A Double Gas‐Hydrate Related Bottom Simulating Reflector at the Norwegian Continental Margin

• Published in: Annals of the New York Academy of Sciences
• Main Authors: ANDREASSEN, KARIN, MIENERT, JÜRGEN, BRYN, PETTER, SINGH, SATISH C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2000
• Subjects: Norway; Methane hydrate
• Online Access: http://dx.doi.org/10.1111/j.1749-6632.2000.tb06766.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1749-6632.2000.tb06766.x; https://nyaspubs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1749-6632.2000.tb06766.x

A bstract : An unusual pattern of two bottom simulating reflections (BSRs) has been observed on seismic profiles from the continental margin offshore Western Norway. One of these reflections (BSR1) extends over large areas and has the characteristics of the classical BSR, that is a phase‐reversed reflection from the base of the gas‐hydrate stability zone. The second BSR (BSR0) occurs at approximately 70 ms two‐way travel time beneath BSR1 and is here called a double BSR. The distribution of BSR0 is more local than that of BSR1 and it does not show the phase‐reversal relative to the sea floor reflection that is characteristic for a BSR at the gas hydrate‐free gas boundary. Results from an industrial borehole, from full waveform inversion of multichannel seismic data, from high‐frequency ocean bottom hydrophones, and interpretation of seismic profiles, clearly indicate that BSR1 is reflected from the base of the methane hydrate equilibrium field. Results from full waveform inversion indicate that BSR0 corresponds to a 16–20 m zone where the velocity drops from about 1.8 km/s to a minimum of 1.4 km/s and then increases again. The low velocity of 1.4 km/s suggests the presence of free gas. The results support the hypothesis that BSR0 is a reflection from the base of gas hydrates containing hydrocarbons with a heavier molecular weight in addition to methane gas. Interference of reflections from the top and base of the low‐velocity zone associated with BSR0 explain why BSR0 is not phase‐reversed.