Introduction to Aeromagnetic Surveys

In aeromagnetic surveys, magnetic measurements are made from low-flying airplanes flying along closely spaced, parallel flight lines. This is a common type of geophysical survey carried out using a magnetometer aboard or towed behind the aircraft. The principle is similar to a magnetic survey carried out with a hand-held magnetometer, but allows much larger areas of the Earth's surface to be covered quickly for regional reconnaissance. The aircraft typically flies in a grid-like pattern with height and line spacing determining the resolution of the data (and cost of the survey per unit area). Additional flight lines are flown in a direction perpendicular to the main transect to assist in data processing. As the aircraft flies during a survey, the magnetometer records tiny variations in the intensity of the ambient field which is the sum total of the earth’s field (with its regional variations), the local effects of magnetic minerals in the crust, as also the temporal effects due to the constantly varying solar wind.   By subtracting the solar and regional effects, the resulting aeromagnetic map shows the spatial distribution and relative abundance of magnetic minerals (most commonly the iron oxide mineral magnetite) in the upper levels of the crust.  The huge volumes of data acquired through aeromagnetic surveys are processed into a digital aeromagnetic map.

Because different rock types differ in their content of magnetic minerals, aeromagnetic maps allow a visualization of the geological structure of the upper crust in the subsurface, particularly the spatial geometry of lithounits and the presence of folds and faults.  Aeromagnetic surveys are particularly useful where bedrock is obscured by surface regolith, soil or water. Aeromagnetic data was once presented as contour plots, but now is more commonly expressed as colored and shaded computer generated pseudo-topography images. The apparent hills, ridges and valleys are referred to as aeromagnetic anomalies. A geophysicist can use mathematical modeling to infer the shape, depth and other properties of lithounits responsible for the anomalies.

Aeromagnetic surveys are widely used to aid in the production of geological maps and are also commonly used in mineral exploration. Some mineral deposits are associated with an increase in the abundance of magnetic minerals, and occasionally the sought after commodity may itself be magnetic (e.g. iron ore deposits).  The data from aeromagnetic surveys are processed and plotted at a map scale that will allow the flight lines to be properly discriminated.  At 1:50,000­scale, for example, a flight-line spacing of I km is satisfactorily represented by 2 cm on the map.  The final maps are often reduced to 1:100,000 or 1:250,000 for matching with regional topographic and geologic maps. 

Aeromagnetic anomalies so slight that they rise to only a few gammas (gamma 10-1 gauss) above the regional background may be significant in a mapping program.  In a more direct connotation, magnetic anomalies may rise to 10,000 or 50,000 gammas over an iron orebody.  Among the most common magnetic minerals, magnetite, ilmenite, pyrrhotite, and specular hematite, magnetite has by far the highest magnetic susceptibility and is the most common accessory rock mineral.  A strong aeromagnetic anomaly may therefore be associated with a variety of rock conditions, such as a tactite zone or a magnetite-rich mafic intrusion or volcanic flow bordered by felsic intrusions, by rhyolitic volcanics, or by most kinds of sedimentary rocks.  Some sedimentary rocks, such as ferruginous shale and "ironstone," will of course show a magnetic response. Metamorphic derivatives of ferruginous sedimentary rocks cause some of the strongest magnetic responses.  Precambrian banded iron formations have a particularly high magnetic susceptibility.   

Aeromagnetic anomalies are best interpreted by incorporating geologic mapping and other geophysical information (gravity, seismic-reflection) where available. Interpretations often involve both map-based information (e.g., a fault map) and three-dimensional information (e.g., a geologic cross section).  Revelation of subsurface structure of the upper crust is perhaps the most valuable contribution of aeromagnetic surveys.

Aeromagnetic surveys are also used in reconnaissance mapping of unexploded ordnance (UXO).  For this purpose, the magnetometer is flown on a helicopter, since the sensors must be close to the ground (relative to mineral exploration) to be effective. Electromagnetic methods are also used for this purpose.

Three Dimensional Current Flow

When a current is introduced into the ground a three dimensional current flow results (Fig 1). This assumes no variation in conductivity.  The current density (the total current crossing any surface normal to the lines of current) is very high around the electrodes so the resistance around the electrodes is very high compared to an area far removed from the electrode. This is called the contact resistance of the electrode. This produces large potential differences in the vicinity of the input electrodes. To avoid this problem two additional electrodes are introduce at points where the potential gradient is negilgible (Fig 1). To further limit any possible electrode potential with the detector electrodes, a pot is used as the electrode. The contact is thus a liquid/liquid contact and no electrode potentials are developed.

 

 Fig 1: Lines of current flow and equipotential in an homogeneous conductor.

Fig 2: The effect of a body of high conductivity on lines of current flow and equipotential.
If  the conductivity of the ground is constant the ideal current flow is as shown by Fig 1.  However, if a body with differing conductivity occurs in the surveyed area then the lines of current floe will be distorted as shown in Fig 2. Plotting the equipotential lines is relatively simple. Two porous pots are connected by constant length of wire to a galvanometer and with one electrode fixed, the other is moved until the galvanometer registers a zero reading. Both elec trades must therefore be on a line of equal potential. If all such locations are plotted then a map of equal potential lines (and also lines of equal current flow) can be prepared. This can then be examined to see if any anomalously conductive material is present.

Reference: Economic and applied geology: an introduction.  William George Shackleton

Potential due to a Point

Since the electrical resistivity ranges of different earth materials overlap, the resistivity measurements cannot be directly related to the type of soil or rock in the subsurface without direct sampling or some other geophysical or geotechnical information.  Resistivity methods however allow determination of the subsurface layers of the rock, the location of ground water table, the location of fault zone.  This can be determined from the measurement of potential distribution in the earth’s surface and from the geometric characteristics of the probe used to measure this distribution.

Consider a single current electrode on the surface of a medium of uniform resistivity. The Current flows radially away from the electrode so that the current distribution is uniform over hemispherical shells centered on the source.  Lines of equal voltage (equipotentials) intersect the lines of equal current at right angles.  The voltage drop between any two points on the surface can be described by the potential gradient.  dV/dr is negative because the potential decreases in the direction of current flow.

 

 Fig 3. Current flow for a single surface electrode.

 

 

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