Time-Domain vs. Frequency-Domain CSEM: Implications for Marine Exploration

The frequency-domain marine controlled source electromagnetic (CSEM) method is now routinely applied to map resistive hydrocarbons buried beneath the seabed in deepwater. Alternatively, it has been suggested that time-domain CSEM methods may offer improved resolution of difficult targets such as deeply buried reservoirs. Furthermore, time-domain methods may overcome a sensitivity limitation imposed by the airwave saturation that is experienced for shallow-water frequency-domain CSEM. In order to examine and test these claims, we have developed a modeling code for computing time-domain responses for layered 1D models with arbitrarily located and oriented transmitters and receivers. Our code extends the open-source frequency domain code Dipole1D by efficiently computing the time-domain step-on and impulse responses by Fourier transformation of the frequency-domain kernels. By applying a realistic noise model to synthetic data generated from this code, we systematically examine the sensitivity and resolution of time-domain and frequency-domain CSEM to representative targets of interest for offshore hydrocarbon exploration and exploration surveys of seafloor volcanic and hydrothermal systems. These studies have practical implications for marine EM survey systems that use either towed or stationary transmitters and receivers.