Seismometer – The basic Principlesigel/Lectures/Sedi/sedi... · Seismology and the Earth’s Deep...

Seismology and the Earth’s Deep Interior Seismometry

Seismometer – The basic PrinciplesSeismometer – The basic Principles

u ground displacementxr displacement of seismometer massx0 mass equilibrium position

The motion of the seismometer mass as a function of the ground displacement is given through a differential equation resulting from the equilibrium of forces (in rest):

Fspring + Ffriction + Fgravity = 0

for example

Fsprin=-k x, k spring constant

Ffriction=-D x, D friction coefficient

Fgravity=-mu, m seismometer mass

using the notation introduced the equation ofmotion for the mass is

tutxtxtx grrr

−=++

)()()(2)(

ϖϖε

ϖε &&&&&

From this we learn that:

- for slow movements the acceleration and velocity becomes negligible, the seismometer records ground acceleration

- for fast movements the acceleration of the mass dominates and the seismometer records ground displacement

Seismometer – examplesSeismometer – examples

Seismometer – QuestionsSeismometer – Questions

x0 xr1. How can we determine the damping

properties from the observed behavior of the seismometer?

2. How does the seismometer amplify the ground motion? Is this amplification frequency dependent?

We need to answer these question in order to determine what we really want to know:The ground motion.

Seismometer – Release TestSeismometer – Release Test

x0 xr1. How can we determine the damping

properties from the observed behavior of the seismometer?

0)0(,)0(0)()()(

xxxtxtxhtx

&&& ϖϖ

we release the seismometer mass from a given initial position and let is swing. The behavior depends on the relation between the frequency of the spring and the damping parameter. If the seismometers oscillates, we can determine the damping coefficient h.

0 1 2 3 4 5-1

1F0=1Hz, h=0

0 1 2 3 4 5-1

1F0=1Hz, h=0.2

0 1 2 3 4 5-1

1F0=1Hz, h=0.7

Time (s)

0 1 2 3 4 5-1

1F0=1Hz, h=2.5

Time (s)

x0 xrThe damping coefficients

can be determined from the amplitudes of consecutive extrema akand ak+1We need the logarithmic decrement Λ

⎟⎟⎠

⎞⎜⎜⎝

⎛=Λ

The damping constant h can then be determined through:

224 Λ+

Seismometer – FrequencySeismometer – Frequency

The period T with which the seismometer mass oscillates depends on h and (for h<1) is always larger than the period of the spring T0:

1 hTT−

Seismometer – Response FunctionSeismometer – Response Function

tirrr eAtxtxhtx ϖϖϖϖ 0

2200 )()()( =++ &&&

2. How does the seismometer amplify the ground motion? Is this amplification frequency dependent?

To answer this question we excite our seismometer with a monofrequent signal and record the response of the seismometer:

the amplitude response Ar of the seismometer depends on the frequency of the seismometer w0, the frequency of the excitation w and the damping constant h:

+⎟⎟⎠

⎞⎜⎜⎝

⎛−

Seismometer – Response FunctionSeismometer – Response Function

+⎟⎟⎠

⎞⎜⎜⎝

⎛−

Sampling rateSampling rate

Sampling frequency, sampling rate is the number of samplingpoints per unit distance or unit time. Examples?

Data volumesData volumes

Real numbers are usually described with 4 bytes (singleprecision) or 8 bytes (double precision). One byte consists of 8 bits. That means we can describe a number with 32 (64) bits. We need one switch (bit) for the sign (+/-)

-> 32 bits -> 231 = 2.147483648000000e+009 (Matlab output)-> 64 bits -> 263 = 9.223372036854776e+018 (Matlab output)(amount of different numbers we can describe)

How much data do we collect in a typical seismic experiment?Relevant parameters:- Sampling rate 1000 Hz, 3 components- Seismogram length 5 seconds- 200 Seismometers, receivers, 50 profiles- 50 different source locations- Single precision accuracy

How much (T/G/M/k-)bytes to we end up with? What about compression?

