Comparison of pseudo‐random binary sequence and square‐wave transient controlled‐source electromagnetic data over the Peon gas discovery, Norway

Abstract

ABSTRACTWe discuss the problem of source control in controlled‐source electromagnetic (CSEM) surveying and compare and contrast equal energy transient square‐wave and transient pseudo‐random binary sequence source signatures for the same towed‐streamer electromagnetic survey line over the Peon gasfield in the Norwegian sector of the North Sea. The received response of the transient square‐wave data was 11 dB greater than that of the pseudo‐random binary sequence data, due to diffusive attenuation of higher frequencies present in the more broadband pseudo‐random binary sequence signature. Deconvolution of the pseudo‐random binary sequence data recovers the total impulse response function, increases the signal‐to‐noise ratio by 32.6 dB and separates most of the air wave from the earth impulse response by the causality principle. The recovered impulse responses have more detailed information in the frequency domain than the transient square‐wave data. The pseudo‐random binary sequence data were acquired with a 10 Hz source bit rate but contain no information about the Peon gasfield at frequencies above 2 Hz. The bit rate could have been reduced to 4 Hz, increasing the signal energy below 2 Hz by 150% and thus, potentially, increasing the signal‐to‐noise ratio by a further 4 dB.Because the total earth impulse response can be recovered from the broad‐bandwidth pseudo‐random binary sequence data, further time‐domain processing may be applied, including correlated noise removal, which can increase the signal‐to‐noise ratio by as much as 20 dB, and air wave removal using the causality principle. The information in the arrival time of the peak of the earth response provides the potential for traveltime to resistivity mapping to provide a starting model for inversion.