State-of-the-art in passive seismic array methodology, And ... · Why arrays? Techniques...

transcript

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

Offset

Surface waves Decay = 1/r

Body waves Decay = 1/r2

Surface Waves

Bensen et.al. (2007)

Surface Waves

Love Waves Rayleigh Waves

Sensitivity

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

Frequency

Coherency

Array Techniques

l  Sensor separation → wavelength sensitivity

Array Techniques

l  Complex array design, 10+ sensors

l  Array beamforming

l  Wavefield characterisation

l  Velocity vs depth

Array Techniques

l  Array design and array response

Kennett, et.al. (2015)

Array Techniques

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)

Array Techniques

State of the art methodology

l  Three component constraints

Love Waves Rayleigh Waves

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

Depth (m

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...

Documents