In order to test the results that different grid orientations had on the results over linear features, a single 30 metre grid was surveyed at three different angles (test area 3). The first was a normal north-south traverse, then the grid was turned through 45 degrees, and a south-west/north-east traverse was adopted. The same grid was then used to carry out a south-east/north-west traverse. This technique meant that only the centre of the initial north-south grid was surveyed in every orientation, but this still allowed the test to be carried out across a linear trackway.
The results of this test can be seen in above, with the images marked normal sensor. What this means is that the sensor was being carried at about 25-30cm above the ground. The same grid was then surveyed after raising the sensors by 20cm. What is immediately apparent is that the trackway located in the centre of the test grid is detected with varying degrees of success, depending on the orientation of the sensors. It is least visible when walking along its axis (the SW-NE test). After we had been using the machine for three days, we began to realise that the height of the sensor above the ground was affecting the raw data returns, particularly when comparing the difference in the data after changing from one surveyor to another.
The image above shows the different response of the Grad 601-2 across an area, where the darker and lighter areas (here contrast stretched to enhance the differences) indicate different operators (and thus different sensor heights). It was after this survey that we realised that we would have to adjust the sensor height when changing operators, so that the survey was collecting comparable data. We also discovered that the new operator had to rezero the instrument after the sensor height had been adjusted, otherwise the data became even more skewed. The actual difference is shown in test area 3 , where the sensors were raised by 20cm and the grids were re-surveyed. In both cases, the data appears to become much smoother, that is there is less magnetic contrast in the images. This is part of a known trade-off, where sensitivity increases the nearer the sensor is to the ground, but where there is a corresponding increase in background “noise”, caused by a combination of disturbed ploughsoil and iron objects in the upper soil.
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