The basic seismograph
Schematic diagram of an inertial seismometer
The mass is suspended on a spring. This gives a relative free of movement.
Position of mass to the respect of soil can be read on the scale.
R is the damping element needed to reduce the harmonic motion of the spring and
allow easier readings of the displacement.
IMAGE FROM: Instrumentation in Earthquake Seismology Jens Havskov, Institute of Solid Earth Physics University of Bergen Norway and Gerardo
Alguacil Instituto Andaluz de Geofisica University of Granada Spain 2004
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SEISMIC RECORDING
A - MEASUREMENT OF SEISMIC SIGNALS
All seismic measurements belongs to a wider field of application known as vibrometry or
acoustics. Any elastic wave propagating on a media (solids, liquids or gases) can be read
as a vibrometric measurement.
- the basic seimograph
- measurement units
- the black box seismograph
- how a seismograph works
- kinds of seismic signals
In geophysics the measurement is made using specific measurement units, for example:
arrival times of a wave from a source to the receiver, dispersion of the frequencies to the
respect of the survey line, phase variation, etc. All these measurements usually requires
much more care than other kind of measurements.
?
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A measurement of a length can be made with a ruler or a caliper or other suitable device
according to the precision you require. As it is made you can trust on the measurement to
be within the uncertainty specified by the manufacturer of the measuring device.
In vibrational or geophyiscal field of application, measurement can be much more tricky
especially when involve element of relative big dimensions and weight (buildings,
machines, soils). You can't count on a reference point (like lenght measurement from
here, the reference point, to there, the measurement point).
Almost all vibrational equipment relies on inertial sensors that, althoug highly precise,
relatively high measurement uncertainties (error) are introduced and some time
uncertainties of the error itself.
The key is describe mathematically as much complete as possible your measurement
device.
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In seismometry and geophysics we use these basic
measurement units
Basic Measurement units
Lenght:
meter
Position:
meter
Frequency:
hertz
Time:
second
m
m
Hz
s
Displacement/Acceleration/Velocity
(oscillation per second)
Composed Measurement units (involving more than one measure)
Velocity:
meter per second
m/s
Acceleration:
meter per second per second
m/s/s , OR, m/s*s, OR m/s2
Other measurement units used often because more practical under certain circumstances
Period:
1 / Hz → secondi
s
(used for frequencies below 1Hz)
Magnitude ratio:
decibel
dB
First two waveforms (Original) are from a P wave of an earthquake recorded by a
velocitymeter (S) and an accelerometer (A)
The second and third couple of seismograms are derived by derivative and integration
of the firsts as acceleration and displacement.
All waveforms are, as expected, perfectly correlated.
Abbreviation of fraction of the measurement units often used in geophysics
mm
millimeters
0.001 m
1 x 10 -3
μm
micrometers (microns)
0.000 001 m
1 x 10 -6
nm
nanometers
0.000 000 001 m
1 x 10 -9
IMAGE FROM: Instrumentation in Earthquake Seismology Jens Havskov, Institute of Solid Earth Physics University of Bergen Norway and Gerardo
Alguacil Instituto Andaluz de Geofisica University of Granada Spain 2004
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Example of vibrometric measurement
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Vibrometry in seismology
Basic vibrational measurement units:
displacement, acceleration and
velocity
a = absolute displacement, 10 mm
b = velocity, 5 mm/s for 2 secondi
c = acceleration, about 70 mm/s2 for
a very short time
d = relative motion of mass of a
mechanical sensor with
eigenfrequency of 1Hz
e = output voltage of an equivalent
velocity transducer with a sensitivity
of 200V/m/s
f = output of an accelerometer having
an egienfrequency of 20Hz and a
position transducer with 1000V/m of
sensitivity
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IMAGE FROM: Instrumentation in Earthquake Seismology Jens Havskov, Institute of Solid Earth Physics University of Bergen Norway and Gerardo
Alguacil Instituto Andaluz de Geofisica University of Granada Spain 2004
A signal from an earthquake of M3.0 recorded by a 1Hz sensor.
Three waveforms report corrected seismograms with a peak displacement of 1300nm,
a velocity peak of 35000 nm/s and an acceleration peak of 1680000 nm/s/s
IMAGE FROM: Instrumentation in Earthquake Seismology Jens Havskov, Institute of Solid Earth Physics University of Bergen Norway and Gerardo
Alguacil Instituto Andaluz de Geofisica University of Granada Spain 2004
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How these recordings are obtained?
