Underwater controlled source electromagnetic sensing: Locating and characterizing compact seabed targets

The detection of conductive or magnetic objects of interest obscured in coastal and marine sediments has motivated the need for advanced marine geophysical technologies suited for relatively small scale search and characterization missions. Targets include both anthropogenic objects such as marine infrastructure associated with undersea cables, seabed foundations for windfarms, and unexploded ordnance and other munitions hazards as well as shallow natural and geologic objects (e.g, freshwater lens, gas hydrates, mineral ore, and heterogeneous sediment deposits). Successes achieved by large scale marine controlled source electromagnetic systems used for hydrocarbon and mineral exploration have paved the way for potential adaptation of sensing strategies and scaled array configurations to detect and characterize these shallower and smaller targets. Building on established marine electromagnetic theory and based on the use of existing electric and magnetic field sensing designs, we analyze the electromagnetic fields emitted from excited targets in the frequency range between 100 Hz and 200 kHz. We present the results of numerical modeling and experimental studies to develop potential design strategies for implementing both magnetic (B) and electric (E) field sources and receivers. Application of three-dimensional numerical simulations (via the method of auxiliary sources and finite element methods) in addition to one-dimensional analytical models (integral dipole approximations) yield optimal arrangements for a potential advanced electromagnetic sensing system. We study the electromagnetic field distributions from both electric (voltage-fed dipole) and magnetic field (encased and submerged induction coil) active sources. Controlled source experiments in laboratory and open water settings reveal the effects of signal attenuation, target scattering, and influence of the sea bottom.