state-of-the-art in passive seismic array methodology, and ... · why arrays? techniques...

State-of-the-art in passive seismic array methodology,

Nick Smith PassiveX Pty. Ltd.

And useful application for mineral exploration

Overview Wavefield Why arrays? Techniques Applications

l  Microtremor wavefield l  Why use arrays? l  Array techniques l  State of the art methodology l  Applications

Overview

Microtremor Wavefield

Microseism

Microtremor

Central Australia North Chile Overview Wavefield Why arrays? Techniques Applications

Surface Waves

Time

Offset

Surface waves Decay = 1/r

Body waves Decay = 1/r2


Surface Waves


Bensen et.al. (2007)

Surface Waves


Love Waves Rayleigh Waves

Depth

Sensitivity

10 Hz

5 Hz

1 Hz

Surface Waves


Surface wave phase velocity sensitive to seismic

velocity structure (Vp, Vs and Density) l  Most sensitive to Vs l  Vs insensitive to fluids (e.g. groundwater)

Why Use Arrays?


l  Fundamental information on cover! −  Cover +/- bedrock velocity structure −  Stratigraphic variability

Techniques


Array concepts: l  Seismic interferometry

—  Retrieve coherent wavefield propagating between

two sensors —  Greens function

l  Array beamforming —  Delay and sum, power distribution

—  Wavefield composition, directionality and velocity

Array Techniques


SPAC l  Circular array, 4+ sensors l  Ave. coherency for sensor pairs l  Independent of wavefield directionality l  Surface wave dispersion

l  Shear-wave velocity vs depth.

Array Techniques


SPAC l  Cross correlation / coherency

l  Isotropic wavefield OR azimuthal average

Azim. Ave. coherency

Coh

eren

cy

Frequency

Coherency

Array Techniques


SPAC

l  Sensor separation → wavelength sensitivity

r1 r2

r3 r4

Array Techniques


FK

l  Complex array design, 10+ sensors

l  Array beamforming

l  Wavefield characterisation

l  Velocity vs depth

Array Techniques


FK

l  Array design and array response

Kennett, et.al. (2015)

Array Techniques


FK

l  Array response steering by coherent signal

— Basic concept: delay and sum of signals

—  In practise: calculate power for different

combinations of back azimuth and slowness

Array Techniques


PSAR SQspa

Kennett, et.al. (2015) FK

Array Techniques


Gal, et.al. (2014)

FK

Array Techniques


State of the art methodology

l  Three component constraints

Love Waves Rayleigh Waves

Depth

Sensitivity

Array Techniques



l  Inversion techniques

—  e.g. Neighbourhood Algorithm

—  e.g. Markov Chain Monte Carlo NA Velocity versus Depth

Array Techniques



l  Markov Chain Monte Carlo

—  Bayesian inference

—  Correct propagation of uncertainty into modelling

result

—  Does not require explicit regularisation or

parameterisation

—  Trans-dimensional

Array Techniques



l  Markov Chain Monte Carlo

Bodin, et.al. (2012)

Applications


Turning noise into signal is really interesting

BUT….. Why is this important for mineral exploration?

Applications


1.  Shear wave velocity vs depth profiles for

greenfields passive seismic work

2.  Assessment of the changeability of “1)” throughout the project area

3.  Assessment of complex geology

4.  Assessment of complex wave propagation phenomena for best modelling results

Applications


1) and 2): Average Vs variation

- H/V resonance frequency depth conversion

- Geological structure

Applications


3) Complex geology

- Multiple large impedance contrasts

- Velocity inversion El

liptic

ity

Azi

. Ave

. Coh

Depth (m

)

50

150

Stra

tigra

phy

500 1500

SPAC SPAC + H/V

Vs (m/s) Frequency (Hz)

Applications


4) Complex wavefield - Higher modes - Sites with large Vs contrasts, or velocity reversals - Source characteristics

n = 1 n = 2 n = 4

niV < ni+1

V < ni+2V...

Applications


Higher modes SPAC using model based “effective mode” and amplitude response for assumed harmonic vertical point sources

Ikeda, et.al. (2012)

Summary


Passive seismic arrays are not just interesting... They are also useful for: l  Characterising the wavefield l  Characterising the geology l  Validating assumptions And are critical for l  Accurate and adaptive large scale passive seismic

surveying

state-of-the-art in passive seismic array methodology, and ... · why arrays? techniques...

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