THE SEISMOMETER IS A COMPLEX BOX
?
IMAGE FROM: Instrumentation in Earthquake Seismology Jens Havskov, Institute of Solid Earth Physics University of Bergen Norway and Gerardo
Alguacil Instituto Andaluz de Geofisica University of Granada Spain 2004
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THE SEISMOMETER IS OFTEN A BLACK BOX
LET'S TRY TO CLARIFY
AT LEAST A LITTLE
AND HAVE A MORE CLEAR IDEA
ON HOW A SEISMOGRAPH RECORD
AND ADVANTAGES OF MODERN
DIGITAL RECORDING SEISMOGRAPHS
?
the following is what was in use
some decades ago
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paper roller
and pen mechanism
A - MEASUREMENT OF SEISMIC SIGNALS
THE MOST AND COST
EFFECTIVE DEVICE TO
RECORD SOIL VIBRATION
IS THE
GEOPHONE
starter input
BASED ON INERTIAL
PRINCIPLE OF A MOVING
COIL (MASS)
SURROUNDED BY A
MAGNETIC FIELD
IT GENERATES SIGNAL BY
INDUCTION OF AN
ELECTRIC CURRENT
paper speed
adjustment
spring charger
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STIFFNESS OF THE
SPRING AND WEIGHT OF
THE MASS INFLUENCE THE
NATURAL FREQUENCY OF
THE GEOPHONE
inertial mass
for pen driving
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HOW A SEISMOGRAPH WORKS
SO WHAT HAPPENS ?
K
?
Filtro
a/d
Unità di controllo,
memoria,
comunicazione.
Y
X
THE GEOPHONE CONVERT THE VIBRATION IN ELECTRIC SIGNAL
AN AMPLIFIER “K” INCREASE THIS SIGNAL IN A USEABLE LEVEL
A FILTER AVOID SOME SIGNAL TO ENTER IN THE SYSTEM
THE A/D CONVERT IN AMPLITUDE SLICES THE SIGNALS
THE CONTROL UNIT CONVERT SLICES THE TIME
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Signal conversion
Double amplitude resolution
A WAVEFORM IN THE CONTINOUS TIME DOMAIN
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Conversion in discrete time and amplitudes
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Double time and amplitude resolution
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SNR Signal/Noise Ratio
Quadruple time and amplitude resolution
a
b
specified in dB: log (a / b) x 20
examp: a=40 b=3 -> SNR = 22dB
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Noise over a signal
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Planetary noise distribution in frequency
Oceanic waves noise 0.7-0.15Hz
Anthropic
noise
atmospheric
pressure
variations
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Earth tide
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Typical oceanic noise
- (24 hours plot)
Seismic waves, natural and artificial
?
?
Nuclear test,
North Korea, 12 febbraio 2013,
M5.1
Earthquake
California/Nevada del 13 febbraio 2013
M5.2
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Anthropic noise + barometric noise + seismic (earthquake)
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SEISMIC WAVES – body waves
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P waves: fastest, compressional
Particle motion same direction of wave
motion
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S waves: slower, particle motion is
perpendicular to the wave motion
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Image ©2000-2006 Lawrence Braile, used with permission.
Surface waves
A - THE PARTICLE MOTION
?
Combination of P and S waves
incident on the free surface of the
propagation media (soil)
P WAVES
THE PARTICLE (see red dots) moves forward and backward following the
compressiona and dilatation of the material.
In EXPLORATION SEISMOLOGY the P waves are used with the REFRACTION
and REFLECTION methods.
Generated by the incidence of S wave on
the free surface plane of the propagation
media
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Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State
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Image ©2000-2006 Lawrence Braile, used with permission.
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A - MEASUREMENT OF SEISMIC SIGNALS
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A - THE PARTICLE MOTION
ALL THESE KIND
OF SEISMIC
WAVE ARE
ALWAYS
PRESENT
IN THE FORM OF
SO CALLED
AMBIENT NOISE.
TO DETECT
EACH KIND OF
WAVE IS NEEDED
A SENSOR WITH
PROPER
SENSITIVITY
S WAVES
THE PARTICLE (try to follow the motion of a dot) moves as a pure translational
motion in the direction of the polarization of the wave. It may be polarized at any
angle. Anyway the S waves are not compressional and so they generate usually a
wider displacement of the particle.
In SOIL EXPLORATION the S waves are used in SH refraction method.
