Multi-Parameter Surveys
Exploration success is vastly improved by using a multi-parameter approach,
whereby several complementary geophysical techniques are used together to
maximize subsurface information. Sander Geophysics owns and operates a DHC-6
Twin Otter survey aircraft that is capable of acquiring multiple geophysical
parameters simultaneously. These parameters include frequency domain
electromagnetics, gravity, magnetics and gamma-ray spectrometry, which can all
be used together to provide a more complete geological picture. Associated
aircraft GPS data as well as radar and laser altimeter data are also recorded.
Figure 1: Reid-Mahaffy test site data showing
multiple data compone
Multi-Parameter Data
The electromagnetic data is provided by SGL's SGFEM system, which is designed
for high resolution mapping, and consists of a four frequency (912, 3,005,
11,962, and 24,510 Hz) transmitter mounted in a wingtip pod and a receiver
mounted in a pod on the opposite wing.
Gravity data is provided by AIRGrav, a total field gravimeter designed and
built by SGL specifically for the airborne environment, which is mounted
inside the aircraft cabin.
A total field magnetometer mounted in a rigid tail stinger provides the
magnetic data, and a gamma-ray spectrometer with up to 3 boxes (50.4 litres
downward facing, 8.4 litres upward facing) of sodium iodide crystals provides
the radiometric data.
SGL´s de Havilland Twin Otter on survey in Ireland
Reid-Mahaffy Geophysical Test Site
A survey using this multi-parameter system was flown over the Reid-Mahaffy
geophysical test site in Ontario to demonstrate the system's capabilities over
a typical mineral exploration target with known geology and good depth
control. Using a primary (traverse) line spacing of 200 m, a total of 315 line
kilometres were flown over the test site and surrounding area at a speed of 90
knots (167 km/h) and at an altitude of 52 m.
Specialized Processing & Interpretation
The Reid-Mahaffy data have been processed using SGL's suite of processing
tools, and an analysis of the data has been performed. Figure 1 shows the
complementary nature of multiple instruments. The first grid is the apparent
conductivity derived from the lowest frequency (912 Hz) which tends to target
deeper conductors. The principal conductors of the Reid-Mahaffy area are
clearly visible and delineated by the east-west striking conductive zones in
the centre of the area. The magnetic anomaly grid shows magnetic bearing
targets, especially useful to delineate certain dykes. The first vertical
derivative of the Bouguer gravity grid supports the other data sets by imaging
density contrasts which can help map geological units and contacts. The
radiometric data can potentially identify geological units by measuring the
quantity of radioelements (potassium, uranium and thorium) common in many
rocks where they are outcropping or where in situ soil is present. Finally, a
high quality digital terrain model can be derived from the laser altimeter
showcasing very detailed terrain.
Figure 2 shows a cross section of Line 40 (location shown in Figure 1) that
crosses over the conductors of interest. The top pane shows the radiometric
data, and the second pane shows the first vertical derivative of the Bouguer
gravity combined with the magnetic anomaly data. The bottom pane is a
Conductivity Depth Image (CDI) whereby all frequencies from the SGFEM system
are used to generate a suite of conductivity values with associated depths.
The result is a relatively fast product that can help localize targets of
interest, and can even map overburden thickness due to the great near surface
resolution of the SGFEM system. The CDI clearly shows multiple conductors that
correlate with the apparent conductivity data.
In some settings, various parameters may correlate to each other (either
directly or inversely), or one may image a feature that the others do not by
mapping a different rock property. A much better interpretation can be
generated by having all of these tools to provide a more complete geological
picture.
Figure 2: Test line showcasing multiple parameters
in profile plus CDI generated from SGFEM system
|