It's very recent technique to get 1-D shear wave velocity structure even in aseismic zone.

2.  Ambient noise or background noise is a wide spread noise
presents everywhere. The following are the causes of
generation of ambient noise :
 Oceanic excitation
 Atmospheric activities(Wind, Storm etc.)
 Human Activities
 The ambient seismic noise consists mostly of surface wave.
Low frequency waves (below 1 Hz) are called microseisms
genrates due to natural causes mainly ocean waves.

3.  Primary Wave (P Wave)
 Shear or Secondary Wave (S Wave)
 Most important waves associated with ambient noise correlation
that is Surface wave. There are two types of Surface wave.

4.  The phase velocity(C) of wave is the velocity with which the phase of wave
propagates in medium.
 The group velocity(U) of wave is the velocity with which the overall shape of
the wave , known as envelop of the wave propagates through medium.

5.  The unique property associated with the Surface waves is
dispersion which means the velocity of these waves depends
on wavelength.
 Short wavelength surface waves, closer to the surface, travel
slower than long wavelength waves.
 Rayleigh waves with long wavelength penetrate more deeply in
to the Earth than short wavelength.
 As velocity generally increases with depth, deeper penetrating
long wavelength travel with faster seismic velocity than short
wavelengths. So, Rayleigh waves are dispersive.

6.  In this technique empirical Green's function can be generated
between any pair of seismic station from a seismic network by
cross correlating the continuous noise wave field recorded at
those stations.
 The Green's function obtained from noise correlation consists
of the surface wave which is similar of the surface waves
generated from an earthquake.
 surface waves can be extracted along each station pair of the
network also in an aseismic zone from this technique.

7. Fundamental
Principle

8. Theory

9.  The Koyna–Warna region in the Western Ghats of India is a
globally well-known site of reservoir-triggered seismicity.
 Over the past 50 years the region has experienced 22 M ≥5
earthquakes.
 The seismicity followed the impoundment of the Koyna Dam
in 1962 and subsequently the Warna dam situated about 25
km southeast in 1985.
 The area is covered by Deccan traps, mainly comprising
volcanic basalt, with thickness estimated to vary from 1 to 2
km.
CASE -STUDY

10. Fig. The Koyna- Warna region in western India , comprising the Deccan trap cover and indicating the net work
of 11 seismic broad-band stations (inverted triangles ), along with the station codes and seismisity (M>3) of
the region in the last seven years . The inset shows the location of Koyna-Warna region .The lines joining the
station pairs represent the ray path of Green’s functions , whereas the thick dashed lines show the ray paths
between earthquake epicenters and the seismic stations that were used for dispersion’s analysis.

11.  In the present study, continuous waveform data from a
seismic network of 11 broadband stations in the Koyna -
Warna region of western India (Fig. 1) are used.
 Data is recorded in continuous time series of 12 months
duration on the vertical component of each station are cross
correlated with that of every other station.

12. Raw seismic Data
Remove instrument irregularities, remove
earthquake effect, and cut to length of one day
Cross Correlation (CC)
Stack daily correlations for total
duration
Extract Group Velocity
Inversion for 1D shear velocity
model

13. Fig. (a) Daily Cross correlations of ambient noise between stations SKP and MRT indicating
consistent arrival times of the empirical Green’s functions ,which are stacked (the gray trace at
the top).Asymmetric Green’s functions indicate directivity of noise source , mainly from the
Indian ocean in the south.(b ) Empirical Green’s functions from the correlation of ambient noise
,with respect to station separation (DIST) in the Koyna - Warna region

14. Fig. (a)Example of a cross-correlation function generated for five months for
the station pair SKP-MRT
(b) The peak arrival time of the Rayleigh wave is similar to that of the
conventional surface wave from local earthquake in the region

15. Fig (a) and Fig (b) describing Dispersion curves and best fit corresponding to (a) all the
station pairs used for ambient noise cross correlation are ABG–GKL, KOK–ABG, KOK–WAG,
KOK–GKL, MRT–GKL, MRT–WAG, WAR–ABG, and WAR–WAG (b) For five large recent
earthquake ,The abscissa shows the wave period, and the coordinate is the group velocity of
Rayleigh waves.

16. Fig. The shear wave velocity models obtained along each station pair ,
Numbers are given to each station pair
1 ABG.GKL
2 KOK.ABG
3 KOK.WAG
4 KOK.GKL
5 MRT.GKL
6 MRT.WAG
7 WAR.ABG
8 WAR.WAG

17.  Conventional surface-wave dispersion analysis is carried out
using selected earthquake data to compliment the results of
velocity structure using ambient noise data.
 The results obtained are quite similar, although deviations are
expected in view of variations in the paths, focal depths, and
lateral heterogeneities in the structure.
 The thickness of the basaltic layer is about 1.0 km.
 As the epicentre distances for local earthquakes in the
Koyna– Warna region is very small. it is apparent that the
surface-wave dispersion analysis can only resolve the upper
few kilometres.

18. Depth(km) Shear velocity ( Km/s)
0.0 3.0
0.8 3.3
2.4 3.6
6.5 3.9
Table-2
The Best Shear Wave Velocity model From the Ambient Noise
Cross Correlation Study
Table-3
The Average shear wave velocity model Using Earthquake Data
Depth(Km) Shear velocity (km/s)
0.0 2.9
1.0 3.1
2.1 3.6

19.  The average Deccan trap thickness in the Koyna - Warna region
is estimated as 0.8 km, with a shear-wave velocity of 3 km/s
underlain by a layer with a velocity of 3.3 km/s, which probably
represents a weathered granitic layer.
 A massive granite-gneissic basement is observed beneath the
weathered part with a shear-wave velocity of 3.6 km/s.
 Ambient noise cross correlation technique is very useful
technique to get shear wave velocity structure even in aseismic
zones, which is useful in assessing earthquake hazard. and
near surface corrections for oil and mineral survey.