Sub-basalt exploration Addtitional information.pptx - Statoil Innovate

Innovate challenge:
Sub-basalt exploration
Additional material
Peter Hanssen

Geo-scientific classification of basalts
Planke & Haugen, 2001, SeismicImaging and Interpretationof Volcanic Constructions.

Single basalt flow
Vp (km/s)
2 4 6
Altered Flow Top
Upper Zone Transition
Characteristic Properties of Subaerial Flood Basalts
(ODP Sites 642/917)
Average Vp = 4.0 km/s
Vp/Vs = 1.8-2.0
Massive
Interior
Seismic Anisotropy = 10-25%
Average Density = 2.5 Mg/m3
2-15 m
Average basalt thickness = 6-7 m
Single flow not a problem => Corrib
Shoulder
Zone Lower Transition
Planke& Haugen, 2001, Seismic Imaging and Interpretation of Volcanic Constructions.

Several basalt flows
• 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
Planke & Haugen, 2001, SeismicImaging and Interpretationof Volcanic Constructions.

P-wave velocity over flow thickness
V p (km/s)
5
4
3
1
3
+
4
917A: MCS Average
642E: VSP Average
0 5 10 15 20 25
Unit Thickness (m)
2
YT-2: VSP Average
1 917A Logging Unit 1 (V p-stc)
+
Average of
Characteristic
Velocity Distribution
642E: Fine Grained Flows
917A - all Logging Units (V p-stc)
• The thicker the single basalt
flow, 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?
Planke & Haugen, 2001, SeismicImaging and Interpretationof Volcanic Constructions.

Vp/Vs ratio
Planke & Haugen, 2001, SeismicImaging and Interpretationof Volcanic Constructions.

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.

Step-back function of diving-waves
• 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?
Richardson et al., 1999, Petro. Geosci., Vol. 5.

Are locally-converted waves feasible?
• 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?
Hanssen, 2002, PhD thesis, University of Edinburgh.

Any solutions?
• Low frequency response

Thank you.
Sub-Basalt Imaging
– Additional Material
Peter Hanssen
Principal Geophysicist
PetHan@Statoil.com
www.statoil.com