b'Minerals geophysics Minerals geophysicsprocessed as per normal. Inversion of thesewell, and there were strong indications for generated a credible layered resistivitysimilar mineralisation within prominent pattern; there was even a small, discretestructural trends through the area. IP anomaly at moderate depth. But thingsHowever, the colour stretch used was did not quite look right: unexpectedly,histogram equal area, the default option resistivities decreased with depth. It wasin many imaging packages, because it only after considerable detective work thatrequires no operator intervention and we realised that the array was corrupttheproduces attractive-looking maps. By local pot wire had remained common forcontrast, our image produced using a all eight receiver dipoles, giving us a dipole- simple linear colour stretch was equally bipole array with a 1600 m n = 0.5 receiverspectacular, but for all the wrong reasons. bipole, 1400 m n = 1.5 receiver bipole,The mineralisation-related response 1200 m n = 2.5 receiver bipole, etc. Withremained, but all other responses in the Terry Harveythis knowledge, the contractor was ablesurvey area were seen to be nowhere Associate Editor to recover field results appropriate to ournear the required magnitude. All our for Minerals geophysics planned hybrid dipole-dipole array; usingnew mineralisation had vanished! As a firstname.lastname@example.org these we generated an inversion patternsimple compromise, I settled for a clipped where the resistivities increased with depthrange linear image which emphasised the as expected. Significantly for drill-targeting,absence of other significant responses, The flip side the IP anomaly was now deeper-seatedbut did retain the structural information, Within our exploration team, one of theand displaced to the east. One take-homealbeit, unfortunately, un-mineralised. geophysicists responsibilities is to advancemessage is that if something doesnt lookThe take-home message here is to check geologists understanding (particularlyright, investigate further! what colour stretch has been used in that of our graduate geologists andthe presentation of results; by their very new starters) of mineral geophysics andData processing nature, histogram equal area colour of its role in our mineral explorationReviewing pre-existing data for a projectstretches will always deliver a significant programmes. This can either take the formarea unearthed a semi-detailed gravityproportion of apparently anomalous of a series of one-on-one discussions,survey, the data for which had beenresponses (e.g. coloured red), whether they or, more commonly, course-stylereportedly tied to the national datumare truly anomalous or not.presentations. Some basic geophysicaland corrected to Bouguer values by the theory is included, but the main emphasiscontractor (this was the only data supplied),Interpretationis on the application of appropriatethen further processed to generate a seriesOne of a series of trial lines of 2D reflection geophysical techniques. Relevant caseof images by the consultant. There wasseismics surveyed in an area of structurally histories, which are an integral part ofa strong regional north-south gradientcomplex hard-rock geology generated a these presentations, typically featurein the Bouguer gravity results, effectivelydisordered pattern nearer surface with a geophysical success. I wondered whetherremoved by taking a residual, whichpersistent, well-defined shallow-dipping problematic geophysics might also have ashowed credible local patterns. However,reflector at depth. The contractors role to play in the education process. Ivethe few Geoscience Australia (GA) gravityinterpretation identified the source of selected one example from each of fourvalues available in the survey area werethis feature as a major sub-horizontal stages in geophysical exploration: surveytotally at odds with these results, meaningthrust fault, the presence of which would conduct, data processing, presentation ofthat we could not reasonably incorporatehave significant economic implications. results, and interpretation. the semi-detailed survey into the broaderHowever, while drill-testing of this feature GA coverage. After some trial and error, wedid encounter a structure at around Survey conduct deduced that the contractor had appliedthe right depth, it was just one of many We had planned to survey a series of linesthe latitude correction in the reverse sense.within an unchanging rock type and had with 200 m n = 0.5-7.5 hybrid dipole-dipoleWe recalculated a latitude correction forno particular geological significance. IP-resistivity using a roll-along array, 400each station, doubled it, then subtractedAn alternative interpretation in this m transmitter dipole leading; one-twistthat from the contractors value. Thestructurally complex area could be that the multicore cables automatically handledsemi-detailed survey results could then besource of this feature might actually relate the receiver dipole connections. At theincorporated into the broader GA coverage.to off-line sampling of a steeply dipping start of the line, the array built up from oneOne take-home message is dont discardreflector striking sub-parallel to the survey receiver dipole to the full eight receiverapparently incompatible survey data out ofline. Mapped geology confirmed the dipole array in seven moves. The weatherhandfind the explanation and you maypresence of a major steeply-dipping fault was extremely hot, conditions for the crewbenefit there-by. zone striking near-parallel to the survey were tough, and it was nearly Christmas.line. A seismic line surveyed transverse Contrary to normal survey practice, thePresentation of results (adapted fromto this fault zone may not directly image receiver operator elected to remainMineral geophysics Preview 190) such a steeply dipping reflector; a 3D stationary at the rear of the array duringWe were surveying an area with sub-audioseismic survey should provide a definitive the build-up, rather than moving forwardmagnetics looking for extensions to knownanswer. The take-home message here 200 m each time, reasoning that the one- structurally-controlled mineralisation. Theis that conventional sedimentary basin twist multicore cables would take care ofimage of the survey results as deliveredstyle seismic interpretation may not be the array wiring. The results from the firstby the contractor was quite spectacular;appropriate in structurally complex hard-line were duly delivered by the contractor,the known mineralisation had respondedrock geological environments.FEBRUARY 2023 PREVIEW 44'