BCGS Technical Talk – February 18, 2016
Speaker: Sergio Espinosa, S.E. Geosciences & Exploration
Title: Anisotropy Effects in the Geophysical Exploration and Characterization of Deposits
Date/Time: Thursday, February 18, 2016 @ 4:30pm
Location: 4th Floor Conference Room, Room 451, 409 Granville St. (UK Building at Granville and Hastings), Vancouver
Anisotropy Effects in the Geophysical Exploration and Characterization of Ore Deposits
Sergio Espinosa Ph.D., P.Geo, S.E.Geoscience & Exploration, Vancouver, Canada
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material’s physical or mechanical properties (wording taken from Wikipedia).Anisotropy occurs anywhere and anytime, and not only in nature. It could be described e.g. as someone walking in a city where streets are busy with traffic. How easy, or how difficult, will be for that person: to walk along the traffic on the sidewalk? or to cross the same busy street? His walking speed will definitely be reduced when trying to cross the street where traffic goes in one direction.
Mathematically, anisotropy can be quantified with tensor algebra and analysis. Anisotropy is described as a tensor where the elements represent one specific property changing in different directions. In Physics, anisotropy tensors are found e.g. in Thermodynamics, Continuum Mechanics (hydraulic and elastic waves), Magnetism, and Electromagnetics. For example, the deformation in a specific direction of elastic bodies, when subjected to a force with a different direction, is described in the the Law of Hook. The stiffness tensor describes the elastic properties of the material being deformed.
Also, materials are a lot of times electrically anisotropic. This means they have different electrical properties when current flows in different directions. For example, a crystal of graphite consists microscopically of a stack of sheets, and current flows very easily through each sheet, but moves much less easily from one sheet to the next. In an analogy, this was described above as someone trying to cross a busy street. The conductivity tensor in the Law of Ohm describes the direction-dependency of the electric properties of the material when being exposed to a current flowing in different directions.
In Geology, anisotropy can be observed in many scales: from crustal scales (fault systems and unconformities), to deposit scales (veining due to hydrothermal fracturing), and to microscopic scales, since most common rock-forming minerals are anisotropic, including quartz and feldspar.
In the exploration of oil and gas, geophysicists have been using regularly, and since a long time, the concept of anisotropy. Geological formations with distinct layers of sedimentary material can exhibit electrical anisotropy, meaning this that electrical conductivity in one direction (e.g. parallel to a layer), is different from that in another (e.g. perpendicular to a layer). That property, sometimes called channeling, is being used to identify hydrocarbon-bearing sands in sequences of sand and shale. Hydrocarbon-bearing sands have high resistivity, whereas shales have lower resistivity.
The geological scale where mineral explorers are mostly interested in is the deposit scale. A stock-work, for example, a geological expression being common in many ore deposit types such as in porphyries, is a complex system of structurally-controlled or randomly oriented veins, which are many times sulphide-bearing and very conductive. That vein system, allowing electrical current in one direction, but restricting it in another direction, should have a specific signature in the geophysical data acquired over the deposit.
Is anisotropy an effect that mineral geophysicists prefer to avoid? Could that effect help us find and characterize an ore deposit?