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Characterisation of Overburden Rock Mass and Top Coal Delineation Using Triaxial Drill Bit VSP Method

Open Cut » Geology

Published: May 14Project Number: C19020

Get ReportAuthor: Shivakumar Karekal, Nobukazu Soma and Toshiyuki Tosha | CSIRO, National Institute of Advanced Industrial Science and Technology (AIST), Japan

Coal loss and coal dilution is a serious problem in open pit coal mines. Poor structural characterisation of rock mass can lead to poor fragmentation which could hamper the productivity of draglines often used in cast blasting to remove the overburden. Coal loss and dilution can amount to 8-10%, leading to substantial revenue losses for coal mining industry. Therefore, identification of underlying rock mass structures including delineation of major fracture and fault planes and surface morphology of the top surface of the coal seam is important. The location of the surface of the mining seam in advance of drilling enables driller to stop the blast hole at a standoff distance, thereby reducing coal loss.  There is a clear need for developing cost effective techniques to delineate the top surface of the coal seam.

 

We have studied a low cost exploration technique using passive seismic signals generated during the drilling of blast holes to identify the subsurface structures including the coal seams. The method adopted is one of the Seismic While Drilling (SWD) known as Tri-axial drill-bit VSP (TAD-VSP) which is based on a time-spatial cross correlation analysis method, and has many similarities with the seismic interferometry in terms of the signal processing.

 

Numerical modelling was employed prior to conducting SWD field trials to gain more insight into the wave propagation and to optimise the location of the 3-Component downhole sensor. This was followed by the field trials at two different mine sites.

 

Field trial results for the identification of subsurface features are presented in this report. The subsurface features were compared with the geophysical data for ground truthing and are discussed. The results suggest the method has a potential use to detect subsurface features.

 

In addition, a new drill model was established to characterise the rock mass based on the principles of cavity expansion theory with a material softening constitutive equation. The model is modified to account for various drill parameters such as weight on bit, revolution of drill bit per minute, number of drill bits in contact with rock and rate of penetration to estimate the compressive strength/hardness. An advanced cluster technique was adopted to characterise the overburden layers based on the strength distribution. A good correlation was obtained between the model predicted layers and that of core-log data. The methodology developed here would facilitate the characterisation of overburden strata using drilling data for cast blasting applications.

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