b'FeatureDiscovery and geophysics of the Khamsin depositPlate 4 in Figure 12.2 seems not to require polarizationConclusionproperties to simulate the datapossibly because galvanic currents associated with its higher conductance are suppressingAn important step in the discovery of the Khamsin deposit was or overwhelming any polarization effects. Plate 4, however,recognition that the double peaked gravity anomaly could be deserves mention because it is almost coincident with a minorcaused by a low density palaeochannel within the cover rocks magnetic high and a flattening of the gravity gradient seenand passing over the source of the central gravity high. It led to in Figure 12.1, and it might represent dense, magnetic andthe development of a model comprising dense and magnetic conductive mineralisation that has not yet been tested.ovoids arranged so as to suggest the classic IOCG scenario, while The plate-in-host model may not be the best for the presentstill simulating the data, and thereby elevating the geological purpose, but it is the only code to which we have access thatcredibility of the model to a point where drilling between the includes the full effects of the conductive host on the response ofpeaks seemed worthy of the risk. Gravity and magnetics are well a target body. Figure 8.1 hints that the anomaly attenuation dueestablished techniques in the search for IOCGs; however, the to increasing depth-to-top, is rapid (compared to potential fieldKhamsin experience indicates that explorers need to be alert anomalies), and it might be argued that it is the upper surface into features that can disrupt the apparent coincidence of gravity contact with the cover rocks that would dominate the response ofand magnetic anomalies. a block-like body. The thin plate model could therefore be betterIt seems that part of the deposit can be seen in the conductivity than what might be expected. However, the thin plates createdepth-slices and cross-sectionsat least when the gravity and a dilemma for resistive bodies. Setting the plate conductancemagnetics hint at where to look in the images. This at least to zero in a 0.025S/m halfspace yields a weak conductivityleaves room for optimism in applying TEM in the search for low in the CDI; however, this might not be enough to simulateIOCGs elsewhere. some resistive features. Normally one would simply increase the plate thickness but since this option is not allowed by the thinWe believe that the joint interpretation of magnetic and gravity plate model, we have had to dip into the non-physical region ofdata remains the most important method in the search for IOCGs. negative conductances to simulate some features in the data. However, a qualified success at Khamsin suggests that TEM can Figure 12.1. Response of nine flat-lying plates in a 0.025 S/m halfspace. Figure 12.2. When Plate 1 is given zero conductance, the full extent of Plates5and 9 are evident. Table 2. TEM model with non-interacting plates in a non-polarisable 0.025 S/m halfspace. Plate Length Width Depth Dip Sig-P Cole-Cole ParametersS m tau c1 1500 1450 70 0 8.0 0.0 0.001 0.252 400 470 75 0 5.0 0.0 0.001 0.253 900 950 85 0 6.0 0.0 0.001 0.254 500 670 420 0 62.0 0.0 0.001 0.255 1500 550 460 0 11.0 1.4 0.008 0.256 400 550 50 0 -0.98 0.0 0.001 0.257 1300 300 420 0 -30.0 0.0 0.001 0.258 900 900 250 0 8.0 0.0 0.001 0.259 1500 820 460 0 12.0 1.1 0.005 0.2550 PREVIEWFEBRUARY 2024'