Funnel-Shaped Seismic Anomalies: Characterization and Emplacement Processes
Patrice Imbert1, Sutieng Ho1, and Patrick Le Quellec2
1Total, Pau, France
2Total, Paris, France
V-shaped seismic reflectors have been seen on seismic for several decades, but were interpreted as “erosional features” until 3-D seismic became available. The advent of 3-D revealed that a number of these, especially in the North Sea, were actually closed features, i.e. funnels rather than valleys. As the quality of survey increased, a distinction could be made between erosional funnels, which initially developed as craters on the seafloor, and conical intrusions. The distinction is based on the comparison between the inner and outer part of the funnel: in the former case, the infill bears no resemblance to the surrounding sediments, while conical intrusions just push up a keystone of sediment, which can be perfectly correlated with the outside of the anomaly.
A third category was recently observed on seismic surveys from a passive continental margin: funnel-shaped anomalies are filled with sediments that can be correlated with what is deposited around the anomaly, but these sediments have collapsed into the funnel after being deposited, implying that some material removal took place below sedimentary cover. The model proposed involved the emplacement of pingo-like massive hydrate structures followed by their dissociation, leaving only a weld at the end of the process. The volumes involved are quite high, on the order of a few km3. Multiple episodes of funnel-shaped anomalies are stacked, suggesting a rhythmic succession of hydrate growth and dissociation over several My.
It is therefore crucial to correctly correlate the inner and outer sides of conical seismic anomalies in order to be able to discriminate between the three possible mechanisms mentioned above. All three are related with fluid expulsion, but they have quite different meanings in terms of fluid nature and process.
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