Instrumentation in Earthquake Seismology

364 Pages·2010·8.552 MB·English

by  Jens Havskov, Gerardo Alguacil (eds.)

INSTRUMENTATION IN EARTHQUAKE SEISMOLOGY
MODERN APPROACHES IN GEOPHYSICS
VOLUME 22
Editorial Advisory Board
B.L.N. Kennett, Research School of Earth Sciences,
The Australian National University, Canberra, Australia
R. Madariaga, Laboratoire de Geologie, Ecole Normale Superieure,
Paris, France
R. Marschall, Hannover, Germany
R. Wortel, Department of Theoretical Geophysics,
University of Utrecht, The Netherlands
For other titles published in this series, go to:
http://www.springer.com/series/6248
INSTRUMENTATION
IN EARTHQUAKE
SEISMOLOGY
By
JENS HAVSKOV
University of Bergen,
Norway
and
GERARDO ALGUACIL
University of Granada,
Spain
AC.I.P. Catalogue record for this book is available from the Library of Congress.
ISBN-10 1-4020-2968-3 (HB)
ISBN-13 978-1-4020-2968-4 (HB)
ISBN-10 1-4020-2969-1 (e-book)
ISBN-13 978-1-4020-2969-1 (e-book)
Published by Springer,
P.O. Box 17, 3300 AADordrecht, The Netherlands.
www.springer.com
Printed on acid-free paper
First published 2004
Reprinted with correctio ns 2006
Revised reprint 2010
With permission SEISAN and SEISLOG software can be downloaded
from http://www.geo.uib.no/seismo/software/software.html and
http://extras.springer.com
All Rights Reserved
© 2004, 2006, 2010 Springer
No part of this work may be reproduced, stored in a retrieval system, or transmitted
in any form or by any means, electronic, mechanical, photocopying, microfilming, recording
or otherwise, without written permission from the Publisher, with the exception
of any material supplied specifically for the purpose of being entered
and executed on a computer system, for exclusive use by the purchaser of the work.
Printed in the Netherlands.
Table of contents

Preface...........................................................................................................................xi

CHAPTER 1
Introduction....................................................................................................................1
1.1 Sensor.....................................................................................................................2
1.2 Recorder.................................................................................................................6
1.3 Stations and networks.............................................................................................7
1.4 Arrays.....................................................................................................................9
1.5 Instrument correction and calibration.....................................................................9

CHAPTER 2
Seismic sensors..............................................................................................................11
2.1 The standard inertia seismometer.........................................................................12
2.2 Seismometer frequency response.........................................................................14
2.3 Seismometer frequency response, alternative solution.........................................17
2.4 The velocity transducer........................................................................................20
2.4.1 Electromagnetic damping..............................................................................22
2.4.2 Polarity convention........................................................................................23
2.5 Instrument response curves, different representation...........................................23
2.6 Sensor response to transient signals......................................................................26
2.7 Damping constant.................................................................................................27
2.8 Construction of seismic sensors............................................................................30
2.8.1 The Wood Anderson short period torsion sensor.............................................31
2.8.2 Long period sensors.......................................................................................31
2.8.3 Garden-gate...................................................................................................32
2.8.4 Inverted pendulum.........................................................................................34
2.8.5 LaCoste.........................................................................................................34
2.9 Sensor calibration coil..........................................................................................36
2.10 Active sensors.....................................................................................................38
2.11 Accelerometers...................................................................................................39
2.12 Velocity broadband sensors................................................................................40
2.13 Extending the frequency response, inverse filtering and feed back....................42
2.14 Theoretical aspects of active sensors.................................................................43
2.14.1 General response of a feedback system.......................................................43
2.14.2 The basic Force-Balanced Accelerometer...................................................44
2.14.3 Broadband feedback seisometers.................................................................47
2.14.4 Other feedback techniques...........................................................................49
2.15 Sensor self noise.................................................................................................51
2.16 Noise in passive sensors coupled to amplifiers, theoretical aspects...................55
2.17 Some new trends in seismic sensors...................................................................60
vi
2.17.1 Seismometers with electrochemical transducers..........................................60
2.17.2 Micromachined accelerometers and seismometers ....................................62
2.18 Sensor parameters...............................................................................................63
2.18.1 Frequency response.....................................................................................64
2.18.2 Sensitivity....................................................................................................64
2.18.3 Sensor dynamic range..................................................................................65
2.18.4 Sensor linearity............................................................................................66
2.18.5 Sensor cross axis sensitivity........................................................................66
2.18.6 Sensor gain and output.................................................................................67
2.19 Examples of sensors...........................................................................................67
2.19.1 Exploration type 4.5 Hz geophone...............................................................68
2.19.2 Short period sensor, the L4-C.......................................................................68
2.19.3 Accelerometer, the Kinemetrics Episensor...................................................68
2.19.4 Broadband sensors, the Streckeisen STS-1 and STS-2 and Güralp
CMG-3T..................................................................................................................69
2.19.5 Negative feedback sensors, Lennartz LE-3D................................................71
2.19.6 Borehole sensors...........................................................................................71
2.20 Summary of sensor specifications......................................................................72
2.21 Which sensor to choose......................................................................................74

