This paper addresses the physics of the marine time-domain controlled source electromagnetic (TDCSEM) method. Like the marine frequency-domain controlled source electromagnetic (FDCSEM) method, the TDCSEM method is sensitive to resistive hydrocarbon reservoirs. Employing forward modeling techniques is helpful to better understand the physics of the TDCSEM method.
The traditional seismic reflection method is a principal tool to prospect marine hydrocarbon reservoirs. However, it is known well that the associated drawback of using the seismic reflection method solely lies in defining whether the potential hydrocarbon reservoir contains hydrocarbon or seawater (Ellingsrud et al, 2002). From this point of view, marine electromagnetic (EM) methods have the potential to reduce the risk of drilling dry wells because they can distinguish seawater-saturated reservoirs (low electric resistivity) and hydrocarbon-saturated reservoirs (high electric resistivity). Like the FDCSEM method, the TDCSEM method is in another favorable EM system to complement the seismic reflection method for marine hydrocarbon exploration. However, the direct comparison of the TDCSEM method to the FDCSEM method has not yet been investigated in detail. In this study, using plots of currents below the source as well as the fields that would be measured on the seafloor at different distances from the source, we explore the detailed physics of the TDCSEM method for 1-D and 3-D hydrocarbon reservoirs, and directly compare its sensing ability to that of the FDCSEM method.
In this study, three models are employed. 1) The background model consists of a 1km thick, 0.3 Ohm-m seawater layer and 0.7 Ohm-m half space seafloor. 2) A 1- D reservoir model has a 100m thick, 100 Ohm-m infinite layer embedded in the background model at a depth of 1km below the seafloor. 3) The 3-D reservoir model employed is shown in Figure 1. The TDCSEM modeling employs the 250m long, 100 Ampere,
-directed, horizontal electric dipole (HED) which is placed 950m below the air-water interface. The source produces a step-off waveform with measurements made while the current is off. The FDCSEM method utilizes the same survey configuration as the TMCSEM method employs for consistent comparison, but is energized with a 0.63Hz sinusoidal waveform. The time domain results were computed using the parallel finite-difference algorithm of Commer and Newman (2004), while a modified version of the algorithm of Newman and Alumbaugh (1995) that incorporates a 1D solution developed by Ki Ha Lee was employed to compute the frequency domain response.
The TDCSEM in-line,
-component electric fields (EX) and y-component magnetic fields (dBY/dt) are shown in Figure 2. The EX measurements sense the presence of the different reservoirs configurations, but the observed differences between the background and 1-D model are a) relatively small, and b) primarily exist in the early time DC limit rather than during the transient. These responses are verified by comparing the current distribution in the background model (Figure 3) to that in the 1-D reservoir model (Figure 4). In both models, the largest currents which dominate the response are horizontal and confined to the conductive seawater.
