b'Minerals geophysics Minerals geophysicsequipment for the work. We decideddig larger deeper holes for the receiver that a geophysicist on site would notelectrodes and pre-water them to be necessary; getting personnel to siteimprove stability.was difficult, the contractor was very experienced and data quality could beThe survey continued with some monitored remotely. Logistics were animprovements to data quality, but important consideration and timing wasproblems persisted. Faulty line segments critical: we needed to clear lines prior towere re-surveyed, generating many survey commencement, transport therepeat readings. However, some of these geophysical equipment and crew to therepeat readings differed markedly from remote site, then complete the surveyvalues that wed earlier thought to be prior to the onset of the rainy season;OK. Were positional mismatches in areas Terry Harveyreturning next year was not an option. of sharp resistivity gradients a problem? Associate Editor forThe terrain was locally very steep with There were problems from the start.highly resistive silica caps on some of the MineralsgeophysicsLine clearing was delayed when thehilltops. The rainy season had started (it terry.v.harvey@glencore.com.au available labour were re-prioritised tobegan early that year) and we wondered other work - not everyone shares yourwhether moisture in connectors was view on the role of geophysicsanda cause of some of our problems. We When good surveys go bad mobilisation to site of the geophysicalinstigated procedures to minimise water crew and equipment was pushed back.ingress and pushed on.Geophysical surveys do not alwaysA miscommunication between the go to plan. Sometimes the techniquecontractors supervisor, who couldnt beAlthough it took twice as long as is found to be inappropriate or theon site because of an injury, and the crewexpected, we did complete the survey. equipment inadequate, sometimesleader resulted in the use of an incorrectVery late in the programme we found out its the environment that causesdipole size for the first line, which thenthat, unbeknown to us, the contractor unforeseen problems, and sometimeshad to be re-surveyed. was using stainless steel plates rather its the conduct of the survey itself.than the porous pots used in previous Timely recognition of such problemsWith the survey finally underway, resultssurveys as receiver electrodes (dont can save wasted effort and expenditure,began to flow in. However, all was notassumealways check!). We theorised but its not always that simple.well. Negative receiver voltages forthat interactions between the plates and Modification of survey parameters andsome dipoles indicated problems withthe humus layers may have generated practices can often be done on the fly,the bundled cables. Where a negativelocalised voltage differences which but with a firm commitment to survey,voltage dipole was juxtaposed to acompromised many of the IP decay and work already underway, there maydipole with a higher than expectedcurves, but it was too late to run checks.be an understandable reluctance tovoltage, it was probable that just two abandon the survey altogether. Somewires were interchanged and data couldWhy did we persist for so long? There was of these considerations and theirbe reprocessed accordingly. Wherealways the thought that the problems impacts can best be illustrated by anmultiple misconnections were present,could and would be solved. Some never example. recovery of usable data was effectivelywere. So much effort had been invested impossible. There were errors in next- in getting the survey underway that Our particular Survey from Hell wasday overlap repeat readings as well. Atabandonment did not seem a viable a rolling dipole-dipole array inducedour request, all bundled cable segmentsoption, and time constraints meant that polarisation (IP) -resistivity surveyand take-outs were checked, and severalwe could not come back the next year. planned for a rugged jungle-coveredfound to be incorrectly wired wereHaving a supervisor or geophysicist area off-shore from Australia. Somediscarded. Not the best of starts! on site would have been preferable, short lines of IP-resistivity had beenbut it just wasnt possible. Much of completed in the area twenty yearsThere were problems with the IP decaythe resistivity data and some of the IP previously and appeared to work; morecurves too. Many were noisy, there weredata appeared usable, although our recently, extensive IP-resistivity had beenlots of negative (reversed) decay curves,confidence had been shaken by the poor successfully surveyed elsewhere in theand some otherwise normal IP decaysrepeatability in some areas. And we did region. Tenders were called and the workshowed overshoot where a positivehave a possible explanation for the weird awarded to a geophysical contractorcurve would decay past zero to negativeIP decay curves. In the end we made do whod worked well for us in the regionvalues and reversed decay curves wouldwith what we had, but it was not a happy several years previously. We weredecay past zero to positive values. Inexperience.confident that the IP-resistivity surveyshort they were a mess! We doubled the was both feasible in this difficult terraintransmitter dipole size to increase signalSo, when youre next stewing over yet and appropriate to the explorationstrength, modified survey procedures toanother weather-related stand-by day, target sought, and that wed engagedensure that maximum possible currentstake some comfort in the fact that things the right contractor with the rightwere being used, and asked the crew tocould be a lot worse.43 PREVIEW JUNE 2021'