CHAPTER 3
Seismic noise.................................................................................................................77
3.1 Observation of noise.............................................................................................77
3.2 Noise spectra........................................................................................................80
3.3 Relating power spectra to amplitude measurements.............................................80
3.4 Origin of seismic noise.........................................................................................85

CHAPTER 4
Analog to digital converter..........................................................................................87
4.1 Example of a simple analog to digital converter, the Flash ADC.........................88
4.2 Basic ADC properties...........................................................................................89
4.3 A typical ADC, the Ramp ADC...........................................................................92
4.4 Multi channel ADC..............................................................................................93
4.5 Digitizers for a higher dynamic range..................................................................94
4.6 Oversampling for improvement of the dynamic range.........................................95
4.7 Sigma Delta ADC, SDADC.................................................................................98
4.7.1 How sigma-delta improves digitization noise: theory..................................102
4.8 Aliasing..............................................................................................................103
4.9 Anti alias filters..................................................................................................104
4.10 Examples of digitizers......................................................................................108

CHAPTER 5
Seismic Recorders.......................................................................................................113
5.1 Analog amplifier.................................................................................................113
5.1.1 Differential input-output..............................................................................114
5.2 Analog filters......................................................................................................115
5.2.1 Amplifier specifications and noise..............................................................117
5.3 Analog recording................................................................................................119
vii
5.4 Introduction to digital recorders.........................................................................122
5.5 Digitizing............................................................................................................123
5.6 Time stamping of data........................................................................................124
5.7 Storage media and recording in a ring buffer.....................................................127
5.8 Seismic triggers..................................................................................................128
5.9 Summary of trigger parameters and their settings..............................................132
5.10 Communication and data retrieval....................................................................135
5.11 Public domain data acquisition systems...........................................................137
5.12 Seismic recorders in use...................................................................................138
5.13 The next generation recorder............................................................................140
5.14 Examples of recorders......................................................................................141
5.15 Which recorder to choose.................................................................................147

CHAPTER 6
Correction for instrument response..........................................................................151
6.1 Linear systems....................................................................................................153
6.2 Spectral analysis and the Fourier transform........................................................156
6.3 Noise power spectrum........................................................................................163
6.4 General instrument correction in frequency and time domain............................165
6.5 General representation of the frequency response function................................167
6.6 Anti alias filters..................................................................................................171
6.7 Instrument correction and polarity......................................................................172
6.8 Combining response curves................................................................................173
6.9 Common ways of giving response information..................................................176
6.9.1 GSE..............................................................................................................177
6.9.2 SEED...........................................................................................................178
6.9.3 SEISAN.......................................................................................................180

CHAPTER 7
Seismic stations...........................................................................................................183
7.1 Geographical location of a seismic station.........................................................184
7.2 Site selection and seismic noise survey..............................................................185
7.3 Installation of the seismic station.......................................................................187
7.4 Sensor installation...............................................................................................188
7.4.1 Broadband sensor installation.......................................................................190
7.4.2 Borehole installations...................................................................................194
7.4.3 What is a good VLP station..........................................................................195
7.5 Temporary installation of seismic stations.........................................................196
7.6 Lightning and over-voltage protection...............................................................197
7.7 Power..................................................................................................................198
7.7.1 Stations running off the main AC supply.....................................................199
7.7.2 Batteries........................................................................................................200
7.8 Power sources.....................................................................................................204
7.8.1 Solar cells.....................................................................................................204
7.8.2 Wind generators............................................................................................208
7.8.3 Fuel cells.......................................................................................................209
viii
CHAPTER 8
Seismic networks........................................................................................................211
8.1 Purpose of seismic networks..............................................................................212
8.2 Network geometry..............................................................................................213
8.3 Network configuration: physical and virtual networks.......................................217
8.4 Physical seismic networks..................................................................................218
8.4.1 Communication standards...........................................................................221
8.5 Virtual seismic networks....................................................................................223
8.6 The choice between physical and virtual seismic systems.................................227
8.7 Seismic data transmission...................................................................................227
8.8 Analog data transmission....................................................................................228
8.9 Radio links..........................................................................................................230
8.9.1 Simplex-duplex............................................................................................230
8.9.2 Point to point or point to multi point radio networks...................................231
8.9.3 Spread spectrum..........................................................................................231
8.9.4 Radio link construction and equipment.......................................................232
8.9.5 Radio links with repeater stations................................................................235
8.10 Telephone and satellite.....................................................................................238
8.11 Digital data transmission protocols and some examples of their use................240
8.11.1 Serial data communication........................................................................240
8.11.2 TCP/IP communication.............................................................................243
8.11.3 Compression of digital data.......................................................................245
8.11.4 Error correction methods used with seismic signals..................................246
8.11.5 Seismic data transmission and timing........................................................247
8.11.6 Communication and graphics....................................................................247
8.12 Examples of networks......................................................................................248
8.13 Running a seismic network...............................................................................251
8.13.1 Physical operation......................................................................................252
8.13.2 Data processing..........................................................................................253
8.14 How to make your own network......................................................................255

