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SEISMIC MONITORING OF LARGE-SCALE KARST PROCESSES

IN A POTASH MINE

A.B. Ivanov, C. Petrov

Mining Institute, Perm, Russia

A. Johnson

Mining Constultants Ltd., Stellenbosch, Republic of South Africa

In October 2006 a serious accident happened in Berezniki-1 mine of the world’s second largest Verkhnekamskoye potash

deposit. The integrity of the waterproof stratum which separates mine workings from aquifers was broken and an uncontrolled

flooding of the mine had begun. The accident resulted in the appearance of several potential problems associated with karst

processes in the rock mass above the mine, i.e. hazardous subsidence of the earth’s surface and formation of a sinkhole. The

exact position of the water inflow zone was unknown due the inaccessibility of the excavations within the problematic part of

the mine. The applied complex of geophysical and geotechnical methods made it possible to localise the inflow zone, to

estimate the size of the expected sinkhole and to monitor its development in time. Seismic monitoring played an essential role

in these investigations. This paper summarises the details of the seismological observations in Berezniki-1 mine and the main

results thereof.

INTRODUCTION

Mining in the Verkhnekamskoye Potash Deposit

The Verkhnekamskoye potash deposit is situated in the

northern part of Perm region (Russia) and comprises of

seven minefields six of which were in development in

2006. Productive layers in sylvinite and carnallite zones

(Figure 1) are 140-420 m below the earth’s surface. The

room-and-pillar mining method is used in all mines.

Figure 1. Schematic geological section of the

Verkhnekamskoye deposit

The main requirement for safe mining in the

Verkhnekamskoye deposit is the keeping of integrity of the

waterproof stratum. This stratum consists of salt-clay rocks

which are laid above the productive layers (Figure 1) and

prevent them against solving by weakly mineralized water

or brines from the upper horizons. Penetration of the brines

through the waterproof stratum may cause unavoidable

mine flooding and forming of a karst depression or sinkhole

on the surface. Such situation had already taken place in the

Verkhnekamskoye deposit previously - in Berezniki-3 mine

in 1986.

An Emergency Situation in Berezniki-1 Mine in 2006

On 18 October 2006 a loss of integrity of the

waterproof stratum and penetration of brines from upper

layers to the mining excavations were identified according

to indirect indicators (dynamics and chemical composition

of brines in mine) in the western part of Berezniki-1 mine.

This area was mined out in the sixties-seventies and most of

the excavations there were partially backfilled or collapsed

which restricted the access to the area. For this reason the

place of brines inflow was unknown within the area of

800x1000 m size. The problem of localizing the inflow

zone was solved promptly by applying a complex of surface

geophysical and geotechnical methods (including

seismological observations). As the result, a 400x500 m so-

called risk zone was contoured.

The following techniques were applied:

• Active surface seismic survey to track the changes in

elastic properties of the near-surface layers.

• Passive seismic monitoring to detect any dynamic

processes within the risk area.

• Borehole and surface hydrodynamical and

hydrochemical observations to monitor the flow of brines

and groundwater in different horizons.

• Land-surveying to measure surface subsidence.

From October 2006 to July 2007 the karst processes

had no visible manifestations. However, the application of

several techniques pointed to significant variations of the

SEISMIC MONITORING OF LARGE-SCALE KARST PROCESSES IN A POTASH MINE - 2 -

controlled characteristics and properties (i.e. gradual

decrease of seismic wave velocity for different horizons

within the risk area, anomalous subsidence (with rates up to

270 mm/month) in the eastern part of the risk zone,

formation of local areas with high concentration of methane

in soils).

At 3 PM of 28 July 2007 a sinkhole had appeared on

the earth’s surface 150 m to south-west from the centre of

the risk zone1. The initial size of the sinkhole was 55 x 80

m. A huge gas and rock blowout had happened next

morning.

During the following several months the sinkhole had

actively grown mostly in the northern and north-eastern

directions toward the federal railway (Figure 2a-b). The

growth decelerated significantly during the winter months,

but nevertheless the eastern slope of the sinkhole reached

the old federal railway in December. During February and

April 2008, a short-time increase in the rate of the sinkhole

expansion was noted in the eastern and south-eastern

directions (Figure 2c).

At the end of 2008 the flooding of Berezniki-1 mine

was completed and the growth of the sinkhole virtually

stopped. Its size had stabilized at approximately 440 x 320

m (Figure 2d).

The Use of Seismic Monitoring for the Study of Karst

Processes

It is plausible that the fracture of waterproof stratum

and karst cave(s) development may be accompanied by

dynamic events leading to the radiation of seismic waves in

the ambient rockmass. We may assume two possible types

of such events:

1) Rupture of tensile and shear cracks.

2) Rock falls from roof and walls of the cave(s).

Registration and processing of seismic signals from

such events may provide information about their basic

source parameters such as: location, time and radiated

seismic energy. An analysis of the spatial and temporal

characteristics of the events may lead to conclusion about

the current state and tendencies of the karst processes. This

is the traditional way of using seismic monitoring for the

study of karst processes (Wust-Bloch and Joswig2006

).

DATA

Regional Seismic Network

A regional seismic network was in operation in

Berezniki-1 mine since 1998. The network consisted of 6

underground and 1 surface vertical geophones installed 1-2

1 Risk zone is an area where sinkhole appearance had been

anticipated.

km apart from each other. The network allowed to reliably

detect all events with seismic energy above 1000 J within

the whole area of the mine (about 50 km2) (Table I).

Table I Characteristics of the regional and the local

seismic networks used in Berezniki-1 mine

Characteristics Regional

network

Local

network

Controlled area [km2] 50 0.5

Average distance between

sensors [m] 1000-2000 150

Minimal seismic energy of

events [J] 1000 10

Frequency range [Hz] 0.5-100 10-1000

The system registered several dozens of events per

year. The largest ones had seismic energy of about 105 J.

The analysis of the source mechanisms showed that most of

the events could be associated with the falls of ground (rock

falls) in the excavations (Ivanov2005

).

The regional seismic network of Berezniki-1 mine has

been in operation until 28 October 2006, when the mine-

wide power supply system was switched off and the access

to the mine was restricted.

It was the termination of the regional monitoring and

the need of more detailed seismic information which

initiated the organization of local seismic monitoring in the

risk zone (Malovichko et al.2009

).

RESULTS

A time lag ( T∆ , days) between the periods of

increased seismic activity and the moments of accelerated

growth of the sinkhole was found. The delay was

proportional to the square of the sinkhole diameter L and it

depended on the direction of the growth (N, E, S or W):

±=∆

±=∆

10.20+220

4,10+150

2

,

2

,

LT

LT

SW

NE

(1)

ACKNOWLEDGEMENTS

The authors would like to thank Alexander Zverev and

Alexander Peresypkin for help in carrying out of the

seismic monitoring in Berezniki-1 mine. We wish to

acknowledge the support of the leading specialists of JSC

”Uralkali” Alexander Shumaher and Nikolay Kuznetsov.

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Figure 2. Evolution of the sinkhole

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