VERTICAL or
HORIZONTAL
SENSITIVITY
GEOPHONE
Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State
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Image ©2000-2006 Lawrence Braile, used with permission.
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A - THE PARTICLE MOTION
A - MEASUREMENT OF SEISMIC SIGNALS
LOVE waves
Love waves are like S waves but the propagate on surface.
They can be used for Soil Exploration.
A variant of MASW can use Love waves.
A VERTICAL SENSITIVITY GEOPHONE
Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State
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A - THE PARTICLE MOTION
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A - MEASUREMENT OF SEISMIC SIGNALS
AN HORIZONTAL
SENSITIVITY GEOPHONE
Oriented for MASW using
Rayleight waves
RAYLEIGH WAVES
THE PARTICLE (try to follow the yellow dot) have the typical retrograde elliptical
motion. The motion contain many information of the subsoil structure.
Rayleigh waves can be recorded with both vertical and horizontal geophones; so
you can pick radial or longitudinal waves.
AN HORIZONTAL
SENSITIVITY GEOPHONE
Oriented for SH survey or
MASW in Love Waves
Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State
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B - NEAR SURFACE EXPLORATION
SINGLE CHANNEL SEISMOGRAPH
WITH EMBEDDED GEOPHONE
DYNAMIC RANGE = 35db
TIME RESOLUTION = 0.01 seconds
ENERGIZATION = EXPLOSIVE
- APPLICATIONS
- INTERPRETATION METHODOLOGIES and
MODELIZATION
Downhole/Crosshole
Refraction P waves
Refraction SH waves
Dispersion of Rayleigh waves (MASW)
Dispersion using ambient noise (microtremor)
Horizontal/Vertical Spectral Ratios (HVSR)
DATA PROCESSING:
Ruler, pencil, patience...
?
SINGLE CHANNEL SEISMOGRAPH
WITH DIGITAL TELEMETRY
DYNAMIC RANGE > 110dB
TIME RESOLUTION = 50 microseconds
ENERGIZATION = 5 kg HAMMER
DATA PROCESSING:
IMMEDIATE TIME RESULT
FEW CLICK FOR 1st LAYER
DEPTH HORIZONT
- LIMITS OF METHODOLOGIES
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B - NEAR SURFACE EXPLORATION
APPLICATIONS
Near surface geophysics can help in several fields.
This list is not exaustive and imply the use of different
tecniques and instruments;
?
All listed application can benefit of the study of elastic
waves in soil, both artificial and natural (ambient noise).
www.sara.pg.it
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B - NEAR SURFACE EXPLORATION
B - NEAR SURFACE EXPLORATION
APPLICATIONS (basic)
IMPORTANT NOTICE BEFORE CONTINUE!
- DETECTION OF LAYERS DEPTH (and thickness)
NON-UNIQUENESS!
NO SINGULARIDAD!
- RECONSTRUCTION OF 2D/3D SECTIONS
?
?
- MEASUREMENT ELASTOMECHANIC
PROPERTIES OF SUBSOIL LAYERS
All results obtained with
an exploration method
potentially can come from
more than one model
-
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B - NEAR SURFACE EXPLORATION
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B - NEAR SURFACE EXPLORATION
CROSS-HOLE MEASUREMENT
APPLICATIONS (advanced)
- VyXX measurements
- Measurement resonance frequency in soil
and structures
- Calculation of landslides volumes
- Water search (even at several hundreds of meters)
- Assessment of structural stability and integrity
- Extimation of bedrock depth
- Archeological research
- etc...
