Geophysics holding the geology to account – siting mine infrastructure

Australian Institute of Geoscientists > Applied Geoscience > Geophysics holding the geology to account – siting mine infrastructure

Geophysics holding the geology to account – siting mine infrastructure

Chris Wijns, Minerals co-chair, ASEG-PESA 2015 Conference and Exhibition

Planning infrastructure for a mining project depends on knowing ground conditions over the site. The main geotechnical approaches used, drilling and test pits, suffer from severe areal undersampling. Point measurements are expected to represent large areas and engineers extrapolate and interpolate from the geological logging of few drillholes to characterise sites for various elements of mine infrastructure. Errors in site characterisation can result in costly extra earthworks or wholesale changes in infrastructure layout.

Ahead of infrastructure planning at the Kevitsa mine in northern Finland, the most pressing issue was where to locate the crusher. The mine lease was tight, and amid waste dumps, the tailings dam, and operational considerations of distance from the pit, the crusher had to be located on a solid bedrock high in order to avoid costly excavation through thick, glacial till or very fractured bedrock. Drillholes were sparse outside of the resource area, so the company turned to airborne EM for consistent coverage of the whole mine lease. Prior petrophysical logging of resistivity (Figure 1) showed a sharp contrast between the conductive overburden (glacial till) and the very resistive fresh rock below. There is very little weathering on this site, but where it exists, in highly fractured bedrock, the resistivity occupies an ambiguous middle ground. Inversion of the EM signal into conductivity versus depth served as the basis for picking an interface that would represent the transition from overburden to fresh rock. A system that could record very early time channels was chosen in order to be able to map the very near surface.

Representative petrophysical log

Figure 1: (Top) Representative petrophysical log of resistivity showing a large jump between overburden (glacial till) and fresh rock below. (Bottom) Resistivity statistics from the entire petrophysical database illustrating the large contrast between overburden and fresh rock, plus the ambiguous middle ground of fractured and weathered bedrock.


Figure 2 illustrates the map of overburden depth using a cut-off of 500 ohm·m in resistivity. This ensures that, according to the petrophysical statistics, highly weathered and fractured bedrock is included in the overburden category, since such ground would be unsuitable as a foundation for the crusher. All drillholes at the time of planning are also included in Figure 2, with logged depth of overburden in the same colour scale as the EM-derived depth. Darker colours indicate deeper weathering. Two facts are obvious: there were no drillholes around the crusher location at the time, and the EM shows a deepening of the overburden past 30 m, which is far beyond the 15 m that the engineers were ready to accept for excavation. Based on the existing drillholes with logged overburden to about 10 m depth, and a number of test pits that recorded overburden to no more than 5 m depth around the planned plant and crusher site, the engineers were ready to interpolate between these to assume a suitable site for construction. The EM results threw this planning into doubt, and it was obvious that more geotechnical holes needed to be drilled. These were placed as shown in Figure 3, and confirmed the EM results. Logging comments are included on the figure. The primary conclusion is that previous logging, and especially test pits, recorded the extent of glacial till up to the first instance of bedrock, whether or not this was followed by intense fracturing and weathering. In this area of very deep unstable ground, the later geotechnical holes recorded alternating fractured bedrock and clay/sand layers, which would never be seen in a test pit that stops when the excavator shovel hits first bedrock. Most strikingly, at the original crusher site, these periodic clay/sand layers and fractured bedrock persist to 50 m depth. This would have been a showstopper for the construction phase.

Overburden depth

Figure 2: Overburden depth at the plant site derived from airborne EM data and drilling. Drillholes are coloured with the identical scale as the EM-derived depth, where black is more than 30 m.


On the strength of the EM supported by new geotechnical holes, the plant infrastructure was shifted about 150 m to the northwest, as shown in Figure 3. This brought the crusher onto shallow bedrock according to both EM and drilling results, which now satisfied the engineering team. The EM mapping exercise cost $75,000 for data collection, processing, and inversion for bedrock depth, and saved the company from placing $300M worth of crusher and plant infrastructure in a bad spot.

EM derived overburden depth

Figure 3: EM-derived overburden depth around the crusher with follow-up geotechnical holes and logging results that confirm the EM story. The overburden depth uses the same colour scale as in Figure 2. The final crusher location is 150 m northwest of the originally planned location.


One of the main points of education about geophysical mapping, and in particular this example, is that precise correspondence with drillhole data, on a hole-by-hole basis, is unreasonable. This is apparent in Figure 2, for example. The footprint of a single airborne EM reading is over 300 m2 at the surface (from a loop about 18 m across). The “footprint” of the core from a PQ geotechnical drillhole is 57 cm2 or 0.0057 m2. The EM reading represents a very large volume average and provides a qualitative way to interpolate between drillholes, as well as to verify how well a single hole may represent the rock volume around it. Another ubiquitous caveat on comparing geological logging to geophysical mapping is that visual geological logging is inconsistent, and the greater the number of geologists involved, the truer this is. Geophysical measurements, to their advantage as well as their detriment, are entirely consistent across space and time. (The detrimental aspect is the inability to make informed and adjustable decisions about, e.g., slightly resistive overburden vs. equally slightly resistive fractured bedrock.) In the present case of overburden mapping, the challenge is to determine what the electromagnetic data are logging versus the geological boundary required and logged for in drillholes.

Site characterisation from sparse geotechnical drilling requires a lot of interpretation, from deciding what is important to log, to deciding where to place holes and how to interpolate data between them. The consequences of faulty characterisation can be anything from annoying to disastrous, with the price tag attached. Engineers hate uncertainty, and geophysical mapping of the subsurface can offer a cheap way to reduce this uncertainty beyond what geological mapping (logging) can do. Where there is a distinct physical property contrast to map, the geophysical dataset can be used to hold the geological/geotechnical model to account and make sure it doesn’t miss what is between the drillholes. From February 14-19, 2015, the Australian Society of Exploration Geophysicists, in partnership with the Petroleum Exploration Society of Australia, is hosting the ASEG-PESA 2015 Conference and Exhibition in Perth, plus associated workshops before and after. I encourage geologists to think of case studies where the geophysics was held to account by geological observations, and submit working titles for their presentations at, followed by full abstracts from June 1, for an opportunity to show the geophysicists what they missed between the drillholes.

This article also appeared in the May 2014 issue of AIG News, AIG’s quarterly member newsletter.