Shallow water inversion
Where the water depth is less than about 2 times the depth from seabed to target resistor, the signal in the offset range which should be sensitive to the target resistor will be dominated by energy that is sensitive to the air. This effect is colloquially known as the “airwave effect” and has previously rendered the CSEM technique ineffective in such circumstances.
OHM’s proprietary shallow water inversions take full account of the resistive structure below the source and receivers, as well as the resistive structure above them. By accurately accommodating all modes of signal in the CSEM solution, the inversion is able to correctly assign the CSEM signal to its various causes and accordingly to provide a true image of the subsurface.
“The airwave” is a complex interaction of responses from the subsurface, the air, and the water column. Simple approaches like trying to separate “upgoing” and “downgoing” energy do not accurately describe this interaction and will not lead to accurate solutions of the subsurface.
The shallow water inversion module performs a complex calculation of the source and receiver diffuse fields when the water is shallow. The algorithms accurately predict every mode of signal and lead to a true forward prediction for the CSEM response generated by the subsurface in these conditions; this is then projected into the 2.5D inversion engine enabling the inversion to solve for the correct subsurface structure.
Careful survey design, especially in the frequency domain, is important for effective shallow water surveys and exceptional care should be taken at this stage.