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Ground Magnetic Surveys –
Seeing Beneath the Rocks

Although ground magnetic surveys measure rock magnetism, they can help find minerals that aren’t
magnetic, including gold. In fact, the areas that aren’t magnetic can be as significant as those that are.
This is why ground magnetic surveys are important for far more than finding iron ore.

Why Is A Ground Magnetic Survey Useful?
Ground magnetic surveys map the magnetism of underlying rocks. The most common magnetic
minerals found are pyrrhotite, (iron sulphide), and magnetite. Magnetite when found with sufficient
purity and quantity may become an iron ore deposit.
Pyrrhotite is important because of the minerals it’s often associated with including pyrite, another iron
sulphide mineral, which may contain gold. Other valuable minerals often associated with pyrrhotite
include; chalcopyrite (copper sulphide), sphalerite (zinc sulphide), and pentlandite (nickel sulphide).
Pyrrhotite is key to the ability of ground magnetics to reveal hidden zones of potentially valuable non-
magnetic minerals.
Ground magnetic surveys are also used to understand the general structure of underlying rock ,
identifying faults and folds otherwise hidden beneath cover rocks and for identifying demagnetised
zones associated with hydrothermal activity.
The heat of, hydrothermal activity demagnetises rocks. These demagnetised zones may be identified
using ground magnetics on a local scale, or by aeromagnetics at a regional level. This technique can
identify epithermal gold deposits. In the goldfields around Charters Towers, Queensland, Australia,
aerial and ground magnetic surveys helped identify demagnetised associated with swarms of
hydrothermal quartz veins, under younger sedimentary cover.

Conducting a Ground Magnetic Survey
A survey is usually done by two operators. While walking across the landscape an operator can
encounter numerous obstacles, but the idea is to walk as straight as possible. In heavily vegetated areas,
walking can be quite slow and concentration has to remain high to avoid hazards such as snakes, rabbit
holes or even obscured mine shafts. On good ground, operators can walk up to 15 km a day, so a high level
of fitness is required. If you are not fit at the start of the survey, you will be by the end!
Each operator walks across the survey area with a magnetometer and GPS in a backpack. Changes in
the earth’s magnetic field are recorded along the length of each line. GPS coordinates for the readings
are also recorded. A plot of these coordinates reveals that the actual paths walked are normally not
perfectly straight but wriggle across the landscape. Luckily, these minor deviations do not significantly
affect the result.

Planned ground magnetics with a line spacing of 25m (red). Walked lines by
Operator 1 (white) and Operator 2 (black) are shown. Base image: Google Earth.

Along with the magnetic field changes, the elevation of the land surface is also
recorded by the GPS. This data will later be used by software to generate 3D
model of the topography of the survey area.
The spacing between successive survey lines can vary between 100m to 20m
depending on the resolution required. The narrower the line spacing, the higher
is the resolution of the resultant magnetic image. The highest resolution images can reveal subtle
structures including subtle features in faults and shears, which may point to high grade ore shoots.
A base station is needed for every survey. The station records daily variations in the earth’s magnetic
field caused by electric currents in the upper atmosphere, by solar activity. These daily fluctuations are
later corrected for, otherwise incorrect magnetic images can result.

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