Regional scale imagery is useful for understanding how the pairs of paddocks are different, where soils are derived from and how they may have been transported. Local scale imagery is necessary to understand how paddocks might be different from eachother, and what soil variation may be present within paddocks.
Here local scale images are presented at a range of scales, depending on the distance between focus paddocks. The legends used are the same as for the regional data, apart from elevation, which has been set up in the order blue (low)<yellow<red (high), to show the range in elevation in a 20km square centered on each pair of paddocks.
Patchewollock
The Patchewollock paddocks are both on lunettes, but the No-Till paddock (east) is on a relatively major lunette, much further from the present lake than the Till paddock, which is on a minor lunette (Figure 16). The sample site for the No-Till paddock is on the west side of the lunette, whereas the sample site for the Till paddock is on the flat between the minor lunette and the major lunette further east. Soils from the Till paddock are likely to have been more recently deposited (underwater) than those from the No-Till paddock, although the sample site in the No-Till paddock has higher thorium levels. Both sites have similar potassium levels,and a similar texture profile.
Figure 16. Closer views of elevation, aerial imagery, thorium and potassium for the Patchewollock paddocks. An EM survey was not done at this site.
Sea Lake
The Sea Lake paddocks are on what is now a billabong, but may once have been the main channel of the Tyrrell Creek (Figure 17). The No-Till paddock (west) is on the main channel of the billabong, whereas the Till paddock is up on the bank. The hourglass-shaped low area in the No-Till paddock, where the sample area was chosen, has a sodic crust at 10-15cm, and a layer of coarse sand at 70-80cm. The sample area in the Till paddock is in a low part of that paddock, but is a more conventional Mallee soil, with lime nodules to depth.
The EM survey on the No-Till paddock picks out that the northern part of the hourglass-shaped low area is high (salty), whereas the southern part of the flat is less salty. The sample area for the Till paddock is also relatively low (less salty) than other parts of the flat in the paddock. In both cases this may be because of runoff leaching these soils more than those surrounding them. The thorium and potassium images both show the sample area in the No-Till paddock to be higher sand, lower clay than in the Till paddock.
Figure 17. Closer views of elevation, vertical dipole EM, thorium and potassium for the Sea Lake paddocks.
Yaapeet
The elevation image shows that Lake Albacutya has, in the past, been much larger, together with Lake Hindmarsh eroding a strandline that probably once went to the west of it. The raised area to the south of the lake (in the bottom left of the elevation image, Figure 18) is probably the lunette of the terminal lake west of Rainbow, rather than a strandline. The no Till paddock is thus on the shores ofan old lake. The Till paddock is on the west flank of a low strandline.
The Till paddock did not have the same range in topography as the No-Till paddock, and an attempt was made to match sampling areas on a soil type basis, with EM to guide uniformity. The sampling area on the No-Till paddock is quite heavy but also lower in the paddock and may have been leached, leading to the lower EM. The sampling area in the Till paddock is on a rise but has relatively high EM. The Till paddock is, on the whole, much higher in EM.
On the basis of thorium and potassium images, the two sample areas are quite similar.
Figure 18. Closer views of elevation, vertical dipole EM, thorium and potassium for the Sea Lake paddocks.
Culgoa
The Culgoa area appears relatively flat on the regional scale, but at local scale some features stick out (Figure 19). The Tyrrell creek runs to the east of both paddocks. The rise to the west appears to be a lunette off the floodplain of the Richardson River extension north of Birchip. The No-Till paddock is on the Tyrrell creek floodplain, whereas the Till paddock is further from the creek, slightly higher and probably flooded less often.
The No Till paddock has a sandy rise in the center and is relatively lower in EM, whereas the Till paddock is flatter and higher in EM. The EM of both sampling areas is similar. Unfortunately the Culgoa area is in the part of the gamma-radiometric data with east-west artifacts. The No-Till paddock sits in the middle of one of these and may be lower in both potassium and thorium than implied by the colours.
Figure 19. Closer views of elevation, verticaldipole EM, thorium and potassium for the Sea Lakepaddocks.
Donald
The Donald paddocks are at the southern end of one of the strandlines, which appears to finish in the high ground at the northwest of the image in Figure 20. The high ground just northeast of the No-Till (northern) paddock is the lunette of a small lake. The paddocks themselves are quite flat, with low ground in the area between the sample sites.
There are no EM maps for these paddocks, but both potassium and thorium images imply that the sample area for the Till paddock is on soil with higher clay. The north end of the No-Till paddock appears sandy on the Potassium image.
Figure 20. Closer views of elevation, aerial imagery, thorium and potassium for the Donald paddocks. An EM survey was not done at this site.
Minyip
The Minyip paddocks are just east of the Dunmunkle creek (Figure 21). The Till paddock is closer to the current path of the creek, whereas the No-Till paddock appears to be on the edge of the former creek channel. There is another, more recent former creek channel between the current creek and Minyip. The southeast corner of the Till paddock contains a small sandy rise, typical of the country to the southeast of the paddocks. The higher ground along the west of the Till paddock appears to have blown from the creek channel.
Both paddocks are relatively high in EM. The high EM patches in the Till paddock probably reflect break-of-slope water from the sandy rises. The sample areas are in parts of the paddocks with similar EM.
The thorium image shows higher levels for the sample area in the Till paddock, indicating that it has been flooded recently compared to the No-Till paddock.
Figure 21. Closer views of elevation, vertical dipole EM, thorium and potassium for the Minyip paddocks.
Summary
The focus paddocks cover a range of southern Mallee landforms and soils. Paired paddocks are certainly not identical in soil type, but in most cases the sample areas chosen are as close a match as could reasonably have been obtained. This analysis should help farmers and agronomists seeking to interpret elevation, EM and gamma-radiometric surveys.