High-Resolution AeroMagnetic (HRAM) data

Example 3 – The causes of magnetic anomalies within the sedimentary section

Since the advent of HRAM surveys, the understanding of the causes of sedimentary anomalies has lagged far behind our ability to image them. Igneous and metamorphic rocks have magnetic susceptibilities with a typical range of 1x10^-4 – 10SI units. By comparison sedimentary rocks have typical susceptibilities of 0 – 1x10^-3SI units, resulting in magnetic anomalies that are orders of magnitude weaker. Consequently, surveys must be flown close to the ground in order to detect these weak anomalies (i.e. HRAM or S-HRAM survey type). The example right shows a litholog with associated magnetic susceptibility. It is apparent that the different geological facies/lithologies have different magnetic properties. This information can be useful for mapping surface contacts using magnetic data.

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Magnetite is often cited as the most important magnetic mineral due to its high susceptibility. But only few sedimentary rocks contain an appreciable quantity of magnetite. Studies have shown that other ferromagnetic minerals, such as maghaemite and griegite, and even paramagnetic minerals such as siderite, hematite, and glauconite are important contributors to sedimentary rock susceptibilities. In the example left, the correlation between the percentage of glauconite in the rock (left) and measured magnetic susceptibility (MS, right) is striking.

Measurements of many rock samples from the Western Canada Sedimentary Basin have resulted in some broad ranges of magnetic susceptibility assigned to various rock types. Because common rock-forming minerals such as quartz and calcite are non-magnetic, pure quartzites and limestones are also non-magnetic. The factors that govern rock susceptibility are broad, but include, mineral species included, grain size, redox state, and Fe content.