Marine Electromagnetic Sounding Research Articles

Mathematical Modeling of Mobile Marine Electromagnetic Soundings

The article models a mobile method for marine electromagnetic sounding in which the field source and the field detector are both towed above the sea bottom with the objective of discovering hydrocarbon deposits under the sea bottom. The problem of determining the electromagnetic field in a nonhomogeneous medium is solved by the integral equation method. We show that the highest efficiency is achieved in a symmetrical setup with simultaneous field excitation by a horizontal electrical dipole and a vertical magnetic dipole.

A mobile time-domain sounding system for shallow water

We discuss a new method of marine electromagnetic sounding in shallow water using a horizontal electric dipole as a source of current and a vertical electric dipole as a receiver. The measurements are performed in the time domain at short transmitter-receiver offsets. The locality, sensitivity, resolution and detection depth of the proposed method were studied using a number of simple three-dimensional model examples. A new approach to determining the electrical characteristics of a hydrocarbon reservoir based on seismic ‘skeleton’ fixing the boundaries of the geological formations was demonstrated on the Luva Discovery electrical model. The new method enables surveying in depths of water in the range of 25 to 100 m using a towed source and a vertical four-electrode gradient receiver.

Preliminary Results of Marine Electromagnetic Sounding with a Powerful, Remote Source in Kola Bay off the Barents Sea

We present an experiment conducted in Kola Bay off the Barents Sea in which new, six-component electromagnetic seafloor receivers were tested. Signals from a powerful, remote super-long wave (SLW) transmitter at several frequencies on the order of tens Hz were recorded at the six sites along a profile across Kola Bay. In spite of the fact that, for technical reasons, not all the components were successfully recorded at every site, the quality of the experimental data was quite satisfactory. The experiment resulted in the successful simulation of an electromagnetic field by the integral equation method. An initial geoelectric model reflecting the main features of the regional geology produced field values that differed greatly from the experimental ones. However, step-by-step modification of the original model considerably improved the fit of the fields. Thereby specific features of the regional geology, in particular the fault tectonics, were able to be corrected. These preliminary results open the possibility of inverse problem solving with more reliable geological conclusions.

Bimodal quasi-one-dimensional method for the inverse problem of marine electromagnetic sounding

A quasi-one-dimensional method is applied to solve two-dimensional inverse problems with a plane wave source and observations for two electromagnetic field polarizations. The structure of the medium is represented by a model commonly used in marine sounding, with the electromagnetic field observed at the lower boundary of a conducting top layer modeling the sea.

Mathematical modeling of marine electromagnetic sounding of a three-dimensional nonhomogeneous medium

The article considers a method for three-dimensional mathematical modeling of the effect of a threedimensional nonhomogeneity on the electromagnetic field in marine sounding. The electromagnetic field source is a moving electric cable towed by a vessel over the surface of the sea. The field is measured at the seabed using popup sensors. The field component most responsive to the nonhomogeneity is identified. The problem is solved by the integral equation method.

海底電磁探査の近年の進歩とメタンハイドレート検出への適用

Marine electromagnetic (EM) sounding methods were developed originally for imaging the deep mantle, partial melt below mid-oceanic ridges, active faults in the crust around subduction zones, etc. Recently, marine EM sounding has been applied extensively for oil, gas, and methane hydrate exploration. In this paper, we introduce various marine EM sounding methods and case studies. Natural fluctuations of EM fields from the ionosphere can be observed on the seafloor using ocean-bottom electromagnetometers (OBEMs), and the marine magnetotelluric sounding method can be applied for imaging the deep structure (> several km below the seafloor). However, the natural signal is less powerful in the higher frequency band and provides less resolution for near-seafloor structures. A high-frequency controlled electromagnetic source on/near/below the seafloor is necessary for evaluating the shallow structure. Marine controlled-source EM (CSEM) sounding using a horizontal electric dipole, towed near the seafloor and sending an electric signal, and OBEMs as receivers settled on the seafloor is widely used for oil and gas exploration, and occasionally for methane hydrate explorations. A vertical electric dipole is also used for magnetometric resistivity (MMR) sounding when exploring hydrothermal systems around ridges. On the other hand, both sources and receivers attached to a cable can be towed near the seafloor for a continuous survey. Towed-CSEM sounding with a time-domain analysis is used especially for methane hydrate exploration. Another type of towed-CSEM with magnetic signal obtains porosity information on near-seafloor sediments. A deep-towed marine DC resistivity survey has been applied near the seafloor, and successfully imaged shallow methane hydrate distributions in sedimentary layers. Thus, various marine EM soundings can illuminate sub-seafloor methane hydrate, and are useful for discussing the accumulation/dissolution process of methane hydrate and the potential of methane hydrate as a new energy resource.