High Resolution
Airborne Magnetometer Surveys
High resolution airborne magnetometer surveys can play an important part in
your exploration program. Improvements in magnetometer design have resulted in
more accurate data, which combined with GPS, yield significantly better
aeromagnetic maps. The overall improvement in accuracy often warrants
resurveying areas which were flown using instrumentation and methods now
considered obsolete.
High resolution aeromagnetic maps reflect the underlying geology regardless
of the degree of exposure. They play such an important role in interpreting
structures and rock type distribution that high resolution magnetometer
surveying can truly be called a basic, universal exploration tool. Moreover
with the relatively low cost of aerial magnetometer surveys they are
particularly attractive in the early stage of an exploration project, when
large areas need to be covered rapidly.
Airborne Magnetometer Surveys at SGL
Sander Geophysics Limited (SGL) of Ottawa, Canada, specializes in flying high
resolution magnetometer surveys worldwide. To maintain our position as a
leader in the field, SGL's engineers and scientists are actively involved in
developing better instruments and survey methods.
One of SGL's Cessna Grand Caravans
We use optically pumped cesium magnetometers with a sensitivity of 0.005 nT,
and computer controlled real-time digital compensation, giving an overall
system resolution of 0.01 nT. The magnetometer sensors in the aircraft and in
the reference station are identical, ensuring that all magnetometer data sets
are equivalent in terms of sensitivity and noise envelope. The sampling rate
can be adjusted depending on survey requirements, with most surveys delivered
at 10 Hz.
Fixed Wing Magnetometer Surveys
All of our aircraft have been extensively modified to accommodate geophysical
instruments and to reduce the aircraft's magnetic field. Typical Figures of
Merit (FOM) for SGL's fixed wing aircraft are less than 1 nT. Our standard
configuration is to have a single magnetometer sensor mounted in a "stinger"
rigidly attached to the tail of the aircraft. Alternatively, several magnetic
sensors can be used simultaneously to directly measure the horizontal and/or
vertical gradient of the magnetic field by separating the sensors horizontally
on booms attached to each wing of the aircraft, and separated vertically on
the tail stinger.
Helicopter Magnetometer Surveys
Magnetic data can be acquired using a “bird” towed beneath the helicopter,
which incorporates two total field magnetic sensors that are separated
horizontally. The bird maintains a constant separation of 4.9 m between the
magnetic sensors, which allows recording of the measured horizontal gradient.
A GNSS (dual frequency GPS) antenna mounted directly on the bird provides
accurate sensor position data.
SGL "bird" in Greenland
Alternatively, a single total field magnetometer can be mounted in a stinger
rigidly attached to the front of the helicopter, eliminating the need for the
towed bird. This option can be useful in built-up areas where towed loads may
not be permitted. If there is no regulatory restriction, we usually prefer to
use a towed bird because it is magnetically quieter and yields higher quality
data.
C-FXPL with stinger
Navigation, Data Recording and Processing
SGL’s aircraft are equipped with GNSS receivers (dual frequency GPS)
integrated into a proprietary navigation and flight path recovery system. This
system, called SGNav, allows for
excellent navigation and provides an accuracy of better than 1 m in
post-flight recovery. SGNav is used in
conjunction with our pre-planned, computer-aided drape flying system, SGDrape.
This system allows us to produce a drape flying surface which is optimal for
safety and data quality. Using a drape surface ensures that adjacent flight
lines and control lines are flown at comparable levels, resulting in better
quality magnetic data.
SGL’s reference stations automatically record diurnal fluctuations of the
earth's magnetic field. Both airborne and ground magnetometer data acquisition
computers use a temperature compensated quartz crystal oscillator and a
counting circuitto provide real-time clocks. These clocks are synchronized to
the GPS time strobe, which has an absolute accuracy of 0.5 x 10-6.
The company’s data centre in Ottawa is equipped for processing and
interpretation of high resolution magnetic data. An array of products designed
to represent the geology, as expressed through the magnetic data is available.
SGL provides complete interpretational services by experienced geoscientists,
enhancing the value of your high resolution airborne magnetic survey.
Airborne Magnetometer Survey System
Airborne Instruments |
Magnetometer Sensor |
Geometrics |
Strap-down, optically pumped, cesium split beam |
Sensitivity: 0.005 nT |
Sensor noise level: <0.02 nT |
Sampling rate: 10 Hz |
Compensator |
Sander Geophysics - AIRComp real-time digital
compensation |
Range: 20,000 to 200,000 nT |
Resolution: 0.001 nT |
Sampling rate: 160 Hz |
Data Acquisition System |
Sander Geophysics - SGDAS airborne computer |
Capable of recording unlimited number of channels at
variable intervals, and digital scrolling chart display of the data.
Data is recorded on a vibration tolerant removable drive. The system
clock is a quartz time standard automatically synchronized to UTC by the
GPS signal to an accuracy of 1 millisecond. |
Video Imaging System |
Sander Geophysics - SGDIS digital video |
Radar Altimeter |
|
Resolution (m) |
Calibrated to |
Range
(m) |
TRT AHV8 |
0.5 |
1% |
0 to 3,050 (10,000 ft) |
King KRA-10 |
0.1 |
1% |
0 to 760 (2,500 ft) |
FreeFlight |
0.5 |
1% |
0 to 760 (2,500 ft) |
Barometric Altimeter |
Sander Geophysics Digitally Recording Barometric Altimeter |
2.0 |
±4 m |
0 to 10,000 (33,000 ft) |
Reference Station Instruments |
Magnetometer Sensor |
Same as airborne |
Magnetometer Interface |
Sander Geophysics - SGRef |
Range: 20,000 to 100,000 nT |
Resolution: 0.01 nT |
Sampling rate: 2 Hz |
Data Acquisition System |
Sander Geophysics - SGRef |
This system runs SGL data acquisition software capable of
recording an unlimited number of channels at variable intervals, and
includes a digital scrolling chart display of the data. Data is
recorded on a vibration tolerant removable drive. The system clock is a
quartz time standard automatically synchronized to UTC by the GPS signal
to an accuracy of 1 millisecond. |
Power Source |
12 VDC can be wind or solar powered |
Navigation Instruments |
Global Positioning System |
NovAtel - GNSS (Global Navigation Satellite
System), reference and airborne |
Sampling rate: 20 Hz |
Real-Time Differential GPS |
Satellite link to the aircraft for real-time in-flight
differential GPS (RDGPS), if required |
|