Borehole
Impulse
source
0
15 ﾠ
Vx = d / (ta-t0)
ta
t0
d
Borehole
Receiver
(geophone)
30
CROSSHOLE TESTING
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B - NEAR SURFACE EXPLORATION
B - NEAR SURFACE EXPLORATION
CROSS-HOLE MEASUREMENT - pros
DOWN-HOLE MEASUREMENT
Surface
impulse source
measured as t0
VAn = path / (ta-t0)
Vx = Van - Van-1
- Samples of rock are collected and can be tested in
laboratory after coring
- The most accurate measurement of velocity of
subsoil
- Report information from direct observation of
rocks and stratigraphic layers
- Measurement of layer depth is with high precision
(centimeters)
P waves
t0
0
15 ﾠ
ta
S waves
approximated
path
Borehole
Receiver
(geophone)
real path
BOREHOLE TESTING
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B - NEAR SURFACE EXPLORATION
S waves
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- borehole field setup
CROSS-HOLE MEASUREMENT - cons
- Require drilling of at least two holes
- Require heavy machines
- Not always machines can enter in the test place
- Not long distance can be achieved with small
energization device
- Obtained results are very limited in area (distance
between holes)
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B - NEAR SURFACE EXPLORATION
- borehole field setup
B - NEAR SURFACE EXPLORATION
This calculation it is not from
data of the previous slide
P waves
S waves
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- borehole field setup
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B - NEAR SURFACE EXPLORATION
DOWN-HOLE MEASUREMENT - pros
- Can be used on a single hole
- Easier than Cross-Hole
- Effective in result
- Samples of rock are collected and can be tested in
laboratory after coring
- Almost direct measurement of ground velocities
- Report information from direct observation of
rocks and stratigraphic layers
- Measurement of layer depth is with high precision
(centimeters)
S waves only dx
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S waves dx and sx
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B - NEAR SURFACE EXPLORATION
B - NEAR SURFACE EXPLORATION
REFRACTION - Snell's law
DOWN-HOLE MEASUREMENT - cons
- Drilling is expensive
- Require heavy machines
- Not always machines can enter in the test place
- Measurement is over a single vertical line
Image from IRIS
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B - NEAR SURFACE EXPLORATION
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B - NEAR SURFACE EXPLORATION
Refraction P waves
REFRACTION MEASUREMENT
d2
d1
SEVERAL KIND OF DIFFERENT METHODS AND
INTERPRETATION METHODOLOGY BELONGS TO
THE REFRACTION MEASUREMENT, FOR EXAMPLE
TOMOGRAPHY.
d0
t0
t0
REFRACTION SURVEYS ARE NORMALLY
EXECUTED ALONG A STRAIGHT LINE
INVOLVE A NUMBER OF GEOPHONES
(RECEIVERS) AND CAN INVOLVE FROM ONE TO
SEVERAL SHOTS (ENERGIZATIONS) OF THE SOIL
:
ta
?
Soft layer
Soft layer with increasing hardness
Harder layer
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B - NEAR SURFACE EXPLORATION
B - NEAR SURFACE EXPLORATION
Refraction P waves
Refraction P waves in the field (array)
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B - NEAR SURFACE EXPLORATION
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B - NEAR SURFACE EXPLORATION
Refraction P waves (with seismograms)
Refraction P waves in the field (energization)
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B - NEAR SURFACE EXPLORATION
B - NEAR SURFACE EXPLORATION
Refraction P waves
Refraction P waves seismogram
- Array geometry must be accurate!
- Straight line is mandatory
- Array length: 3 times the required depth
- Vertical sensitivity sensors are used
- Level topography must be known (if not flat)
- Multiple shots needed for 2D profiling
- Special software needed for tomography
?
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B - NEAR SURFACE EXPLORATION
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B - NEAR SURFACE EXPLORATION
Refraction P waves seismogram
Refraction SH waves - Follow the same principle of
refraction in P waves but the energization is different.
?
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B - NEAR SURFACE EXPLORATION
B - NEAR SURFACE EXPLORATION
Refraction SH waves
For best results energization should be made on
two directions and pick the arrival using overlapped
representation of the seismogram
Refraction pros and cons
INFORMATION OBTAINED USING THE REFRACTION
ARE ALWAYS USEFUL AND A REFRACTION
RECORDING SHOULD BE EXECUTED
IN ALL SITES
IT CAN AN GIVE VERY USEFUL INFORMATION AND
IT IS LESS SENSITIVE TO INTERPRETATION ERRORS
THAN OTHER METHODS.
