Michael Asten
Email: [email protected] <**> Michael Asten was a Professor (Research), part-time, in the School of Earth Atmosphere and Environment, Monash University, Melbourne, retiring in 2015, and continuing as an Adjunct Senior Research Fellow until 2021. He is now Research Director for Earth Insight, in Melbourne, Australia. He gained a PhD in geophysics from Macquarie University in 1977, worked in academia and the mineral exploration industry in Australia and overseas to1997, and joined Monash part time in 1997. He is recipient/co-recipient of five innovation awards from the Australian Society of Exploration Geophysicists, CSIRO and BHP in the years to 1997. While at Monash he has been recipient of two ARC Linkage grants, two US Army SERDP grants, four US Geological Survey and Geoscience Australia grants. He has current collaborative projects on the use of passive seismic methods for earthquake hazard studies and assessing thickness of soft cover in mineral exploration. These projects involve collaboration with the US Geological Survey, the North-American Consortium of Organizations for Strong Motion Observation Systems (COSMOS), the Middle East Technological University, Turkey, and an Australian resources company.Over the past decade he has also applied methods of time-series analysis to the study of natural cycles of climate change. In 2020 he was an Expert Reviewer for the IPCC Second Order Draft of the 6th Assessment Report (area of expertise natural cycles of climate change and climate sensitivity). He is also a member of the panel of experts for the European Geophysical Union.He served for 3 years as the Australian Geoscience Council representative on the Australian Academy of Sciences UNCOVER Executive Committee, a group providing guidance for development of next-generation technology in Australia for mineral exploration.He has served twice on ARC panels reviewing key centres. Prof. Asten has published as author or co-author 209 scientific papers (including 110 peer-reviewed to the ARC C1 and E1 standards) on a wide range of topics in mineral exploration and earthquake hazard. He has published on climate sensitivity as deduced from deep-ocean records of the Eocene period, and is part of an international group of scientists investigating centennial and millennial natural cycles of climate change and the hypothesis that they are in part subject to astronomical control via cosmic ray flux on the Earth’s atmosphere. He is a regular commentator on climate-change science with 45 OpEds and letters and comments published in newspapers geoscience journals, since 2009.
Phone: +61(0)412348682
Address: 8 Rae st, Hawthorn
Vic 3122
Australia
Phone: +61(0)412348682
Address: 8 Rae st, Hawthorn
Vic 3122
Australia
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assessment of the region. This classification is based on a combination of seismic cone penetrometer tests (SCPTs),
supplemented with microtremor horizontal/vertical spectral ratio studies and published borehole data. This paper
compares shear-wave velocities from SCPTs with shear-wave velocities derived by measuring microtremor phase
velocities using small seismic arrays. Processing the array data using the multi-mode spatially averaged coherency
(MMSPAC) method proves effective in providing direct measurements of sand/clay thickness and a shear-wave
velocity profile which is comparable with SCPT measurements.
The shear-wave velocities derived from microtremor array measurements are found to be within 10% of the SCPT
estimates in the top 30 m of sand/clay. The array data is able to correctly identify low-velocity soft clays under
sands. The method is also effective in regions where a surficial layer of calcareous sandstone would typically
preclude SCPT measurements.
thickness and low-strain shear strength of sediments for purposes of earthquake hazard site
classification. The spatial autocorrelation (SPAC) method of processing data is effective in
yielding wave velocity data from multi- and omni-directional sources. A model study of single
pair, triangular, hexagonal and square arrays shows that the triangular or hexagonal arrays provide
adequate azimuthal averaging for application of the SPAC method to studies of microtremors,
provided the plane-wave sources have an azimuthal spread of order 60o. Plots of the imaginary
part of the averaged coherency provide an additional indicator of quality of the averaging, with
the hexagonal array having a further advantage that the imaginary part is zero for all frequencies
for noise-free data, and hence deviations from zero provide a measure of statistical noise in the
coherency estimates. The use of single pairs of geophones, square or linear cross arrays requires
that sources have an azimuthal spread exceeding 90o, and, even with such a spread of sources,
such arrays may yield averaged coherencies which deviate strongly and variably from the ideal J0
curve. Routine plotting of the imaginary part of the averaged complex coherency provides a
useful quality control on SPAC methods.
thickness and low-strain shear strength of sediments for purposes of earthquake hazard site
classification. The spatial autocorrelation (SPAC) method of processing data is effective in
yielding wave velocity data from multi- and omni-directional sources. A model study of single
pair, triangular, hexagonal and square arrays shows that the triangular or hexagonal arrays provide
adequate azimuthal averaging for application of the SPAC method to studies of microtremors,
provided the plane-wave sources have an azimuthal spread of order 60o. Plots of the imaginary
part of the averaged coherency provide an additional indicator of quality of the averaging, with
the hexagonal array having a further advantage that the imaginary part is zero for all frequencies
for noise-free data, and hence deviations from zero provide a measure of statistical noise in the
coherency estimates. The use of single pairs of geophones, square or linear cross arrays requires
that sources have an azimuthal spread exceeding 90o, and, even with such a spread of sources,
such arrays may yield averaged coherencies which deviate strongly and variably from the ideal J0
curve. Routine plotting of the imaginary part of the averaged complex coherency provides a
useful quality control on SPAC methods.
