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Practical Magnetotellurics
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 (ISBN-13: 9780511079191)



Contents

Prefacepage xi
Symbolsxii
1Introduction1
1.1Magnetotellurics as a passive electromagnetic exploration method and its relation to active electromagnetic and geoelectric methods1
1.2Problems for which EM studies are useful: a first overview of conduction mechanisms5
1.3An historical perspective10
1.4MTnet14
1.5Books that cover other aspects of EM14
2Basic theoretical concepts15
2.1Assumptions of the MT method16
2.2Time invariance as a consequence of the plane wave assumption16
2.3Why EM speaks volumes: MT as a vector field method18
2.4The concepts of transfer function and penetration depth21
2.5Induction in a layered half-space: the concept of apparent resistivity and phase23
2.6Induction at a discontinuity: a simple two-dimensional (2-D) world and the concept of E- and B-polarisation28
2.7Adjustment length31
2.8Induction arrows32
2.9The impedance tensor and a preview of three-dimensionality34
3Planning a field campaign37
3.1Target depths and choosing the right sensors and equipment37
3.2Target area and spatial aliasing47
3.3Arrays versus profiles: further considerations of dimensionality48
3.4Resolving power and the duration of a campaign49
3.5Sources of noise external to the Earth and a preview of processing schemes50
3.6Economic considerations53
3.7Suggested checklist of field items54
3.8A step-by-step guide to installing an MT station54
4From time series to transfer functions: data processing58
4.1Fourier transformation, calibration and the spectral matrix59
4.2Least square, remote reference and robust estimation of transfer functions65
4.3‘Upwards’ and ‘downwards’ biased estimates71
4.4Displaying the data and other deceptions74
4.5Through data interpolation to pseudosections76
4.6Pseudosections versus Cartesian graphs78
5Dimensionality and distortion79
5.1The discontinuity revisited: the concept of static shift80
5.2Rotating the impedance tensor81
5.3A parade of general models and their misfit measures83
5.4Problems with decoupling E- and B-polarisations: farewell to the 2-D world98
5.5Decomposition as an hypothesis test: avoiding the black-box syndrome101
5.6How many strike directions are there?102
5.7The concepts of anisotropy, skew bumps, and strike decoupling106
5.8Static shift, the long and the short of it: ‘correcting’ or modelling?111
5.9Current channelling and the concept of magnetic distortion115
6Numerical forward modelling117
6.1Why forward modelling, and which scheme?117
6.2Practical numerical modelling: gridding rules, boundary conditions, and misfits revisited119
6.3From computed electric and magnetic fields to transfer functions125
6.4Avoiding common mistakes126
7Inversion of MT data130
7.1Forward, Monte Carlo and inverse modelling schemes131
7.2D+ optimisation modelling versus least-structure philosophy, or a treatise on the difference between the mathematical and the physical viewpoint133
7.3Artefacts of inversion137
7.4Introducing a priori information142
7.5Forward modelling versus inversion142
7.6Avoiding common mistakes143
8The general link to other geosciences: conduction mechanisms146
8.1Laboratory measurements under in-situ conditions147
8.2How field measurements and laboratory measurements can or cannot be combined152
8.3Multi-phase systems and mixing laws: interdependence of mixing ratios and connectivity155
8.4Bulk conductivity versus direction-dependent conductivity159
8.5Scaling, random resistor networks and extreme heterogeneity161
9The special link to other geosciences165
9.1EM and seismics166
9.2EM and seismology167
9.3EM and geodynamics170
9.4The oceanic mantle: ocean-bottom and island studies172
9.5Oceanographic applications176
9.6Industrial applications and environmental studies177
10Other EM induction techniques181
10.1Magnetometer arrays and magnetovariational studies182
10.2Equivalence relations revisited in spherical co-ordinates: introduction to induction by the daily magnetic variation and the Z / H technique183
10.3The horizontal gradient technique and inhomogeneous sources at high- and low-latitudes188
10.4Vertical gradient techniques190
10.5Active induction techniques191
Appendix 1Theorems from vector calculus193
Appendix 2The transfer function in the wavenumber-frequency domain and equivalence transfer functions195
Appendix 3Probability distributions199
The normal (Gaussian) distribution199
Samples200
The chi-suqared (χ2)distribution funtion201
The Fisher F-distribution201
Appendix 4Linear regression203
Bivariate regression205
Appendix 5Fourier analysis208
Appendix 6Power and cross spectra212
Glossary214
References229
index246

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