introducing a unique new product · introducing a unique new product: ... just noise, appears in...
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Introducing a unique new product:
Magnetic depth transects Calculation of depths to magnetic layers has been automated, allowing a comprehensive set of magnetic depths across the Northern Territory.
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Magnetic depths transects are available, running along lines of latitude, every 5 km southwards. We are looking at magnetic layers, in the top 1 km, stretching across one and a half degrees. In this case the transect runs across the top of the Larrimah map sheet.
West-to-east transects run across the 1:250 kx map sheets. There is also a south-to-north version, with different parameters. Both sets are available on a DVD from NTGS' Geoscience InfoCentre
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North is on the right. We are looking at a transect running from south to north across the Hale River map sheet. I dare say we are looking at the Arunta diving underneath the sediments to the south. There is a set of parameters which can be optimised for any one location. However these runs are automated to run across the entire Territory. It would be a good idea to check both sets over your area of interest because these are different parameters.
Individual depth profiles can be accessed as an overlay on the NTGS’ Geophysical Image Web Server
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The raster is in this case, the geology map of Darwin. I also recommend the greyscale vertical derivative image with the red depth profiles overlaid. These are revised from the ones you saw at AGES last year. in particular, the noisy shallow layers have been cut off. What you are looking at in each profile is the probability of a single layer occurring at any one of these depths between zero and 1000 m, centred on the zero.
Depth profiles derived from the power spectrum of 20 x 20 km of TMI.
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The red square is a cookie cutter dragged across the NT magnetic stitch. Each 20 x 20 sample is Fourier transformed, and the power spectrum is analysed with a formula obtained from modelling. The area is Birrindudu, near the Western Australian border. The image is the vertical derivative of the TMI, showing extensive layer of basalts near the surface. The red dot in the centre is a borehole by Ausquest, passing through 500 m of basalt into the Precambrian sediments below. We are going to see through the top layer to the layers below.
The slopes along each power spectrum provide its depth profile.
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By fitting a series of line segments along the power spectrum, the slopes are obtained. A formula provides the probability of depths. In the second graph you can see that there are probabilities layers at 2000 m etc, however there are many hits in the vicinity of 500 m. Note that I said “in the vicinity of”, this is after all magnetic depths that we are talking about. There is also what appears to be a strong signal between 300 m and the surface. However it is heavily contaminated by noise. The density of hits at each depth to 1000 m is expressed in the depth profile in the third figure here. In mass production, you cannot pause to look at the second figure here to judge subjectively whether there is too much noise. In mass production runs, you must cut out the surface layers.
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The Ausquest borehole logged the magnetic susceptibility, which you see here on the left. On the right is the depth profile with the surface layers removed, rendered as a curve. It is conservative to assume that only one layer is being predicted by the depth profile. The strongest slope in the power spectrum and thus the most likely depth will be given by the layer which is a combination of being the closest, the most magnetic, or the most lumpy of the magnetic layers . I say “most lumpy” because a smooth homogeneous magnetic layer does not give a slope on the power spectrum. However even basalts are sufficiently heterogeneous.
Transect running east from the borehole. With the top layer stripped off, we can trace the next strongest layer.
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The borehole is here, the first depth profile. The transect runs eastwards from the area where the basalts are thickest. This is the layer below the top basalt you see in the vertical derivative image. There is a fairly good probability that we looking at a single layer, that we can trace for hundreds of kilometres. The depth signal breaks up towards the west as if this layer is terminating, or a second layer is competing in the spectra.
Transect running north from the borehole. Knowing that it is a stack of basalts, we see them interfering.
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Away from the thickest basalt stacks, the appearance of “breakup” usually indicates the layer is terminating. Here it is interference between the layers. Possibly there are other structures too, breaking up the profile.
Beyond the basalt, it picks out a lesser mag layer or just becomes noisy.
