Introduction to Controlled-Source Electromagnetic Methods: Detecting Subsurface Fluids

Description

This volume describes how controlled-source electromagnetic (CSEM) methods are used to determine the electrical conductivity and hydrocarbon content of the upper few kilometres of the Earth, on land and at sea. The authors show how the signal-to-noise ratio of the measured data may be maximised via suitable choice of acquisition and processing parameters and selection of subsequent data analysis procedures. Complete impulse responses for every electric and magnetic source and receiver configuration are derived, providing a guide to the expected response for real data. 1-D, 2-D and 3-D modelling and inversion procedures for recovery of Earth conductivity are presented, emphasising the importance of updating model parameters using complementary geophysical data and rock physics relations. Requiring no specialist prior knowledge of electromagnetic theory, and providing a step-by-step guide through the necessary mathematics, this book provides an accessible introduction for advanced students, researchers and industry practitioners in exploration geoscience and petroleum engineering. Contents Preface Notation and Conventions 1 Introduction 1.1 Ohm’s Law and Resistivity 1.2 Resistivity of Rocks 1.3 Resistivity Anisotropy 1.4 Effect of Hydrocarbons on Resistivity: Archie’s Law 1.5 Example Well Logs: P-Wave Velocity and Resistivity 1.6 Controlled-Source Electromagnetic Surveys 1.7 Seismic and Electromagnetic Propagation 1.8 One-Dimensional Example of a Buried Resistive Layer 1.9 One-dimensional Example of a Buried More-Conducting Layer 1.10 Extraction of Resistivities from CSEM Data: The Problem of Inversion 1.11 Outline of the Book 2 Sources, Receivers, Acquisition Configurations and Source Time Functions 2.1 The Current Dipole Source 2.2 Receivers 2.3 Source–Receiver Configurations 2.4 The Air Wave Problem 2.5 Sources of EM Noise 2.6 Source Time Functions 3 Fourier Analysis and Linear Filters 3.1 Temporal and Spatial Fourier Transformation 3.2 Example of a Plane Wave 3.3 Resolution and Bandwidth 3.4 Similarity Theorem 3.5 Impulse Function (δ) 3.6 The Sifting Property 3.7 Parseval’s Theorem and the Energy in a Signal 3.8 Convolution and the Convolution Theorem 3.9 Linear Filters and Impulse Response 3.10 Earth as a Linear Filter 3.11 Cross-Correlation, Autocorrelation and Time-Reverse 3.12 Derivative Theorem 3.13 Wavefield Transformation 3.14 Sampling and Aliasing 3.15 Sampling Theorem 3.16 Discrete Fourier Transform (DFT) 3.17 Filtering of Sampled Signals: Discrete Convolution 3.18 Frequency Domain Deconvolution 3.19 The Wiener Filter 3.20 Time Domain Deconvolution 3.21 Laplace Transform 4 Electromagnetic Fields in a Horizontally Layered VTI Medium 4.1 Basic Equations 4.2 The Electromagnetic Field for a Source in a VTI Whole Space 4.3 The Electromagnetic Field of a Source in a VTI Half-Space 4.4 The Electromagnetic Field for Marine CSEM 4.5 The Electromagnetic Field for Land CSEM 5 Numerical Examples 5.1 The Electric Field in a VTI Whole Space 5.2 The Electric Field in a Homogeneous Half-Space 5.3 The Electric Field in a Marine CSEM Setting 5.4 The Electric Field in a Land CSEM Setting 6 Source Control 6.1 The Convolutional Model in CSEM 6.2 Pseudo-Random Binary Sequence 6.3 Convolution and Deconvolution with a PRBS 6.4 Effect of Noise and Deconvolution Gain 6.5 Heaviside Function, or Step Function 6.6 Square Wave Function 6.7 Special Periodic Functions 7 Deep Water CSEM 7.1 Introduction 7.2 Attenuation of Electromagnetic Signals in Sea Water 7.3 Acoustic Positioning 7.4 Deep-Towed Current Dipole Source 7.5 Ocean-Bottom Receiver Node 7.6 In-line and Broadside Responses 7.7 Receiver Orientation 7.8 Acquisition Geometries 7.9 Source–Receiver Synchronisation and Data Processing 7.10 Amplitude versus Offset Example 8 Land CSEM with a Transient Source Signal 8.1 Introduction 8.2 Acquisition of 2D and 3D CSEM Data 8.3 Deconvolution and Removal of the Air Wave 8.4 Isotropic Half-Space Response 8.5 Signal-to-Noise Ratio of MTEM Data 8.6 Attenuation of Cultural Noise 8.7 CSEM Survey Over an Underground Gas Storage Site in France 8.8 Apparent Resistivities from Time to Peak of Impulse Response 8.9 Resistivities from Step Responses 9 Shallow Water CSEM with a Transient Source Signal 9.1 2D Data Acquisition with an OBC 9.2 Examples of OBC Data 9.3 Removal of Spatially Correlated Noise 9.4 Time-Lapse Marine OBC Data Repeatability 9.5 Towed-Streamer Marine CSEM Data 10 Forward and Inverse Modelling of CSEM Data 10.1 Forward Modelling 10.2 Inverse Modelling 11 Recovery of Resistivities from CSEM Data 11.1 Effect of Offset 11.2 Attenuation and Model Parameterisation 11.3 Resistivities from Seismic Velocities 11.4 Example from North Sea Harding Field 11.5 Test of Methodology Using Real CSEM Data 11.6 Implications for Electromagnetic Survey Planning 11.7 Example Inversion of Deep Water 3D CSEM Data 12 Efficient CSEM 12.1 General Considerations 12.2 Land 12.3 Shallow Water Marine 12.4 Deep Water Marine 12.5 Source Time Function 12.6 Conclusions Appendix A The Electric Field in a VTI Whole Space Appendix B The Electromagnetic Field in a VTI Layered Medium Appendix C Green’s Functions and Their Derivatives Appendix D The Final Value Theorem References Index