sub-basalt exploration addtitional information

8/7/2019 Sub-Basalt Exploration Addtitional Information

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Innovate challenge:

Sub-basalt explorationAdditional material

Peter Hanssen



Geo-scientific classification of basalts

P l a n k e & H a u g e n , 2

0 0 1 , S e i s m i c I m a g i n g a n d I n t e r p r e t a t i o n o f V o l c a n i c C o n s t r u c t i o n s .



Single basalt flow

Characteristic Properties of Subaerial Flood Basalts

(ODP Sites 642/917)

Average Vp = 4.0 km/s

Vp/Vs = 1.8-2.0

Seismic Anisotropy = 10-25%

Average Density = 2.5 Mg/m3

Average basalt thickness = 6-7 m

Single flow not a problem => Corrib

2 4 6

Vp (km/s)

2-15 m

Shoulder

Massive Interior

Altered Flow Top

Upper Transition Zone

Lower Transition Zone

Planke& Haugen, 2001, Seismic Imaging and Interpretation of Volcanic Constructions.



Several basalt flows

P l a n k e & H a u g e

n , 2 0 0 1 , S e i s m i c I m a g i n g a n d I n t e r p r e t a t i o n o f V o l c a n i c C o n s t r u c t i o n s .

• Usual flow thickness in the several

meter scale

• Often sedimentary rocks in between

single flows / eruptions

• Lava cools fast from top, less fast from

bottom of a single flow

• Gradient to the core of the flow

Additional erosion on top

• Velocities correlated with resistivity,

density and mostly porosity

• Can form reservoir and cap rock

• Connection of inter-basalt reservoirs

questionable

• Main targets are sediments below

the basalt sequence



P-wave velocity over flow thickness

P l a n k e & H a u g e

n , 2 0 0 1 , S e i s m i c I m a g i n g a n d I n t e r p r e t a t i o n o f V o l c a n i c C o n s t r u c t i o n s .

3

4

5

V p ( k m / s )

0 5 10 15 20 25Unit Thickness (m)

917A: MCS Average

YT-2: VSP Average

642E: VSP Average

1

2

3

4

+

1 p-stc

p-stc

917A Logging Unit 1 (V )

917A - all Logging Units (V )+

A v e r ag e o f

C h a r ac t e r i s t ic

V e loc i t y D i s t r i b u

t io n

642E: Fine Grained Flows

• The thicker the single basaltflow, the higher its velocity

• Is there a general trend and

is the relation the same for

different provinces?

• In which cases can we use

average velocities for the

whole basalt sequence?



Iceland VSP summary

• The average VSP velocity down to 1.8 km is typically 4.5-5.1 km/s below

the weathering layer. The measured velocity is close to the expected velocity

of >4.5 km/s for basalt flows of ~15 m average thickness.

• There are no good correlations between interpreted refraction interfaces

and major geological and/or interval velocity changes in the boreholes.

• Horizontal refraction velocities are consistently higher than the measured

VSP velocities. Measured anisotropy is 6-18% (typically 10%), and increases

with depth due to increased ray path bending of diving waves.

• Dikes act as high-velocity wave-guides and lead to increased heterogeneities

and more complex wave propagation.



• At very far offsets the diving wave reaches

the base of the basalt sequence

• Due to the lower velocity of the underlying

sediments the diving wave discontinues

• This can be observed as a step-back function

as shown on the left at around 18km offset

• Can this be used to construct the general

structure of the area?

• And why do refracted and reflected wave

velocities often not convert? Is this only

explainable by anisotropy?

Step-back function of diving-waves

Richardson et al., 1999, Petro. Geosci., Vol. 5.



• Is there a chance to utilize body waves

which are converted to shear waves only

on their way through the basalt sequence?

• Should we record PS or pure S-waves

on the seabed?

• How do we combine different P and

S-waves during the processing?

• Should we use sources on the seabed?

• And how do we broaden the frequency

response from beneath the basalts?

Are locally-converted waves feasible?

Hanssen, 2002, PhD thesis, University of Edinburgh.



Any solutions?

• Low frequency response <30Hz from sub-basalt sequence

• Short term:

– Low-frequency setup of airguns and streamers

– Deep towed streamers and bigger/deeper airguns or bubble tuningare common practice

• Long term: – Are new offshore source-designs for frequencies below 3Hz feasible?

• Do electromagnetic methods help to improve the seismic image?

• What is the best way to jointly invert gravity and seismic data?

• Are there other methods, survey designs or instrumentation more suitable

for sub-basalt imaging around?

• Next to demultiple and preserving low-frequencies, are there other

important processing steps?

sub-basalt exploration addtitional information

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