(Relative) Dynamic range(Relative) Dynamic range

What is the precision of the sampling of our physical signalin amplitude?

Dynamic range: the ratio between largest measurableamplitude Amax to the smallest measurable amplitude Amin.

The unit is Decibel (dB) and is defined as the ratio of twopower values (and power is proportional to amplitude square)

What is the precision of the sampling of our physical signalin amplitude?

Dynamic range: the ratio between largest measurableamplitude Amax to the smallest measurable amplitude Amin.

The unit is Decibel (dB) and is defined as the ratio of twopower values (and power is proportional to amplitude square)

In terms of amplitudes

Dynamic range = 20 log10(Amax/Amin) dB

Example: with 1024 units of amplitude (Amin=1, Amax=1024)

20 log10(1024/1) dB 60 dB

In terms of amplitudes

Dynamic range = 20 log10(Amax/Amin) dB

Example: with 1024 units of amplitude (Amin=1, Amax=1024)

20 log10(1024/1) dB 60 dB

Nyquist Frequency(Wavenumber, Interval)

The frequency half of the sampling rate dt is called theNyquist frequency fN=1/(2dt). The distortion of a physicalsignal higher than the Nyquist frequency is called aliasing.

The frequency of thephysical signal is > fNis sampled with (+) leading to theerroneous blueoscillation.

What happens in space?How can we avoidaliasing?

A cattle gridA cattle grid

Signal and NoiseSignal and Noise

Almost all signals contain noise. The signal-to-noise ratio isan important concept to consider in all geophysicalexperiments. Can you give examples of noise in the variousmethods?

Discrete ConvolutionDiscrete Convolution

Convolution is the mathematical description of the changeof waveform shape after passage through a filter(system).

There is a special mathematical symbol for convolution (*):

Here the impulse response function g is convolved withthe input signal f. g is also named the „Green‘s function“

Convolution is the mathematical description of the changeof waveform shape after passage through a filter(system).

There is a special mathematical symbol for convolution (*):

Here the impulse response function g is convolved withthe input signal f. g is also named the „Green‘s function“

)()()( tftgty ∗=

,,2,1,00

migi ,....,2,1,0=

njf j ,....,2,1,0=

Convolution Example(Matlab)

0 0 1 0

>> conv(x,y)

0 0 1 2 1 0

0 0 1 0

>> conv(x,y)

0 0 1 2 1 0

Impulse responseImpulse response

System inputSystem input

System outputSystem output

Convolution Example (pictorial)Convolution Example (pictorial)

x y„Faltung“

0 1 0 0

0 1 0 01 2 1

0 1 0 0

0 1 0 01 2 1

0 1 0 0

DeconvolutionDeconvolution

Deconvolution is the inverse operation to convolution.

When is deconvolution useful?

Deconvolution is the inverse operation to convolution.

When is deconvolution useful?

Digital FilteringDigital Filtering

Often a recorded signal contains a lot of information thatwe are not interested in (noise). To get rid of thisnoise we can apply a filter in the frequency domain.

The most important filters are:

• High pass: cuts out low frequencies

• Low pass: cuts out high frequencies

• Band pass: cuts out both high and low frequencies and leaves a band of frequencies

• Band reject: cuts out certain frequency band and leaves all other frequencies

Often a recorded signal contains a lot of information thatwe are not interested in (noise). To get rid of thisnoise we can apply a filter in the frequency domain.

The most important filters are:

• High pass: cuts out low frequencies

• Low pass: cuts out high frequencies

• Band pass: cuts out both high and low frequencies and leaves a band of frequencies

• Band reject: cuts out certain frequency band and leaves all other frequencies

Digital FilteringDigital Filtering

Low-pass filteringLow-pass filtering

Lowpass filteringLowpass filtering

High-pass filterHigh-pass filter

Band-pass filterBand-pass filter

Seismic NoiseSeismic Noise

Observed seismic noise as a function of frequency (power spectrum).Note the peak at 0.2 Hz and decrease as a distant from coast.

Instrument FiltersInstrument Filters

Time Scales in SeismologyTime Scales in Seismology

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