Moment tensor analysis of Moment tensor analysis of acoustic emission data from a acoustic emission data from a triaxial laboratory experimenttriaxial laboratory experiment

D. Kühn, V. Vavryčuk, E. Aker, F. Cuisiat, M. Soldal, K. D. V. Huynh

3rd Annual AIM Project Meeting, 10 – 12 Oct 2012, Smolenice, Slovakia

Motivation

• growing interest in induced seismicity to monitor reservoirs (petroleum, geothermal, CO2 storage)

• need to better understand induced seismicity associated with – rock deformation– failure of rock– reactivation of faults– reservoirs/surrounding formations

• field data: – inadequate instrumentation– insufficent knowledge of velocity structure– poor data quality

• laboratory experiment:– full control over receiver coverage– possibility of repeated measurements of axial and radial P-

and S-wave velocities during experiment

Objectives of the lab experiment

• study initiation of micro-fractures and development of borehole breakouts

• relate resulting acoustic emissions (AE) to:• fracturing process

• associated source mechanism

• macroscopic deformation of the sample

• numerical modelling of sample behaviour/acoustic emissions using a particle flow code (not shown here)

Triaxial experiment

• 12 1-C piezoceramic receivers: full waveforms recorded good coverage for event location and

moment tensor inversion

• dominant AE frequency: 500 kHz• sampling frequency: 10 MHz• typical AE wavelength: 10 mm• source dimension: < 1 cm• moment magnitude: < -4

• axial and radial P- and S-wave velocity: monitor anisotropy• axial and radial strain measurements: relate AE activity to deformation

• sleeve and pinducers from Ergotech• sensor layout adapted from W. Pettitt (ASC) & P. Young (Univ.Toronto)

Test sample

• outcrop sandstone from quarry in the Vosges mountains, France

• intact and perforated sample with a 5 mm horizontal borehole

• porosity: 20%

• uniaxial compressive strength: 48 MPa

• horizontally layered

• length: 12.7 cm

• diameter: 5.08 cm

• testing on dry sample

Four stages of AEs:1)no AEs before point A2)slow linear increase (A to B) development of borehole breakouts

(global contracting behaviour of sample)

3)sharp increase (B to C) localisation of macroscopic shear bands (dilatancy of sample)

4)accelerated phase (after C) catastrophic failure

1 500

2 000

2 500

3 000

3 500

4 000

0 10 20 30 40 50 60 70 80 90

Veloc

ity [m

/s]

Vertical stress [MPa]

VP0

VP90

VSV

VSH

(vertical vP)

(horizontal vP)

(horizontal vS, normal to bedding plane)

(horizontal vS, parallel to bedding plane)

Four stages of AEs:1)no AEs before point A2)slow linear increase (A to B) development of borehole breakouts

(global contracting behaviour of sample)

3)sharp increase (B to C) localisation of macroscopic shear bands (dilatancy of sample)

4)accelerated phase (after C) catastrophic failure

1 500

2 000

2 500

3 000

3 500

4 000

0 10 20 30 40 50 60 70 80 90

Veloc

ity [m

/s]

Vertical stress [MPa]

VP0

VP90

VSV

VSH

(vertical vP)

(horizontal vP)

(horizontal vS, normal to bedding plane)

(horizontal vS, parallel to bedding plane)

Four stages of AEs:1)no AEs before point A2)slow linear increase (A to B) development of borehole breakouts

(global contracting behaviour of sample)

3)sharp increase (B to C) localisation of macroscopic shear bands (dilatancy of sample)

4)accelerated phase (after C) catastrophic failure

1 500

2 000

2 500

3 000

3 500

4 000

0 10 20 30 40 50 60 70 80 90

Veloc

ity [m

/s]

Vertical stress [MPa]

VP0

VP90

VSV

VSH

(vertical vP)

(horizontal vP)

(horizontal vS, normal to bedding plane)

(horizontal vS, parallel to bedding plane)

Four stages of AEs:1)no AEs before point A2)slow linear increase (A to B) development of borehole breakouts

(global contracting behaviour of sample)

3)sharp increase (B to C) localisation of macroscopic shear bands (dilatancy of sample)

4)accelerated phase (after C) catastrophic failure

1 500

2 000

2 500

3 000

3 500

4 000

0 10 20 30 40 50 60 70 80 90

Veloc

ity [m

/s]

Vertical stress [MPa]

VP0

VP90

VSV

VSH

(vertical vP)

(horizontal vP)

(horizontal vS, normal to bedding plane)

(horizontal vS, parallel to bedding plane)

Four stages of AEs:1)no AEs before point A2)slow linear increase (A to B) development of borehole breakouts

(global contracting behaviour of sample)

3)sharp increase (B to C) localisation of macroscopic shear bands (dilatancy of sample)

