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| Titel |
Seismic wave propagation through an extrusive basalt sequence |
| VerfasserIn |
Oliver Sanford, Richard Hobbs, Richard Brown, Nick Schofield |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250133505
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| Publikation (Nr.) |
 EGU/EGU2016-14120.pdf |
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| Zusammenfassung |
| Layers of basalt flows within sedimentary successions (e.g. in the Faeroe-Shetland
Basin) cause complex scattering and attenuation of seismic waves during seismic
exploration surveys. Extrusive basaltic sequences are highly heterogeneous and contain
strong impedance contrasts between higher velocity crystalline flow cores (∼6 km
s−1) and the lower velocity fragmented and weathered flow crusts (3-4 km s−1).
Typically, the refracted wave from the basaltic layer is used to build a velocity model by
tomography. This velocity model is then used to aid processing of the reflection data
where direct determination of velocity is ambiguous, or as a starting point for full
waveform inversion, for example. The model may also be used as part of assessing
drilling risk of potential wells, as it is believed to constrain the total thickness of the
sequence.
In heterogeneous media, where the scatter size is of the order of the seismic wavelength
or larger, scattering preferentially traps the seismic energy in the low velocity regions. This
causes a build-up of energy that is guided along the low velocity layers. This has implications
for the interpretation of the observed first arrival of the seismic wave, which may be a biased
towards the low velocity regions. This will then lead to an underestimate of the
velocity structure and hence the thickness of the basalt, with implications for the
drilling of wells hoping to penetrate through the base of the basalts in search of
hydrocarbons.
Using 2-D acoustic finite difference modelling of the guided wave through a simple
layered basalt sequence, we consider the relative importance of different parameters of the
basalt on the seismic energy propagating through the layers. These include the
proportion of high to low velocity material, the number of layers, their thickness and the
roughness of the interfaces between the layers. We observe a non-linear relationship
between the ratio of high to low velocity layers and the apparent velocity of the
first arrival suggesting that such a sequence may cause a reduction of the apparent
velocity by as much as 1 km s−1. We also find that the rate of amplitude decay in the
higher velocity layer is related to the interface roughness between the basalt layers. |
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