22nd March, 2018
A Can of Worms?
“A can of worms” - Metaphorically speaking, to open a can of worms is to examine or attempt to solve some problem, only to inadvertently complicate it and create even more trouble. When I saw this image (Figure A) in Shearwater’s showrights library I thought “Where is it from? What is it showing? How did those worm shapes form?” This image puts us in a privileged position to view part of the largest submarine fan in the world (3000km long and 1000km wide) - The Bengal Deep Sea Fan.
The image is a timeslice (horizontal slice) through a volume of seismic data acquired to aid the search for hydrocarbons along trend from the large Shwe Gas Field in offshore Myanmar, which presently exports gas to China via pipeline. The survey is located offshore Myanmar (see approximate regional position shown by the red circle on Figure B). The shapes that we see are a series of channels that interfere with each other, cross-cut each other, and overlay each other.
The Bengal Deep Sea Fan consists of sands and muds that originated by erosion in the Himalayas, which are transported 1000’s of kilometres via the Ganges and Brahmaputra river systems to the coastal/delta/shelf/shallow marine part of the system. At times of low sea level, river/fluvial systems are eroded and coarse grained sediments can be rapidly transported via canyons and collapse erosion to the deeper water areas.
Figure C shows a simplified diagram depicting the features of the Himalaya-Ganges-Brahmaputra-Bengal Fan relationship.
A seismic interpreter’s job is to come up with a 3D geological model using the seismic data volume. However even using a single horizontal slice image (Figure A) can allow us to zoom in on selected areas and start to understand the processes which yielded the particular shapes we see. Figures D1 & D2 show the characteristics of cross-cutting older channels and the roll of deposition and erosion on the inner and outer regions of the meanders respectively.
These channel processes are taking place 100s of metres below sea level, driven by the force of gravity and marine currents. When studying geography over 50 years ago, I never thought that one day I would earn a living studying meander processes which occur100s of metres below sea level.
David Jackson - Principal Geologist Shearwater