Ch, . Ilmenau University of Technology Department of Electrical Engineering and Information Technology

M-sequence based ultra-wideband radar and its application to crack detection

in salt mines

Dipl.-lng. Ralf Herrmann

Dissertation zur Erlangung des akademischen Grades

Doktor-Ingenieur (Dr.-Ing.)

vorgelegt der Fakultät für Eletrotechnik und Informationstechnik der

Technischen Universität Ilmenau von Dipl.-lng. Ralf Herrmann, geb.

am 19.09.1978 in Schmalkalden

Gutachter:

Vorgelegt am:

Verteidigt am:

URN:

1.

2.

3.

Prof. Dr.-Ing. habil. Reiner S. Thomä

Prof. Dr.-Ing. Motoyuki Sato

Prof. Dr.-Ing. Reinhard Knöchel

07.07.2011

21.10.2011

urn:nbn:de:gbv:ilm1 -2011000344

Contents

Acknowledgement III

List of Figures VIII

List of Tables XI

List of Abbreviations and Symbols XII

Abstract XVII

Abstrakt XIX

1 Introduction to non-destructive testing 1

1.1 Non-destructive testing — an emerging research field with many applications . . 2 1.2 A challenging task — inspection of the disaggregation zone in salt mines . . . . 4 1.3 Properties of the disaggregation zone in salt rock and NDT sensors 6 1.4 Current radar sensors in salt mines and GPR 8

2 Theoretical analysis and design goals for a new sensor system 11 2.1 Propagation conditions for electromagnetic waves within salt rock 11 2.2 Bandwidth and frequency 14

2.2.1 Scattering of thin gaps or layers 15 2.2.2 Scattering of small volume defects 19

2.3 Mode of operation — electromagnetic wave interaction with salt rock 21

2.3.1 Electro-magnetic surface waves 22 2.3.2 Radar sensor using antennas 23 2.3.3 Antenna polarisation 25

2.4 Dynamic range requirements 27 2.4.1 Dynamic range for UWB time domain signals 27

2.4.2 Setup for a typical measurement situation 29 2.4.3 Simplified model for dynamic range simulation 30 2.4.4 Results of dynamic range simulation for thin gaps 32

2.5 Summary of design goals for the new UWB sensor 37

3 U W B system design for detection of sub-mm disaggregation in salt 39 3.1 Overview of available UWB electromagnetic sensor technologies 39

3.1.1 CW/FMCW principle 41

3.1.2 Vector network analysers 43 3.1.3 Impulse radar 44

R. Herrmann

V I Contents

3.1.4 Noise and pseudo-noise principles 46 3.2 The starting point — review of the M-Sequence UWB sensor principle 49

3.2.1 RF chipset of basic M-Sequence devices 50 3.2.2 Digital backend of basic M-Sequence devices 54

3.3 New 12 GHz bandwidth M-Sequence UWB sensor 55

3.3.1 Extended M-Sequence stimulus 56 3.3.2 Software-defined dense equivalent time sampling with four receivers . . . 60 3.3.3 Frontend for measuring full two-port S-parameters and calibration . . . . 63

3.4 Summary of system design for a 12 GHz bandwidth M-Sequence sensor 65

4 Uniform dense equivalent t ime sampling 67

4.1 General aspects of sampling for UWB sensors 69

4.2 Analysis of accuracy requirements for uniform dense sampling 71

4.2.1 Periodic non-uniform sampling with sine signals 72

4.2.2 Periodic non-uniform sampling with an UWB stimulus 76

4.3 Calibration of phase states for uniform dense sampling 81

4.3.1 Phase shifter control hardware and timing 81

4.3.2 Phase shifter control calibration 84 4.4 Summary of phase shifter control calibration for uniform dense equivalent time

sampling 92

5 Device clutter removal by full 2-port calibration 95

5.1 Recapitulation of basic coaxial calibration methods 96 5.1.1 Example: 3-term calibration 98

5.2 Full two-port calibration of M-Sequence UWB-sensors with 8-term method . . . 101 5.2.1 Choice of calibration method 101

5.2.2 Formulation and implementation of 8-term correction 105 5.2.3 Performance figures of 8-term correction 110

5.3 Accuracy analysis of calibrated MLBS system 116 5.3.1 Calibration repeatability and long-term effects 116 5.3.2 Comparison of calibration performance 123

5.4 Summary of coaxial calibration for the new M-Sequence system 129

6 Selected salt mine measurement results 133

6.1 Measurement setup in salt mines 133

6.1.1 Antenna type and arrangement 135 6.2 Data processing for detection of disaggregation zone 139

6.2.1 Removal of antenna crosstalk 140 6.2.2 Suppression of surface reflection 142

6.2.3 Visualisation of disaggregation zone 146 6.2.4 Typical data processing flow for disaggregation in salt rock 149

6.3 Measurement results for an old cylindrical tunnel 150 6.3.1 Measurement setup for Bernburg 151 6.3.2 Selected scans in 2D visualisation 153

6.3.3 3D visualisation of a tunnel section 154

6.4 Measurement results for a newly cut tunnel in Borth 157

R. Herrmann

Contents VI I

6.4.1 Subsidence analysis of measurement results 158

6.4.2 Development of the disaggregation zone in a new tunnel stub 160 6.5 Comparison of UWB and ultrasonic sensor results 164

6.5.1 Properties of ultrasonic prototype sensor for inspection of the disaggrega­tion zone 164

6.5.2 Ultrasonic and radar results for a reference profile in Bernburg 166 6.6 Summary of real-world measurements in salt mines 167

7 Summary and outlook 171

7.1 Summary of UWB sensor development for salt rock inspection 171 7.2 Summary of salt rock disaggregation measurement results 175 7.3 Contributions to the state of the art 177 7.4 Aspects of further technical development 179 7.5 Outlook for UWB sensor application in salt mines 180

7.6 A near-future vision: a new family of UWB sensors for your favourite application 182

A Algorithms for calibration of equivalent t ime uniform sampling 185 A . l Definition of cost function 186 A.2 Nelder-Mead based optimum search 189 A.3 Successive approximation search 190

A.3.1 Implementation and parameters of approximation search 191 A.3.2 Reduction of number of test signal measurements 191

8 Summary of full two-port coaxial calibration for S-parameters 193 B.l Calculation of measured S-parameters 195 B.2 Two-port calibration using all 16 error terms 197 B.3 Reduction of the 16-term error model to 8 terms 200

Bibliography 203

Publications with own contributions 214

Theses 217

Thesen 219

Erklärung / Statement of Origin 221

R. Herrmann