A seismic survey is a method of investigating the subterranean structure, primarily used in search of oil and gas deposits. Marine seismic surveys have been performed since the 1950s when chemical explosives were used to create sound waves. In the 1960s airguns were developed and are now currently used for almost all seismic surveys. Airguns are less destructive than explosives to the environment, and are commonly used to initiate the seismic waves (a high-level, low-frequency sound), which have largely replaced the use of explosives. Airguns work by suddenly releasing compressed gas (generally air) and the sudden rush of gas across the outlet ports creates an acoustic signal. Typically, the acoustic signal is primarily aimed downward but unfortunately, as sound travels so well in seawater, a significant amount of the energy travels outward and can be heard for many hundreds of kilometres. In order to find oil and gas deposits in the offshore ocean floor, an array of airguns is typically used. Generally, a small ship will tow the airguns in an array, which are fired every few seconds. Behind the airguns, hydrophones are attached to long cables to pick up the signal reflected off the sea floor. Based on the return time of the reflected or refracted impulses to the hydrophones, the subterranean structure can be mapped.
But why is this relevant? There is a company that wants to do a seismic survey in the Pletmos Basin, off the Southern Cape. The area of interest is 2500km2 in size, and ranges between Plettenberg Bay and St Francis. The target area is situated 12km from shore and up to 60km out to sea. Now before they can begin the survey itself, there is a process they need to go through. For now, the initial scoping report is closed for public comments, but a full environmental impact assessment will follow. A number of parties have expressed their concern and have opposed the seismic survey of the Pletmos Basin.
This is the start of a series of blogs which will aim to look into seismic surveys in a marine environment.
Written by: Minke Witteveen
For further reading:
- Jasny, M. 2010. Boom, Baby, Boom: The Environmental Impacts of Seismic Surveys. Accessed: 2017-01-14. URL: https://www.nrdc.org/resources/boom-baby-boom-environmental-impacts-seismic-surveys
- Reuther, S., du Toit, J. 2016. 3D Seismic Survey in the Pletmos Basin, Southern Cape. Scoping Report 510482. Prepared by: SRK Consulting. For: Sungu Sungu Oil (Pty) Ltd.
- Richardson, W.J., Greene, Jr. C.R., Malme, C.I., Thomson, D.H. 1995. Marine Mammals and Noise. Pp137. California: Academic Press.