Operations ready, large-scale Passive DD Tomography for mines:
Visualise stress and damage in your changing rock mass

Passive Tomography takes advantage of the precision of modern seismic systems, using the events as seismic sources to compute 3d velocity models of mines.  Changes in the velocity models infer stress change and damage.

The problem in the  past has been coverage and resolution, but using modern seismic systems and double difference tomography, BAE and our associate experts in the field have developed applications of the technology using existing software that are sufficient for assisting in day to day  geotechnical decision making.

How does it work?

When a seismic event is located using a modern seismic system, a simple velocity model for the rock mass is assumed. This results in an error in the calculated event location.

Double difference tomography recomputes the solution for the event location by iterating to produce a new estimate of the velocity model. What is achieved is a 3d ‘block’ model of varying velocity that minimises the error in event location.  The procedure can be repeated as often as needed to compute changes in the mine over time, provided there have been enough sources (seismic events or blasts)  during the period of interest. The number of events, and the distribution within the volume of interest determine the resolution.

Fig. 1: Figure: Passive tomography results showing areas of increased (solid) and decreased velocity (light colours) on a long section through a panel cave.

How is it used?

Increases in velocity imply  increases in elastic strain. Decreases imply density drops—if they are small the changes could be due to de-stressing, while larger decreases imply rock mass damage.

BAE have undertaken back analysis studies and numerical experiments to quantify what these changes mean, and can adjust these  as needed for new environments and applications.

At a number of mines, BAEs associates in this field have been able to achieve useable models on  a  precinct scale, with entire work areas of a mine covered on a month-by month basis.

The results are then:

• Compared to the data from BAEs numerical models to  improve calibration
•  Used to test  for developing problems in the mine
• In some cases, sophisticated numerical models are  used with the tomography results to back analyse the effectiveness of preconditioning or de-stressing.

An example application is shown above. The tomography result shows increases in velocity (stress) and decreases (damage) between two operating areas. The tomography was undertaken for monthly periods over 12 months and provided valuable insight into the monthly changes in the rock mass induced by rapid development of an undercut for a block cave.

BAE sees regular 3d tomography as an important tool in the implementation of simulation aided engineering in mines.

Is your mine ready?

DD Passive tomography makes use of existing data already collected by mines with seismic systems. If you have a quality seismic system, your mine is ready.

The process requires a small effort and cost to undertake on a frequent basis, but can be used to  leverage significant value from existing seismic systems. Passive DD tomography is the closest thing we have to a monitoring tool  for looking inside a rock mass in 3d.

Any mine with an existing seismic system can benefit from implementing regular  tomography to better understand the changes in their mine in 3D.  BAE can assist with a complete tomography service, or arrange parts of the workflow for mines with different needs.

Contact mpfitzner@beckarndt.com.au for more details