Airborne Geophysical
Survey Maps
Reduced-to-Pole Magnetics
To compensate for the shift of the true anomaly position over the causative
source, due to the magnetic inclination and declination, the magnetic data was
recomputed so that magnetic anomalies will appear as they would if located at
the north magnetic pole. The result of this operation is that in theory, the
magnetic anomaly is located directly overtop of the causative source. The
computation is referred to as "reduced-to-the-pole" (RTP). The RTP not
only shifts the anomalies to their correct position with respect to the
causative magnetic bodies, but assists in the direct correlation and comparison
of magnetic anomalies, trends, structural axis, and discontinuities with mapped
geologic surface expression.
First Vertical Derivative (1VD)
Vertical derivatives compute the rate of change of the magnetic field as it
drops off when measured vertically over the same point (upward continuation).
The First Vertical Derivative (1VD) has the effect of sharpening anomalies,
which allows for better spatial location of source axes and boundaries
Second Vertical Derivative (2VD)
To enhance local anomalies and to outline the edges of anomalous bodies
within the data, a Second Vertical Derivative (2VD) is computed. The 2VD is a
powerful interpretive tool and that is used to assist in the delineation of
causative bodies and to accurately locate changes in the magnetic field
gradients. Better definition of discontinuities and their relationship to
geology can be gained from the use of this tool. A 2VD will show steep gradients
over faults and positive closures over the "up thrown" blocks.
Apparent Resistivity
Apparent resistivity maps are extremely valuable for geological
mapping and interpretation. Apparent resistivities can be calculated from each
electromagnetic frequency used in the survey. However, the horizontal coplanar
coil configurations are most often used because they have maximum coupling to
horizontal layering. The apparent resistivity map shows the results for the 6600
Hz data. |