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Phase-reversed seabed reflections in seismic data: examples related to mud volcanoes from the South Caspian Sea

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Abstract

The sediment–water interface is usually marked by an increase in acoustic impedance and is therefore displayed in a seismic section as a positive polarity reflection. Here, we use the term “seabed phase reversal” to describe areas of seafloor which are instead expressed as a negative polarity reflection in seismic data. We describe in detail a number of examples of seafloor phase reversals and use a simple one-dimensional geophysical model to test the hypothesis that they are the result of the presence of gas within the seafloor sediment. Our examples are all related to seismically imaged mud volcanoes located within the South Caspian Sea. Sections of phase-reversed seafloor at the summit area of these volcanoes have been mapped to reveal the existence of seafloor mud pools (salses) and recently erupted mud flows which show a strong similarity to smaller-scale features at onshore volcanoes in Azerbaijan. Synthetic geophysical modelling indicates that under the physical conditions likely to occur when the seabed sediment is gas-bearing, the seafloor will be expressed as a strong negative polarity reflection. Unlike other indicators of seafloor gas, such as pockmarks, which merely record the transient expulsion of fluids from sedimentary basins, seafloor phase reversals indicate the presence of gas in marine sediment at the time of survey acquisition. They therefore are of significance to engineering and site survey operations as well as the identification of biological communities and gas flux calculations.

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Acknowledgements

The authors would like to acknowledge the National Environment Research Council (NERC) for provision of award number NER/S/C/2004/12724 which supports this research. Also acknowledged are BP Azerbaijan for provision of seismic and well data, and Schlumberger Geoquest for provision of IESX and GeoViz interpretation software. We are grateful to Alan Judd and another anonymous reviewer for their helpful comments. Andrew Hill, Eric Deville and Mads Huuse are thanked for help with data provision and technical discussion.

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Authors and Affiliations

  1. 3DLab, School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building, Park Place, Cardiff, CF10 3YE, UK

    Robert J. Evans

  2. BP Azerbaijan, Chertsey Road, Sunbury on Thames, Middlesex, TW16 7LN, UK

    Simon A. Stewart

  3. CeREES (Centre for Research into Earth Energy Systems), Department of Earth Sciences, University of Durham, Science Labs, Durham, DH1 3LE, UK

    Richard J. Davies

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  1. Robert J. Evans
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  2. Simon A. Stewart
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  3. Richard J. Davies
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Correspondence to Robert J. Evans.

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Evans, R.J., Stewart, S.A. & Davies, R.J. Phase-reversed seabed reflections in seismic data: examples related to mud volcanoes from the South Caspian Sea. Geo-Mar Lett 27, 203–212 (2007). https://doi.org/10.1007/s00367-007-0073-3

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  • DOI: https://doi.org/10.1007/s00367-007-0073-3

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