Enhancing Microseismic Monitoring through Strategic Sensor Placement in Long Boreholes (1km+)

Enhancing Microseismic Monitoring through Strategic Sensor Placement in Long Boreholes (1km+)

The integration of advanced technology, including artificial intelligence, plays a crucial role in optimizing microseismic monitoring performance. Yet, the foundation of an effective monitoring system lies in the stragegic placement of sensors around the monitoring area. Optimal sensor arrays lead to superior data quality and deeper insights.

The challenge in underground mining environments often lies in the limited access for sensor installation constrained by the mine geometry. Relying solely on short boreholes deployed from excavations underground may result in a suboptimal sensor array, prone to location artifacts. In contrast, array mixing sensors deployed from the underground excavation and sensors deployed in long boreholes, either from the surface or underground, can significantly enhance the performance of a seismic system.

For boreholes less than 300 meters, the installation is relatively straightforward. The risks of borehole collapse are minimal, sensors can be lowered without mechanical assistance, and grouting can typically be completed within a single shift using basic equipment.

However, installations in boreholes exceeding 300 meters are more complex and require meticulous planning. Key considerations include:

1. Sensor Performance: Longer cable lengths can affect sensor response due to transmission line effects, influenced by the sensor’s output resistor and cable capacitance.

2. Mechanical Assistance: For deeper installations, mechanical devices are often necessary to lower sensors.

3. Borehole Stability: The walls of longer boreholes, especially when not vertical, are more prone to collapsing.

4. Grouting Challenges: Grouting in deeper boreholes demands specialized mixes and cannot always be completed within a shift, requiring appropriate planning and more advanced techniques than conventional grouting.

5. Geological Considerations: The feasibility of drilling must account for local geology to avoid major or problematic structures.

As the superintendent responsible for the deployment and commissioning of the geotechnical system, I assembled a team of highly skilled professionals to develop and execute a novel and innovative procedure. This collaborative endeavor led to the successful installation of 90 triaxial sensors in 15 inclined boreholes, ranging from 800 to 1200 meters, and the strategic placement of additional underground sensors and Elexon cave tracking systems in 30 boreholes, each about 1100 meters long. The success of this project highlights the critical role of teamwork and expertise in advancing geotechnical monitoring practices.

This drilling and installation campaign achieved a 100% success rate, with no sensor damage or losses. Only a few boreholes that needed to be drilled through the orebody required redrilling due to the fractured nature of the rock.

Our installation procedure was straightforward and robust:

1. PQ holes (123 mm) were diamond drilled, extending 15 meters beyond the target depth.

2. After drilling, the rods and casing were removed, and new HQ casings were installed.

3. HQ casings were prepared with a perforated section (~10 meters) at the bottom.

4. The custom-designed sensor string was lowered using a weighted 6 mm wireline.

5. After securing the sensor string, with the last sensor positioned 15 meters from the hole’s toe, grouting began.

6. The grout, prepared with a 24-hour retardant and fluidifier, was transported by concrete trucks.

7. Grouting was performed through the perforated casing, using a specially designed manifold to connect the casing with the grout hose while allowing the sensor cable and wireline to remain in place. The grout then returned around the casing annulus.

8. Grouting continued until the returning grout’s density matched the pumped grout’s density.

In summary, installing seismic sensors in long boreholes can significantly enhance microseismic monitoring, allowing for more accurate event localization and characterization. However, this process is intricate and requires expertise to avoid costly errors.

Acknowledgements: I extend my heartfelt gratitude to the exceptional team whose dedication and expertise were pivotal to the successful sensor installation at the Oyu Tolgoi mine. Special thanks go to Shaun Hogan, Matthew Metz, Christopher Carlson, Batdelger T, Erdenetogtokh Bayar, Enkhtur Lonjid, Munkhtsolmon Munkhchuluun and the entire Oyu Tolgoi geotechnical monitoring and major drilling teams, whose professionalism and camaraderie have left an indelible mark on my memory. Their collective effort was instrumental in the project’s success.

I would also like to acknowledge gary tubridy and the Avalon Sciences Ltd team for providing the seismic sensor strings we installed. The lighthweigh design and the company manufacturing agility allowed us a the high level of customization needed to successfully deliver this challenging project.

Lastly, while this article primarily focuses on seismic sensor installation, I feel it is important to acknowledge the Elexon Mining team. My gratitude is not out of obligation, but born from the genuine pleasure of collaborating with them on the deployment of the cave tracking and smart markers system. Their presence in the post picture is a testament to our joint efforts. Special thanks to Marcus Bigault, Harley Goodman, Jacob Lloyd and Simon Steffen , with whom I sincerely enjoyed working

Below is a hand drawn depiction of the installation proceedure for your enjoyment.