Geological interpretation of geophysical models

This might seem obvious but one thing I have learned about geophysics over the years is that all of the different geophysical methods only tell you about the specific physical property of the rock mass that are being measured by that method. For example, if you are measuring magnetism, if the rock is not magnetic in any way (normal or reverse) it is in a sense, opaque to the method. Similarly, if a rock is not conductive, electrical geophysics will be limited to telling you the rock is not conductive. Variations in seismic wave speed could be related to any number of factors, structural, lithological or mineralogical so all that a velocity model tells you is the speed of seismic waves in any given location.

Leão-Santos et al. 2022. Total magnetic intensity of Carajás Mineral Province.

It's almost so simple that it goes without saying but still, it’s worth saying. Why? Because we geologists need to remember this when we interpret geophysical data. There might well be a fault in the rocks or a porphyry intrusion or some other feature but if there is no contrast in physical properties between that feature and the surrounding rocks, it won’t be picked up.

Interpretation involves determining the geologic significance of geophysical data and generally integrates all available geologic and geophysical information.

SEG, "Fundamentals of Geophysical Interpretation"

The most powerful way of using geophysics is to combine multiple data sets. You have to consider what each is telling you about the rocks in any given location.

In an example I came across recently, the seismic velocity went from high to low across an area, but the magnetic signature was unchanged. It forced me to think about what process could preserve magnetic minerals yet destroy or reduce the seismic velocity. Brittle fracturing of the rock would be a likely cause, so I'm planning to look at the drill logs to see if the geologists on the project have noted poor rock quality caused by fracturing. Let's see what we find.

This approach of using multiple data sets to tackle a problem is of the reasons geology is such a fascinating science. It is also one of the reasons why we need people with good lateral thinking skills who can see connections across data different data types. Geoscientists will take multiple datasets and synthesise them to produce a conceptual model for an area. With well placed geophysical data we can take geological observations and link them across three dimensions in order to understand the subsurface.

Seismic velocity structure and interpretation of the upper subduction zone of Tocopilla, north Chile. Contreras-Reyes, 2018