anisotropy effects in geophysical data

Anisotropy Effects in Geophysical Data

Sergio Espinosa, Ph.D., P.Geo Principal Consulting Geophysicist S E G eoscience and Ex ploration

Sergio Espinosa, Ph.D., P.Geo Principal Consulting Geophysicist; S E G eoscience and Ex ploration SEG.and.Exploration@gmail.com

Presented at BC Geophysical Society. Thursday February 18th, 2016

Objective of this Talk- Create awareness among mineral explorers.

- Start a discussion group.

- Answer following questions:

- (1) How can we integrate anisotropy in inversion calculations?

- (2) How can we use those effects to discover, delineate, and characterize ore deposits?

What is Anisotropy?

- let’s google !!!

Wikipedia:- 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 (absorbance, refractive index, conductivity, tensile strength, etc). An example of anisotropy is the light coming through a polarizer. Another is wood, which is easier to split along its grain than against it.

Encyclopaedia Britannica:

- Anisotropy, in physics, the quality of exhibiting properties with different values when measured along axes in different directions. Anisotropy is most easily observed in single crystals of solid elements or compounds, in which atoms, ions, or molecules are arranged in regular lattices. In contrast, the random distribution of particles in liquids, and especially in gases, causes them rarely, if ever, to be anisotropic.

Encyclopaedia Britannica:

- … A familiar example of anisotropy is the difference in the speed of light along different axes of crystals of the mineral calcite. Another example is the electrical resistivity of selenium, which is high in one direction but low in the other; when an alternating current is applied to this material, it is transmitted in only one direction (rectified), thus becoming a direct current.

Motivation for this Talk

- let’s rewind a bit !!!

Freiberg (Germany)

- 1988: B.Sc. in Applied Geophysics in Engineering * material sciences: concepts of homogeneity and isotropy

- 1989: M.Sc. in Geothermal * thermal flow in different directions

- 1992: Ph.D. in Hydro-fracs and Micro-seismicity * using S-wave splitting, quantification of anisotropic degree of rock mass after hydraulic fracturing event

Berlin and Potsdam (Germany)

- 1993-1995: Postdoc and research in Seismology and Earthquake Hazards

* velocity anisotropies in the Andes

Examples of Anisotropy:

- In daily life !!!

thinking about …

Our thoughts are most of the time

anisotropic

going to the PDAC?

or going to the beach?

thinking about …

thinking about electric currents and

other flows

or thinking about Radiometrics?

thinking about electric currents and

other flows

different speed when walking in different

directions

different speed when walking in different

directions

Examples of Anisotropy in the natural world:

- In Mathematics !!!

Scalars …

- are real numbers defined by a point in space (or in time)

e.g. temperature Talso: mass density Rho-m

Vectors …

- are defined by two points in space (or in time)

e.g. velocity v = ( v1v2

)or any other vector field

- those two points define magnitude and direction

Tensors …

- are defined by at least two vectors in space

e.g. material property P = ( P11P21

P12P22

- or more commonly by three vectors in space

P = (P11 P12

P21 P22 P23P31 P32P33

- a tensor is anisotropic when all nine elements have different values

Tensors …

- (furthermore)… describe relations between scalars (tensors 0th order), vectors (tensors 1th order), and other tensors (2nd order and more).

excitation

- can describe e.g. relations between excitations and responses

Tensors …

- In Physics !!!

