Gas Escape Structures Revealed by High Resolution Geophysics, Offshore Egypt

Dupré, Stéphanie¹, John Woodside¹, Ingo Klaucke² (1) Vrije Universiteit, Amsterdam, Netherlands (2) IFM Geomar, Kiel, Germany

The Nile deep sea fan system presents a rich variety of fluid escape structures, gas chim­neys, pockmarks and several types of mud volcanoes. These seep related structures were explored for the first time using the Nautile submersible during the Nautinil expedition in 2003 and are characterized by high thermal gradients and highly gas-saturated sediments. More recently, high resolution side scan sonar data acquired during the Mimes expedition (2004) brings more detail to the geophysical imagery. The EdgeTech DTS-1 deep tow sonar was deployed around 100 m above the seafloor and operated at a frequency of 75 kHz. In addition to the sidescan sonar sensors, the DTS-1 contains a 2-16 kHz chirp subbottom pen­etrator. Several gas plumes were detected in the water column above Isis and Amon mud volcanoes and the Centre Nile pockmarks. These observations confirm the intensity of the present-day activity offshore Egypt in terms of seepage associated with gas emissions and its continuity through time. High-backscatter patches related to these seeps have been observed in all of the explored sites. Structures associated with mud flows, such as concen­tric ridges and gullies, and subsurface carbonate crust structures were identified on numer­ous lines. High variability in the backscattering observed at most of the sites greatly assists post-processing interpretation, e.g. seafloor geological mapping. The high-resolution DTS­1 data provide the possibility to identify and map fluid emission areas associated with dif­ferent types of lithology, mud flows, carbonate crust and gas plumes when the backscatter signal is calibrated with in-situ observations.