STAR: l’anisotropia sismica come indicatore delle variazione delle proprietà

elastiche della crosta

Ancona, 23-04-2013

Pastori M., Piccinini D., Guerri M., Margheriti L., Barchi M.

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Equivalent to the birefringence in optics

The main technique to study Seismic Anisotropy is based on the Shear Wave Splitting (SWS) phenomena

Anisotropic parametersAnisotropic parameters

Fast Polarization DirectionFast Polarization Direction ΦΦ typically polarized parallel to the direction of the cracks-fractures

Delay Time (Delay Time (t)t),, the lag of the slower wave, it is a measure of intensity and/or thickness of the anisotropic medium

(Cra

mp

in &

Lo

vell, 1991)

Shmin

Vertical Stress

Shear Wave Splitting

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(Extensive Dilatancy Anisotropy Model;

Crampin,1993)

2.2. Stress open Stress open crack EDAcrack EDA

Fluid-filled micro-cracks aligned or ‘opened’ by

active stress field

1.1. Inherited fracture Inherited fracture fieldfield

3.3. Time dependent Time dependent APEAPE

Zatsepin & Crampin, 1995)(Anisotropic Poro Elasticity Model;

(Zinke and Zoback, 2000)

Possible crustal anisotropic causesPossible crustal anisotropic causes

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Pic

cin

ini

et a

l. C

om

pu

ter

and

Geo

scie

nce

20

13

OUTPUTS

• Fast direction and delay times

• Errors on the parameters

• Cross correlation

Anisomat+ is a set of MatLab scripts that use the cross-correlation method to compute anisotropic parameters: fast direction and delay time

1. Three-component seismogram in SAC format (having the same length)

2. P & S picking

3. Event and Station Coordinates

4. Test incidence angle

5. Test signal quality

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Cru

stal

Sei

smic

An

iso

tro

py

in t

he

reg

ion

Comparison between the average values of the anisotropy for the 2010 seismicity (Guerri et al 2012 and the 2000-2001 CDC experiment (Pastori et al., 2012). the comparison show a robust agreement both for the average fast directions and for the average delay time values. Green and purple bars represent fats direction and their length is proportional to the delay time.

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Fast Null

Fast directions vs

Shmin

Guerri et al. 2012

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Pie

tral

un

ga

Sei

smic

seq

uen

ce 2

010

Anisotropy in space and time

Waveforms picking and locatio by AN

Why are ATPI and ATPC different? Lithology? Local stress? 90°flip due to fluid overpressure?

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Manual pickers vs automatic pickers>> >>Anisotropic parameters

ATPCATPIManual pickings

Automatic pickings

1-10hz 0.3s

ATPC

1-10hz - 0.35s

ATPI

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NULLE NULLE

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Manual pickers vs automatic pickers>> >>Anisotropic parameters

Manual pickings- 1-10hz 0.3s ATPCATPI

ATPCATPI

1-10hz - 0.35s

Automatic pickings

NULLE NULLE

Andameno della sismicità 2010

3.8

Tem

po

ral

vari

atio

n o

f an

iso

tro

pic

par

amet

ers

ATPCATPI

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Manual pickings- 1-10hz 0.3s window

Tem

po

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vari

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n o

f an

iso

tro

pic

par

amet

ers

ATPCATPI

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Automatic pickings- 1-10hz 0.35s window

Wo

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ress

…Final remarks

Real Time monitoring of anisotropic parameters?

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The dense seismic network allow to locate many small earthquakes in the “S window” of each station, giving results all the back-azimuths and well sampling the rock volume

Manual pickings seems to give more stable results but on averages the completely automatic procedure give consistent results

We should test selection of the results on the base of picking weight or…?

At Taboo thanks to the geological-and structural knowledge of the area we should be able to understand why we observe different anisotropic parameters in space and time!

Yes go ahead,

Let’s try it!!!

90°flip for high luid pressure; can we see them?