Geophysical Magnetic Survey Visualization products

What are all these maps telling us?

Geophysical surveys are, and will continue to be, a data product that is critical to success in mineral exploration. Unfortunately, finding or acquiring the data is the easy part. Geophysical surveys must be correctly interpreted to be effective exploration tools. Commonly new surveys will be delivered with some interpretive notes by a qualified geophysicist however, this is a luxury geologists do not always have. Often in our industry, mineral exploration is informed by compilations of historical or open-source data not accompanied by qualified interpretations.

Beginning my career, I found myself woefully uneducated in the practical applications of exploration geophysics. I had learned a lot of math and understood the fundamentals of the earth’s magnetic field but that was not enough to understand or appreciate what the magnetic map on the wall of my office was telling me. It was only through strong mentorship and dedicated professional development efforts that I have achieved a level of aptitude interpreting geophysical surveys that I can be proud of. In discussions with my peers, I have found that my experience has not been unique. For this reason, I wanted to share some brief notes about how common magnetic survey products affect the display of the survey and how you can better understand what your maps are telling you.

I am an exploration geologist, not a geophysicist. Therefore, this article is not intended to deliver an expert level understanding of magnetic geophysical surveys and their data products. My hope is to present a brief and fundamental review of common magnetic data processing products to provide young and/or inexperienced geologists with a new or enhanced perspective.

The Basics

All geophysical surveys are measurements of some physical property variation within the survey area. It is important to note that the data gathered is an imperfect representation of the measured property variation. Magnetic surveys detect variations in magnetism. Magnetism is a vector quantity which has a magnitude and direction. For this reason and a few others there is a complicated relationship between sources and responses which is further influenced by the global location of the survey. The data quality of a magnetic survey can be affected by both environmental and methodological factors. These environmental factors can be geological and non-geological. Among other things, non-geological environmental factors include cultural items like powerlines, pipelines, etc. For this reason, it is prudent to investigate the area for such things prior to interpretation. Of course, the degree of influence will be a function of scale.

The intensity of a feature on a magnetic survey map is referred to as the amplitude. The display of these intensities is both relative and subjective. Something represented by the colour pink on one map does not mean that a feature is the same magnetic intensity as another pink feature on a map of a different survey type, even in the same survey area. In addition, features of the same magnetic intensity may not display the same amplitude as each other since their respective intensities are a function of depth as well as size and shape of the source, among other things. This brings me to wavelength. The wavelength of a feature affects the sharpness of its display. As the separation between source and detector increases, so does the wavelength of the response. Therefore, deeper features appear in some visualization products as somewhat amorphous or low resolution 

TMI - Total Magnetic Intensity

For all intents and purposes this is as close to a representation of the raw data as one can achieve. As intensities are relative to the dataset the highest amplitude features in the map will often wash out or disguise the detail in lower amplitude regions of the map. This is exaggerated if the spread between the highest and lowest intensities is great. Because magnetics has a magnitude and a direction the response of features on a TMI map is not actually directly over the source unless the surveys were taken directly at the North or South magnetic poles. In the Northern hemisphere the response is shifted to the south of the source and in the Southern hemisphere the response is shifted to the north of the source. This is a critically important thing to know if one is planning exploration drill holes from a TMI map which is not reduced to pole.

RTP- Reduced To Pole

RTP processing transforms the data to create a direct vertical relationship between source and response, as though the survey was conducted at the magnetic pole in your hemisphere. This is processing that can be applied to TMI imagery as well as all other data visualization products. This specifically corrects the issue North and South shifted responses identified in raw TMI imagery.

VD - Vertical Derivative

Vertical derivatives transform the data to appear as though the survey was taken at a greater source – detector separation by comparing data from multiple elevations (upward continuations) to calculate gradients. This reduces “noise” by filtering out long wavelength features. The resulting image, post transformation, emphasizes the short wavelength, near surface features.

HG - Hotizontal Gradient

This process converts the display to represent the slope of the response, or the change in magnetic intensity over distance. The signals are therefore centered over the margins of magnetic features. For this reason, it is an excellent determinant of where the edges of a specific feature truly are. The intensity of the response reflects the rate of change, not the magnitude of the magnetism of the feature

AS - Analytical Signal

The analytic signal is technically defined as the square root of the sum of the squares of the vertical and the two horizontal derivatives of the total magnetic field. But this is not a math textbook so what does that mean to you when you have a map in front of you? This method produces a signal vertically above the magnetic contacts creating an “edge detector” comparable to the horizontal derivative; however, the apparent intensity or amplitude of the analytical signal response is directly related to the strength of the magnetism of the source as opposed to the rate of change over distance as in horizontal derivative processing.

Tilt

Tilt derivative processing simplifies the relationships between sources and signals by creating a positive response over magnetic sources and a negative response elsewhere. This effectively levels the amplitudes of all magnetic features. This is an effective data product for structural interpretation as it enhances the visibility of low amplitude features which would have been not well resolved in a TMI or TMI-RTP map. The most concise way to describe Tilt is that it is an amplitude equalizer.

Conclusion

Magnetic geophysical surveys are almost a ubiquitous feature of all mineral exploration projects in Canada. Even if there is no property specific survey, provincial governments across the country have conducted large airborne surveys resulting in troves of publicly available data. One of the most compelling things about magnetic surveys is their usefulness as a tool for mapping structure and pseudo geology. I say pseudo geology because one can only map units of similar magnetic character which may or may not be lithologically identical. If you are inexperienced in the interpretation of magnetic survey data it is my hope that this article will allow you to look at the mag map on the wall of your office a little differently, as well as afford you a new appreciation of what can be gained from a thoughtful and qualified interpretation of your survey results.

David Murray P.Geo

President and Principal Consultant

Resourceful Geoscience Solutions Inc.