b'Minerals geophysics Minerals geophysicsevening, calculating values with a slideblocked transmitter current injection\x08 We rule or basic calculator then plottingneeded to penetrate the silcrete\x08 Would profiles and contouring plans andexplosives do the job?pseudosections by hand\x08 ContouringThis proved surprisingly easy to organise\x08 resistivity values in log space becameA case of gelignite was purchased over second nature\x08 the counter at the local Burns Philp Store This was a time of fierce rivalry betweenin Mt Isa, along with a box of detonators IP equipment manufacturers - timeand a roll of safety fuse\x08 The gelignite domain versus frequency domain\x08and safety fuse went into the back of the Time domain equipment tended totruck, the detonators into the glove box\x08 be less portable, so the gradient arrayTesting in the field produced marked Terry Harveywas favoured\x08 EM coupling was moreimprovements\x08 We developed quite a Associate Editor forproblematic in the frequency domain,routineopen up a crack in the silcrete Mineralsgeophysicsso the dipole-dipole array was favoured\x08with a crowbar, carefully charge the hole terry.v.harvey@glencore.com.au We were in the frequency domainwith half a stick of gelignite primed with camp, and our receivers were reallydetonator and a metre of safety fuse, just bespoke AC voltmeters\x08 To take aback-fill the hole with rubble, pile rocks, The Wild, Wild West reading, the instrument was nulled withanthills and dead logs on top to contain a potentiometer for a 3 hertz transmittedthe explosion, light the safety fuse, then As with many of you, much of the earliestsignal, the deviation from a seconddrive 100 metres to the next site\x08 Safety part of my career as a geophysicist wastransmitted signal at 0\x083 hertz wasprecautions consisted of taking extra care spent in the field conducting groundmeasured on a dial, then a repeat readingcrimping the safety fuse to the detonator, geophysical surveys\x08 Unlike most ofwas taken at 3 hertz to check for drift\x08using only a wooden skewer to pierce you, this time frame spanned the lateWith these values, plus knowledge of thethe gelignite to accommodate the 1960s and early 1970s\x08 We did things atransmitted current and the dispositiondetonator, not wiping your forehead after bit differently back then! My employerof the four electrodes, apparent resistivityyoud handled gelignite, and ducking was Mines Exploration, the explorationand IP frequency effect values couldyour head inside the vehicle when the subsidiary of Broken Hill South, whichbe calculated\x08 One transmitter-receiverexplosion went off because at 100 metres at that time was still mining the maindipole pair at a time\x08 we were still in fly rock range\x08lode at Broken Hill\x08 We had a mandateThen the rains came\x08 Despite the to explore for both metallic and non- Broken Hill South had extensive ground inimprovements in current strength metallic resources throughout Australia;northwest Queensland over the Cambrianafforded by the wet ground, we decided over those initial years I got to see a lot ofBeetle Creek Formation for explorationto continue using explosives\x08 Things the country\x08 for rock phosphate\x08 Proterozoic rockswent smoothly until one day we bogged Prior to the arrival of the MPPO-1 and thebeneath this were considered prospectivethe vehicle next to the charged hole\x08 development of the SIROTEM time domainfor base metal mineralisation and IP- By the time wed exhausted the easy electromagnetic (EM) instrumentation,resistivity offered a means of searchingoptions and got the doors open, the induced polarisation (IP) was the principalfor blind base-metal deposits in thisburning safety fuse had disappeared electrical geophysics technique used inenvironment\x08 Accordingly, in the earlybelow ground level\x08 We retired to a safe mineral exploration in Australia\x08 Mines1970s, I was working way north of Mountdistance, abandoning the vehicle, then Exploration typically had one or twoIsa in northwest Queensland, surveyingrealised that the box of detonators was IP crews working throughout the year\x08long lines of 100m, n = 3 & 4, dipole- sitting on the front seat and there was a Initially, these were McPhar Geophysicsdipole IP-resistivity\x08 The countryside couldcase of gelignite in the back\x08 Fortunately crews contracted on a yearly basis; latercharitably be called rolling woodlands,there was no sympathetic explosion of the company formed their own crewsand lines were cleared with a lightdetonators and gelignite, but the vehicle using in-house personnel and equipment\x08 bulldozer blade to improve access\x08was showered with mud and debris\x08 After However, it soon became apparent thatthat, we moved the vehicle to the next By todays standards the equipmentour geophysical equipment was not upsite before somebody lit the safety fuse\x08we used was primitive\x08 Transmitterto the task\x08 The problem was inadequate output was limited, receivers were singletransmitter current strength\x08 Much ofNone of this would be possible in todays channel, and results were read fromthe ground surface was highly resistivecomprehensive safety regime, thank dials and hand-written on field sheets\x08silcrete, which, despite lots of effortgoodness, but it did add extra interest to Crew leaders processed the results eachin electrode preparation, effectivelythe survey!31 PREVIEW APRIL 2021'