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B - NEAR SURFACE EXPLORATION
B - NEAR SURFACE EXPLORATION
Refraction SH waves
For best results energization should be made on
two directions and pick the arrival using overlapped
representation of the seismogram
Refraction pros
- Direct reading of velocity (P or SH)
- Rather easy identification of the first horizont depth
- Possible identification of 2nd and 3rd layer
- With just two shot it can identify tiltness of layers
- If refraction seismograms are difficult to read this is
a direct indicator of “something strange” to be better
evaluated with care
- Waterbed can be identified with reasonable accuracy
(p waves jump to >= 1500 m/s)
P WAVE
S WAVE
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B - NEAR SURFACE EXPLORATION
B - NEAR SURFACE EXPLORATION
Refraction cons
Refraction example with
single or multiple shots
0 m
-5
-15
-10
- Relatively long arrays are needed
- Sensitive to inversion of velocity (layers below the
slower layer are hidden)
- In P waves the waterbed hide lower layers
- High energies are needed if the soil is very soft
- In S waves horizontal energization is needed, this is not
always so easy
PROFONDITA' RIFRATTORI
0.1
-35
-30
0.125
-25
0.15
-20
s
DROMOCRONE ORIGINALI
0.075
0
25
50
75
100
125
150
175
m
150
175
m
25
50
75
100
125
150
175
m
-25
0
-20
0
-15
-10
0.025
-5
0.05
0 m
SEZIONE VERTICALE
s
-30
0.15
-35
DROMOCRONE TRASLATE
0.125
0
25
50
326.5 m/s
75
100
1658.3 m/s
125
3901.1 m/s
0.1
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0.075
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0.025
0.05
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0
B - NEAR SURFACE EXPLORATION
PROFONDITA' RIFRATTORI
0 m
25
50
75
100
125
150
175
m
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PROFONDITA' RIFRATTORI
0 m
-5
-5
Refraction example with
single or multiple shots
-15
-15
-10
-10
Refraction example with
single or multiple shots
0
-35
-30
0.125
-30
-25
-25
0.15
-20
-20
s
DROMOCRONE ORIGINALI
75
100
125
150
175
m
0
50
75
100
125
150
175
m
150
175
m
SEZIONE VERTICALE
-15
-20
0
-10
-15
0.025
-10
-5
-5
0.05
0 m
SEZIONE VERTICALE
25
0 m
50
0.1
25
0.075
0
25
50
75
100
125
150
175
m
-30
0.15
-35
-25
s
DROMOCRONE TRASLATE
-30
-20
-25
0
0
25
50
75
1610.1 m/s
100
125
3893.0 m/s
150
175
m
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0.1
25
5
0
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347.7 m/s
0.125
346.6 m/s
50
75
1529.4 m/s
100
125
3875.8 m/s
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B - NEAR SURFACE EXPLORATION
B - NEAR SURFACE EXPLORATION
Multichannel Analysis of Surface Waves - Dispersion
Multichannel Analysis of Surface Waves - Dispersion
Almost all energy released on soil by the energization propagate in soil as
surface waves (mainly Rayleigh waves). The MASW method is based on
the modelling Reyleigh waves from a given stratigraphy.
The experimentally recorder Rayleigh waves are treated in the frequency
domain allowing the software to split the signals in different frequencies.
The analysis accomplished on all receivers allow to identify at what speed
each frequency moves along the survey line.
This effect is called "dispersions".
This is a quite bad seismogram for refraction.
Useable for MASW
A dispersion curve can be observed and "picked" from the
Obtained multichannel spectrum.
Using the mathematical model of Rayleigh waves a synthetic dispersion
curve can be calculated, and compared.
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B - NEAR SURFACE EXPLORATION
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B - NEAR SURFACE EXPLORATION
Multichannel Analysis of Surface Waves - Dispersion
Multichannel Analysis of Surface Waves
(Dispersion analysis)
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Yellow line: P waves refracted signals
Area between yellow and red line: Dispersion of energy
(in frequency and time) along the array
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B - NEAR SURFACE EXPLORATION
B - NEAR SURFACE EXPLORATION
Multichannel Analysis of Surface Wave
MASW
- Length of the array can be reduced
- Require less number of receivers (8 receivers can be ok)
- Insensitive to velocity inversion (see below softer layers)
- Require less energy than refraction (use surface waves)
- Averages the velocity of all the subsoil structure
- ...
Dispersion spectrum
?
MASW cons
- Generates 1D profiles (a single vertical stratigraphy)
- ...
Dispersion curve
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B - NEAR SURFACE EXPLORATION
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B - NEAR SURFACE EXPLORATION
Multichannel Analysis of Surface Wave
Surface Wave analysis (dispersion) with ambient noise
The MASW method use energization to produce a
train of Rayleigh waves
Surface Wave Analysis can also use ambient noise.
This tecnique is known as REMI (Refraction/Microtremor)
and ESAC/SPAC
ESAC = Extended Spatial AutoCorrelation
derive from
?
