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?

Since the early 90’s, SJ Geophysics has been continually developing and experimenting with unconventional IP array designs. Current customized arrays utilized with the Volterra acquisition system make use of cross-line dipoles to improve the azimuthal distribution of the data. A key aspect of any IP survey, the selected array design must meet the survey objectives determined at the start of the project.In the mineral exploration industry, survey design rarely gets the time and attention that it deserves. Rather, surveys are acquired by essentially “shooting in the dark” using last week’s parameters in the hopes that the target of interest will become illuminated and the picture more clear. Unfortunately, this approach simply doesn’t work. A well defined survey objective and a-priori geologic information must be taken into consideration during the survey evaluation and design process in order to get the best results.

All to often the survey objectives are overlooked by the potential client when requesting a survey and replaced with “How deep can I see?” Their hope is to have an answer in 30 seconds. In trying to answer this question for clients, the importance of good array design becomes important.

Simple 2D-inline arrays have evolved into complex customizable arrays such as the diamond array that incorporate dipoles in multiple directions to maximize signal coupling and improve surface resolution.  An examination of receiver arrays and how they affect the resulting data collected will be discussed. Data examples from real world surveys will be provided to illustrate the benefits of 3DIP array designs. A key example is the Ootsa Property, owned by Gold Reach Resources. In 2013, SJ Geophysics took the initiative to re-survey the Seel deposit to compare their new 3DIP acquisition equipment and survey methods against an older IP survey acquired using conventional 2DIP and early 3DIP systems. The benefits of advanced 3D array designs is evident in the resulting inversion models.

The Collahuasi Mine is located at 4,800 meters above sea level on the Andean Plateau in the Tarapaca Region of northern Chile. Commercial activity at the mine dates back to 1880 when its systems of high-grade copper and silver veins began to be exploited. These operations continued for fifty years until their interruption by the Great Depression. Work in the area resumed in 1978 when key components of the Rosario deposit were identified. Although the layout of many of the abandoned tunnels and stopes are relatively well documented by Collahuasi, confirmatory drilling showed that the information from historical records is neither exhaustive nor always complete (the result of lost or forgotten records). Because of the transit of heavy machinery, these conditions are a cause of concern for the continued safe conduct of today’s operations. In addition, these cavities must be taken into account when a mine block is designed for blasting/exploitation, since they may noticeably decrease the ore grade of the block.

Over the course of the past 3 years, Golder Associates and Collahuasi have worked closely together to developed a quick and reliable method for detecting potential underground voids/abandoned tunnels at the site. The method uses high resolution 3D electrical resistivity imaging (ERI), microgravity and GPR, in combination with systematic confirmatory drilling of anomalies to achieve a rate of success of nearly 70% in voids detect.

As a separate topic, if time allows, we will also look at the results of a non-conventional approach used at Collahuasi for blast damage characterization of the rock mass using borehole geophysical techniques.

Jean-Philippe Mercier is co-owner of Advanced GeoScience Imaging Solutions (AGSIS) and until recently was the Golder lead seismologist. He currently focuses primarily on mine seismology in the context of deep and high stress mining. Jean-Philippe’s main interest consists in developing processing techniques and monitoring approaches to exploit the untapped potential of seismological analysis in extracting information on the spatial and temporal response of the rock mass to mining. Dr Mercier earned a Ph.D. from UBC in crustal seismology and has co-authored multiple papers published in peer-reviewed journal and conference proceedings. He is the principal force behind, and developer of, the techniques presented today.