Geophysics

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This article was written by Snowden's Principal Consultant, Jeremy Peters.


Geophysics refers to the measurement of some property of the earth: gravitational field, magnetic field, radioactivity, ability to transmit vibration and electricity, amongst others. There’s a myriad of geophysical techniques to confuse the investor, each with different applications.

It’s important to remember that none of these techniques tells the geologist what's in the ground but they may identify a potentially good place to drill. The geologist or interpreting geophysicist is usually looking to identify potentially mineralised structures in the rock.

All geophysical techniques are subject to the interpretation of the geophysicist. Several times I've drilled a geophysical target at one orientation for no result, only to hit something at another orientation using an alternative interpretation. Other times, what has appeared to be a strong conductor of electricity, possibly massive sulphides, has turned out to be a conductive layer, such as graphite or carbonates. There may well be sulphides in the hole, but these are barren pyrite, not the masses of pentlandite or chalcopyrite that the geologist (and investors) hoped for.

Success requires the best data that technology can acquire and a persistent, competent geophysicist with an open mind, prepared to reinterpret the data as new evidence is secured.

Common techniques

Magnetics: an important airborne or ground-based family of techniques that measures the natural magnetic field of the rock. Most rock contains varying quantities of magnetic minerals. Faults and other structures destroy these minerals and appear as magnetic lows. These techniques are easy to interpret and are very useful for seeing through cover and identifying potentially mineralised structures.

Electromagnetics (EM): another popular family of techniques that can be ground-based or airborne. The concept is similar to IP – a loop is used to induce an electromagnetic field in the ground and another loop measures the characteristic of the decay, frequency, magnetic susceptibility and other characteristics of the field. This is useful for many styles of mineralisation because it can cover a large area quickly and yields information about numerous rock types and their structure (faults, fractures, folds).

Induced Polarisation (IP): a popular, more sophisticated subset of EM techniques, typically applied to high-sulphide targets. Current is passed into the ground and a voltage decay or polarisation measured across an electrode array.  Useful for homogeneous and finely disseminated sulphide orebodies, including porphyry-related deposits.

Magnetotellurics (MT): is an EM variant that relies on natural variations in the earth’s magnetic field to induce fields in the ground. This technique is not always easy to interpret but is valued for its ability to penetrate deep beneath the surface. As near surface deposits become harder to find, explorers increasingly rely on relatively low-cost MT techniques to help identify mineralisation at depth.

Resistivity or mise-la-masse: a family of techniques, often used post-discovery, that simply pass a current through the ground and measure conductivity. These techniques can be applied to massive sulphide targets, which are typically more conductive than unmineralised rock. A simple way for a geologist to test the suitability of a rock for resistivity survey is to take a sample and test its resistivity with a multimeter.

Radiometrics: measures the naturally occurring radiation associated with (usually) uranium, potassium and thorium minerals. This is useful in identifying lithology and is invaluable for mapping.

Gravity:  measures variations in the earth’s gravitational field and is useful for large-scale or deep identification of lithology and intrusives. Historically confined to the petroleum sector as a reconnaissance tool in sedimentary basins, but increasingly used by the mineral sector to identify dense intrusive-related deposits such as iron oxide copper gold deposits (IOCG).

Seismic: measures the behaviour of either naturally occurring or induced vibration in the earth’s crust. Ubiquitous in hydrocarbon exploration, is increasingly used in mineral exploration to identify lithology and structure, particularly at a large scale or at depth.

In my view, the modern explorer will not be successful without appropriate geophysical data, competently interpreted. The cost of geophysics is highly variable, depending on the technique and location, but it is prudent to budget around $0.15M for meaningful preliminary geophysical prospecting and analysis of a previously unexplored prosepct in remote Australia. My experience is that this investment repays itself within the first drilling campaign by reducing the number of metres required while better targeting those metres that are drilled.


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