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The red line is the traverse running from south to north for 200 km. At least two, perhaps three layers of basalt can be picked out by eye: the surface layer , a deep layer in the north, and possibly a non-basalt layer towards the south of the line. The borehole appears to the north-east.
Kudinga basalt surfaces appear on the west, noise to the east…
Suppressing the surface reveals deep behaviour…
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This transect runs from west to east along the top of the Elkedra map. The area is folded, but the layers still give depth signal. ��Our geologists have mapped the Kadinga basalt as surfacing in the west (here), and diving down with the other layers, (here). When the service layer is included in the transect, the basalt comes up clearly in the west, while a distinctively different pattern of near surface noise, just noise, appears in the east. By cutting off the near surface signals much clearer image emerges. In the west of the bottom figure the second layer appears below the mapped basalt. At centre two layers of (what is probably) basalt appear. The basalt when appears to dive steeply down, but be mindful that the vertical exaggeration is about 40:1. Patterns in the far east may support interpretations from other data.
If needed, Euler depths could be used to fill the near-surface gap, as it can discriminate between shallow layers and noise.
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Euler uses a completely different technique that picks out magnetic bodies within the noise. The red bars have a width proportional to the probability of a layer at that depth. We do not supply Euler depths, you would have to get that analysis from your own geophysicist or a consulting geophysicist. The second figure is the geologists interpretation of 1989, off the Frew River map.
A strongly magnetic layer can drown other signal from above and below.
The image shows a distinct near-surface layer that is removed from the transect. The layer that dominates the transect only shows through the image as smudges.
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With some confidence, you can trace a continuous unit. In this case the unit appears to travel underneath a second layer visible as a smudge on the far east of the transect.
Two layers can often be tracked.
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Often, a second layer becomes visible in the transect. It should be somewhat stronger (in magnetically lumpy terms!) to appear on the same depth profile as the upper. Note that the lower layer appears to peter out near the centre of the transect, but that does not imply its absence, just that it is sinking down out of range. �However the technique cannot tell you whether the layers lie above each other in the TMI sample, or whether they are alongside each other. That is, you may be looking at the same layer , vertically thrown. Check the vertical derivative images for poor quality, in which case the signal may fail to appear. On the far west at the start of the Traversimages
Interpreting effect of widely spaced survey data and sample overlap
Overlap of 20 km samples @5 km means a compact body appears as a 4-cell object
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In the far west, old widely spaced data damages the appearance of a second layer in the transect. Then the traverse runs through some good closely spaced data to show a double layer on the transect. Then the traverse runs through some poor data, but the good signal persists for 10 km, because the TMI samples are 20 km wide for the 5 km step-out, the overlap provides sufficient signal. At centre, a distinct four-column repetition of character reveals a compact body. Its signal dominates each of the four depth profiles where is included by the overlap. The apparent depth may be misleading. It is quite evident in the vertical derivative image that very widely spaced data is east of centre. however it is long wavelengths are intact and two deeper layers can be interpreted. Along the far west of the line, the 20 km samples sufficiently overlap the closely spaced data to the south to give a good transect.
Kalkarinji Basalts run at depth further than their interpreted presence.
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Received wisdom says that early in the Cambrian, crustal extension north of the Kimberleys flowed all the way across the Northern Territory into Queensland. The magnetic depth transects show that they are more or less continuous from our western border to the eastern border.
Near-surface basalts show up in the fine detail of the magnetic images, but deeper basalts such as in Arnhem Land are harder to see.
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Already, you can see that basalts run all the way to the coast of the Gulf of Carpentaria. Just not close enough to the surface to be mapped by our geologists. However the basalts do peter out to the south.
Free-air anomaly in the gravity shows the NT sedimentary basins.
