• GT-Grid
  • Kel Ex example
  • OGS example
  • Dyke Model
  • Pipe Model
GT-Grid: Gradient Tensor Gridding
 
A proprietary technique developed by SHA. Creates a high-resolution grid that honours the measured TMI as well as the measured horizontal magnetic gradients.
 
GT-Grid: Kel Ex Development example.
Kimberlite Detail  
1 2
This illustration presents the GT-GRID total magnetic field map of the Alpha kimberlite at 80 metre terrain clearance and 150 metre line spacing together with the corresponding ground magnetic contours. The northern extension, trending near parallel to the flight direction, is correctly rendered by GT-GRID.
Modern & Paleo-Rivers  
3

GT-GRID of total magnetic field

Terrain clearance 80 m.

Line spacing 150 m.

Contour interval 2 nT.

4

Vertical Magnetic Gradient

Calculated from the GT-GRID of total Magnetic field.

5

Surface Magnetic Component

Calculated from the GT-GRID of total magnetic field.

The lower illustration presents the total field, calculated vertical gradient and calculated surface magnetic component. The sinuous EW anomaly is related to modern drainage that cuts into and removes magnetic glacial till, creating a negative anomaly. The sinuous NS anomaly is related to Paleo-drainage that has cut a channel into the underlying rock surface and provided a keel of thickened till to produce a positive anomaly. These anomalies of about 2 nT amplitude are clearly rendered by the GT-GRID process.
GT-Grid: KEL-EX example.pdf
GT-Grid: Ontario Geological Survey example.
Horizontal gradient measurements were acquired on part of this total field aeromagnetic survey flown at 100 m terrain clearance and 200 m line spacing. This gradient information has been corrected, geo-reference and used to apply the Gradient Tensor, GT-GRID technique to construct an improved total field magnetic map. This illustration presents the total field and calculated vertical gradient maps as published by the OGS on the left together with their GT-GRID counterparts on the right. The improvements in resolution and accuracy provided by measured horizontal gradients with GT-GRID processing are readily apparent.
Published OGS Version
GT-Grid Version
TMFOGSTotal Magnetic Field (OGS)
TMGTTotal Magnetic Field (GT-Grid)
CVGOGSCalculated Vertical Gradent (OGS)
CVGGTCalculated Vertical Gradient (GT-Grid)
GT-Grid: OGS example.pdf
GT-Grid: Dyke Model.
Aeromagnetic surveys are a valuable component of most exploration programs and represent a significant exploration investment. While aeromagnetic survey lines must be close enough to resolve the anomalies and structure of interest; over-specification can increase cost with very little added benefit. The new Gradient Tensor methodology, using horizontal magnetic gradients, can provide higher resolution and greater accuracy at wider line spacing. The example presented here illustrates the technical and economic value of the GT-GRID process.
1True Image

On the left is the modeled response of parallel, 25 metre wide, magnetic linears, with fault offsets of 25, 50 and 75 metres, 100 m below the aircraft. The peak amplitude is 100 nT and the contour interval 10 nT.

Geologic structure is often revealed by dislocations of magnetic axes and contacts. Points of dislocation can often be traced from one magnetic unit to another to define the axis of associated fault plane. The horizontal fault displacement can also be measured from the magnetic axis offset. Vertical displacements may be revealed by a change in the anomaly flanks; the down-thrown side will have a broader response reflecting greater depth. Vertical displacements can also bring a wider or narrower cross section to basement surface and the fault may be revealed by a sudden change in apparent width.

The ability to recognize these often subtle structural details is limited by the quality of the magnetic map. Accurate flight line positioning is a critical necessity but the quality of the griding and map making process is also of fundamental importance. To illustrate, a model scenario consisting of narrow parallel dykes, with fault off-sets of 25, 50 and 75 metres, was created. The corresponding profile data, at 100, 140 and 200 metre line spacing, was compiled using a conventional minimum curvature method as well as the new GT-GRID technique. At 100 m. line spacing, there is a little chatter in the conventional map but both can marginally resolve even the 25 m. displacement. At 140 m. line spacing the interpolation oscillations in the conventional map have visibly increased and by 200 m. are very pronounced. It can be argued that the fault displacements are still notable along the anomaly ridge, particularly when the model is presented for reference; however, the GT-GRID presentation clearly presents the dislocations not only along the ridge but also along the flanks of the anomalies.

The new Gradient Tensor griding process uses measured horizontal magnetic gradients to improve anomaly definition and provide a clear and accurate map presentation well suited to structural interpretation. In the examples presented the consistency between the true image and all of the GT-GRID examples, from 100 to 200 metre line spacing, is quite remarkable. For structural interpretation, the Gradient Tensor method is clearly a cost effective alternative to excessively high flight line density.

2Conventional 100m

3GT-Grid 100m
4Conventional 140m
5GT-Grid 140m
6Conventional 200m
7GT-Grid 200m
GT-Grid: Dyke Model.pdf
GT-Grid: Pipe Model.
Aeromagnetic surveys are a valuable component of most exploration programs and represent a significant exploration investment. While aeromagnetic survey lines must be close enough to resolve the targets of interest; over-specification can increase cost with very little added benefit. The new Gradient Tensor methodology, using horizontal magnetic gradients, can provide higher resolution and greater accuracy at wider line spacing. The example presented here illustrates the technical and economic value of the GT-GRID process.

1

True Image (200nT)

Fight line spacing is always a critical survey specification. The appropriate range is a function of the height of the aircraft above the magnetic sources of interest. A minimum, very tight line spacing would be equal to the height, and a maximum, very loose spacing would be 3 times the height. A good choice would be in the range of 1.5 to 2.5 times the height of the aircraft above source.

A conventional total field aeromagnetic survey will detect the true amplitude of a small target if it is directly over-flown. Should the target occur between flight lines, its true amplitude will not be measured and at a sufficiently large line spacing, its presence or significance will no longer be recognized. The possibility of missing an anomaly can be minimized by closely spaced flight lines but this can be an expensive solution.

The new Gradient Tensor griding process provides a cost-effective alternative. It uses measured horizontal magnetic gradients to improve small anomaly recognition without decreasing the line spacing. The example on the left presents the true anomaly together with examples of the mapping outcome with flight line intervals of 160, 200 and 240 metres. The peak amplitude associated with each example is presented for comparison. Even at a tight line spacing of 160 metres there is a recognizable difference between GT-GRID and the conventional process. The true amplitude is 200 nT, the GT-Grid gradient process presents 164 nT and the conventional total field process presents 123 nT. The ability of the conventional approach to measure and display true amplitude diminishes rapidly in comparison to the Gradient Tensor process.

The amplitude presented by GT-GRID at 240 metre line spacing is essentially the same as that of a conventional survey at 160 metre line spacing. Also note that the true circular nature of the anomaly is preserved by the GT-GRID process, even at the wide 240 m. line spacing.

2Conventional 160 m (123 nT)
3GT-GRID 160 m      (164 nT)
4Conventional 200 m (97 nT)

5GT-GRID 200 m      (143 nT)

6Conventional 240 m (74 nT)

7GT-GRID 240 m      (120 nT)

GT-Grid: Pipe Model.pdf