Introduction to Geomagnetic method

1. GEOMAGNETIC METHOD

2. INTRODUCTION
• The magnetic method is a very popular and inexpensive approach for
near-surface metal/iron detection.
HOW DOES IT WORK?
• When a ferrous material is placed within the Earth's magnetic field, it
develops an induced magnetic field.
• The induced field is superimposed on the Earth's field at that location
creating a magnetic anomaly.
• Detection depends on the amount of magnetic material present and
its distance from the sensor.
• The anomalies are typically presented on colour contour maps.

3. • The magnetic field values are normally expressed in gamma or
nanoTesla.
• 1 gamma = 1 nanoTesla

5. INDUCED MAGNETIZATION
• When a magnetic material, say iron, is placed within a magnetic field,
H, the magnetic material will produce its own magnetization. This
phenomenon is called induced magnetization.
• 3 types of magnetic material:
- Diamagnetism
- Paramagnetism
- Ferromagnetism

6. REMANANT MAGNETIZATION
• If the magnetic material has relatively large susceptibilities, or if the
inducing field is strong, the magnetic material will retain a portion of
its induced magnetization even after the induced field disappears.
This remaining magnetization is called Remanent Magnetization.
• Remanent Magnetization is used to map the motion of continents
and ocean basins resulting from plate tectonics.
• The only way to measure the remanent magnetic component of a
rock is to take a sample of the rock back to the laboratory for analysis.
This is time consuming and expensive. So, we typically assume there
is no remanent magnetic component in the observed magnetic field.

7. VARIATIONS IN EARTH’S MAGNETIC FIELD
• The Earth’s magnetic field varies with time, i.e. it is not constant.
• As the Earth rotates, the outer layers of the ionosphere interact with
the solar wind to cause minor fluctuations in the magnetic field.
• Depending upon the frequency, duration and intensity of these
fluctuations, they are given different names.

8. SECULAR VARIATION
• Geomagnetic secular variation refers to changes in the Earth’s
magnetic field on time scales of about a year or more.
• These changes mostly reflect changes in the Earth's interior, while
more rapid changes mostly originate in the ionosphere.

9. DIURNAL VARIATION
• Fluctuations with a period lasting of several hours to one day are
called diurnal variations.
• However, they are not predictive and are usually not a problem when
conducting magnetic surveys.
• This diurnal drift can cause a variation of the order of 50 nT per hour.

10. NORMAL FIELD, TERRAIN AND ELEVATION
CORRECTION
• Normal Field Correction – variation in field with latitude and
longitude
• Terrain Correction – only comes into play near the base of a steep
slope of high k material
• Elevation Correction – generally not required for ground surveys, only
in airborne when large changes in elevation is experienced

11. MICRO-PULSATION
• Erratic, short-term blips or spikes in the magnetic field are called
micro-pulsations.
• These can range in intensity from a few through to tens, or even
hundreds, of nanoTeslas in intensity.
• These variations can present a problem when we are surveying where
it may appear similar to anomalies caused by buried objects.

12. MAGNETIC STORMS
• When the amplitude and duration of micro-pulsations becomes
severe it is then called a magnetic storm.
• Typical micro-pulsations last a few hours whereas magnetic storms
can last for days.
• It is not recommended to conduct a total-field survey during a
magnetic storm, as you may not be able to remove all of the rapidly
changing variations in the magnetic field, giving rise to perhaps false
anomalies.

13. FIELD INSTRUMENTS
• Fluxgate Magnetometer
• Proton Precession Magnetometer
• Optically Pumped Magnetometer
• Overhauser Magnetometer

14. APPLICATION OF MAGNETIC SURVEY
• Mineral Exploration
• Geothermal Exploration
• Fault study
• Mapping buried utilities
• Locating buried tanks or drums

15. MAGNETIC SUSCEPTIBILITY OF ROCKS