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The free air anomaly shows the deviation from neutral buoyancy. The values are inverted, so that in blue (here, here and here) appear the basins, which are large volumes of less-dense material, having positive buoyancy. although the basalts are strongly magnetic, their mass access is often less than the mass deficit due to the sediments above and below them. I may be corrected on this point, but to my eye there are fewer or thinner basalts over some of the Proterozoic (Precambrian) basins. This may be simply due to the buoyancy of sediments below them causing them to dome up and be eroded down. Where the basin overlies the basalt, the magnetic depth profiles can be used to track the basement. However, I haven’t had much success so far. Perhaps if you have the interest, you may find it profitable to sift through our supply of magnetic depth transects. Or you may get your geophysicist to rerun the profiles with different parameters to track your basement interest.
In the south of the NT, tectonic effects have been largely north-south, so use the south-to-north transects
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The image is of the gravity worms, from the NTGS’ Geophysical Image Web Server. They are lineaments derived from our gravity data. In this case I am inferring compression. In a series of compressions across the Proterozoic, the old sediments of the Territory have been reworked, often changing their magnetic susceptibility. Hunting through the south-to-north depth transects will pay off with the intermittent images that allow an event to be tracked… As the Proterozoic sediments have been compressed by a long history
Evidence of the Arunta overthrusting onto the Amadeus sediments. VE ~ 50
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This north-dipping layer appears repeatedly in transects across the Arunta Block. It just begs for explanation. Certainly it is tempting to say this is the bottom of the Arunta. There is detail in the south part of the transect, characteristic of sediments. Note a similar soft character as the transect runs into the Gnarlia Basin.
Sediments are usually magnetically transparent, but that also allows relatively weak signals to emerge from the low noise levels. Here a syncline is evident on several adjacent transects. The transect has been taken from south to north to cross the strike here in the Amadeus Basin.
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The syncline runs from the south-east to the north-west and can be tracked across successive depth transects. It appears to be a single layer within the syncline, and may be useful to identify the stratigraphic unit, so that the transects pick can be used to trace it through the structure of the Amadeus Basin sediments.
Where there is magnetic survey data offshore, the transects can aid the hydrocarbon explorers.
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The image shows shows some ground being released for exploration on the Australian continental shelf. The Timor trench is in the top north-west corner, the Tiwi Islands are hidden in the south-east. Recently, we ran a magnetic survey across the Tiwi Islands…
Under the Tiwi Is, the magnetically transparent Cretaceous sediment overlies the Pine Creek Orogen.
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Two boreholes on Melville Island went to 500 m in Cretaceous sediments without hitting the basement, expected to be the Pine Creek Orogen. The subsequent magnetic survey clearly shows a magnetic basement descending in a series of half grabens. Note the flat area (here) in the north east.
Magnetic layer, believed to be a dolerite in the Pine Creek Orogen. - stepping is apparent
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We are now looking at the westmost of the south-to-north traverses across Melville Island. Note the depth scale goes down to 1600 m, a case of choosing the parameters for the area of interest. The multiple layers to the south are almost certainly faulted versions of the same magnetic layer.
- next eastward
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Again, the layer is tracked northwards. Because the strike is diagonal, the double representation of the layer has appeared further northwards. There is ferruginous material on Melville Island, including strands of heavy sands. However the near surface layer on the north end can be ascribed to noise, due to the otherwise non-magnetic material at depth. The very last depth profile might have some significance.
– continuing
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The layer can be traced down to 1500 m. Rarely can significance be ascribed to signal this deep.
Half graben is clearly picked out at 1350 m
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This is the flat area apparent on the image of the vertical derivative. It is tempting to trace the signal to 1600 m in the north most depth profile.
Magnetic depth transects are a new product, unique to NTGS. They may be obtained on DVD from the Geoscience InfoCentre, and some DVDs are available today at the booth. The magnetic depth profiles can be accessed individually over the web, as overlays on the Geophysics Image Web Server.
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The transects are packed into MPEG movie files, so that the twenty-odd transects on a 1:250 kx map can be stepped through easily and a selected image can be captured. There are 50,000 individuals depth profiles. Although they can be viewed individually on the Image Web Server, I do recommend the transects so that each depth profile can be placed in the context of its neighbours.