Plotting, regional trends and processing

This page is an introduction to many of the subjects related to presenting large magnetic field data sets. Raw data are not usually presented directly. Choices of contour plotting parameters must be made; features not related to targets might be removed; and data or image enhancement processing might be employed. Here we introduce some aspects of these topics.

The most common form of magnetic survey data involves "total field" measurements. This means that the field's magnitude along the direction of the earth's field is measured at every location. To the right is a total field strength map for the whole world (a full size version is in the sidebar mentioned in the Earth's field section).

At the scale of most exploration or engineering surveys, a map of total field data gathered over ground with no buried susceptible material would appear flat. However, if there are rocks or objects that are magnetic (susceptible) then the secondary magnetic field induced within those features will be superimposed upon the Earth's own field. The result would be a change in total field strength that can be plotted as a map. A small scale example is given here:

Total field strength is measured along six lines covering an area of 15 x 50 metres. With no susceptible material underground, all values would be the same (about 56,000 nT near Vancouver, BC.) Values recorded will vary if susceptible material exists. These variations in total field strength can be displayed as a contour plot. Filling the contour plot helps visualize the magnetic field variations. Colour contour maps are now the preferred form of plotting raw "total field intensity" data. Data along one line is often plotted as a graph in order to display more details (click here ). A more complete summary of the data set shown here is provided on a single 11" x 17" sheet in PDF format.

Large data sets are commonly gathered using airborne instruments. They may involve 10⁵ to 10⁶ data points to show magnetic variations over many square kilometres. An example of a large airborne data set is shown to the right, with a larger version, including alternative colour scale schemes, shown in a sidebar.

Such data sets will be too large to invert directly, but they can provide extremely valuable information about geology and structure, especially if some processing is applied to enhance desirable features and/or suppress noise or unwanted features.

Removal of regional trends

In order to interpret the magnetic data in terms of magnetic features and structures at depth, the anomalous field caused by buried features of interest must be isolated. In other words, we must try to remove the contribution to measurements consisting of the earth's field combined with fields due to geologic features larger than the actual survey area. This is accomplished by estimating and subtracting the regional, or large scale field. If we designate magnetic fields as B, then

Estimates of the regional field may be obtained from:

• the IGRF (International Geomagnetic Reference Field) discussed in the next section;
• a constant value selected by the interpreter (when survey areas are small);
• a more sophisticated polynomial (map) generated by a computer using least squares (or other) analysis of data;
• it is also possible to use inversion at a large scale to define a regional field.

To illustrate the process, when data are collected along a line, the removal of a regional trend can be managed graphically, as shown here:

For magnetic maps (data collected over an area) the choice of a regional trend may not be particularly easy, but it is critical to get it right if a correct interpretation of subsurface distribution of susceptibility is to be obtained. Here is an example showing the regional magnetic map and a local anomalous field taken from a survey in central British Columbia.

Regional field. Airborne magnetic data gathered over a 25 square km area around a mineral deposit in central British Columbia. Some geological structural information is shown as black lines. The monzonite stock in the centre of the boxed region is a magnetic body, but this is not very clear in the data before removing the regional trend. Local anomalous field. Anomalous total magnetic field strength in the boxed area of the large-scale map, after the regional SW-to-NE trend has been removed. Now the signature of the monzonite stock is more clearly visible.

Processing options

There are numerous options for processing potential fields data in general, and magnetics data specifically. One example (figure shown here) is provided in a sidebar. The processing was applied in this case in order to emphasize geologic structural trends.

Some other good reasons for applying potential fields data processing techniques are listed as follows:

• Upward continuation is commonly used to remove the effects of very nearby (or shallow) susceptible material.
• Second vertical derivative of total field anomaly is sometimes used to emphasize the edges of anomalous zones.
• Reduction to the pole rotates the data set so that it appears as if the geology existed at the north magnetic pole. This removes the asymmetry associated with mid-latitude anomalies.
• Calculating the pseudo-gravity anomaly converts the magnetic data into a form that would appear if buried sources were simply density anomalies rather than dipolar sources.
• Horizontal gradient of pseudo-gravity anomaly: gravity anomaly inflection points (horizontal gradient peaks) align with vertical body boundaries;  therefore, mapping peaks of horizontal gradient of pseudo-gravity can help map geologic contacts.

The effects of these five processing options are illustrated in a separate sidebar on processing of magnetics data.