Service: Find where magnetization history is influencing your data
IN MANY cases you can successfully assume away the remanent magnetization of rocks in your exploration area of interest. But if strong remanence rears its head, run-of-the-mill magnetic survey interpretations become unreliable. We can detect where that's likely to be the case.
A magnetic survey acquires data on the total magnetic field present at spots along survey lines in the area of interest.
The total magnetic field at any given spot is a combination of a few contributing sources. Those contributors are (1) the earth's magnetic field at that spot, (2) the magnetic field induced in magnetically susceptible rocks at that spot by the earth's field, (3) any remanent magnetism in the rocks at that spot, and (4) other magnetic fields such as those stemming from the sun, the earth's atmosphere, and architectural infrastructure.
These contributions combine directionally through vector addition to produce the observed total magnetic field. Usually the fourth contributor mentioned above, 'other' magnetic fields, are not as significant as the other three sources (solar and atmospheric influences are corrected before magnetic data are process and interpreted).
Remanent magnetism is the net magnetization present in a mineral or lithology when no external magnetic field is present. Remanence is sometimes described as a material's magnetic memory, because it represents the sum of the material's magnetization history.
Processes contributing to the remanence seen in a 'remanence-susceptible' material include:
- cooling from an initial temperature that exceeds the Curie (magnetic lock-in) point in the presence of an external magnetic field (e.g., the cooling of a magmatic body)
- chemical changes in the presence of an external field (e.g., the crystallization of magnetite ore in a banded iron formation)
- deposition of grains in water in the presence of an external field
Remanence seen in the mineral exploration context is typically due to first process listed above, called thermo-remanent magnetization.
The minerals that typically are abundant enough to be responsible for the induced magnetism or the remanent magnetism, or both, of their parent rocks are the iron oxide magnetite, the iron sulfide pyrrhotite and the iron-titanium oxide ilmenite.
Magnetite and pyrrhotite are ferrimagnetic. The crystal structures of magnetite and pyrrotite are arranged in a way that produces a net magnetic moment, which means they can produce their own magnetic field. Therefore, one property of a ferrimagnetic material is that it can be remanently magnetized.
Ilmenite's crystal structure is antiferromagnetic, which means induced magnetism is possible via its magnetic susceptibility. The crystal structure has no net magnetic moment, so there is no possibility of the pure mineral producing its own magnetic field. 
Interestingly, however, microscopic blade-shaped growths of ilmenite in hematite can become strongly and stably remanent. This is an example of remanence arising from chemical changes.
Under the right conditions, the imenite blades, also called lamellae, collectively possess a net magnetic moment. The intensity of this magnetism is proportional to the density of lamellae and contact layers in the sample, according to work by Geological Survey of Norway researcher Suzanne McEnroe and her collegues. The poster they presented at an European Space Agency science meeting in Germany last year provides a good summary of their research. 
Their findings support the concept of lamellar magnetization — magnetization arising at chemical interfaces inside mineral grains hosting exsolution textures.
McEnroe and her co-workers said in their ESA meeting poster that they had studied in the past decade numerous localities hosting remanence-controlled anomalies. The localities were in Scandinavia and North America.
"Temperature-pressure experiments at 10 gigapascals and 580 degrees Celsius showed that lamellae in the ilmenite-hematite system are stable, and not resorbed as would be inferred from the 1 atmosphere phase diagram, thus indicating that lamellar magnetism of considerable strength may persist deep in the crust," they said.
"We have found that lamellar magnetism ... is a common source for remanent magnetic anomalies."
Note that Fathom Geophysics can assess the likelihood that remanent magnetization will affect the interpretation of magnetic data. This is useful because it indicates whether magnetic lows in the data are in fact due to non-magnetism, or whether they are remanently magnetic. See Figure 1 and Figure 2, which show a likelihood assessment of remanence issues in a synthetic dataset.
FIGURE 1: The location of bodies (white outlines) in this synthetic dataset, superimposed over color-scaled total magnetic intensity (TMI). The bodies have varying depth, varying magnetic susceptibility and varying remanence. Note that because remanence is a directional property, it has inclination and azimuth components (both of which we have varied in this synthetic data).