SPAC = SPatial Auto Correlation
based on research of Prof. Aki at end of 1950s
INVERSION
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B - NEAR SURFACE EXPLORATION
B - NEAR SURFACE EXPLORATION
REMI / ESAC-SPAC pros
- Depth can be increased up to 5 times the aperture of
the survey geometry
- Insensitive to velocity inversion (see below softer layers)
- Require no energy (use ambient noise)
- Averages the velocity of all the subsoil structure
- ...
Surface Wave analysis (dispersion) with ambient noise
It can use a single receiver line (straight line) REMI
Or
a 2D receiver positioning: ESAC/SPAC method
REMI / ESAC-SPAC cons
- Generates 1D profiles (a single vertical straigraphy)
- Don't work in presence of small impedance contrasts
- Longer time in deployment
- Longer time of execution (from 30 to 600 seconds)
- Requires high number of receiver minimum 16 units
- If the place is very silent it could not work well
both methods relies on the fact the ambient noise is
stochastic in the whole volume of analyzed soil
(the same is assumed with HVSR method, we see below)
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B - NEAR SURFACE EXPLORATION
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B - NEAR SURFACE EXPLORATION
Horizontal/Vertical Spectral Ratios (HVSR)
Surface Wave analysis (dispersion) with ambient noise
REMI
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Yutaka Nakamura in 1989 proposed the technique, generally
referred to as the H/V method.
Nakamura indicated that the horizontal-to-vertical (H/V)
spectral ratio of microtremors at a site roughly equals the Swave transfer function between the ground surface and
bedrock at a site.
This means that the H/V peak period and peak value itself
correspond to the natural site period and amplification factor
respectively.
This method does not require any boreholes or long array
deployment. Lermo and Chavez-Garcia showed the
applicability of Nakamura’s method of microtremor in site
effect prediction.
ESAC/SPAC
use arbitrary geometries, even random
?
N
THIS METHOD IS VERY SENSITIVE TO ERROR OF EXECUTION SO
DETAILED GUIDELINES HAS BEEN PROVIDED AS “SESAME protocol”.
SARA electronic instruments s.r.l.
45
47
SARA electronic instruments s.r.l.
47
B - NEAR SURFACE EXPLORATION
B - NEAR SURFACE EXPLORATION
Horizontal/Vertical Spectral Ratios (HVSR)
Horizontal/Vertical Spectral Ratios (HVSR)
The peak in the HVSR curve result from the thickness and
impedance of subsoil layers. The peak results evident if the
impedance difference between layers is over 25%
AN HVSR SURVEY CAN BE RUN WITH A
SINGLE TRIAXIAL SEISMIC STATION.
THE STATION HAVE TO BE HIGH
SENSITIVITY AND ULTRA LOW NOISE.
SENSORS MUST HAVE THE SAME
TRANSFER FUNCTION
If velocities increases gradually without consistent increases or
variation of the impedance it would have no clear peaks on the
HVSR curve and consequently the method don't give good results.
?
?
SENSORS MUST BE PERFECTLY LEVELLED
AND ORTHOGONAL.
WIND AND OTHER MAN-MADE NOISE HAVE
TO BE AVOIDED
The equation show the relation between
Vs = averaged velocity of Rayleigh waves in the H thickness having
a T1 period of oscillation.
ELSE THE H/V CURVE WOULD BE BIASED
AT CERTAIN FREQUENCIES MAKING THE
MEASUREMENT UNRELIABLE
SARA electronic instruments s.r.l.
48
50
B - NEAR SURFACE EXPLORATION
50
B - NEAR SURFACE EXPLORATION
Horizontal/Vertical Spectral Ratios (HVSR)
?
SARA electronic instruments s.r.l.
Horizontal/Vertical Spectral Ratios (HVSR)
HERE SOME
GUIDELINES, MOST
OF THEM ARE
PRESCRIBED BY
THE SESAME
PROTOCOL AND
MANY OTHER
CODES FOR HVSR
MEASUREMENTS
The equation is so simple that people are attracted and suppose it
can solve any problem of stratigraphic modelling.
As always this is too beauty to be true.
?
Anyway it can be VERY useful if joint modelling and analysis is
performed;
Using refraction + masw + hvsr + possibly downhole survey
The user can benefit of the best of each method and have a very
good result.
For known stratigraphy (i.e. from a previous made coring) both
MASW and HVSR can take huge benefit and become very robust
interpretation and subsoil modelling methods.
SARA electronic instruments s.r.l.
49
51
SARA electronic instruments s.r.l.
