HIGH PROFILE
Micro seismic tailings monitoring
Surewave micro seismic technology can disregard the
environmental and man-made signals to detect the release
of energy from strata under stress under tailings dams
he industry desperately needs more reliable
and accurate sensors and technology
solutions for tailings dams, not just general
plans to do things better under existing codes of
practice. Before this years Brumadinho disaster at
Vale’s Córrego do Feijão iron ore mine in Brazil, on
November 5, 2015, the Samarco Fundão tailings
dam collapsed, releasing over 32 Mm3 of tailings
into Fundão River, a tributary of Rio Doce River,
overtopping Santarém Dam, and causing what at
the time before the latest incident was considered
to be Brazil’s worst ever environmental accident.
Since this failure, new and sophisticated
technologies have been used to allow for real-
time monitoring of tailings storage facilities, in
order to increase safety on these structures.
One of the most troublesome causes of dam
failures is internal erosion due to uncontrolled
flow of water. This can cause distress to an
embankment dam, and most embankments
exhibit some seepage. However, this seepage
must be controlled in velocity and quantity.
Seepage occurs through the earthen
embankment or dike and/or through its
foundation. Seepage, if uncontrolled, can erode
fine soil material from the downstream slope or
foundation, and continue moving towards the
upstream slope to form a pipe or cavity to the
pond or lake, often leading to a complete failure
of the embankment. Such a failure is usually
referred to as internal erosion or “piping.” Silt and
fine sands – as is usually the case in iron ore
tailings – are most prone to piping, because they
erode easily.
SureWave Technology Ltd, a privately owned
T
64 International Mining | JUNE 2019
UK company, has developed a “unique” micro
seismic technology able to disregard the
environmental and man-made “signals” to detect
the release of energy from strata under stress.
Liquid movement that generates micro seismic
”waves” can also be detected. This can be utilised
to monitor a structure or area for stress and/or
movement of material and liquid.
The system is a new application of a unique
technology predominantly used to monitor mine
stability. The success in monitoring the build-up of
stress during mine operations is invaluable, if
advanced notice to a structural failure can be
given. By analysis of the source mechanics
calculated by the system from the micro seismic
events, it can be demonstrated that stress within
the structure is releasing seismic energy at a
microscopic level. This technique has been used
to forewarn of structural failure: in one example
some 18 weeks warning was able to be given. The
benefits of this are clear in protecting life, assets
and the opportunity for the mine owners to take
preventative measures to prevent the failure.
SureWave Technology with the assistance of
Samarco and Fundação Gorceix, both from Minas
Gerais, Brazil, initially set up a mine monitor
(Model MM2Tri) six channel, at the top of
Germano Dam, within Samarco site. Data was
recorded from 7 December 2016. The system has
solar power and a 3G ‘dongle’ modem connected
to the internet. The cables enable the sensors to
be placed 300 m apart across the dam wall. The
system was moved to the second step from the
bottom on the 7 February 2017. During this time,
significant site activity prevented collection of
meaningful data. This included a hammer drill rig
working up the dam for several weeks. There was
also some mobile network outages, and initial
solar power issues. Between these issues, good
data was collected with micro seismic events
clearly evident both from the dam and the water
reservoir.
The Germano Pit Tailings Storage Facility was
chosen as the ideal TSF for the tests to be
performed. It is a 60 m high upstream dam
designed so as to rehabilitate the old iron ore pit.
It is used for the disposal of iron ore coarse
tailings; typically sand or silt tailings that present
high permeability and allow free draining of the
TSF. The TSF was built with a 5 m high compacted
laterite starter dyke, and subsequent 5 m high
raisings, until it reached its current 60 m height.
Its drainage system is made up of a draining
blanket that was built underneath the TSF starter
dyke. Samarco required information on any
movements within the dam structure, as well as
tracking any moving liquid around the dam wall.
The location of any moving water was of
paramount interest.
The system rapidly started to detect and plot
micro seismic events from the first day of
installation. The locations were an uncertain
factor, as the sand/silt nature of the dam creates
a different velocity model for the system. In
addition, the automatic process of ”picking” the P
and S waves (or body and surface waves) needed
significant and time-consuming changes. This
work was undertaken between December 2016
and June 2017. Also, it was important to make the
system universal, as it must be capable of being
deployed at any such dam without further user
adjustment or application changes (beyond
loading a new site image and such like).
“These requirements for a universal system
have been achieved. Seismic events and moving
liquid were clearly identified, and their locations
plotted on a 3D site image. The system also
creates a CSV file for further analysis if required.
The site velocity model is loaded from a simple
text file. However, the exact event distance,
calculated from the Primary (P wave) and Surface
(‘S’ or Love / Rayleigh) wave time of arrival
difference relies on the exact automatic detection
of the start of these waves. In this application, the
first arrival of these waves is very difficult to
detect, and depends on the amount of
liquification of the material. In this regard, the S
wave will not travel through liquid, and instead we
get surface waves. These have a different velocity.
The movements detected were plotted according
to the source characteristics.”
Material movement is plotted in red, whereas
other (non-P & S) wave is in blue and is assumed