Marine controlled source electromagnetic method used for the gas hydrate investigation in the offshore area of SW Taiwan

Constraints on a shallow offshore gas environment determined by a multidisciplinary geophysical approach: The Malin Sea, NW Ireland

“…This is clearly observed in line C (Figure ), as the instrument enters the basin both measurements of conductivity rise until the instruments entered a pockmark. Sediments charged with gas should generally show a decrease in their conductivity, as has been seen elsewhere [ Hsu et al ., ]. The results shown in Figure can be interpreted as a (1) change in sediment facies, with coarser sands outside the basin and finer, muddy sediments in the interior [ Monteys et al ., ], (2) in certain locations, the presence of microbial activity that is known to increase the conductivity of sediments [ Atekwana et al ., ], and (3) caused by the water influx as gas has been released refilling sediment pores [ Sultan et al ., ], which will cause an increase of conductivity as the resistive phase (gas) is removed from sediments.…”

“…There are very few EM surveys of areas affected by pockmarks [e.g., Müller et al ., ; Minshull et al ., ; Hsu et al ., ], but some of the conclusions drawn from these works are that shallow EM profiling can significantly improve understanding of sediment distribution and vertical zonation of seepage processes. It has been postulated that the walls of pockmarks act as pathways for gas flow [ Newman et al ., ] and that sediments are disturbed within the pockmark as a consequence of the violent gas release mechanism [ Judd and Hovland , ].…”

“…Within the basin several areas are characterized by conductive anomalies associated with fluid flow processes and potentially the presence of microbial activity, as suggested by previous work. Pockmark structures show a characteristic electrical signature, with high-conductivity anomalies on the edges and less conductive, homogeneous interiors with several high-conductivity anomalies, potentially associated with gas-driven microbial activity.There are very few EM surveys of areas affected by pockmarks [e.g., M€ uller et al, 2011;Minshull et al, 2012;Hsu et al, 2014], but some of the conclusions drawn from these works are that shallow EM profiling can significantly improve understanding of sediment distribution and vertical zonation of seepage processes.…”

“…There are very few EM surveys of areas affected by pockmarks [e.g., M€ uller et al, 2011;Minshull et al, 2012;Hsu et al, 2014], but some of the conclusions drawn from these works are that shallow EM profiling can significantly improve understanding of sediment distribution and vertical zonation of seepage processes.…”

Two types of marine controlled source electromagnetic transmitters

“…, 2011; Schwalenberg and Engels , ; Hsu et al . ). The most extensive applications of MCSEM to date are the pre‐drill appraisal of seismically identified direct hydrocarbon indicators to avoid drilling dry holes associated with structures characterized by strong seismic reflections.…”