Geologic setting

FEAVO effects are caused by focusing through velocity or absorption heterogeneities smaller than the Fresnel zone Spetzler et al. (2004) - too small to be resolved by velocity analysis methods currently employed in production settings, too small to send the energy outside the aperture of the seismic survey, but not so small and sharp that they would simply cause a diffraction. This means roughly ``a few tens of meters''. White et al. (1988) shows analytically that it is more likely that the interfaces are smooth rather than sharp. Highly visible FEAVO effects appear easily in the presence of velocity contrasts as small as 2%, and no absorption Vlad (2004a). Examples of such heterogeneities include:

(1) Irregular interfaces between spatially extended media with different velocity and/or absorption characteristics: (a) channels on the sea bottom caused by currents, former rivers, or glaciers; (b) positive landforms on sea bottom such as moraines (North Sea); (c) Irregular thickness of permafrost; (d) low-velocity eolian, fluvial or marine sediment covering karst features or other irregular erosion surfaces; (e) interfaces of plastic clay or salt bodies. In this case FEAVO can be hard to see because of the much more powerful illumination effects caused by interface undulations of a larger spatial wavelength than those which cause FEAVO.

(2) Small lenticular bodies of contrasting properties with the surrounding medium. They may be filled with gas, in which case absorption would play an important role. They can be small in all directions, as would be the case with filled peat bogs, or they can be elongated along one direction, such as gas sand-filled river channels (small in cross-section) or lenses formed by gas-liquid contact inside a fold associated with normal listric faulting, (left panel in Figure ).

 

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Figure 1 Examples of geologic settings which may cause focusing. From Sheriff and Geldart (1995).

(3) Termination of a relatively thin layer of highly contrasting properties with the surrounding medium, either by tapering off stratigraphically (right panel in Figure ), or by ending abruptly against a fault Hatchell (1999, 2000a,b). The latter case, illustrated by Figure , is interesting becayse it may occur much deeper (thousands of meters) than the previously described ones, and the bodies causing the FEAVO anomalies may have much sharper interfaces.

 

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Figure 2 Top: Velocity model and isochrones for a shot at (0,0). The background is 1830m/s and the slab is 1647m/s. No absorption, pseudospectral two-way method. Bottom: The shot is downward continued through the velocity model with the slab and without the slab to simulate the seismograms that a horizontal strings of geophones would record at a depth of 6000m. At each x location, the plotted value is the ratio between the highest amplitude obtained at that location without the slab and with the slab. The end of the slab induces focusing. The dispersion is just a numerical effect. From Vlad and Tisserant (2004).

Needed: realistic modeling of these FEAVO-causing geologic settings with a variety of plausible parameters (shape, size, depth, velocity, Q), in 2-D and 3-D, in order to estimate the range of parameters which results in FEAVO anomalies (source too small to be resolved by state-of-the-art velocity analysis methods used in production, yet too large and smooth to cause diffractions). This is a computationally-intensive task. The datasets obtained from modeling could be used as benchmarks for FEAVO detection and removal.