Figure 1.58: Resistivity anomaly across Skrugard obtained by high-resolution 3D inversion of 2011 CSEM data (Nguyen et al., 2013). (Statoil and licence partners Eni Norge and Petoro. Data courtesy of EMGS.)
Figure 1.59: Principle of CSEM surveying. The vessel is towing an electromagnetic source such as a horizontal electrical dipole after receivers have been placed on the seafloor. The source emits a low frequency current signal that penetrates below the water bottom. Hydrocarbon-bearing layers, characterised by elevated electrical resistivity, can be detected by the receivers. Mathematical inversion is used to estimate subsurface resistivity values from the CSEM data. (Figure from S. Constable, Geophysics, 2010.)
Air (resistive) CSEM Transmitter 25-100 m
100-300 m dipole, 100-1000 amps
Electric and magnetic field recorders Oil, Gas (resistive) (variable conductivity) Seafloor
Salt, carbonate, volcanics (resistive)
Natural-source Magnetotelluric Fields
Seawater (very conductive)
Figure 1.60: Frequency spectrum of square-wave signal of fundamental frequency f =0.25 Hz. The signal has harmonics (‘overtones’ or multiples of the fundamental frequency) of various amplitudes.