4)accelerated phase (after C) catastrophic failure

1 500

2 000

2 500

3 000

3 500

4 000

0 10 20 30 40 50 60 70 80 90

Veloc

ity [m

/s]

Vertical stress [MPa]

VP0

VP90

VSV

VSH

(vertical vP)

(horizontal vP)

(horizontal vS, normal to bedding plane)

(horizontal vS, parallel to bedding plane)

3D X-ray CT scan (post test)

red: pore volumegreen: fractured volume

Data analysis: work steps• 2551 events classified as AE (detected on a minimum of 3 receivers)

• 1072 events located recorded on 6 or more receivers:– grid search method

– transverse isotropic velocity model

– InSite software

• correct amplitudes according to calibration

• assume cosine sensitivity function accounting for receiver sensitivity varying with incidence angle

• subset of 305 events with clear P-wave arrivals recorded on at least 10 receivers analysed by moment tensor analysis:

– volume changes during triaxial laboratory experiments interpreted as volume expansion by tensile opening of fractures

– P-wave first onset amplitudes

– Green’s functions assuming a homogeneous velocity model

– linear inversion performed in time domain

– neglecting time-dependence of source-time function

• resulting in 162 moment tensors fulfilling quality requirements

black triangles: pinducersred triangles: transducers (measuring velocities)

left: coloured according to origin timeright: coloured according to phase of experiment green: development of borehole breakouts blue: localisation of macroscopic shear bands red: catastrophic failure of the sample

Decomposition of moment tensors

• irregular distribution• events occurring close to borehole:

higher fraction of isotropic percentage• events occurring in fracture wings:

higher fraction of DC percentages

Decomposition of moment tensors

events close to borehole events located in the fracture wings

• majority of events situated in upper half (opening fractures)• events with small seismic moment: mechanism tensile • events with large seismic moment: mechanism close to pure shear

coloured according to seismic moment

Interpretation of t-k coordinates

Hudson (t-k) plots

events close to borehole events located in the fracture wingscoloured according to origin time

Hudson (t-k) plots

phase 4phase 2 phase 3

coloured according to distance from borehole

Hudson (t-k) plots

Summary

• majority of events indicates tendency of opening fractures

• seismic moments increase with time• double couple percentages increase with time• isotropic percentages decrease with increasing

seismic moment events with small seismic moments are rather related to

tensile mechanisms of explosive character, whereas events having a large seismic moment are related to more ”pure shear-type” mechanisms

• events close to borehole show higher fraction of isotropic percentage compared to events in fracture wings (higher fraction of DC percentages)

• source mechanisms coincide with orientation of fractures developing • strike of nodal line close to strike of borehole• fractures almost vertical => focal mechanisms: normal/reverse faulting• focal mechanisms in fracture wing A (closer to borehole): more scattered,

most likely due to complicated stress conditions close to borehole• significant non-double couple components• positive values of ISO and CLVD: tensile fracturing• vp/vs ratios retrieved correspond to values measured

Challenges

• records too noisy to automatically pick P-wave amplitudes• unknown amplification of sensors (due to unknown coupling)• changes in P- and S-wave velocity throughout experiment

complicate interpretation of non-DC components• anisotropy may cause artificial ISO and CLVD percentages even

for pure shear cracks (Vavryčuk, 2005)• difficult interpretation of moment tensors: correlation of

mechanisms with time, seismic moment, location in sample is inconclusive

• waveforms affected strongly by presence of horizontal borehole and probably specimen’s wall rendering moment tensor solutions computed employing P-wave first onset amplitudes and homogeneous velocity model potentially erroneous

• outlook: full waveform inversion taking into account full geometry of sample will be applied using SEM to compute Green’s functions

• no S-wave distinguishable due to proximity of source and receiver (in most cases)• complex waveforms• long coda waves due to interaction with surface of borehole/specimen’s walls

event close to borehole event located in the fracture wings

strike-slip source with 900 kHz main frequency and ~4mm wavelength

CUBIT software for meshing of 194 710 hexahedral elements

Spectral element method(Komatitsch and Tromp, 1999)

borehole walls: reflectingsample outer walls: absorbing

Direct P wave

Direct S wave

S-to-S reflected wave (borehole walls)

(Komatitsch and Tromp, 1999)Spectral element method

Conclusions

• advanced laboratory equipment to measure acoustic emissions from stressed rock

• initiation and coalescence of microcracks detected from AE around a small borehole

• complex source mechanisms close to borehole, and shear/tensile opening along macroscopic failure plane

• successfully shown that non-double couple events occurred during experiment

• the larger the events the larger the DC component

• SEM is a strong tool to compute waveforms in complex media and thus to improve understanding on potential sources of errors during full moment tensor inversion

Thank you Thank you

for yourfor your

attention!attention!