Proceedings of the 12th SEGJ International Symposium, 2015
Scenario to Optimize Nickel Laterite Exploration using ERT with Gradien
Configuration.
Sabrianto Aswad(1), Yudi Surawan(1), Muh. Altin Massinai(1), Wanni(2)
(1)
Hasanuddin University (sabri@science.unhas.ac.id), (2) Exploration Department, PT. Vale Indonesia.
ABSTRACT
Some researcher showed tramendeous result how ERT
could identify zones or lateric deposits profile, more
special in nickel laterite exploration. Mostly the results
claim ERT data used as complementary technique
from existing exploration data, which are drill core
data or test pit. it means that ERT survey done after
collecting drill core data or test pit. This ERT data then
correlated with drill core data or test pit for geological
interpretation on identify nickel laterite profile. In this
research we go forward not only use ERT as
complementary technigue but also the most important
things used ERT become primary data in guidance for
collecting drill core or test pit data. ERT images reveal
nickel laterit deposit profile, which are limonite zone,
saprolite zone and bed rock zone. This result will use
as guidance to choice the position of drilling and how
deep shoul be drilled. Configuration used in this
researh was gradient configuration.
KEY WORDS: ERT, nickel laterite, drill core,
gradient configuration
INTRODUCTION
Nickel laterite resulted as product of lateric weathering
and supergene nickel enrichment developed on
ultramafic rock. It generally happen on tropical and
substropical climate with extensive weathering. The
routine procedure to mine this nickel laterite by using
drilled hole, mostly cored to get imaging of subsurface.
In early stage of exploration drilled hole perform with
sparse interval, for an example every 200 metres. Then
if there was good indication of nickel laterite reserve
drilled hole interval decrease become smaller. At the
eksploitation stage this drilled hole interval becomes
more smaller to get subsurface information more detail
and precisely. Off course more smaller drill hole
interval it cost will be increased. The limitation of
drilled hole data is the limit of coverage area and the
data only show variation of layer vertically. To get
continuity of layering in horizontal perspective should
be done by using correlation or geostatistical anaysis.
Another problem rise from drilled hole data was on
determine bed rock. In this case there is requirement
should be fullfill to decide bed rock, for an example
assumed after drill fresh rock 3 metres means reach
bed rock. In reality some of field has unserpentinised
peridotite as source rock, which has rock boulder
proportion. In this block it could found the thickness of
rock boulder more than 3 metres. It means the
assumption on determine bed rock unvalid. This error
will affect on exploitation technique and reserve
calculation.
Because of those problem we should have an
alternative approach as a guidance related with drilled
hole interval and how deep should be drilled. One
solution that geophysical method offer is direct current
(DC) resistivity
method. Electrical method was
choosen because the resistivity of material ranges over
saveral orders of magnitude allow to distinguished
limonite, saprolite and bed rock optimally. Some
researcher showed satisfy result using DC resistivity
method on identify lateric deposits (Robain et al., 1996
; Ritz et al., 1999 ; Beauvais et al., 1999). Savin et al.
(2003) dan Robineau dkk. (2007 ) has applied this
method for nickel laterite exploration and gave
satisfactory result . There is a good fit between ERT
and Drilled hole data in the same block but difference
field on identify zones nickel laterite profile
(Aswad et al., 2014).
DC resistivity method employ an artificial source of
current injected to the ground the electodes. The
procedure is to measured potential at others eletrodes
in the vicinity of current flow (Telford, 1990). By
understanding geometrical factor and integrate it in
certain formula with current and potential information
to calculate apparent resistivity as physical properties.
Distribution of apparent resistivity measure on the
surface should be tranform by inversion to become
distribution of resistivity below the surface. This
information will make easy to interpret related with
subsurface material and to know the boundary each
layers of nickel laterit profile.
ERT cross section will help to point the position of
drilled hole. The interval of drilled hole not should be
same but depend on some geological variation. ERT
cross section gave also information how deep should
be drill every drilled hole.
Configuration used in this reasearch is Gradient array.
The gradient array can be done by multielecrodes data
acquisition, significantly improve the speed of data
acquisition and in the same time give higher data
density (Dahlin and Zhou, 2006).