FIGURE 2: Results of the Fathom Geophysics likelihood assessment for remanence. The TMI data has been reduced to the pole, with anomalies detected outlined in black. The anomalies have been shaded according to the likelihood that remanent magnetism will affect interpretation of the data. A white anomaly indicates the strongest likelihood that remanence will be an issue, while a black anomaly indicates the weakest likelihood. Anomalies with varying shades of gray indicate intermediate degrees of likelihood that remanence will affect interpretation. Note that the central diamond-shaped body in the synthetic data is (expectedly) not picked up as an anomaly. This is because the body was defined as having relatively low magnetic susceptibility and zero remanence.
It's possible to process and interpret magnetic data in areas where remanent magnetization is a significant contributor to the area's magnetic signature.
The synthetic data we use here is a stand-in for airborne magnetic data over an area where remanence is a significant contributor to the local magnetic signature.
By and large, most processing and interpretation of magnetic data within the minerals exploration industry is done with an ostrich-like approach to remanence. Essentially the approach is equivalent to just wishing remanence wasn't there.
It involves the simplifying assumption that (1) either no remanence is present at all to toy with the area of interest's magnetic signature, which is induced in the surrounding magnetically-susceptible minerals by the presence of the earth's magnetic field, or (2) that any remanence present always has the same direction as the magnetic signature and therefore is of no real consequence.
Under this simplifying assumption, the reduction to the pole (RTP) process** is carried out on total magnetic intensity (TMI) data using the same magnetic inclination and declination across the whole area of interest.
Taking this route is convenient because methods of working around remanence in data can be computationally intensive — a problem for explorers who are time-constrained or dealing with very large datasets, or both.
While assuming remanence away may be an appropriate thing to do when remanence is in fact absent or insignificant, unfortunately this isn't always the case.
For instance, as we mentioned earlier, findings by Geological Survey of Norway researcher Suzanne McEnroe and her collegues suggest that ilmenite-hematite exsolution textures — which cause remanent magnetism to arise — may appear relatively commonly in rocks.  The presence of those textures may therefore mean that magnetic anomalies are more frequently affected by remanence than previously thought. Over the years, other researchers have also noted that other mineralogical sources of remanence may be more significant and more prevalent than they hadd seemed at first blush. 
As we mentioned, remanent magnetism is the net magnetization present in a mineral or lithology when NO external magnetic field is present.
To cut a long story short, a remanently magnetized material has its internal magnetic domains lined up in the same direction. A strongly remanent rock possesses 'locked-in' domains that resist alignment induced by an external magnetic field.
The Koenigsberger Ratio, a measure of remanence, is the ratio of remanent magnetization to induced magnetization for a given material. A relatively high figure for this ratio indicates that a material's remanence is locked in to a relatively greater degree than other materials.
The problem with remanence
The effect of remanence on your reduced-to-the-pole (RTP) magnetic data is something of a continuum, going from being barely noticeable through to making your data behave in a fashion that is the total opposite of what it should look like.
Generally speaking, in the absence of remanence, a known vertically-plunging magnetic body should generate an anomaly in the RTP magnetic data that has a peak perfectly coinciding with the body's mapped location. If there is any offset between the peak and the mapped location, you've got some degree of remanence present. The greater the offset, the greater the significance of the remanence.
In the worst cases of remanence, this offset is so extreme that instead of seeing a peak in the RTP data over the mapped location of your known magnetic body, you're staring at a 'trough' — an anomalous low. In these instances, you know you're dealing with not only strong remanence, but also remanence possessing a direction that's very different to the local magnetic signature.
Extreme remanence effects on RTP data are relatively easy to spot — look for deep RTP lows that are peaks at the same locations in the analytic signal (also known as total gradient) map.
The presence of less extreme remanence may be more problematic for explorers, by going unnoticed and damping the important signals explorers are searching for in their magnetic data. 
However, the good news is it's possible to account for all of the possible degrees of remanence, from the subtle through to the severe, during data processing. Even better news is that to do this there's no need to measure remanence directly. 
The synthetic data
Here we attempt to come up with an acceptable estimate of total magnetic field for each of the anomalies present across a synthetic dataset (the same one we used in our previous article on remanence).