Examples of Anisotropy in the natural world:

Thermodynamics:

- q = - k * grad T

- T: temperature

- q: heat flow

- k: thermal conductivity coefficient

- Law of Fourier

Continuum Mechanics:

- S = - k * d

- d: displacement

- S: applied stress

- k: elasticity coefficient

- Law of Hook

Magnetics (Maxwell):

- M = - k * H

- H: magnetizing vector field

- M: magnetized vector field

- k: magnetic susceptibility

- Law of Magnetization

Electrical (Maxwell):

- j = - k * E

- E: Electric vector field

- j: Current density vector field

- k: electric conductivity

- Law of Ohm

Physical property of materials…- the tensor K might be thermal conductivity coefficient, elasticity

coefficient, magnetic susceptibility, electric conductivity… whatever it is, it is a material property, it is a rock property, it is a physical rock property

K = (k11 k12

k21 k22 k23k31 k32 k33

- in this example of a tensor of 2nd order, a material is anisotropic when all 9 elements have different values

Scale-invariance of Anisotropy:

- In Astrophysics - In Regional Geology - In Mineralogy and Microscopy - In the Exploration of Oil / Gas - Why not in Geophysical Exploration of Ore Deposits?

In Astrophysics and Cosmology:

- Cosmic microwave background radiation caused by primitive light of the early universe

In Regional Geology:

- Any topographic map is an example of anisotropy, since water will flow in preferred (or most probable) directions.

In Mineralogy:

In Oil & Gas:- Although a lot of

times oil and gas reservoirs can be modelled in 1D (layered Earth), anisotropy effects play an important role in geophysical data.

In Ore Deposits:- Ore deposits are

geometrically very complex and are normally modelled in 3D. However, mineral / mining geophysicists do not normally talk about anisotropy of ore deposits.

In Ore Deposits:

- The Bingham Canyon Porphyry Cu-Mo-Au Deposit

In Ore Deposits:

- Porphyry stockwork veins in sericitized monzonite (Northparkes, Australia)

- visiting the core shack at Roundup 2016

2/17/2016 Magnetic anisotropy and remanent magnetism in hemo-ilmenite from ore deposits at Allard Lake, Quebec - Hargraves - 1959 - Journal of Geophysical Researc…

http://onlinelibrary.wiley.com/doi/10.1029/JZ064i010p01565/abstract 1/3

Go to old article view

Journal of Geophysical Research Explore this journal >

Magnetic anisotropy and remanent magnetism in hemoilmenite fromore deposits at Allard Lake, Quebec

First published:

October 1959 Full publication history

10.1029/JZ064i010p01565 View/save citation

Cited by:

54 articles Refresh Citing literature

Robert B. Hargraves

Abstract

The anisotropism of magnetic susceptibility of nearly pure hemoilmenite ore from deposits in the

Allard Lake area consists of a distinct plane of maximum susceptibility (defined by maximum and

intermediate axes along which the susceptibility is of similar magnitude) with a minmum susceptbility

axis at right angles. This plane coincides with a preferred crystallographic grainorientation indicated

by the parallelism of titanhematite lamellae which have exsolved on the basal plane of the

ferrianilmenite host grains. Measurement of the remanent magnetism (RM) shows a striking tendency

for the RM vectors to lie in or near this preferred plane. On the assumption that hematite (if not

ilmenite as well) is potentially ferromagnetic in the basal plane only (paramagnetic parallel to the caxis), it was inferred that the RM vectors could represent the resolved component of the magnetizing

field which tended to lie in the plane of maximum susceptibility. Under this hypothesis, it is possible to

resolve the broad spread shown by the RM vectors of samples from one deposit in terms of a single

Volume 64, Issue 10

October 1959

Pages 1565–1578

View issue TOC

- we are definitely not discovering the wheel !!!

- Do we see the reality (show) of geological anisotropy in our geophysical data?

Questions:- (1) How can we integrate anisotropy in inversion

calculations?

- (2) Should we add anisotropy as a standard physical rock property?

- (3) How can we use those effects to discover, delineate, and characterize ore deposits?

What do we need?:- We need a geophysical flow (e.g. electrical, EM wave,

etc) illuminating the subsurface in all possible directions (we already do it).

- We need to measure responses in all possible directions (we also do it a lot of times).

- We need to think “out of the box”. Our physical rock properties are not scalars, they are tensors.

Proposal:

- Let’s start a brain storming group and let’s invite economic and structural geologists to these discussions.

Thanks !!!

anisotropy effects in geophysical data

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