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Seabed logging (SBL) gathers the electromagnetic methods of marine subsoil exploration and more specifically those dedicated to the exploration of oil and gas at sea.

Appeared in 2000, these techniques, with more than 500 industrial jobs, present after 15 years of commercial success a discovery record rate of nearly 90 % and seem now to turn the world in the offshore exploration field.

Proposing a serious index of the presence of hydrocarbons , electromagnetic SBL coupled with seismic reflection survey is probably the first reliable method for direct detection of hydrocarbons. Complementing the structural concepts of oil exploration used since the 1920s, the SBL now radically modifies the approach and the philosophies of exploration especially those then including drilling and well logging activities.

Electromagnetic Seabed Logging: a new tool for oil and gas prospecting, which original publication in French was in 2012, presents these methods, its principles, advantages, limitations, instruments, modeling and applications.

It is also designed to be a tool for a reflection on the use of electromagnetic energy for the exploration in a conductive medium as sea water thus setting the theoretical and practical limits of these investigations for future developments. <

This book is intended of course for the geophysicists and the petroleum geologists, but also for the earth scientists, the reservoir engineers and the log analysts

Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction

Abstract
After a preamble, this introduction first places seabed logging in the global economy. After a succinct reminder of petroleum geology the aspects of its research and discovery are discussed. Indirect (also called structural) prospecting is of course mentioned, thus introducing the benefits of direct prospecting today offered by electromagnetic exploration. The latter uses the electrical properties of rocks and especially the conductivity, which is the best indicator of the facies and lithology and consequently of the presence of hydrocarbons. A history of electromagnetic prospecting techniques is given for information, followed by a study of the seabed logging market and its environmental impact.
Stéphane Sainson

Chapter 2. Principles and Methods

Abstract
This chapter first recalls the various laws and equations that govern the propagation of electromagnetic energy in the more or less electrically conductive media that are seawater environments, marine sediments and hydrocarbon deposits. From these theoretical elements, principles and methods of detection of oil and gas can be proposed. They make apparent that the seabed recording of lateral and in-depth variations of the electromagnetic fields (electric and/or magnetic fields) induced by a natural or artificial source, locally modified by the distribution of the electrical conductivity of the subsoils, allows highlighting of the presence of hydrocarbons more resistive in more conductive sediments (because they are saturated with seawater). It is also apparent that the variable current and dipole–dipole type controlled source methods in different configurations (in line and broadside) are then more favorable for the detection of hydrocarbons, which is, if certain conditions are met such as a resistivity contrast, able to be measured with a good signal-to-noise ratio.
Stéphane Sainson

Chapter 3. Metrology and Environment

Abstract
This chapter describes the physical environment where measurements will be made. It then presents the electromagnetic properties of the concerned media (electrical conductivity, magnetic permeability and dielectric permittivity) depending on the characteristics of seawater and subsoil rocks (facies, lithology). It then discusses the frequency and temporal aspects of the detection method depending on the propagation media and the background noise in the deep sea (several \( \mathrm{p}\mathrm{V}/\mathrm{m}/\sqrt{\mathrm{Hz}} \)). It defines in substance the skin effect, the energy attenuation, the investigation depth, the magnitude of the amplitudes of the fields accessible to measurement (about 1 \( \upmu \mathrm{V}/\mathrm{m}/\sqrt{\mathrm{Hz}} \) or, if normalized, about 10−12 V/A.m2), the signal-to-noise ratio and the modes and propagation/diffusion conditions in the presence or absence of oil. Then it proposes data acquisition systems to establish the intrinsic characteristics of the measuring instruments and especially the power of the transmitters and the receptor sensitivity. This chapter ends with a description of optimal conditions of detection and favorable field procedures.
Stéphane Sainson

Chapter 4. Instrumentation and Equipment

Abstract
This chapter describes in detail (electrical, electronic, mechanical and signal processing characteristics) the various measuring instruments used in prospecting, i.e., the electrical current sources and field receivers. The transmitters are mobile dipoles of very low frequency able to deliver currents of several hundred amperes under the water, or even telluric sources caused by magnetic storms (solar wind) or atmospheric storms (Schumann resonance). Seabed fixed receivers are vector electrometers and magnetometers able to record the horizontal components of electric and magnetic fields in a bandwidth covering the frequencies of 0.01 to 10 Hz. The accuracies of these low noise instruments (several \( \mathrm{nV}/\sqrt{\mathrm{Hz}} \)) are respectively about one \( \upmu \mathrm{V}/\sqrt{\mathrm{Hz}}/\mathrm{m} \) and one nT. Generally the signal-to-background noise ratio is very favorable to the detection and can reach a factor of 1000. Finally, we briefly mention the operational means and procedures with the different equipment and a few elements of signal processing used during the seabed logging survey.
Stéphane Sainson

Chapter 5. Interpretations and Modeling

Abstract
This chapter describes the various methods of interpretation of electromagnetic data. After a succinct recollection of the approach to solve the forward problem determined from the Maxwell equations and Ohm’s law, several methods of resolution are proposed. These involve either analytical 1D models considering some (quasistatic) approximations or numerical models for higher-order dimensions. Analytical equation resolutions have been favored for their educational value considering geological canonic models and relatively simple integration methods. Then we recall some data inversion techniques, which allow us to directly access specific resistivity values of the subsoil and therefore to detect the presence or absence of hydrocarbons. Finally we describe the analog models that concretely allow us to establish special detection devices or check some assumptions.
Stéphane Sainson

Chapter 6. Geological Applications

Abstract
This last chapter is devoted to geological applications. It corresponds to a tour of geologically favorable cases. After a succinct recollection of the data-processing process, seven prospecting campaigns borrowed from the technical literature are exemplified. They reflect very different (historically, geographically and geologically) types of prospects. To simplify reading and comprehension, these studies are presented following the same scheme—with their geographical, geological and technical context—and then the interpretation results are presented as profiles, sections or maps. The chapter concludes with an outline of the ongoing developments and an epilogue dealing with commercial success since 2000.
Stéphane Sainson

Backmatter

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