Ocean Electro
177
Part A | 8.1
8. Ocean Electromagnetics
John J. Holmes
Even though acoustic waves can travel long distances in the sea with little attenuation, ocean
electromagnetics has important applications in
the areas of geophysical surveys and searches of
the seafloor and sub-bottom, communications
across the sea–air boundary, and high data transfer rate at short ranges. Unlike in-air propagation
of electromagnetic fields, the finite conductivity of seawater results in a frequency-dependent
phase velocity, attenuation, intrinsic impedance,
and reflection and transmission coefficients at
the ocean’s surface. After giving a short summary of the electric and magnetic properties of
the ocean, this chapter begins with Maxwell’s
equations and develops the mathematical descriptions of electromagnetic fields and dipole
sources within a conducting media. The differences
between plane wave reflection and transmission
at the surface of fresh water and seawater are
used to highlight how electromagnetic propagation within the electrically conducting ocean is
so very different than the more familiar radio frequency transmissions in air. In addition, equations
are presented that describe the fields from sub8.1 Electromagnetism
in an Ocean Environment ...................... 177
8.2 Electromagnetic Field Theory................. 178
8.3 Plane Wave Propagation. ....................... 180
8.4 Reflection and Transmission
of a Plane Wave at the Surface
of Fresh Water ...................................... 182
8.5 Plane Wave Incident on Seawater .......... 184
8.6 Magnetic and Electric Dipoles
in an Unbounded Ocean ........................ 186
8.7 Magnetic and Electric Dipoles
in a Bounded Ocean .............................. 188
8.8 Electromagnetic Propagation
in the Ocean at Optical Wavelengths ...... 193
References................................................... 195
merged electric and magnetic dipoles that are located both far and near the sea surface. These
formulations are valid over the frequency range
from 0 Hz to a few MHz. Finally, a brief discussion
of ocean electromagnetics at optical wavelengths
is given at the end of this chapter.
Even though acoustic waves can travel long distances
in the sea with little attenuation, ocean electromagnetics has several important applications. For example;
electromagnetic surveys, searches, and communication
systems can operate effectively in acoustically noisy environments, they can cross the sea–air boundary and be
received by high speed airborne sensors, and they have
much higher data transfer rates at short ranges. In addition, electromagnetic surveys of seafloor gas hydrate
pockets can determine their methane content, a shortfall
of seismic techniques. Moreover, the short range limitations of underwater electromagnetic field propagation
are actually beneficial when pinpointing submerged targets during naval operations.
8.1 Electromagnetism in an Ocean Environment
The oldest known application of electromagnetism in
an ocean environment is the magnetic compass used
for navigation. Today, electromagnetic sources and sensors are useful tools found in many diverse areas of
oceanographic study such as physical oceanography,
seafloor geophysics, marine chemistry, and biology.
Advances in the understanding of electromagnetism
within the boundaries of these scientific disciplines
have found application to engineering systems for energy generation, oil and gas exploitation and recovery,
commercial fishing, weather prediction, Tsunami warning, archeology, and the military. This chapter will
177
Part A | 8.1
8. Ocean Electromagnetics
John J. Holmes
Even though acoustic waves can travel long distances in the sea with little attenuation, ocean
electromagnetics has important applications in
the areas of geophysical surveys and searches of
the seafloor and sub-bottom, communications
across the sea–air boundary, and high data transfer rate at short ranges. Unlike in-air propagation
of electromagnetic fields, the finite conductivity of seawater results in a frequency-dependent
phase velocity, attenuation, intrinsic impedance,
and reflection and transmission coefficients at
the ocean’s surface. After giving a short summary of the electric and magnetic properties of
the ocean, this chapter begins with Maxwell’s
equations and develops the mathematical descriptions of electromagnetic fields and dipole
sources within a conducting media. The differences
between plane wave reflection and transmission
at the surface of fresh water and seawater are
used to highlight how electromagnetic propagation within the electrically conducting ocean is
so very different than the more familiar radio frequency transmissions in air. In addition, equations
are presented that describe the fields from sub8.1 Electromagnetism
in an Ocean Environment ...................... 177
8.2 Electromagnetic Field Theory................. 178
8.3 Plane Wave Propagation. ....................... 180
8.4 Reflection and Transmission
of a Plane Wave at the Surface
of Fresh Water ...................................... 182
8.5 Plane Wave Incident on Seawater .......... 184
8.6 Magnetic and Electric Dipoles
in an Unbounded Ocean ........................ 186
8.7 Magnetic and Electric Dipoles
in a Bounded Ocean .............................. 188
8.8 Electromagnetic Propagation
in the Ocean at Optical Wavelengths ...... 193
References................................................... 195
merged electric and magnetic dipoles that are located both far and near the sea surface. These
formulations are valid over the frequency range
from 0 Hz to a few MHz. Finally, a brief discussion
of ocean electromagnetics at optical wavelengths
is given at the end of this chapter.
Even though acoustic waves can travel long distances
in the sea with little attenuation, ocean electromagnetics has several important applications. For example;
electromagnetic surveys, searches, and communication
systems can operate effectively in acoustically noisy environments, they can cross the sea–air boundary and be
received by high speed airborne sensors, and they have
much higher data transfer rates at short ranges. In addition, electromagnetic surveys of seafloor gas hydrate
pockets can determine their methane content, a shortfall
of seismic techniques. Moreover, the short range limitations of underwater electromagnetic field propagation
are actually beneficial when pinpointing submerged targets during naval operations.
8.1 Electromagnetism in an Ocean Environment
The oldest known application of electromagnetism in
an ocean environment is the magnetic compass used
for navigation. Today, electromagnetic sources and sensors are useful tools found in many diverse areas of
oceanographic study such as physical oceanography,
seafloor geophysics, marine chemistry, and biology.
Advances in the understanding of electromagnetism
within the boundaries of these scientific disciplines
have found application to engineering systems for energy generation, oil and gas exploitation and recovery,
commercial fishing, weather prediction, Tsunami warning, archeology, and the military. This chapter will