The monitoring of dam safety, including identifying and minimising geotechnical risks, is a challenge faced by geotechnical engineers, especially in the mining and water supply industries. Microseismic technology provides a continuous solution for the monitoring of geotechnical structures.

Tailings dams are some of the most challenging structures for geotechnical engineers to manage due to the following: 

  • A dam’s body construction is often built with residual materials from mining operations.
  • The continuous production of tailings demands successive dam raisings.

As a result, it has been estimated that the failure rates of tailings dams are 100 ties higher than other dams (Azam and Li in the Tailings Dam Failures (2010): A Review of the Last One Hundred Years article).

On a global scale, tailings dam failures are becoming more frequent, causing significant damage to the environment and loss of life. To mitigate these catastrophic events in the future, Tetra Tech developed a microseismic technology that can monitor the structural stability of tailings dams and identify parameters that may lead to failure. 

 Seismic station from a microseismic monitoring system. This station stores part of the installed equipment in the field for online data transmission.

Microseismic technology

Tetra Tech provides consulting and engineering services and is based in California with operations on all continents. The company has been developing innovative solutions to support operational safety of geotechnical structures, such as microseismic technology.

Tetra Tech signed a partnership with the Institute of Mine Seismology in Australia to develop an innovative technology to be applied in the monitoring of tailings dams in Brazil. Presently, Tetra Tech has installed this technology in over 20 mining dams on an operational scale of 24/7 monitoring regime.

Tetra Tech’s microseismic technology uses one array with multiple possibilities. Based on two monitoring approaches — conventional seismic monitoring and the innovative seismic ambient noise interferometry — the technology continuously monitors internal changes in a structure, using ground motions and seismic wave velocity propagation, which is a media property. 

Sensitivity analysis performed for a site. The recordable magnitude reaches -2 inside the array, which is the monitoring focus area.

The physical properties of a microseismic monitoring system are the same as those applied in a traditional seismic system. The difference is the equipment’s sensitivity. The employed geophones can record the ground motions with high sensitivity, with a magnitude higher than -2 (considering seismic moment), called microseisms, while a conventional seismic system registers events with magnitude over 0.

In addition, the technology is based on a complex architecture. The installation stage is responsible for ensuring data quality. Due to the high sensitivity of sensors and low theoretical error, uncertainty associated with the installation can overcome the method’s uncertainty. Thus, this stage is critical for the monitoring system success.

The installed system is composed of high sensitivity geophones, analog-to-digital signal converters (NetADC), and pre-processing sensors (NetSP). The data is recorded by these sensors and is sent to a seismic server to be processed and analysed, using two different monitoring approaches.

The conventional approach is based on the registration of ground motions that exceed the sensors’ threshold sensitivity. The occurrence of simultaneous triggers in three or more geophones creates a seismic event. For each event registered, the system calculates source parameters, such as magnitude and location. Such parameters can support the estimation of a ground motion prediction equation (GMPE) that can estimate ground motions for a given energy (or potency) and distance. Therefore, this approach provides measured ground motion data on the monitored structures and supports the prediction of ground motions for simulated events, such as mine blasts.

The second approach is a new methodology called seismic ambient noise interferometry. This method measures the seismic wave propagation velocity change rate (24/7 regime) throughout the medium. This parameter is related to stiffness and specific mass, which are, in turn related to pore pressure, fracturing, deformation, and loading. Therefore, anomalous behavior could be identified with high sensitivity, creating real-time data for geotechnical engineers to make information-based decisions.

The sensors (geophones) must be installed correctly, pointing North, in order for a seismic event to be precisely located by the system.

Tailings dams monitoring 

Following the necessity of new technologies to increase the level of monitoring of tailings dams, coupled with the failures of November 2015 and January 2019, Brazilian miners invested in geophysical methods to investigate dam conditions through imaging inside the structure and 24/7 microseismic continuous monitoring to detect possible risks to the dam physical integrity.

Since 2018, Tetra Tech has commissioned more than 20 systems and installed more than 180 geophones connected to 50 seismic stations. Most of the systems are located in Minas Gerais State, which is the Brazilian mining capital.

These systems are continuously processing collected data to support the safety of miners and dams in multiple locations and ground conditions. It provides an extensive data flow to identify early warnings about the integrity of tailings dams and allows the geotechnical engineers to remotely monitor each dam’s structural stability and mitigate instabilities.

Tetra Tech team installing a seismic station. This structure is customised for each site by Tetra Tech, considering all risks involved. For instance, in sites where vandalism actions could impact the monitoring, Tetra Tech develops taller seismic stations. 

Leading with science

The convergence of geophysics, geology and geotechnics in a multidisciplinary framework is essential in the analysis of dam conditions and highlights the need of intervention to support engineering works. Therefore, in addition to the microseismic method, Tetra Tech operates in other front lines, treating geophysics as an integrative science, in order to evaluate and investigate geotechnical structures. 

The applied geophysics group also works on geophysical-geotechnical interpretation, always seeking to give meaning to the geophysical data. Through shallow geophysical methods, it is possible to assess the safety of the structure, identify points of attention and analyse its hydrogeological behavior.

In addition, with the use of conventional geophones, it is possible to characterise anthropogenic seismic sources, quantify generated vibration and analyse how it attenuates or amplifies in the medium. With such information, it is possible to point out which operational activities can act as instability factors on the structures. 

A microseismic system has a complex architecture. The geophones are grounded in the structure. The data collected is sent to an analog-to-digital converter (NetADC). Next, the data is pre-processed (NetSP) and finally sent to a seismic server at Tetra Tech’s office. Moreover, the system has a GPS to regulate the time acquisition for each geophone.

Finally, using numerical modelling, to consider the physical properties of the medium within a geological-structural context, we can simulate seismic sources, such as hypothetical ruptures or blasts, and predict how the ground motions induced by these blasts would propagate in the medium and reach geotechnical structures.

By leading with science, Tetra Tech is focusing on providing its clients with the technology, tool, and innovative solutions to address the dangers of inadequate geotechnical behaviour within tailings dams to improve safety in the mining industry

 

The authors are Camilla Rodrigues and Lorena Oliveira from Tetra Tech