SRK Exploration realised the full potential of sonic drilling in 2010 when they recommended that sonic rigs should be used at an alluvial platinum project that was struggling to reconcile drilling results and mine production. Sonic drilling was ideal for this project, returning excellent recoveries in soft material while still being able to drill through large boulders in the alluvial profile. All holes were completed without refusal and the results returned grades three times higher than the previous (shell and auger) drill sampling, and allowed the company to realise the full value from its exploration results. Since then, SRK ES used sonic drilling across a wide variety of projects and developed extensive experience and understanding of the advantages and drawbacks of this technique.
Sonic drilling is primarily used to sample cover sequences, but unlike auger, aircore, RAB or RC drilling, it can recover a continuous ‘core’ of intact sample. Returning high quality samples from near-surface unconsolidated or mixed materials is sonic drillings’ main niche. Coupling this sample quality with the ability to drill through hard capping rocks such as silcretes, basalt flows, or large boulders, and return excellent recoveries from underlying loose sediments, makes it ideal for many placer deposits. SRK ES recently recommended sonic drilling for a project in Romania to evaluate dumps that not only contain high grade slag and tailings, but reinforced concrete and other rubble; it is undoubtedly the best drilling technique to sample such a mixed medium.
Sonic drilling uses vibration (in the sonic wavelength) of the drill string and drill bit for penetration, with the driller aiming to achieve maximum vibration (resonance) at the bit where surrounding particles are either fluidised (in loose materials) or fractured (in hard rock). High penetration rates reduce with depth, meaning RC or RAB may become more economic with depths over ~30m. Sonic holes can be drilled dry, but drilling fluids are required through significant hard rock units and in deeper holes. The main drawback of the technique is the high relative cost, being approximately three times the cost of RAB, or one and a half that of RC.
Despite this cost, sonic can be ideal for remote areas where fixed logistics costs are already high, exploration seasons may be short lived and excellent recovery and penetration rates are demanded. In this way, SRK ES have recently helped develop Blue Jay Mining’s Pituffik project, guiding their exploration programme beyond their maiden titanium sands Mineral Resource. Sonic rigs supplied by Eijkelkamp have performed exceptionally well in this medium.
Sonic is certainly not the answer for every project, but it holds the potential of achieving results that prove the feasibility of projects previously unsampled or considered unviable.
 (1)20210330120250934.webp)