Commentary: Integrating geophysics and geology

There always has been, and still is, a problem integrating geophysics into geology.

Not enough geologists have a good understanding of what geophysics measures, how to understand the geological meaning of the measurements and how to use them in an effective exploration project.

On the other hand, geophysicists sometimes do not think or communicate in geological terms. Geophysicists may not understand the geological changes that are relevant to targeting the specific deposit, or will underestimate the complexity of the problem – relying instead on standard methods and unrealistic models. The result will be an ineffective geophysical survey, wasted money or, worse yet, a missed mine.

If the enthusiastic interest in Fugro’s recent “Geophysics for Geologists” and similar workshops is any indication, there is a need for more basic geophysical education for geologists, either at university or for continuing development after graduation, available from the geophysical industry or academia.

Geophysics is a powerful tool for exploration, when used for the right purpose at the right time in the process.

However, it can be compared to logging core in the dark. Imagine that you would have to rely on the hardness, density or smell of the core to determine the rock type. Each test tells you a little more about the rock. It might work if you can distinguish a limited number of important units in your deposit model by these parameters – diabase by the weight, sphalerite by the smell, etc.

Similarly, to use geophysics, explorers must make the connection between geology and the changes in rock property. Just as a geologist knows which minerals can be identified by crystal form, colour and hardness, he or she should know which rocks can be distinguished by magnetic susceptibility, electrical resistivity, density and radioelement concentration. 

Where the rock property information is not available, tests on hand samples or ground geophysical measurements, including previous work, can be used for estimates. Mineral property tables rarely report geophysical properties except density – but they should.

Going into the exploration project, explorers need to predict how the expected geological changes and structure will change the rock properties in a way that might be measurable. What will be the geophysical contrast between the target mineralization and the host geology? What will related alteration do to the host rock properties? Will contrast between layers allow mapping of structure? Once these questions are answered, we can determine which geophysical method and survey parameters can map the changes.

In some cases the target mineralization will have enough contrast and a big enough footprint to be directly detectable – like a volcanogenic massive sulphide deposit. Other times there are associated geological changes in the host geology, such as structural deformation or alteration that provide a much larger target.

Gold deposits are a common example where the target gold mineralization has no measurable geophysical contrast, but the associated alteration and sulphide mineralization may have a detectable contrast. However, the resistivity might be high (quartz-carbonate alteration) or low (sulphide, chlorite or sericite), and the magnetic anomaly is usually low (not high) owing to magnetite reduction. Gamma ray signature might show high potassium counts if there is potassium enrichment.

Mineral explorers can use geophysics to find these deposits, but must understand the mineralization style and understand it will affect the rock properties, both to choose the right exploration method and to interpret the results.

The target will often not be the biggest “bump,” or positive or even a bump at all. That is why it is necessary to understand the geophysical response to the rock property change that is expected, or observed, from the geological model.

Interpretation of the geophysical survey’s results should be a process of interpreting the data to determine the rock property changes that exist and, from that, predicting the geological changes that caused it.

The best interpretations are focused on geology: What geology caused this change in the geophysical data?

The results should be presented as geological descriptions. Not until both stages are complete – geophysics to rock property and rock property to geology – is the interpretation complete.

It is also important to recognize the non-target geological changes that can create geophysical changes that look the same, at least to one method. Multiple methods can be used to measure multiple properties at the same time, each giving a different look at the geological changes they detect.

Interpretation should integrate all the geophysical results and the geological knowledge into a single, cohesive, interpretation of the geology. Where the geophysical results disagree, for example on the apparent location of a contact, it is not necessarily indicating something wrong with the data, but rather the multiple measurements are telling you something more about the geology – like looking at pictures of a single object from different angles. 

Effective use of geophysics is a matter of understanding the effect of geological processes on rock properties, and hence the effect on the geophysical data. It requires combined interpretation of the geophysical data into a single geological description, integrating the available geological information.

Geologists need to understand how changes affect rock properties, and geophysicists need to think in geological terms when interpreting their data.

– Based in Toronto, the author is a professional geoscientist and chief geophysicist at Fugro Airborne Surveys with over 30 years of experience across a broad range of geophysical methods. He can be reached at GHodges@fugroairborne.com. For more information, visit www.fugroairborne.com.