The Most Advanced Helicopter Magnetic System
Others see magnetic gradient as simply additional sensors that may help sometimes in some way.
We Know Magnetic Gradients
How the geophysical theory applies.
How to achieve low noise and high accuracy consistently under all survey conditions.
How to realize the highest map resolution with the data.
Heli-GT technology provides our clients with consistent, assured results on all surveys, over all terrain and under all conditions.WHY SETTLE FOR LESS?
Heli-GT surveying in the Canadian Arctic
Measured magnetic gradients can improve the resolution of any magnetic survey. It can be demonstrated that the use of measured gradients can have an impact equivalent to 30% closer line spacing; a win-win benefit for both resolution and survey cost. Anomaly shapes can be better defined and a wide range of structural directions can be clearly resolved.
Measured magnetic gradients can improve the magnetic map resolution of any survey at any line spacing at any altitude. To realize this potential, the noise level and accuracy of the gradient measurements is critical. As well, the processing software must also be able to make full and effective use of the high quality data collected.
Consider These Fundamental Basics
1) The closer the magnetometer is to the helicopter the higher the noise will be, with or without compensation.
2) Magnetic gradient is a vector with 3 components in the North, East and Vertical directions. If these are not provided, the gradient survey is at best an approximation.
3) If the orientation and attitude of the gradient array is not measured in degrees relative to North, East and Vertical directions, the gradient survey is at best an approximation.
4) If the complete gradient, as defined by 3 gradient components, is not measured, any gradient output of North, East and Vertical components is at best an approximation.
Measurement Approximations Are A Guaranteed Source Of Error & Noise
Since 2007 the Heli-GT system has accurately measured true North, East and Vertical gradient components to the highest resolution, in a low noise environment 25 m below the helicopter.
No Approximations - Lowest Noise & Highest Accuracy By Design
If only 1 or 2 gradient axes are measured the gradient vector is only partly defined and rotation from aircraft measurement coordinates to geographic coordinates is not possible. Approximating or omitting a component from the rotational transform will lead to error and effectively this error becomes orientation noise.
In addition to avoiding orientation noise the true measured vertical gradient is a valuable asset for height correction. Vertical gradient calculated from profile data is a very poor substitute that only provides the correct value over the centre of long anomalies striking perpendicular to the line direction. The error is greatest in complex magnetic areas where accurate vertical gradient values are needed most.
This 3 dimensional aspect and the importance of measurement orientation was understood in 2007 when we introduced the Heli-GT system. The towed bird approach was necessary to have the freedom for 3 dimensional measurement. There is simply not enough clearance between the ground and the helicopter blades to do a reasonable 3rd dimension with onboard stingers.
A towed bird also minimizes helicopter noise and provides the crew with some added ground clearance for safety.
A helicopter or fixed-wing aircraft is not stable. The more a pilot struggles to stay on line the more he has to introduce pitch, roll and yaw motions. In the helicopter example below the attitude changes were all in excess of +/- 10 degrees. The reported FOM was 0.6 nT, implying a noise envelope of about 0.05 nT , or a gradient noise envelope of about 0.005 nT/m. The realized high frequency noise envelope on the transverse gradient profile example below is closer to 0.2 nT/m. Assuming that the FOM is correct this 40-fold increase in actual noise is a result of error/approximation in measurement direction.
The example below is from a survey carried out with a transverse stinger configuration. Pitch, roll and yaw were not measured but rotation of the helicopter is revealed by the direction cosines calculated from the fluxgate magnetometers used for compensation.
High frequency orientation error noise of about 0.2 nT/m is clearly evident in the transverse gradient. Larger, longer wavelength error is also present and led to an almost completely ineffective “gradient enhanced” map.
A 20,000 km magnetic gradient survey disappointment
Often a “gradient enhanced” total field map is essentially the same as the conventional map created from total field profile data alone. Perhaps the “gradient enhanced” benefits are only noticeable in high amplitude areas where gradient amplitudes are high. Typically the “enhanced” gridding processes provide noise level cut-offs that avoid the problems associated with noisy or inaccurate measurements. In contrast our Heli-GT system accurately measures gradients associated with total field anomalies in the order of 1 nT amplitude and our GT-Grid process uses these measured gradients down to the finest level provided.
The example above illustrates the capability of Heli-GT to accurately and cleanly measure the gradients associated with a weak 2 nT anomaly at a sensor altitude of 30 m. The north gradient peak to peak amplitude is only 0.2 nT/m. The east gradient even less. The GT-Grid process makes use of this fine detail to build the accompanying high resolution magnetic map. Note the continuity and sharp definition of trends approaching the flight line direction.
The benefits of the Heli-GT + GT-Grid system are evident in even the finest detail.
The Heli-GT results in a better map for the same line spacing every time.
High magnetic gradient accuracy is accomplished by 2 key design elements. The first is to measure 3 orthogonal gradient components Grad1, Grad2 and Grad3 The second is to measure the true pitch, roll and yaw of the bird using GPS technology. These stable angular measurements are relative to true north and true vertical. Ancillary equipment on the bird includes a radar altimeter to accurately measure terrain clearance and a 3-axis fluxgate magnetometer for magnetic compensation.
In the example to the right - Once gradients 1, 2 & 3 are measured and fully compensated the pitch roll and yaw measurements are used to rotate the birds gradient coordinate system to a geographic north, east and vertical coordinate system, ready for mapping. No matter what the flight line direction or the orientation of the bird Heli-GT can reliably and accurately determine the true magnetic gradients in the north, east and vertical directions
The Heli-GT system was designed to maximize survey results under all survey conditions.
To illustrate the effectiveness of Heli-GT technology under extreme survey conditions we have intentionally forced the bird to swing through excessive pitch, roll and yaw motions, a range of up to 40 degrees.
In the illustration above one can see that after compensation, the measured true gradients in the east, north and vertical directions remain accurate to better than 0.02 nT/m. even in this extreme example.
For perspective; 0.1 nT/m is the gradient amplitude that would be associated with a 2 nT total field anomaly measured at 30 m terrain clearance.
Housing all of the sensors in a non-magnetic bird, towed 25 m below the helicopter, minimizes helicopter interference. In the accompanying illustration a comparison of the magnetometer measurement noise, at high altitude, from a helicopter nose stinger and the Heli-GT system is presented. The stinger noise envelope, after compensation is about +/- 0.2 nT. In comparison the compensated Heli-GT noise envelope is about +/- 0.02 nT, ten times less.
The reported FOM of the stinger system is not supported by the on-line data shown in the illustration to the right. A FOM in the order of 4.8 nT would be more credible.