51
B - NEAR SURFACE EXPLORATION
Horizontal/Vertical Spectral Ratios (HVSR)
?
SARA electronic instruments s.r.l.
52
B - NEAR SURFACE EXPLORATION
REMEMBER HOW A SEISMOGRAPH WORKS
REMEMBER!
NON-UNIQUENESS!
NO SINGULARIDAD!
All results obtained with an exploration
method potentially can come from more than
one model.
?
THE BEST method do not exist!
EXPERT must evaluate and choose
How many, what methods use, and
where have to be used!
SARA electronic instruments s.r.l.
53
K
Filtro
a/d
Unità di controllo,
memoria,
comunicazione.
Y
X
THE KEY COMPONENT OF A SEISMOGRAPH IS THE A/D CONVERTER
IT IS IMPORTANT IT CAN GIVE PROPER PERFORMANCE FOR THE REQUIRED
PURPOSE. BUT THE WHOLE SYSTEM IS WHAT PRODUCE THE RESULTS.
*A GOOD A/D CONVERTER WITH POOR SENSOR WILL LIMIT YOUR RECORDINGS
*A POOR (NOISY) DIGITIZER WILL DESTROY PERFORMANCE OF A GOOD SENSOR
SARA electronic instruments s.r.l.
2
EVERY SYSTEM HAS A REAL SIGNAL RESOLUTION
also called EFFECTIVE NUMBER OF BIT (ENOB)
For near surface exploration, rarely is needed more than 16 bits
microtremor
K
Filtro
a/d
Unità di controllo,
memoria,
comunicazione.
Y
masw
refraction
X
ENOB is almost equal to the real DYNAMIC RANGE
when evaluate a seismograph don't look too much
To the number of declared bit...
check the real dynamic range...
and check if it is real at the specified sampling rate
SARA electronic instruments s.r.l.
Small variation in dynamic limits can be compensated
If the system has a flexible gain management
3
Requisito de ENOB y las frecuencias para los
distintos tipos de medición
seismology
TRIAXIAL SEISMOMETER
microtremor
THE BEST WAY TO KNOW THE PARTICLE MOTION OF THE WAVES
EXCITING THE SURFACE
oversampling
- IT REQUIRES, HIGH SENSITIVITY AND COMPONENTS
HOMOGENEOUSITY
masw
- IT IS IMPORTANT YOU KEEP UNDER CONTROL ALL MOMENTS OF
DATA ACQUISITION CHECKING THE SEISMOMETER
refraction
- IT MUST RESOLVE THE BACKGROUND NOISE (AMBIENT NOISE)
reflection
Sigma-delta
audio / video
SAR
SARA electronic instruments s.r.l.
SARA electronic instruments s.r.l.
Base chart (oval ranges) are from: http://www.tmworld.com/article/CA224083.html
4
5
GeoBox
test demo
SARA electronic instruments s.r.l.
ARRAY OF SENSORS
- NEW GENERATION OF MULTICHANNEL SEISMOGRAPHS HAVE
DIGITAL TELEMETRY = ULTRA LOW NOISE
- MODULARITY
- FLEXIBILITY ON FIELD
- HIGH SENSITIVITY
- RELATIVE HIGH DYNAMIC RANGE
MOST IMPORTANT IS THE CAPACITY TO RESOLVE WEAK
SIGNALS WITH LOW NOISE (16 BIT SAR still the best for this)
7
DoReMi
test demo
IS THE Vs30 PARAMETER MEANINGFUL ?
It is practical to have 1 single parameter to guide
the design of buildings
class
classification sismica del terreno
Vs
Velocity of S waves of near surface layers
f0
resonance frequencies of a site
?
And increase of knowledge of stratigraphy
with possibily avoid misunderstanding?
Not possible to obtain with just a number!
IS THE Vs30 PARAMETER MEANINGFUL ?
WHAT COULD HAPPEN IF YOU WOULD BE
AT THIS SITE?
YES! AND NO!
YOU HAVE TO PUT YOUR EXPERIENCE ON FIELD!
WHAT COULD HAPPEN IF YOU WOULD BE
AT THIS SITE?
CLASS 'A' SOIL?
RESONANCE
RESONANCE
LANDSLIDES
+ RESONANCE
RESONANCE ?
Soil amplification?
f = Vs / 4h
h = Vs / 4f
Vs = f 4h
KNOWING BEDROCK DEPTH
AND
STRATIGRAPHY
IS POSSIBLE TO KNOW
f0, lowest main resonance
frequency of soil
Accurate measurement of f0 allow to detect risk of double
resonance of soil with existing or project of structures.
