If the results of seismic anisotropy can describe the type rocks within earth, so it would be possible make a relation between seismic anisotropy and mineralogy in different depthes.
I specialize in geochemistry, but I think the following paper will be useful
Data Seismic methods in mineral exploration and mine planning: A ...
Best Regards Javan Doloei
I specialize in geochemistry, but I think the following paper will be useful
Data Seismic methods in mineral exploration and mine planning: A ...
Best Regards Javan Doloei
To be honest the only way to know the lithology (and not mineralogy) by seismic is by applying lithocube or genetic inversion calibrated to actual well or outcrop data. Reaching to the mineralogical scale require specific measurements, such as elemental capture spectroscopy (ECS) and if outcrop samples are present for mining a hand held XRD-XRF tool will do the trick. But seismic is a big word, there is 2D, 3D, borehole seismic, VSP, AVO ...... and mostly your resolution is low. The bottom line if you are looking for mineralogy use the designated equipment for it better and run your seismic and then you are really see if you have a link or not.
Dear Javan:
Whatever we have discussed about seismic inversion studies, we assume that the subsurface rock formation is isotropic, and therefore, from a seismic point of view, acoustic velocity is the same in all directions. Often this assumption is not true. Subsurface formation could exhibit anisotropy of different forms.
It is therefore important to study the different types of anisotropy encountered in subsurface formations, their effect on seismic data and then determine how it would affect AVO analysis.
Anisotropic material has physical properties that depend on direction. For example, shale anisotropic and heterogeneous medium if they are formed by layers that were laid out at different times; in carbonates, anisotropy is controlled by fractures and diagenesis. Anisotropy describes variations of physical properties with direction at a given point of the medium.
Also, anisotropy may also be stated as variation of a vector measurement of a rock property with direction. In seismic, the vector is the velocity, on the plane theoretical surface for 2-D and on the volume in 3-D seismic volume.
Let me say you that when we talk about seismic anisotropy we need to mention important issues:
1- The scale. Anisotropy of rocks can be detected only when the wavelength of the waves passing through them is larger than the ordering of elements creating the anisotropy. It means: a stack of layers or horizons in seismic, each of which is homogeneous and isotropic, can be heterogeneous and anisotropic when considered as a single unit.
2- Anisotropy is expected to affect reflectivity changes with offset (AVO effect)
3- Seismic surveys that you use for anisotropy studies must be acquired with a wide – azimuths and long source-receiver offsets.
4- You must process the gathers in azimuths-set, e.g., 4 or 6 data set.
5- The math needed to describe anisotropic wave phenomena is not trivial, therefore, anisotropic velocity model requires determination of several independent elastic coefficients, which can be quite complicated.
6- Anisotropy is a weak value, based on ultrasonic velocity measurements in lab and seismic field data (Thomsen, 1986).
7- Wright (1987) reasoned that since anisotropy means velocity dependence on direction, one could expect that such angular dependence of velocity would influence reflectivity changes with offset. For example: isotropic shale, where shale velocity is lower than sand velocity, the PP reflection coefficient increases with offset, and where the anisotropic shale has horizontal velocity 20 higher than vertical P-velocity, the PP reflection coefficient decrease with offset.
8- The AVO effect from anisotropic influences, is seen to increase with angle of incidence and with the reflectivity at zero offset.
9- The normal hyperbolic moveout NMO correction shows a hockey-stick effect in the gathers in anisotropic media. Therefore, application of higher order moveout (the Eta corrections) corrects the hockey-stick effect. I have examples of this phenomena in the Vaca Muerta Fm, Argentina.
As you can imagine, with all the above considerations, the relationship between anisotropy and mineralogy in subsurface is a hard task for Geophysicists, even in case such as the Neuquina basin, Argentina, where the Vaca Muerta Fm. has a very very good thickness related with vertical seismic resolution.
On the other hand: if you have a gather - set of information and you confirm the existence of anisotropy´s effect, you may think about shale / carbonates facies.
References:
Azimuthally Dependent Anisotropic Velocity Model Update, Koren and Ravve, Geophysics, v. 79, p. C27-C53
Thomsen, L., 1986, Weak elastic anisotropy: Geophysics, 51, No. 10, 1954-1966
Wright, J., 1987, The effects of transverse isotropy on reflection amplitude versus offset: Geophysics, 52, 564–567.
My best wishes, Mario E. Sigismondi
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