b'FeatureDiscovery and geophysics of the Khamsin depositTable 1.Bodies of the density/susceptibility model discussed in the text.App%Mt and App%HtS mean apparent percent magnetite and apparent percent hematite+sulphides respectively.Body (m) Depth Depth extent Mag Sus (SI) Density (t/m**3) App%Mt App%HtS (Felsic) Tot Mass (t) CentroidE NDeep magnetic ovoid28 870 530 0.194 0.123 5.21 0 3.62E+08 729485 6547589Dense non-magnetic bodies making up the funnel-shaped ovoid66 457 411 0 0.73 0 31.06 1.48E+08 729249 654756168 457 411 0 0.7 0 29.79 1.56E+08 729819 654759169 457 411 0 1.01 0 42.98 6.57E+07 729797 654820470 457 411 0 0.76 0 32.34 6.87E+07 729400 654814171 457 411 0 0.93 0 39.57 7.29E+07 729937 654794972 457 411 0 1.17 0 49.79 1.14E+08 729193 654787773 457 411 0 0.84 0 35.74 2.44E+08 729478 6547342Other dense bodies with economic potential75 440 300 0 0.8 0 34.04 1.50E+08 730068 654864376 430 180 0 0.8 0 34.04 8.39E+07 731270 654771382 380 400 0 0.8 0 34.04 5.15E+07 730222 654892085 350 160 0.005 0.6 0.19 25.34 2.18E+07 730467 654821587 330 180 0.005 0.6 0.19 25.34 5.20E+07 730207 654823788 320 250 0.005 0.6 0.19 25.34 4.93E+07 730710 6548165Bodies that simulate the interpreted palaeochannel111 200 50 0 -0.5 - - 2.07E+07 729297 6546412115 200 100 0 -0.7 - - 5.66E+07 729495 6547658119 200 80 0 -0.5 - - 5.45E+07 729909 6549146anomalies, where the induced currents in a target can persist untilshut-off, it predicts horizontal circularly symmetric E-field loops, after the decay of the currents induced in the host, at which timeinitially under the transmitter loop with donut-shaped magnetic they become apparent. In vector displays of the scatter currentsflux linkages that diffuse slowly downwards and outwards, in a on the target, they have a toroidal (or vortex) form, and plate-in- phenomenon he aptly named a smoke-ring. free-space codes can be used to interpret the last few channelsHost currents of the smoke-ring tend to get diverted into (possibly stripped of host effects) of the decay. Thus, the earlyconductive inhomogeneities (as a path of least resistance) workers were essentially correct as far as their understandingand are diverted around features that are less conductive of TEM theory went - because no one seems yet to havethan the host, in a process called Galvanic Current Gathering demonstrated LMI responses over a known IOCG occurrence.(GCG). Vector displays of GCG currents on a plate-like scatterer With the advent of more powerful mainframe computers in theshow filaments flowing from the upstream end (as defined 1960s and 70s, came the efforts of workers like Hohmann (1975),by the direction of host current flow in the host) to the Weidelt (1975), Lajoie and West (1976), Hanneson and Westdownstream end of the conductor. If the currents in the host (1984) and Walker and West (1991) who produced codes thatare subtracted off the current flow pattern, then the residual solve the plate-in-conductive-host problem. They simulate, in thecurrents have return currents that are outside the plate as frequency domain, currents that flow in both the conductive hostillustrated diagrammatically by West and Macnae (1982) and and in plate-like targets, the latter sometimes having conductancesMcNeilletal.(1984). that can range over several orders of magnitude. The improvedThe enhanced (or depleted) currents of this electrical effect understanding was quickly extended to the time domain (Rai 1982)radiate their fields back to the surface where they alter the with Fourier transform algorithms like YVESFT (Holladay 1981)local field being measured, and the effect (at the target) is a that yield the impulse response, and, in Holladays implementation,maximum when the smoke-ring sweeps past. The donut-shaped also carry out the convolution operation that converts the impulsemagnetic flux linkages carried with the smoke-ring induce LMI response to that of any practical transmitter current waveform.vortex currents at the same time, but these dont become the Nabighian (1979) devised an important analytical solution fordominant current form unless the target is conductive enough the time domain currents in a homogeneous halfspace duethat they persist until the E-fields of the smoke-ring have to a circular current carrying loop at the surface. After currentdissipated sufficiently. Also, GCG effects saturate at low plate 46 PREVIEWFEBRUARY 2024'