INCORPORATION OF ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT) DATA IN GEOLOGICAL MODELLING AS METHOD TO INCREASE MODEL ACCURACY IN UNSERPENTINISED ULTRAMAFIC HOSTED NICKEL LATERITE DEPOSIT

• Main Authors: Budhi, Kumarawarman, Wanni, Wanni
• Format: Conference or Workshop Item PeerReviewed
• Language: English
• Published: Departmen Teknik Geologi 2015
• Subjects: Geology
• Online Access: https://repository.ugm.ac.id/135416/1/GEO6%20INCORPORATION%20OF%20ELECTRICAL%20RESISTIVITY%20TOMOGRAPHY%20%28ERT%29%20DATA%20IN%20GEOLOGICAL%20MODELLING%20AS%20METHOD%20TO%20INCREASE%20MODEL%20ACCURACY%20IN%20UNSERPENTINISED%20ULTRAMAFIC%20HOSTED%20NICKEL%20LATERITE%20DEPOSIT.pdf; https://repository.ugm.ac.id/135416/
• Institution: Universitas Gadjah Mada

Nickel laterites develop as residual soils over ultramafic terrain through the processes of chemical weathering. The formation of this residual deposit is controlled by the nature of bedrock, climatic factor, degree of tectonisation and morphology of ground surface. If geological modelling of nickel laterite deposit is carried out solely based on drillholes, no other data support are used to assist the geologist in correlating the drillholes. Therefore, capability of a model to resemble the actual laterite layers surfaces (i.e, limonite-saprolite & saprolite-bedrock contact) depends heavily on the spacing of the drillholes. The denser the drillholes, the closer a model to the actual geological layer surfaces. In addition to that issue, incomplete drilling is also a problem faced by PT Vale Indonesia, Tbk (PTVI). It is one factor contributing to ore gain in resources and reserve reconciliation, which means that the yielded ore tonnage from excavation is greater than the estimated tonnage from a geological block model. It occurs notably for type of ore hosted by unserpentinised ultramafics. In this type of ore, large ultramafic boulders (up to 15 m) may present in saprolite zone and trick the geologists to conclude that they have penetrated the barren underlaying bedrock. So, they stopped the drilling at the saprolite boulders. Consequently, drillholes and the resulted geological models are not deep enough; the resulted top-bedrock (or saprolite-bottom) correlations are inacurrate as well. When deal with these issues, ERT (Electrical Resistivity Tomography) is proven as powerful method. The ERT approach has the ability to image the variation of the geological surfaces irregularity between drillholes, including the top of the bedrock. Result of ERT interpretation can be a guidance for drilling and help geologist in the drillholes correlation as well. Recent study at unserpentinised hosted laterite deposit in one of Sorowako West Block’s area found that involvement of ERT result to the geological modelling increased the accuracy of the geological model. The study was carried out by comparing the geological models that are generated solely based on drillholes, with geological models that involving ERT as additional data. The comparation revealed that limonite and saprolite bottom surfaces of previous model, that are generated solely based on drillholes, was observed much smoother than the ERT guided model. In fact, over-smoothing is clearly observed in the previous model, disregarding the local laterite surface irregularity between holes. In some areas, the interpreted saprolite-bottom from ERT are deeper than the saprolite bottom of the drillholes, suggesting indication of incomplete drilling. The study also resulted in that saprolite volume of ERT guided model in study area was by ratio 119.5 % bigger than the previous model’s. This result alligns with the resource and reserve reconciliation outcome that suggest an ore gain in the unserpentinised saprolitic ore.