ε

ε

ε

εσ

−=

−=

ratio of transverse contraction strain to longitudinal extension strain

in the direction of stretching force

An alternative parameter to describe Vp/Vs characteristics

(Vs/Vp) = √[(0.5 - σ)/(1-σ)],

hence Vs/Vp = 0 - (1/√2), or Vs = 0-70% of Vp

or Vp/Vs = √2 - ∞

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Poisson’s Ratio: σ = 0 - 0.5

≈0.05 for hardest, rigid rocks � ≈0.45 for unconsolidated materials

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Pore fill effectsPore fill effects

Blocking the pore space with clay

The principal cause of seismic

attenuation in sedimentary rocks

is pore fluid movement relative

to matrix - clay fill in pore throats

can have significant effects

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Gassman’s Equations.Convert between brine, oil, and gas pore fill (at fixed pressure), based on knowledge of rock

properties with any one fill, plus ‘laboratory’ values for elastic moduli of grains and other fill types.

In routine use for exploration and for time-lapse seismics.

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Pressure effects

NB: what is pressure an analogue for?

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Shale over gas

sand-ve

-ve

≈0

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1216 m

2432 m

3648 m

4864 m

5472 m

Velocity [m/s]

610 1220 24401830 3048 3658 4267

Shale over oil

sand-ve

≈0

+ve

1216 m

2432 m

3648 m

4864 m

5472 m

Velocity [m/s]

610 1220 24401830 3048 3658 4267

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Gas-oil

contact+ve

+ve

+ve

1216 m

2432 m

3648 m

4864 m

5472 m

Velocity [m/s]

610 1220 24401830 3048 3658 4267

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Oil-water

contact+ve

≈0

≈0

1216 m

2432 m

3648 m

4864 m

5472 m

Velocity [m/s]

610 1220 24401830 3048 3658 4267

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Reminder - the key features