CHAPTER 9
Seismic arrays.............................................................................................................259
9.1 Basic array parameters........................................................................................262
9.2 Array transfer function.......................................................................................266
9.3 Instruments characteristics..................................................................................269
9.4 Field arrangement...............................................................................................271
9.5 Example of a portable array................................................................................274

CHAPTER 10
Calibration and testing...............................................................................................277
10.1 Test equipment and recording of test signals....................................................277
10.2 Sensors..............................................................................................................278
10.2.1 Sensor free period.....................................................................................278
10.2.2 Seismometer damping..............................................................................281
10.2.3 Determining calibration coil motor constant............................................284
10.2.4 Polarity and sensor orientation.................................................................285
10.2.5 Frequency response using harmonic drive methods.................................287
ix
10.2.6 Lissajous figures.......................................................................................290
10.3 Methods for absolute calibration of a sensor....................................................292
10.3.1 Shaking table............................................................................................292
10.3.2 Using the ground as a shaking table.........................................................292
10.3.3 Calibration by stepwise motion................................................................294
10.3.4 Determining sensitivity of accelerometer by tilt.......................................296
10.4 Use of the calibration pulse..............................................................................297
10.4.1. Using pseudo random signals for calibration...........................................301
10.5 Recorder...........................................................................................................303
10.6 Measuring instrument self noise.......................................................................306
10.6.1 Example of a practical noise measurement..............................................307

APPENDIX
Basic electronics..........................................................................................................309
A.1 Basic terms........................................................................................................309
A.1.1 Electric charge..........................................................................................309
A.1.2 Electric potential, current and power........................................................309
A.1.3 Kirchhoff laws..........................................................................................310
A.2 Passive components...........................................................................................310
A.2.1 Resistors, inductors and capacitors...........................................................310
A.2.2 Connection of components in series and parallel......................................314
A.2.3 A passive non-linear component: the diode..............................................314
A.3 DC and AC signals............................................................................................315
A.3.1 Decibels....................................................................................................317
A.3.2 Complex impedances and response..........................................................317
A.4 Power supply.....................................................................................................320
A.5 Common laboratory instruments.......................................................................321
A.6 Amplifiers..........................................................................................................324
A.6.1 Basic amplifier configurations..................................................................326
A 6.2 Active filters.............................................................................................329
A.7 Introduction to logical circuits...........................................................................332
A.7.1 Example: A daily calibration sequence generator for seismometers........334

References...................................................................................................................339
Company references.................................................................................................345

Index............................................................................................................................349
Preface

This book is primarily intended for seismologists and technicians working with
seismological instruments. Seismologists tend to take their data for granted, hoping that
the black boxes of seismographs and processing software will take care of all the nasty
problems, much like driving a car and not worrying too much of how it works. But cars
stop or malfunction and so do seismographs. Thus a basic understanding of seismic
instrumentation is essential, even for the seismologist who is never going to turn a knob
on an instrument.

Instrumentation is not just a topic for seismologists, since most equipment is in fact
installed and maintained by non-seismologists, so this group of professionals has just as
much a need for information on instrumentation.

Early versions of the book have been used for a lab course in seismic instrumentation
given at the University of Bergen since 1999 and also in a few other countries. The
experience from these courses has helped to improve the book, and we think that it is
suitable for students in seismology.

There have been numerous publications on instrumental topics in seismology, with very
special emphasis on particular subjects, but few -if any- general textbooks. Some
overview was made by Lee and Stewart (1981), which, on the instrumental side, mainly
dealt with micro-earthquake networks. The old Manual of Seismological Observatory
Practice (MSOP) (Wilmore, 1979) dealt with all the classical analog seismographs, but
is mostly outdated now. The New Manual of Seismological Observatory Practice
(NMSOP, Bormann (Ed.), 2002, www.gfz-potsdam.de/bib/nmsop_formular.html)) is
the most up to date book on seismic instruments and one of the authors of this book
(JH) has also participated in making NMSOP. So, why another book? The NMSOP
deals with a lot of issues in addition to seismic instruments, and we felt there would be a
need for a book which further expands on the instrumental topics, much more than was
possible within NMSOP, and put it all together in one volume. The intention with this
book is that it should be a practical tool with only the amount of theory needed to
understand the basic principles and that answers to most practical problems should be
found here.

The book particularly deals with seismic sensors, their response functions and how to
make calibration and correct for the instrument. More details of the signal processing
are the matter of Scherbaum’s book ‘Of Poles and Zeros’ (Scherbaum, 2001), published
in the same series as this one. We consider Scherbaum’s a companion book to ours, and
have taken care to have a minimum of overlap.

The book tries to make an overview of some of the current equipment on the market as
well as references to where it can be obtained. We are well aware that this can only be a
snapshot in time due to the fast changes currently taking place and by the time the book