the method of microseismic zonation of sediment thickness and earthquake hazard to
be extended, giving direct estimates of shear-wave velocity and thickness of
sediments over basement. The method is well-suited to built-up areas where cultural
sources of microseisms are spatially distributed, and can be extended to yield useful
data where microseismic energy propagates in higher as well as fundamental modes.
assessment of the region. This classification is based on a combination of seismic cone penetrometer tests (SCPTs),
supplemented with microtremor horizontal/vertical spectral ratio studies and published borehole data. This paper
compares shear-wave velocities from SCPTs with shear-wave velocities derived by measuring microtremor phase
velocities using small seismic arrays. Processing the array data using the multi-mode spatially averaged coherency
(MMSPAC) method proves effective in providing direct measurements of sand/clay thickness and a shear-wave
velocity profile which is comparable with SCPT measurements.
The shear-wave velocities derived from microtremor array measurements are found to be within 10% of the SCPT
estimates in the top 30 m of sand/clay. The array data is able to correctly identify low-velocity soft clays under
sands. The method is also effective in regions where a surficial layer of calcareous sandstone would typically
preclude SCPT measurements.
thickness and low-strain shear strength of sediments for purposes of earthquake hazard site
classification. The spatial autocorrelation (SPAC) method of processing data is effective in
yielding wave velocity data from multi- and omni-directional sources. A model study of single
pair, triangular, hexagonal and square arrays shows that the triangular or hexagonal arrays provide
adequate azimuthal averaging for application of the SPAC method to studies of microtremors,
provided the plane-wave sources have an azimuthal spread of order 60o. Plots of the imaginary
part of the averaged coherency provide an additional indicator of quality of the averaging, with
the hexagonal array having a further advantage that the imaginary part is zero for all frequencies
for noise-free data, and hence deviations from zero provide a measure of statistical noise in the
coherency estimates. The use of single pairs of geophones, square or linear cross arrays requires
that sources have an azimuthal spread exceeding 90o, and, even with such a spread of sources,
such arrays may yield averaged coherencies which deviate strongly and variably from the ideal J0
curve. Routine plotting of the imaginary part of the averaged complex coherency provides a
useful quality control on SPAC methods.
thickness and low-strain shear strength of sediments for purposes of earthquake hazard site
classification. The spatial autocorrelation (SPAC) method of processing data is effective in
yielding wave velocity data from multi- and omni-directional sources. A model study of single
pair, triangular, hexagonal and square arrays shows that the triangular or hexagonal arrays provide
adequate azimuthal averaging for application of the SPAC method to studies of microtremors,
provided the plane-wave sources have an azimuthal spread of order 60o. Plots of the imaginary
part of the averaged coherency provide an additional indicator of quality of the averaging, with
the hexagonal array having a further advantage that the imaginary part is zero for all frequencies
for noise-free data, and hence deviations from zero provide a measure of statistical noise in the
coherency estimates. The use of single pairs of geophones, square or linear cross arrays requires
that sources have an azimuthal spread exceeding 90o, and, even with such a spread of sources,
such arrays may yield averaged coherencies which deviate strongly and variably from the ideal J0
curve. Routine plotting of the imaginary part of the averaged complex coherency provides a
useful quality control on SPAC methods.
the method of microseismic zonation of sediment thickness and earthquake hazard to
be extended, giving direct estimates of shear-wave velocity and thickness of
sediments over basement. The method is well-suited to built-up areas where cultural
sources of microseisms are spatially distributed, and can be extended to yield useful
data where microseismic energy propagates in higher as well as fundamental modes.
capable of yielding information regarding geological structure on both
local and regional scales. Since the source or sources of microseisms
are poorly defined both in time and space, digital processing of data
from an array of at least five seismometers is necessary to extract
velocity information.
Microseisms in the period range 0.1 to 0.5 sec generated by vehicles
or other cultural. sources, have been observed with a 100 m diameter
circular array of seven seismorneters wired to a minicomputer, and processed
to obtain wave velocity using azimuthally averaged coherencies. A search
for two simultaneous propagation velocities at each frequency yields
'velocities which show dispersion characteristics agreeing with theoretical
curves computed for fundamental and first or second higher mode Rayleigh
wave propagation. The results are sufficiently precise to allow inversion,
by comparison of velocity estimates with model curves, to obtain depth
and shear velocity of unconsolidated sediments over rock at the array
site. precision of the depth estimate is of the same order as that obtained
by a seismic refraction survey at the site.