In other words, we do not assume remanence is absent throughout the dataset. Nor do we assume, if remanence is present, that it is inconsequential.
The implication of rejecting these assumptions is that we cannot assume that one combination of magnetic inclination and declination will suffice when performation a reduction to the pole.
The general procedure we used is as follows:
- Obtain the total magnetic intensity (TMI) data for the area.
- Locate the magnetic anomalies in the area. 
- Run multiple reductions to the pole on the TMI data, using a different inclination/declination pair combination each time.
- Produce, from the multiple RTP magnetic datasets created in the previous step, a series of first-vertical-derivative (1VD) maps.
- Produce the analytic signal (AS) map from the half-RTP (i.e., perform an RTP in which only the earth's ambient field is taken into account).
- For the first anomaly in the area, identify the AS-1VD combination of maps that match each other the best (i.e., the pair in which the anomaly peak shows the least offset). The inclination/declination combination used to produce this map combination is assigned to this particular anomaly as its total magnetic field.
- The previous step is repeated for each of the remaining anomalies in the area.
Figure 1 shows the dataset we used, and Figure 2 shows the location of the bodies generating the anomalies we detected. Figure 3 shows the boundary of each anomaly we extracted from the data.
FIGURE 3: Color-scaled map of total magnetic intensity (TMI) for the synthetic dataset used in this exercise.
FIGURE 4: Location of bodies (white outlines) in this synthetic dataset, superimposed over color-scaled total magnetic intensity (TMI). Each of the 12 bodies has its own assigned depth, magnetic susceptibility and remanence. See reference  to see the list detailing these.
FIGURE 5: Positions of anomalies (black boundaries) detected in the reduced-to-the-pole analytic signal of the TMI data. Here, the anomaly boundaries are superimposed on the original TMI data. Note that Body 6 in the synthetic data is (expectedly) not picked up here as an anomaly. This is because the body was defined as having relatively low magnetic susceptibility and zero remanence. Thus, the body has a weak magnetic signature overall.
FIGURE 6: Results for estimating the total field's inclination for each anomaly. The anomalies have been shaded according to the inclination direction estimated (the color spread is histogram-equalized). A perfectly white anomaly indicates the estimated inclination is +90 degrees (straight up, or straight 'out of the paper'), while a full-valued magenta anomaly indicates the estimated inclination is -90 degrees (straight down, or straight 'into the paper'). Anomalies with varying shades of magenta indicate intermediate estimated inclination values. See the tabulated figures given within the discussion for further details.
FIGURE 7: Results of estimating the total field's declination for each anomaly. The anomalies have been shaded according to the declination direction estimated (the color spread is histogram-equalized). The whiter the anomaly, the closer the estimated declination is to -180 degrees. The greener the anomaly, the closer the estimated declination is +180 degrees. Anomalies with varying shades between the white and green endmembers are those with intermediate estimated declination values. See the tabulated figures given within the discussion for further details.
Below are our results based on this procedure.  They show our estimates for the inclination and declination of each anomaly's total magnetic field. The units are degrees. The anomalies have been numbered in the same order as the causative bodies appearing in Figure 2.
- Anomaly 1: inclination -30, declination +45
- Anomaly 2: inclination -80, declination -45
- Anomaly 3: inclination -70, declination 0
- Anomaly 4: inclination +70, declination +15
- Anomaly 5: inclination -60, declination -60
- Anomaly 6: (body has negligible detectable magnetic signature overall)
- Anomaly 7: inclination -25, declination +30
- Anomaly 8: inclination -70, declination +90
- Anomaly 9: inclination -70, declination -75
- Anomaly 10: inclination +70, declination +165
- Anomaly 11: inclination -90, declination -180
- Anomaly 12: inclination -25, declination +75
Note that a negative inclination indicates a direction pointing into the ground (i.e., within the 'subsurface' hemisphere of the spherical coordinate system). Declination is relative to magnetic north. A positive number for declination indicates the direction is measured clockwise from north. A negative number is measured anti-clockwise.
Results of total field estimates
How did our estimates go for each anomaly's total magnetic field inclination and declination?
