A much used transmitter waveform for marine CSEM surveys is the square wave. The square wave has the advantage that maximum energy is transferred to the subsurface, since the transmitter current is running at it’s peak amplitude at all times. The problem with the square wave is that it has a less than ideal frequency spectrum. The frequency domain current amplitudes are proportional to the inverse of frequency, so the amplitudes are reduced with increased frequency. At the same time the absorption of the electromagnetic field increases with frequency. We propose a waveform where the transmitter operate at its peak current at all times, but where the number of switching times within a period may be larger than two, which is the number of switching times per period for a square wave. The method is based on matching desired frequency spectra with spectra obtained from generalized square waves. This is an optimization problem that is solved with a Monte Carlo method. The end results are waveforms that can be used for an electric dipole transmitter and where the frequency spectra are close to predefined desired spectra.
Marine controlled-source electromagnetics (CSEM) as suggested by Cox et al. (1971) and Young and Cox (1981) have later been followed up by contributions from Consta- ble (1990), Constable and Cox (1996), Yuan and Edwards (2000) and MacGregor et al. (2001). These were non- hydrocarbon related studies except for Yuan and Edwards who investigated marine gas hydrates. The application of the method to hydrocarbon exploration, also named Sea Bed Logging (SBL), is described by Eidesmo et al. (2002) and Ellingsrud et al. (2002). The use of the method for hydrocarbon exploration in ExxonMobil is discussed by Srnka et al. (2006). A much used transmitter waveform is the square wave. The square wave has the advantage that maximum energy is transferred to the subsurface, since the source current is running at its peak value at all times except for possible switching intervals. The problem with the square wave is that the current amplitudes are proportional to the in- verse of frequency, so the current amplitudes are reduced with increased frequency. At the same time the absorp- tion of the electromagnetic field increases with frequency. It may be desirable to partially counter act the increased loss with frequency by distributing more power to higher frequencies than is possible with the square wave. We propose a generalized square wave that combines max- imum power with a desired frequency spectrum. The ad- vantage is that more than a few frequencies with appre- ciable amplitude can be acquired in a single tow line. The maximum power is obtained by having an active source at maximum negative or positive current at all times. The method is developed with the assumption that there may be a short period of zero current in the transmitter when switching from a negative to a positive current direction or vice versa.
