February 2013


Geophysical logs useful tool for reporting potash exploration results

By Dave Mackintosh

A recent CIM Magazine Standards column described the benefits of using geophysical logs when reporting resource estimates in coal. They are similarly important in potash exploration. Since they are emitters of gamma radiation, potash-bearing beds are easily discernible using gamma ray logs. Carnallite, normally considered an impurity in sylvite mines, can be identified when gamma logs are used in conjunction with neutron density logs.

As pointed out in the September/October issue’s column, logs from adjacent drill holes can ensure consistent sampling in all holes, and the distinctive trace of each horizon enables demonstration of continuity between holes. Sonic logs provide a necessary link between seismic data and the real world. Geophysical logs can assist in determining core recovery, rock mechanics needs and ore grades.

Core recovery

Core recovery is not usually an issue in potash exploration, where recoveries of 98 per cent and higher are the norm. Problems arise when a brine-based drilling fluid, rather than diesel or mineral-based, is used. Changes in mineralogy can become incompatible with the brines resulting in severely washed core. Where losses do occur, mineralization and recovery can be determined by comparing recovered core length to the geophysical logs.

Rock mechanics implications

Water is a major concern in potash mining. Geophysical logs can help determine whether overlying formations are wet or dry, porous or fractured. Salt cover is another important rock mechanics consideration. Where coring is started within the salt section rather than in overlying formations, geophysical logs are invaluable in determining salt cover thickness and the thickness of immediate beds above potential mine openings.

Ore grade

Questions often arise regarding the use of gamma logs to estimate ore grade. This is fine where a mining operation has amassed a large database of gamma to assay correlations specific to their area. But an exploration geologist rarely has this luxury. Methods proposed by Bannatyne (1983) and Nelson (2007) are commonly cited, but applying these methods does not always produce grade values that are in agreement. In his 2007 paper, “Evaluation of Potash Grade with Gamma-ray Logs,” Philip Nelson, a geophysicist with the Energy Resource Surveys Teams of the U.S. Geological Survey, outlined several problems with estimating potassium oxide (K2O) values from gamma logs:

• Gamma rays penetrate adjacent beds, causing the logging tool to respond to these adjacent beds as well as the target beds, which may result in an over-estimation of grade thickness.

• In uranium exploration, the target beds produce much higher count rates relative to the background count rate. This is not the case in potash exploration where the background count rate can be significant relative to the potash beds and should be subtracted out. Not doing so can introduce errors of as much as 1.5 to two per cent K2O.

• Logging speed reduces count rate.

• Gamma ray attenuation caused by cement, casing, tool diameter and mud is a concern. Crain and Alger (1965) produced curves trying to correct for such attenuation. However, as Nelson (2007) reported, correcting data for borehole condition can create more problems than it solves.

All said, gamma logs are very useful for data verification. One of the first things a Qualified Person might do is plot returned assays versus depth alongside gamma logs. The correlation between count rate and assays is an excellent confirmation of correctly positioned core, accounting for lost core and adequate sample intervals. While there are risks in using potash grades determined from gamma logs, the extent of these risks depends on what use is made of the geophysically determined grades. Using log determinations as an interim measure of potash grade until chemical assays are received during exploration may be appropriate. Restricting the classification of any resource blocks to Inferred until grade is confirmed by chemical assay is prudent. And only do this when there has been a database of exploration drill holes where chemical assay to gamma ray-determined grade can be compared and calibrated. In situations where core does not exist and adequate sampling is not possible, a potential quantity – as stated in item 2.3 of the Standards of Disclosure for Mineral Projects – with a range of K2O values can be reported using gamma log determinations.

As in the September/October column, the author considers it best practice to have the core logged by the geologist using the geophysical logs as a reference.

Dave Mackintosh, an independent consulting geologist, is the author of numerous papers on potash, rock mechanics, mining, and geology.

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