Anomalies 1, 2, 3, 4, 7 and 8 posed no estimation problems at all.
Body 6, expectedly, has no inclination and declination associated with it because its magnetic signature produced no detectable anomaly.
Anomalies 5, 9 and 10 have inclination estimates that succeeded. The estimated declinations don't match the declinations observed at the bodies themselves. This is because compared with the other bodies in this synthetic dataset, these particular bodies are deep-seated. Generally, the deeper the body is, the greater the divergence between the anomaly's estimated declination at the surface and the declination value of the anomaly at the body's depth. Note however, that our declination estimates should be valid at the surface for Anomalies 5 and 9. To peform an RTP, you want to know the anomaly's declination as it exists at the surface, not at few hundred metres' depth. (Note that Anomaly 10's estimated declination is a poor estimate and the reason for this is not immediately apparent).
Anomaly 11 has an inclination estimate that succeeded. The estimated declination is a poor estimate, because the anomaly has an inclination that is essentially vertical (i.e., equal to or in the vicinity of +90 degrees or -90 degrees). The difficulty in correctly estimating the declination of a vertical or near-vertical magnetic field vector is similar to the relatively large uncertainty involved in trying to accurately estimate the direction that a nearly-vertical pencil is pointing in (as opposed to, say, a pencil inclined at 45 degrees). However, the poor declination estimates pose no problem when RTPing: when a body's total magnetic field vector is inclined near-vertical, the declination component of that vector has little influence on the anomaly the body produces. Put another way, in these cases, the shape and position of the pre-RTP anomaly is very close to the shape and position of the RTPed anomaly.
The estimated inclination and declination values at Anomaly 12 likely do not correspond well with the total magnetic field inclination/declination values of Body 12 itself. This is because the body generates a relatively weak anomaly. Nevertheless, our inclination and declination estimates should be valid at the surface at this anomaly's location, as they incorporate the magnetic contribution not only from weakly anomalous body but also from the surrounds.
Overall, we found this exercise on a remanence-affected synthetic test-dataset to have produced acceptable results.
Our method avoids the need for fieldwork to collect oriented hand specimens and oriented drillcore specimens, and therefore also avoids the subsequent need for paleoremanence laboratory analyses, such as hysteresis studies and stepwise demagnetization.
This kind of fieldwork and lab work are laborious and time-consuming, not to mention an overkill, if all an explorer wants to do is to obtain RTPed data that has been stripped of remanence effects for a broad exploration region.
References and Notes
** For a general description of the reduction-to-the-pole process, see the discussion in our write-up on the topic of differential RTP, published in the April 2010 issue of our newsletter.
 See the science news story dated 9 August 2007 at http://www.science20.com/news/magnetic_memory_of_rocks.
 Citation: S. McEnroe, K. Fabian, P. Robinson and L. Brown (2009) "Remanent magnetization in crustal rocks", European Space Agency's Second SWARM International Science Meeting, 24-26 June 2009, Germany, available at www.congrex.nl/09c24/S2_Posters/S2_P06_McEnroe_paper.pdf
 See work cited in: Fathom Geophysics newsletter (Jul-Aug 2010) "Check out our remanent magnetization-spotting capabilities".
 See for example: D.A. Clark and C. Tonkin (1994) "Magnetic anomalies due to pyrrhotite: Examples from the Cobar area, NSW, Australia", Journal of Applied Geophysics, 32, 11-32. The authors said: "It is quite common, therefore, for pyrrhotite-bearing rocks to have a magnetization dominated by an ancient remanence, which dates from the last significant thermal event(s) experienced by the rocks and which may be highly oblique to the present field. Interpretation of magnetic anomalies associated with such rocks may be seriously in error if the effects of remanence are ignored."
See also: R. Pucher (1994) "Pyrrhotite-induced aeromagnetic anomalies in western Germany", Journal of Applied Physics, 32, 33-42. The author said: "Generally it is assumed that a majority of magnetic anomalies is cause by rock bodies in which the carrier of magnetization consists of several iron oxides, in most cases magnetite, maghemite, or hematite. ... However, during the last decade it has been shown that pyrrhotite may be responsible for more magnetic anomalies that previously expected. ... [P]yrrhotite can carry a stable remanent magnetization for several hundred million years."
