Sub-basalt exploration Addtitional information.pptx - Statoil Innovate
Sub-basalt exploration Addtitional information.pptx - Statoil Innovate
Sub-basalt exploration Addtitional information.pptx - Statoil Innovate
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<strong>Innovate</strong> challenge:<br />
<strong>Sub</strong>-<strong>basalt</strong> <strong>exploration</strong><br />
Additional material<br />
Peter Hanssen
Geo-scientific classification of <strong>basalt</strong>s<br />
Planke & Haugen, 2001, SeismicImaging and Interpretationof Volcanic Constructions.
Single <strong>basalt</strong> flow<br />
Vp (km/s)<br />
2 4 6<br />
Altered Flow Top<br />
Upper Zone Transition<br />
Characteristic Properties of <strong>Sub</strong>aerial Flood Basalts<br />
(ODP Sites 642/917)<br />
Average Vp = 4.0 km/s<br />
Vp/Vs = 1.8-2.0<br />
Massive<br />
Interior<br />
Seismic Anisotropy = 10-25%<br />
Average Density = 2.5 Mg/m3<br />
2-15 m<br />
Average <strong>basalt</strong> thickness = 6-7 m<br />
Single flow not a problem => Corrib<br />
Shoulder<br />
Zone Lower Transition<br />
Planke& Haugen, 2001, Seismic Imaging and Interpretation of Volcanic Constructions.
Several <strong>basalt</strong> flows<br />
• Usual flow thickness in the several<br />
meter scale<br />
• Often sedimentary rocks in between<br />
single flows / eruptions<br />
• Lava cools fast from top, less fast from<br />
bottom of a single flow<br />
• Gradient to the core of the flow<br />
Additional erosion on top<br />
• Velocities correlated with resistivity,<br />
density and mostly porosity<br />
• Can form reservoir and cap rock<br />
• Connection of inter-<strong>basalt</strong> reservoirs<br />
questionable<br />
• Main targets are sediments below<br />
the <strong>basalt</strong> sequence<br />
Planke & Haugen, 2001, SeismicImaging and Interpretationof Volcanic Constructions.
P-wave velocity over flow thickness<br />
V p (km/s)<br />
5<br />
4<br />
3<br />
1<br />
3<br />
+<br />
4<br />
917A: MCS Average<br />
642E: VSP Average<br />
0 5 10 15 20 25<br />
Unit Thickness (m)<br />
2<br />
YT-2: VSP Average<br />
1 917A Logging Unit 1 (V p-stc)<br />
+<br />
Average of<br />
Characteristic<br />
Velocity Distribution<br />
642E: Fine Grained Flows<br />
917A - all Logging Units (V p-stc)<br />
• The thicker the single <strong>basalt</strong><br />
flow, the higher its velocity<br />
• Is there a general trend and<br />
is the relation the same for<br />
different provinces?<br />
• In which cases can we use<br />
average velocities for the<br />
whole <strong>basalt</strong> sequence?<br />
Planke & Haugen, 2001, SeismicImaging and Interpretationof Volcanic Constructions.
Vp/Vs ratio<br />
Planke & Haugen, 2001, SeismicImaging and Interpretationof Volcanic Constructions.
Iceland VSP summary<br />
• The average VSP velocity down to 1.8 km is typically 4.5-5.1 km/s below<br />
the weathering layer. The measured velocity is close to the expected velocity<br />
of >4.5 km/s for <strong>basalt</strong> flows of ~15 m average thickness.<br />
• There are no good correlations between interpreted refraction interfaces<br />
and major geological and/or interval velocity changes in the boreholes.<br />
• Horizontal refraction velocities are consistently higher than the measured<br />
VSP velocities. Measured anisotropy is 6-18% (typically 10%), and increases<br />
with depth due to increased ray path bending of diving waves.<br />
• Dikes act as high-velocity wave-guides and lead to increased heterogeneities<br />
and more complex wave propagation.
Step-back function of diving-waves<br />
• At very far offsets the diving wave reaches<br />
the base of the <strong>basalt</strong> sequence<br />
• Due to the lower velocity of the underlying<br />
sediments the diving wave discontinues<br />
• This can be observed as a step-back function<br />
as shown on the left at around 18km offset<br />
• Can this be used to construct the general<br />
structure of the area?<br />
• And why do refracted and reflected wave<br />
velocities often not convert? Is this only<br />
explainable by anisotropy?<br />
Richardson et al., 1999, Petro. Geosci., Vol. 5.
Are locally-converted waves feasible?<br />
• Is there a chance to utilize body waves<br />
which are converted to shear waves only<br />
on their way through the <strong>basalt</strong> sequence?<br />
• Should we record PS or pure S-waves<br />
on the seabed?<br />
• How do we combine different P and<br />
S-waves during the processing?<br />
• Should we use sources on the seabed?<br />
• And how do we broaden the frequency<br />
response from beneath the <strong>basalt</strong>s?<br />
Hanssen, 2002, PhD thesis, University of Edinburgh.
Any solutions?<br />
• Low frequency response
Thank you.<br />
<strong>Sub</strong>-Basalt Imaging<br />
– Additional Material<br />
Peter Hanssen<br />
Principal Geophysicist<br />
PetHan@<strong>Statoil</strong>.com<br />
www.statoil.com