ABSTRACT

To image the internal resistivity structure of the North Alex mud volcano offshore Egypt, the marine electromagnetics group at the Helmholtz Centre for Ocean Research Kiel (GEOMAR) developed and conducted a novel transient marine controlled-source electromagnetic experiment. The system, which was specifically developed to image the mud volcano, is also generally suitable for surveys of other small seafloor targets, such as gas-hydrate reservoirs, fluid-flow features, and submarine massive-sulfide deposits. An electric bipole antenna is set down by a remotely operated vehicle on the seafloor sequentially in two perpendicular polarizations at each transmission station. Two orthogonal horizontal electric field components are recorded on the seabed by an array of independently deployed nodal receivers (RXs). With two transmitter polarizations, the unique acquisition geometry of the system provides a very rich data set. However, for this geometric setup, conventional marine electromagnetic interpretation schemes (such as normalized magnitude variation with offset plots) have been difficult to implement. We have developed a simple imaging technique, which can be used for a first-step mapping of seafloor apparent resistivity with the GEOMAR system. Images can be produced in just a few minutes on a regular laptop computer, and the robustness of the approach was demonstrated using two synthetic data sets from simple seafloor models. The method was then applied to the real data acquired at the North Alex mud volcano in 2008. Results found increased apparent sediment resistivities of up to 4 Ωm near the center of the mud volcano occurring at source-RX offsets greater than 500 m, which mapped to apparent depths of greater than 150 m. This may be caused by large quantities of free gas or freshwater in the sediment pore space.

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