Mafic rocks, which are relatively ubiquitous, can also display strong remnance. For example see: H. Henkel (1994) "Standard diagrams of magnetic properties and density: A tool for understanding magnetic petrology", Journal of Applied Geophysics, 32, 43-53. The author said: "The highly magnetic (due to high magnetite content) rocks of mafic composition ... also show a tendency towards increased [remanent to induced magnetization ratio] values... . The increased remanence is a consequence of ilmenite exsolution lamellae partitioning the magnetite grains into smaller domains, which have a higher coercivity [i.e., a greater reverse external field is needed to demagnetize them]. The orientation of the remanence vector is now an additional factor in shaping the total field magnetic anomalies created by mafic rocks."
 Harnessed the right way, remanence may even prove beneficial to exploration efforts. See discussion in: A.L. Dugdale, C.J.L. Wilson, L.J. Dugdale, C.W. Funk, M. Bosnjak and B. Jupp (February 2010) "Gold mineralization under cover in southeast Australia: A review of an exploration initiative for Stawall-type deposits", Ore Geology Reviews, 37, 41-63. The authors conclude their discussion with: "[Magnetic petrophysics] information combined with orientation of remanence can then be applied to detailed magnetic surveys to further define zones of potential high fluid-flow and hence mineralization."
Remanence is also discussed as being of importance in: M.-L. Airo and S. Mertanen (2008) "Magnetic signatures related to orogenic gold mineralization, Central Lapland Greenstone Belt, Finland", Journal of Applied Geophysics, 64, 14-24. The authors said: "During hydrothermal processes new magnetic minerals can be crystallized or be formed from previously exist[ing] minerals, and a new chemical remanent magnetization (CRM) may be blocked in the rocks. Provided that the newly formed remanent magnetization is stable through time, the age of the magnetization and consequently, the age of the fluid and gold may be defined." Later in their paper, they add: "Particularly in metamorphosed terrains, remanent magnetization may play an important role by influencing the magnetic anomaly intensity and shape."
 When an entire airborne survey needs to be interpreted, comprehensive sample collection and analysis is just not practical, and the method we present here is accurate enough for this kind of application. However, generally, analyses of rock samples is necessary when absolute precision is necessary during the interpretation of magnetic data. See for example: H. Henkel (1994) "Standard diagrams of magnetic properties and density: A tool for understanding magnetic petrology", Journal of Applied Geophysics, 32, 43-53. The author said: "In the case of high [remanent to induced magnetization ratio] values the anomaly shape will reflect the orientation of the remanence vector (in addition to the geometry of the source). ... Therefore, for detailed interpretations, oriented rock samples need to be collected in order to restrict the ambiguity in the interpretation."
 These can be extracted from the dataset's RTP analytic signal of the total magnetic intensity.
 Here is a description of the bodies:
- Body 1: Moderately deep, relatively weak remanence, moderate magnetic susceptibility
- Body 2: Moderately deep, relatively weak remanence, moderate magnetic susceptibility
- Body 3: Relatively shallow, zero remanence, moderate magnetic susceptibility
- Body 4: Moderately deep, relatively weak remanence, moderate magnetic susceptibility
- Body 5: Deep, very strong remanence, weak magnetic susceptibility
- Body 6: Moderately deep, zero remanence, strong magnetic susceptibility
- Body 7: Moderately deep, moderately strong remanence, weak magnetic susceptibility
- Body 8: Moderately deep, moderately strong remanence, weak magnetic susceptibility
- Body 9: Deep, strong remanence, weak magnetic susceptibility
- Body 10: Deep, strong remanence, weak magnetic susceptibility
- Body 11: Relatively shallow, moderately strong remanence, weak magnetic susceptibility
- Body 12: Moderately deep, relatively weak remanence, moderate magnetic susceptibility
The bodies were exposed to an inducing magnetic field that has an inclination of -60 degrees and a declination of +10 degrees. The vector describing the remanence of each of the remanent bodies possessed a direction that was different to the inducing field.