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Anisotropy is not going away

One of the first things geophysics students learn in an introductory seismology course is the wave equation. It is usually introduced under an explicitly stated or tacit assumption of isotropy. This assumption is certainly useful because it allows the students to harness their intuition and even rely on their childhood experience, from which they remember that rocks thrown in water make circles on its surface. Those circles, expanding as the waves propagate, are the wavefronts described by the wave equation; they have circular shapes exactly because water is homogeneous and isotropic.

The properties of homogeneity and heterogeneity are easy to grasp because we readily observe them every day. For instance, while pure water is normally perceived as homogeneous, a hearty stew would be an example of a heterogeneous substance. Physical properties vary spatially in such substances. As the seismology course proceeds, students are introduced to another property of solids — anisotropy. In an anisotropic substance, a measured quantity such as the wave propagation velocity depends on the direction in which it is measured rather than the spatial location of the measurement. Anisotropy is not as readily available to our casual observation as heterogeneity; after all, we see circles and not ovals when we throw rocks in water.

The understanding of anisotropy is critically important for exploration geophysics because the majority of sedimentary formations — through which seismic waves propagate on their way to and from hydrocarbon reservoirs — are shales. They consist of highly anisotropic clay particles, which make seismic anisotropy of shales routinely observable in exploration practice. There are other reasons for the subsurface anisotropy too, notably fractures and non-hydrostatic stresses whose abundant presence is widely documented in the geophysical and geo- logical literature . Ignoring seismic anisotropy, as our industry often does, causes various issues in seismic data processing, some of which might entail substantial financial losses. Among them are:

  • Mispositioned and blurred seismic images, which could lead to missed exploration targets.

  • Improperly placed or poorly imaged faults, which sometimes result in unexpected and expensive-to-fix drilling problems.

  • The inability to extract quality information from converted-wave and shear-wave data.

On top of these things, certain other tasks, such as seismic characterization of fractures, simply cannot be implemented with isotropic assumptions because seismic anisotropy is the very reason for the observed signatures.

As practitioners, what should we do? The advice is straightforward: understand the theory and learn from the practice. There are numerous papers and a few good books that can form a paradigm for applications of seismic anisotropy in exploration and development settings. Embracing such a paradigm is especially important for young geophysicists because, as our data-acquisition technologies improve, we will see more rather than less seismic anisotropy and, as our data processing and interpretation methods mature, we should be able to relate the estimated anisotropy to fluids in the rock and the sub-wavelength rock fabric more and more precisely. 

Don’t ignore seismic attenuation