Controlled Source Electromagnetic Imaging in Shallow Water - A Case Study

CSEM surveys have been used successfully in a variety of settings including West Africa, Southeast Asia, The Gulf of Mexico and the North Atlantic. Early surveys concentrated on deepwater areas. However they represent only a small proportion of potential exploration targets. In particular to date the method has been limited to relatively deep water (300m or more). This is because in shallow water, signals that have interacted with the air can have a severe impact on the recorded signals and can dominate the response. This noise (known as the 'airwave') dominates the CSEM response at source-receiver offsets which are sensitive to resistivity structure at the likely range of depths of hydrocarbon reservoirs. Researchers at OHM have been working on extending the operating envelope of the CSEM method into progressively shallower water. The approach adopted centres on characterising the physics behind the airwave phenomenon, and then using this knowledge to design approaches to data acquisition and processing that mitigate its effect. As with many geophysical problems, there is unlikely to be one silver bullet that will solve the 'airwave' problem in all circumstances, and so a range of concepts were investigated.