Scott Thomson, Scott Adam and Peter Hatherly presented their case for detailed logging of inseam boreholes, and the value of the integration of this data into the geological modelling process, at the recent CRCMining Australian Mining Technology Conference.

The researchers have been developing tailored geophysical logging instrumentation to suit Australian Medium Radius Surface to Inseam and underground inseam boreholes.

To date the traditional forms of exploration – such as vertical boreholes, physical sampling and regional structural determinations largely based on interpretation from geophysical methods such as seismic and magnetic prospecting – have been used on prospective underground coal mining leases.

“Although this methodology has been generally adequate in defining the resource, geological surprises in underground longwall mining still do occur, often with costly outcomes,” the researchers said.

“The recent development of Medium Radius Drilling [MRD], primarily for gas extraction purposes, has provided a hitherto unforeseen opportunity for gaining valuable geological data from the plane of the coal seam itself, rather than relying on surface-based geophysical methods or interpolation between vertical boreholes.”

While in theory the concept is good, the problem lies in the equipment with no specific tool designed to take all the required measurements from horizontal inseam boreholes.

“Geophysical logging of vertical boreholes is commonplace and Measure While Drilling (MWD) and Logging While Drilling (LWD) has been a standard part of oilfield exploration for many years. The challenge is to transfer the techniques and equipment from oil field experience to the specific requirements of horizontal inseam boreholes in coal seams, at a reasonable cost, and to ensure that the data provided can be usefully and conveniently adopted for coal mine exploration,” they said.

While recent ACARP projects have shown the benefits of using inseam logging to assist geological interpretation from inseam boreholes, it has also shown current logging tools are not suitable for deployment in Australian inseam logging configurations.

To date, the best results have come from Deutsche Montan Technologie (DMT) shuttles in Germany. The key is to tailor the tool for Australian conditions.

The researchers said a number of field trials have been conducted over the past two to three years from a highwall at the German Creek mine in Central Queensland to explore an extension of Central Colliery and from an MRD SIS borehole in the Illawarra, NSW.

The German Creek site was expected to traverse a number of igneous intrusions and could therefore potentially provide valuable mine planning data. Two long inseam horizontal boreholes were completed and logged.

According to Thomson, Adam and Hatherly, a range of geophysical techniques was demonstrated at the German Creek site but the most useful proved to be the Directional Gamma System (DGS) and the Coal Combi Sonde (CCS) developed by DMT.

The most useful sondes were directional gamma, gamma-gamma (or density), and acoustic caliper. This suite of tools is enough to elevate inseam drilling geological interpretation from the current status of rudimentary to non-existent, through to a high confidence interpretation that can be used to validate and improve the geological model.

“The DMT shuttles provided valuable data but were clearly not designed for easy application in inseam directionally drilled boreholes of the type common in Australia. A logging tool that addressed this ease of deployment issue is required.

“Other issues that also influenced this decision included the size of the DMT shuttles [large modules with some now out-of-date electronics], limited battery life for pump down and retraction from long Medium Radius Surface to Inseam Drilling boreholes, mechanical engineering problems associated with the deployment of a radioactive source in a non-vertical borehole, and the non-IS [intrinsically safe] nature of the CCS probe.”

Thomson, Adam and Hatherly said the key ingredients for an improved logging system included:

Compatibility with all popular drill pipes (therefore must be capable of being run in drill rods of ‘N’ size at a minimum);

Must be designed IS in order to be used in underground boreholes;

Must incorporate directional gamma, density and acoustic calliper sondes (at a minimum);

Ideally should be linked to drill performance sensors such as Torque/Thrust and the Pressure Logger; and

Must be easily deployed in order to minimise rig down time during data acquisition.

CRC Mining has lodged a patent application for a logging tool that satisfies the above, and incorporates some unique features that specifically address the requirements of logging in a horizontal borehole predominantly drilled in coal.