Azimuthally varying velocity is probably the most significant property of the seismic data that affects the seismic interpretation. A stacking velocity that varies with azimuth creates a list of issues for the interpreter: Loss of frequency content in stacking Character degradation of events Laterally varying non-geologic amplitude changes in the stacked data Poor fault resolution AVO signatures that degrade near faults Amplitude and phase footprints in 3-D data and with 4-D comparisons Poor merge seams between data sets Incorrect azimuthal AVO analysis Poor interval velocity resolution Velocity function misties at the intersections of 2-D lines. And questions for the interpreter: Were the data migrated with the correct velocity field? Do the RMS velocities represent bulk rock velocities? (Can these velocities be used for acoustic inversion and depth conversion)??) Can workable land AVO measurements be correct? Can land AVO be sufficiently accurate to delineate reservoirs, fluids and fractures? Azimuthally varying velocity

Figure 1. Portion of a wide azimuth CMP gather sorted as a) by source-receiver offset and b) far offsets sorted by source-receiver azimuth from North. Figure 2. Portion of a wide azimuth CMP gather. The far offsets were stacked into 5° increment azimuth bins, which were then sorted by source-receiver azimuth. The difference between the fast and slow velocities is 450 ft/s.

Common shot or receiver gather: Basic quality assessment tools in field acquistion. When the traces of the gather come from a single shot and many receivers, it is called a common shot gather. A single receiver with many shots is called a common receiver gather. It is very easy to inspect traces in these displays for bad receivers or bad shots. Common midpoint gather, CMP: The stereotypical gather: traces are sorted by surface geometry to approximate a single reflection point in the earth. Data from several shots and receivers are combined into a single gather. Only shot–receiver geometry is required to construct this type of gather. Common depth point gather, CDP: A more sophisticated collection of traces that takes dipping reflector geometry other subsurface properties into account. CDPs can be stacked to produce a structure stack, and could be used for AVO work, though most authors recommend using image gathers or CIPs [see the update below for a description of CIPs]. A priori information about the subsurface, usually a velocity model, must be applied with the shot–receiver geometry in order to construct this type of gather. [This paragraph has been edited to reflect the update below]. Common offset gather, COFF: Used for basic quality control, because it approximates a structural section. Since all the traces are at the same offset, it is also sometimes used in AVO analysis; one can quickly inspect the approximate spatial extent of a candidate AVO anomaly. If the near offset trace is used for each shot, this is called a brute stack.AVO analysis

Variable azimuth gather: If the offset between source and receiver is constant, but the azimuth is varied, the gather can be used to study variations in travel-time anisotropy from the presence of elliptical stress fields or reservoir fracturing. The fast and slow traveltime directions can be mapped from the sinsoidal curve. It can also be used as a pre-stack data quality indicator.

Figure 8--(a) Seismic record acquired during a survey in Inman, Kansas, with (b) interpreted Rayleigh wave (purple), direct wave (yellow), refraction (blue), and reflection (red).

δδ or delta — the short offset effect — captures the relationship between the velocity required to flatten gathers (the NMO velocity) and the zero-offset average velocity as recorded by checkshots. It's easy to measure, but perhaps hard to understand in physical terms. or epsilon — the long offset effect — is, according to Thomsen himself: "the fractional difference between vertical and horizontal P velocities; i.e., it is the parameter usually referred to as 'the' anisotropy of a rock". Unfortunately, the horizontal velocity is rather hard to measure. γγ or gamma — the shear wave effect — relates, as rock physics meister Colin Sayers put it on Twitter, a horizontal shear wave with horizontal polarization to a vertical shear wave. He added, "γγ can be determined in a single well using sonic. So the correlation with and δδ is of great interest."put it on Twitter Geophysicists often assume that the earth is isotropic. This word comes from 'iso', meaning same, and 'tropikos', meaning something to do with turning. The idea is that isotropic materials look the same in all directions — they have no orientation, and we can make measurements in any direction and get the same result. Note that this is different from homogeneous, which is the quality of uniformity of composition. You can think of anisotropy as a directional (not just spatial) variation in homogeneity. The lower-left image shows a material that is homogeneous but anisotropic. The thin lines are supposed to indicate microfractures, say, or the alignment of clay flakes, or even just stress. So although the material has uniform composition, at least at this scale, it has an orientation. Weak elastic anisotropy. This paper introduced three parameters that we need—alongside the usual VPVP, VSVS, and ρρ—to describe anisotropy. They are δδ (delta), (epsilon), and γγ (gamma), collectively referred to as Thomsen's parameters. Thomsen's parameters

The other bit of jargon you will come across is the concept of transverse isotropy, which is a slightly perverse (to me) way of expressing the orientation of the anisotropy effect. In vertical transverse isotropy, the horizontal velocity is different from the vertical velocity. Think of flat-lying shales with gravity dominating the stress field. Usually, the velocity is faster along the beds than it is across the beds. This manifests as nonhyperbolic moveout in the far offsets, in particular a pull-up or 'hockey stick' effect in the gathers — the arrivals are unexpectedly early at long offsets. Clearly, this will also affect AVO analysis. nonhyperbolic moveoutthe gathersAVO analysis There's more jargon. If the rocks are dipping, we call it tilted transverse isotropy, or TTI. But if the anisotropies, so to speak, are oriented vertically — as with fractures, for example, or simply horizontal stress — then it's horizontal transverse isotropy, or HTI. This causes azimuthal (compass directional) travel-time variations. We can even venture into situations where we encounter orthorhombic anisotropy, as in the combined VTI/HTI model shown above. It's easy to imagine how these effects, if not accounted for in processing, can (and do!) result in suboptimal seismic images. Accounting for them is not easy though, and trying can do more harm than good. If you have handy rules of thumb of ways of conceptualizing anisotropy, I'd love to hear about them. Some time soon I want to write about thin-layer anisotropy, which is where this post was going until I got sidetracked...thin-layer anisotropy