Microseisms in the period range 0.5 to 7 sec, have been observed
with cross arrays of diameter 0.75 km to 3 km. Procedures for expanding
the array have been developed to facilitate measurement of the large range
of wravelengths (l to 20 km) with only seven or five seismometers; the
array cross is asymmetrical and can be expanded while preserving the
shape of the array by moving only two seismometers at a time. Velocity
estimates obtained from high-resolution wavenumber analysis suggest that
higher Rayleigh modes or compressional waves dominate at periods 0.5 to
1 sec. but at longer periods a significant proportion of fundamental mode
Rayleigh waves is present. Provided this deduction is correct, inversion
of the velocity data by comparison with theoretical curves allows depth
of the Sydney Basin at the array site to be deduced. Accuracy of the depth
estimate is +-15% if the basement seismic velocity is known from other methods,
or 30% in the absence of such additional information. Theoretical studies
suggest that this accuracy can be improved if a larger array of diameter up
to 10 km, and a minimum of seven seismometers are used.
capable of yielding information regarding geological structure on both
local and regional scales. Since the source or sources of microseisms
are poorly defined both in time and space, digital processing of data
from an array of at least five seismometers is necessary to extract
velocity information.
Microseisms in the period range 0.1 to 0.5 sec generated by vehicles
or other cultural. sources, have been observed with a 100 m diameter
circular array of seven seismorneters wired to a minicomputer, and processed
to obtain wave velocity using azimuthally averaged coherencies. A search
for two simultaneous propagation velocities at each frequency yields
'velocities which show dispersion characteristics agreeing with theoretical
curves computed for fundamental and first or second higher mode Rayleigh
wave propagation. The results are sufficiently precise to allow inversion,
by comparison of velocity estimates with model curves, to obtain depth
and shear velocity of unconsolidated sediments over rock at the array
site. precision of the depth estimate is of the same order as that obtained
by a seismic refraction survey at the site.
Microseisms in the period range 0.5 to 7 sec, have been observed
with cross arrays of diameter 0.75 km to 3 km. Procedures for expanding
the array have been developed to facilitate measurement of the large range
of wravelengths (l to 20 km) with only seven or five seismometers; the
array cross is asymmetrical and can be expanded while preserving the
shape of the array by moving only two seismometers at a time. Velocity
estimates obtained from high-resolution wavenumber analysis suggest that
higher Rayleigh modes or compressional waves dominate at periods 0.5 to
1 sec. but at longer periods a significant proportion of fundamental mode
Rayleigh waves is present. Provided this deduction is correct, inversion
of the velocity data by comparison with theoretical curves allows depth
of the Sydney Basin at the array site to be deduced. Accuracy of the depth
estimate is +-15% if the basement seismic velocity is known from other methods,
or 30% in the absence of such additional information. Theoretical studies
suggest that this accuracy can be improved if a larger array of diameter up
to 10 km, and a minimum of seven seismometers are used.
capable of yielding information regarding geological structure on both
local and regional scales. Since the source or sources of microseisms
are poorly defined both in time and space, digital processing of data
from an array of at least five seismometers is necessary to extract
velocity information.
Microseisms in the period range 0.1 to 0.5 sec generated by vehicles
or other cultural. sources, have been observed with a 100 m diameter
circular array of seven seismorneters wired to a minicomputer, and processed
to obtain wave velocity using azimuthally averaged coherencies. A search
for two simultaneous propagation velocities at each frequency yields
'velocities which show dispersion characteristics agreeing with theoretical
curves computed for fundamental and first or second higher mode Rayleigh
wave propagation. The results are sufficiently precise to allow inversion,
by comparison of velocity estimates with model curves, to obtain depth
and shear velocity of unconsolidated sediments over rock at the array
site. precision of the depth estimate is of the same order as that obtained
by a seismic refraction survey at the site.
Microseisms in the period range 0.5 to 7 sec, have been observed
with cross arrays of diameter 0.75 km to 3 km. Procedures for expanding
the array have been developed to facilitate measurement of the large range
of wravelengths (l to 20 km) with only seven or five seismometers; the
array cross is asymmetrical and can be expanded while preserving the
shape of the array by moving only two seismometers at a time. Velocity
estimates obtained from high-resolution wavenumber analysis suggest that
higher Rayleigh modes or compressional waves dominate at periods 0.5 to
1 sec. but at longer periods a significant proportion of fundamental mode
Rayleigh waves is present. Provided this deduction is correct, inversion
of the velocity data by comparison with theoretical curves allows depth
of the Sydney Basin at the array site to be deduced. Accuracy of the depth
estimate is +-15% if the basement seismic velocity is known from other methods,
or 30% in the absence of such additional information. Theoretical studies
suggest that this accuracy can be improved if a larger array of diameter up
to 10 km, and a minimum of seven seismometers are used.