Anisotropy Effects on Microseismic Event Location
A
NDREW
K
ING
1
and S
HAHRIAR
T
ALEBI
2
Abstract—
Seismic anisotropy in sedimentary environments is signiFcant—microseismic waveforms often
show strong shear-wave splitting, with differences reaching 40% between horizontally and vertically-polarized
shear-wave velocities. ±ailure to account for this anisotropy is shown to result in large microseismic event
location errors. A method is presented here for determining the Fve elastic parameters of a homogeneous,
vertical transverse-isotropic (VTI) model from calibration shot data. The method can also use data from mining-
induced seismic events, which are then simultaneously located. This simple model provides a good Ft to arrival
times from coal-environment data, and results in dramatic shifts in interpreted event locations.
Key words:
Anisotropy, transverse isotropy, coal mine, microseismic, source location.
1. Introduction
An isotropic material is one whose properties do not vary with the axis along which
they are considered. The elasticity of a homogeneous solid in general is expressed by a
6
·
6 stiffness matrix relating strain and stress components. ±or an isotropic medium, the
elements of the matrix can be deFned by two independent parameters. Most rocks,
however, have material properties that do depend on the axis of observation, hence they
are called anisotropic. The stiffness matrix being symmetric due to simple thermody-
namic considerations, the number of independent elements of the stiffness matrix is
reduced to 21. Thus anisotropy in its most general case can be expressed by 21
independent elastic moduli. This number is further reduced if the material has any axes or
planes of symmetry. The combination of such elements of symmetry can result in eight
anisotropy systems (e.g., G
IBOWICZ
and K
IJKO
, 1994). Transverse Isotropy (TI) is one type
of anisotropy that has been extensively studied in soil and rock mechanics (e.g.,
T
SVANKIN
, 1996; L
IU
et al.,
2000; R
ASOLO±OSAON
, 2003). The existence of one axis of
symmetry in this case, often along the vertical direction (VTI), reduces the number of
independent elastic parameters to Fve.
Most seismological modelling assumes that the earth is isotropic, though anisotropy
studies are becoming an increasingly important part of seismology research (S
HEARER
,
1
CSIRO Exploration and Mining, P.O. Box 883, Kenmore, QLD 4069, Australia.
E-mail: Andrew.king@csiro.au.
2
CANMET-MSSL, 555 Booth St, BCC 10, Ottawa, ON K1A 0G1, Canada.
Pure appl. geophys. 164 (2007) 2141–2156
Ó
Birkha
¨user Verlag, Basel, 2007
0033–4553/07/112141–16
DOI 10.1007/s00024-007-0266-8
Pure and Applied Geophysics