High Resolution
Magnetic Gradiometer Surveys
Sander Geophysics offers low level magnetic gradiometer surveys of very high
resolution and accuracy. The gradiometers use two cesium magnetometers with a
fixed separation to measure the earth's vertical and/or horizontal magnetic
gradient. Total magnetic field measurements are recorded simultaneously.
The vertical magnetic gradient offers a better means of detecting near
surface magnetic sources than total magnetic field measurements, making the
gradient method ideal for mineral prospecting. Gradiometer surveying can
obtain exceptionally detailed data by flying at low altitudes and moderate
speeds.
Horizontal gradients provide detailed near-surface information, and help to
interpolate between survey flight lines. This is especially important if the
survey target has similar dimensions to the survey line spacing. Both
gradients are much less affected by diurnal changes in the magnetic field so
they are ideal for use in areas where significant diurnal activity are
expected.
We use optically-pumped cesium magnetometers with a sensitivity of 0.005 nT,
and real-time digital compensation, giving an overall system resolution of
0.01 nT. The magnetometers in the aircraft and in the ground station are
identical, ensuring that all magnetometer data sets are equivalent in terms of
sensitivity and noise envelope. The sampling rate can be adjusted from 2 to
10 Hz depending on survey specifications.
Cesium magnetometers are ideally suited for gradiometer surveying because of
their very high resolution and exceptional stability. Flight direction
changes do not require reorientation of the magnetometer sensors.
Good navigation and accurate flight path recovery are very important
considerations for high resolution airborne gradiometer surveying. Our
aircraft are equipped with Global Positioning System (GPS) integrated into a
proprietary navigation and flight path recovery system. This system, called SGNav,
allows for excellent navigation providing an accuracy of better than 1 m in
post-flight recovery.
Sander Geophysics is well equipped to provide data processing, a variety of
map presentations and interpretation services to enhance the usefulness of its
vertical gradient surveys.
Magnetic Gradiometer 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 |
| Sensor Separation |
Helicopter: 3 m vertical |
| Fixed-wing: 2.64 m vertical, 19 m horizontal |
| Gradient Noise Level |
Less than 0.02 nT/m vertical |
| Less than 0.005 nT/m horizontal |
| 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 synchronizsed 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 (30,000 ft) |
| Ground 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 synchronised 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 24-channel, dual frequency, GPS receiver, (ground
& airborne) |
| Sampling rate: 1 to 10 Hz |
| Real-Time Differential GPS (if required) |
Satellite link (OmniStar or LandSTAR) to the aircraft for
real-time in-flight differential GPS (RDGPS) |
|
