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Complex 2D deep-water geology (adapted from Lafond et al. 20004)

Animations - Student models Complex 2D deep-water geology (adapted from Lafond et al. 20004). Layers 4, 5, 6, and 7 are the subsalt layers.

An illustration of the free-surface multiples between the basalt top and the free surface, and the internal multiple between the basalt top and the sea floor (36)

Animations - Student models Let us discuss each of the above categories in more detail. Due to the high acoustic impedance contrast between the basalt and the surrounding sedimentary layers, primary waves (P-waves) traveling to the sub-basalt layers and then coming back up have to pass through the large sediment-basalt impedance contrast four times, twice while going down and twice while coming up.

The high-impedance contrast leads to unusually weak sub-basalt primary reflections, as most of the energy is reflected back into the incident medium when the wave hits the sediment-basalt interface. In addition, the presence of a high-impedance contrast at the top of the basalt leads to strong free-surface multiples (FSM) between the basalt top and the free surface, and internal multiples (IM) between the basalt top and another strong reflector, e.g. sea floor. These FSM and IM interface with the primary reflections from the sub-basalt horizons, making interpretation very difficult and sometimes even leading to misinterpretation of various events. Due to ringing, the multiples totally obscure the already-weak reflections from sub-basalt layers.

An illustration of the rough top and bottom of the basalt layer. Due to the roughness, the incident energy is scattered, degrading the signal-to-noise ratio of the data (37)

Animations - Student models To define the second category of basalt complexity, let us look at the top and bottom surfaces of the basalt layer. It is very common in the various basins for the basalt to have a very irregular surface. The roughness of basalt, like the basalt-top roughness and the valley fill at the bottom, etc., cause a significant energy scattering and absorption. These characteristics have a very detrimental effect on the data as multiple scattering from the top and bottom of the basalt layer degrade the signal-to-noise ratio even further.
A shot gather for this model is shown. Note that the reflections from the top of the basalt are scattered due to its rough surface. The rough surface of the basalt also scatters the energy coming from the underlying reflectors thus making the events of the sub-basalt reflectors very weak.

An illustration of small-scale heterogeneities like presence of gas boubbles and fractures in the basalt (38)

Animations - Student models Lastly, the third model contains several small-scale heterogeneities in the basalt, such as the presence of fractures and trapped bubbles (vesicular basalt). When the magma intrudes the sedimentary layers and comes to the surfaces, due to the low temperature at the surface, it undergoes rapid cooling. As a result the gases in the magma do not have enough time to escape and are trapped when the basalt is formed. The layer may also undergo fracturing after deposition due to external forces. These fractures and trapped bubbles act as small-scale heterogeneities that cause a significant absorption and scattering of seismic energy, amking the data noisy.

The seismic image of the model gives a very fuzzy picture of the events below the basalt layer. This is due to scattering of energy when passing through the small-scale heterogeneities present within the basalt layer. The sub-basalt reflections are totally immersed in the noise and are not visible in the seismic section.