Where are existing structures
measurement on soil
and on structure allow to check
the presence of double resonance
and act proper countermeasure
to modify the behaviour of the
structure.
Approximately structure frequency can be computed with:
Where Ted is the period of resonance of building,
n are number of floors
h is the averaged height
and C is a coefficient for:
0.050 bricks & stones buldings
0.075 bricks and reinforced concrete
0.085 reinforced concrete framed building
JOINT ANALYSIS
IS MANDATORY
FOR BEST RESULTS
CORING
+
REFRACTION
+
MASW
+
HVSR
VIDEOS AND ANIMATIONS FROM
- COLLAPSING BUILDING … YOUTUBE VARIOUS SOURCES
- VIDEO ABOUT REFRACTION AND REFLECTION OF WAVES
http://www.iris.edu/hq/programs/education_and_outreach/animations/13
With permission of IRIS , ref. Jenda Johnson.
- ANIMATED GIF OF PARTICLE MOTION
http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html
ADDITIONAL RESOURCES FOR DOWNLOAD
- Geopsy practical guide: (Italian)
http://www.sara.pg.it/scat.asp?idscat=25
- GeoExplorer HVSR:
http://www.sara.pg.it/prodotto.asp?idprod=101&idscatp=10
Credits, information sources, recomended readings.
- Predefined Vs30 Mapping - USGS
http://earthquake.usgs.gov/hazards/apps/vs30/
- Waves and Vibration in Soils: Earthquakes, Traffic, Shocks,
Construction works
J.F.Semblat, A.Pecker, IUSS Oress; ISBN: 978-88-619-030-3
- Surface Wave Methods; Acquisition, processing and inversion
Claudio Strobbia, Politecnico di Torino
- “MASW – Multi-channel Analysis of Surface Waves – La teoria in
pratica” – Manuale
e report - 2005 - www.eliosoft.it
- “Modello geologico-geotecnico” – Luca Martelli - Presentazione Corso
in Microzonazione Sismica - Roma 2009
- “Multistation Methods for geotechnical Characterization using Surface
Waves” – Sebastiano Foti – Dottorato di Ricerca - Politecnico di Torino 2000
- “Shear-Velocity profile across the evergreen basin using microtremor
array studies” – Michael W.Asten – Presentazione nell’ambito de “Third
Annual Northern California Earthquake Hazards Workshop” – USGS
Menlo Park 2006
- “The Surface profiles in Grenoble area determined by the MASW
measurements” – Tsuno, Cornou, Bard – Presentazione QSHA meeting Nizza 2007
- “Vs30 – Tecniche di misura diretta con metodi geofisici” – Pulelli, Furani
- Presentazione Corso Regione Marche - Ancona 2007
- Analisi del rumore ambientale con la tecnica “Refraction Microtremor”
(Re.Mi): alcuni esempi applicativi” – Baldi, De Luca, Giorgi, Mondet Articolo Workshop di Geofisica – Geofluid - Piacenza 2004
- Instrumentation in Earthquake Seismology Jens Havskov, Institute of
Solid Earth Physics University of Bergen Norway and Gerardo
Alguacil Instituto Andaluz de Geofisica University of Granada Spain 2004
- “Array recordings of ambient vibrations: surface-wave inversion” - Marc
Wathelet - Tesi di Laurea Universita’ di Liegi - 2005
- “Indagine ReMi: cenni metodologici” – Luigi Veronese – Articolo 2007
- “Indagini sismiche per l’applicazione delle normative vigenti” – Furani e
Pulelli – Presentazione Corso 2008 - www.progeo.info
- “L’azione sismica: Pericolosita’ e risposta sismica locale” – Roberto
W.Romeo Presentazione Seminario “D.M. 14 - gennaio 2008: Aspetti
progettuali delle nuove norme tecniche in relazione alle competenze
geologiche” - Roma 2008
- GUIDELINES FOR THE IMPLEMENTATION OF THE H/V SPECTRAL
RATIO TECHNIQUE ON AMBIENT VIBRATIONS MEASUREMENTS,
PROCESSING AND INTERPRETATION SESAME European research
project European Commission – Research General Directorate
Project No. EVG1-CT-2000-00026 SESAME
- Observation and modeling of seismic background noise, Jon Peterson ,
USGS, 1993